S r^S^-

SFUND RECORDS CTR

88015738

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGION IX

75 Hawthorne Street San Francisco, CA 94105

April 24, 2002

SFUND RECORDS CTR

0222-01200

MEMORANDUM FOR DISTRIBUTION

FROM: Kathleen Salyer Remedial Project Manager

TO: Distribution List

SUBJECT: Transmittal of the Addendum to the Five-Year Review Report for the Hassayampa Landfdl Superfund Site, April 2002

Transmitted by copy of this memorandum please find the final Addendum to the Five-Year Review Reporl for Hassayampa Landfill Superfund Site, dated April 22, 2002. If you have questions please call me at (415) 972-3267.

Attachment

Distribution List

Nancy Lou Minkler ARIZONA DEPARTMENT OF ENVIRONMENTAL QUALITY 3033 North Central Avenue Phoenix, AZ 85012

Mason Bolitho ARIZONA DEPARTMENT OF WATER RESOURCES 500 North 3rd Street Phoenix, AZ 85004

Keith Bowers HONEYWELL INTERNATIONAL INC., M/S 101-117 402 S. 36'" Street Phoenix, AZ 85034

James G. Derouin, Esq. STEPTOE & JOHNSON LLP Two Renaissance Square, Collier Center 201 East Washington Street, Suite 1600 Phoenix, AZ 85004-2382

Ash Madhok MARICOPA COUNTY DEPARTMENT OF SOLID WASTE MANAGEMENT 2801 West Durango Street Phoenix, AZ 85009

Carl Meier, Esq. HONEYVv'-ELL INC. N'rN56 7171 Ohms Lane Edina, MN 55439

William R.Victor, P.G. ERROL L. MONTGOMERY & ASSOC, INC. 7949 E. Acoma Drive, Suite 100 Scottsdale, AZ 85260

Glenn Hickman HICKMAN EGG RANCH 7403 N. 91" Ave. Glendale, AZ 85305

Doug Wolfe :^MILLER BROOKS.EN\aRONMENTAL=:=:=-_^=::-_..—^ ^ ^ ^ ^ , ^ , ^ _ , , _ __

202 East Earll Dr., Suite 470 Phoenix, AZ 85012

ADDENDUM TO THE FIVE-YEAR REVIEW REPORT

FOR HASSAYAMPA LANDFILL

SUPERFUND SITE MARICOPA COUNTY, ARIZONA

April 2002

Approved by: Date:

Jan^ Diamond Acting Superfund Division Director U.Sy EPA, Region 9

This addendum to the Five-Year Review Report for the Hassayampa Landfill Superfund Site in Maricopa Count}!, Arizona, dated September 27, 2001, documents the U.S. Environmental Protection Agency (EPA) determination that the soil vapor extracfion and treatment portion of the vadose zone remedy is protective. A protectiveness statement for the vadose zone remedy could not be made in the Five-Year Review Report due to concems raised by the Arizona Department of Environmental Quality (ADEQ) on the protectiveness ofthe established soil vapor performance standards for the vadose zone remedy. EPA has examined ADEQ's concems and the existing site data and concluded that the soil vapor performance standards are protective of groundwater, and therefore the soil vapor extraction and treatment system part ofthe vadose zone remedy is protecfive.

The August 1992 Record of Decision (ROD) for the Site required implementation of actions that included two components: remediation of impacted groundwater and remediation of soils and soil vapor above the water table, known as the vadose zone.

The groundwater remedy included extraction of contaminated groundwater, treatment ofthe water using air stripping technology, reinjection ofthe treated water, and continued groundwater monitoring to measure the effectiveness ofthe remedy. The vadose zone remedy included placement of a RCRA cap over the hazardous waste area ofthe landfill, as well as soil vapor extraction, vapor treatment and implementation of access and deed restrictions. The Five-Year Review Report indicated that the groundwater remedy, the deed restrictions and the soil cap portion ofthe vadose zone remedy have remained protective of human health and the environment but deferred making a determination on the soil vapor performance standards until completing a more thorough evaluation.

The selected remedy required that soil vapor cleanup standards be at levels that were protective ofthe groundwater, whereby migration of contaminants fi-om the vadose zone into groundwater did not result in groundwater contaminafion exceeding groundwater cleanup standards. These soil vapor cleanup standards that were set, based on site-specific analytical modeling, have undergone further evaluation since the completion ofthe Five-Year Review Report to determine if they remain protective ofthe groundwater.

ADEQ submitted their concems with the SESOIL analytical modeling used to determine the soil vapor performance standards in a letter dated November 21, 2001 (Attachment 1). The Hassayampa Steering Committee submitted their responses to ADEQ concems on January 11, 2002 (Attachment 2) and provided additional explanation regarding the purpose for and conduct of analytical flow modeling (Febmary 27, 2002). Based on the information provided in these correspondences and our own review ofthe Site data, EPA has concluded that the vadose zone remedy and the associated soil vapor performance standards developed using the SESOIL analytical model are protective of groundwater.

With respect to the groundwater, ADEQ has raised concems that the soil vapor performance standards are not protective based on contaminant concentrations increasing in extraction well EW-4UA. According to the information provided by the HSC, the recently higher contaminant concentrations correspond with the predicted time-fi ame for.higher contaminant concentrations to reach those wells. After careful consideration ofthe data, EPA has concluded that the groundwater monitoring data are generally consistent with location ofthe extraction well with respect to the source area and estimated capture zone ofthe extraction wells. In the event that groundwater contaminant levels continue to increase without diminishing, EPA will reevaluate the protecfiveness of the soil performance standards.

ADEQ also raised concerns that the SESOIL model underestimates the vertical conductivity of the vadose zone, particularly the basalt layer, thereby underestimating the rate at which contaminants migrate fi^om the soil to the groundwater. Based on EPA's evaluation ofthe information provided by the HSC and an evaluation ofthe SESOIL model, EPA has concluded that the input parameters and assumptions used are appropriate. However, if groundwater trends suggest that a significant source remains, EPA will reevaluate the standards.

The HSC has completed four rounds of soil vapor sampling ofthe capped area ofthe vadose zone to demonstrate compliance with the soil vapor performance standards. The HSC provided a detailed evaluation ofthe first three ofthese soil vapor sampling rounds on September 27, 2001 and an evaluation ofthe data for the fourth soil vapor sampling round in Xht Annual Monitoring Report No. 7 for 2001, dated April 1, 2002. All of this data indicates that the soil vapor performance standards have been met in the capped area ofthe vadose zone. However, the soil vapor performance standards have not been met for two contaminants in the uncapped area ofthe vadose zone. Although the contaminant concentrations are low relafive to concentrations beneath the capped area, without the cap these concentrations are predicted by modeling to impact groundwater in excess ofthe groundwater performance standards. Restarting the Soil Vapor Treatment System (SVTS) solely to address this small uncapped area would be problematic and may actually worsen conditions by pulling higher concentrafions of contaminants fi-om the capped area into the uncapped area than may have migrated by passive mechanisms. A passive soil venting pilot test has been initiated and it appears to be a potentially feasible altemative for meeting the performance standards in the uncapped area. EPA is currently evaluating modification to the vadose zone remedy to allow the use of passive venting or low flow soil vapor extraction to complete the remediation ofthe vadose zone.

Protectiveness Statement

"The soirVapof extractionliiSlreatrneiif portion "of the^^ """ Landfill Superfund Site is protective of human health and the environment. The protectiveness ofthe other remedies at the Hassayampa Landfill are documented in the Five-Year Review Report.

This Site requires ongoing five-year reviews to ensure that protectiveness is not compromised. The next review is due September 2006, within five years ofthe completion ofthe Five-Year Review Report.

ATTACHMENT 1

ADEQ COMMENTS

ARIZONA DEPARTMENT OF

ENVIRONMENTAL QUALITY 3033 North Central Avenue • Phoenix, Arizona 85012-2809

(602) 207-2300 • www.adeq.state.az.us

Jacqueline E. Sciiaier Direclor

November 21, 2001 FPU02.062 E-4110.16.11.2

Ms. Kathleen Salyer (SFD-7-1) Remedial Project Manager United States Environmental Protection Agency 75 Hawthome Street San Francisco, Califomia 94105-3901

RE: Hassayampa Landfill EPA Superfund Site, Maricopa County, Arizona

Dear Ms. Salyer;

This letter is in response to your request that the Arizona Department of Environmental Quality (ADEQ) submit in writing the reasons for their dissatisfaction with the on-going attempt to pemianently shut down the Soil Vapor Treatment System at the Hassayampa Landfill site, and the use of SESOIL model in making a final determination on perfomianee standards and the protectiveness of groundwater at the site. ADEQ has, once again, reviewed the responses prepared by Montgomery & Associates on behalf of the Hassayampa Steering Committee (HSC). ADEQ does not agree with some ofthe assumptions and conclusions presented in the report, many of which are rather ambiguous or ambivalent. ADEQ reiterates its earlier positions on the use of SESOIL, as communicated to you in May 2000, July 2000 and September 2001.

The SESOIL model agreed upon in 1996 is no longer adequate for reaching a major compliance decision under the current circumstances. Assumptions about the site have changed since the agreement, and subsequent revelations about the site have demonstrated that the SESOIL model is no longer appropriate. One of our assumptions was that the soil vapor treatment system (SVTS) will operate continuously over a reasonable period of ume until asym.ptoiic levels a;e attained, and SESOIL could then be applied to evaluate risk from residual contamination. To the contrary, the "anaemic" operation ofthe SVTS lasted for only two years, and the system had been shut down since 1998, and "decommissioning" has commenced, in violation ofthe Record

r:orDeeision-that^requiresTthe:5VTS--extract<x)ntaniinated-soil-^vapor in the vadose zone:'=="-~^"'

ADEQ disagrees with the conclusion of HSC in their report Responses to Supplemental Issues, Hassayampa Landfdl, EPA Superfund Site, July 25, 2000, that "groundwater recharge may be the most important vapor transport process for VOCs in the vadose zone", while admitting in the same paragraph that "the magnitude of groundwater recharge at the site is ver>' small, and would result in negligible transport of VOCs in the vadose zone". Currently, contaminated soil vapors are migrating in the vadose zone, and ADEQ contends that density-driven vapor transport (rather than recharge) may be dominant under prevailing conditions at the site. The SESOIL model can

Norllicrn Rogion.TJ Office 1 r>l :'i n.isl Ceci.ir Avenue • Suite F • Fl.igsl.iff, AZ «6f)()4

(320) 77'j ().n3

Southern Region.i! Office 4(10 West Coiv^ress Street • Suite 433 • Tueson, AZ f.S/OI

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Ms. Kathleen Salyer November 21, 2001 Page 2 of 2

not simulate gravity-induced migration that we believe is critical to the site evaluation.

The suggestion by HSC that the Hassayampa site is so complex that only SESOIL can best simulate site conditions lacks credibility. This site is not in any way unique in terms of stratigraphy or hydrology other than the presence of a basalt flow in the vadose zone. Very little data have been presented on soil vapor concentrations within and beneath the basalt, and its interaction with the capillary fiinge. The basalt layer is discontinuous, fractured and may be ver>' porous, as indicated by the easy migration of liquid and vapor-phase contaminants into groundwater despite the basalt 'barrier".

ADEQ is not convinced that the residual contamination in the vadose zone is protective of groundwater quality, and contends that a closure ofthe vadose zone cleanup is premature. Active remediation should resume at the site. Currently, the residual soil vapor concentrations exceed the minimum groundwater protection levels (GPLs). Under the current circumstances, any site-specific perfomianee standards or GPLS established using the SESOIL will be erroneous and misleading. ADEQ agrees that the SESOIL is a robust analytic model that could potentially be used for establishing screening levels, if used appropriately, or if the appropriate parameters were incorporated into this model. Should we make a major compliance decision using a model surrounded by so much doubts about its appropriateness? ADEQ recommends to the EPA that other vadose zone models, including VLEACH, be used to cross-check the SESOIL results. The VLEACH is the most widely used and accepted model in Arizona to evaluate potential groundwater impacts, and its flexibility can accommodate the so-called "complexity" at the Hassayampa Landfill site. ADEQ recommends that the HSC apply the VLEACH model using the data available from this site for comparison, to be reviewed either by ADEQ or the EPA Center for Subsurface Modeling Support (CSMoS) at Ada, Oklahoma.

Please find attached, comments from the ADEQ Remedial Investigation Hydrology Unit regarding this matter. If you have any questions or comments regarding this correspondence, please contact Lou Minkler at (602) 207-4187.

Jaiicy Lou Mbtkjer Project Man^gd^ federal Projects Unit Waste Programs Division • ^ £ 3 '

cc: James G. Derouin, Esq., Steptoe and Johnson William R. Victor, Errol L. Montgomeiy & Associates, Inc. Hugh Rieck, Remedial Investigations Hydrology Unit Project and Reading File

COMMENTS BY THE ADEQ REMEDIAL INVESTIGATION HYDROLOGY UNIT

In consideration of laboratory analytical results from ongoing groundwater monitoring at the Hassayampa Landfill NPL site, the Remedial Investigation Hydrology Unit (RIHU) has reconsidered the continued reliance on previously detemiined soil vapor performance standards designed to protect the quality of groundwater beneath the landfill. Those standards were derived by use ofthe SESOIL vadose zone modeling software. The standards have been stated by the site environmental contractor to be conser\'ative and highly protective of groundwater, and to have become even more so after placement of a low permeability membrane cap over the contaminant source. However, the recently accelerating deterioration of water quality beneath the site despite the standards reportedly having been met, suggest that the standards are inappropriate to meet the groundwater protect! veness objectives set forth in the Record of Decision, or that the compliance vapor concentration sampling data are inaccurate. The situation may result from 1) intrinsic limitationsof the model to accurately portray complex physical relationships of the site, 2) erroneous input values or assumptions necessary to attempt the model simulation, or 3) soil vapor monitoring measurements for detemiination of vapor concentrations not being collected from near the base ofthe vadose zone just above the capillary fringe. The model-derived standards should pertain only to vapor concentrations at the ver>' bottom ofthe vadose zone.

Comments:

1. Vapor phase VOC concentrations in the vadose zone can be assumed to be proportional to the total mass of VOC contaminants present in all phases at equilibrium. Specific vapor concentrations (perfomianee standards) can be derived from conceptual models to ser\'e as a proxy indicator of risk ofthe movement of VOC contaminants from the vadose zone into groundwater. The intent of establishing vapor performance standards using the SESOIL model at the Hassayampa Landfill was to provide & provisional, predictive, and inunediately measurable, remediation goal in the absence of more direct knowledge ofthe potential for groundwater contamination beneath the site. Which particular vadose zone model was chosen to predict protective soil vapor concentrations is not as important as the outcome of the predictions. As with any predictive attempt, new data acquired over fime either confimi

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2. To be acceptable, model results must closely mimic real-world conditions and be consistent with all independently derived data. This is especially the case when specific parameters are used to define critical perfonnance standards. Any reliable model must accommodate all pertinent measured conditions. Model results, and standards derived from any model should be reviewed and revised as necessary as new data from ongoing monitoring become available.

3. The purpose of the recently completed five-year review of remedial efforts al the Hassayampa Landfill was to evaluate, in the light of data unavailable in the past, the interim

results of those efforts. Tliis includes assessing the accuracy of predicted effects and the degree of success achieved by the various actions implemented to protect the environment (including groundwater) and human health. Implicit in the fact that periodic reviews are an accepted part ofthe remedial process is acknowledgment that predicted results may not occur. The five-year review also provides an opportunity to recognize and correct deficiencies in achieving the original objectives.

4. ADEQ's comments on issues of protectiveness in the 5-year review at the Hassayampa Landfill are with regard to long term threat to the environment, primarily to groundwater, and not to any immediate threat to human health. Substantive comments by ADEQ voice concem that remedial actions at the site are proving inadequate in containing hazardous substances to the vadose zone and are not protecting the environment, especially the groundwater beneath the site (see attached graph). Groundwater degradation continues at an increasi:ig rate and current actions (groundwater contairirr.ent only) are inadequate. Foreseeable, long temi, increased demands on the underlying B unit aquifer by development in the area will increase the potential for downward migration and lateral spread of contamination.

5. Of particular concem to ADEQ is the lack of operation ofthe Soil Vapor Treatment System (SVTS) system, and the view supported by EPA's responses that it can be pennanently decommissioned because soil vapor perfomianee standards are said to have met. The rate of VOC migration into unit A groundwater has increased dramatically since operation ofthe SVTS was discontinued in 1998. Although soil vapor standards detemiined from the SESOIL modeling (in 1994, 1996, 2000, 2001) may have been achieved (above the basalt flow, see below), they are clearly not protective ofthe groundwater, as was intended in the ROD.

6. The soil vapor performance standards were, according to modeling results, thought to be protective of groundwater ofthe Unit A aquifer. The standards were provisionally accepted as indicating that VOC contamination would not migrate in significant quantity from the vadose zone to groundwater. The SESOIL model, using the input parameter values that were assumed (i.e. calibration^, fails to accurately describe conditions and processes at the site. Groundwater VOC data strongly suggest that the m.odeling is fl.awed (see attached table ). One likely cause ofthe modeling failure is that it doesn't take density-driven vapor transport into account. Continuing and accelerating VOC contamination of groundwater under conditions predicted by the model to be p^tp^cfivo^st^rongly^suggesJJJiatjtli^n:iodeU nol calibrated correctly.

7. SESOIL cannot simulate density-driven vapor transport of VOCs, although the process is likely to be operating and important at the Hassayampa site. SESOIL assumes that soil gas concentrations within a model layer are evenly distributed and stable. No evidence of mobile NAPL VOCs (free product) has been observed at the site and it is assumed not to be present. Aside from not being able to model density driven vapor transport, which should be considered at the Hassayampa site, the SESOIL model itself is otherwise adequately robust and well demonstrated when correctly calibrated. An attempt to derive soil vapor perfomianee standards as a proxy interim benchmark for protect! veness of groundwater using

SESOIL was not unreasonable. Bearing the aforementioned points in mind, SESOIL should be able to better approximate vadose conditions at the site than the present perfomianee standards indicate.

8. SESOIL considers vapor concentration in contact with the water table surface (capillary fringe) to assess mass movement into solution (Henry's Law /water-air partition coefficient). However, at the site, no vapor concentration data are available for this interface: the water table lies about fifteen feet below the base of a 20 to 30 ft-thick basalt flow. The nearest measured vapor concentrations are from above the basalt. Meaningful soil vapor concentrations have not been, and cannot presently be measured for comparison with the performance standards, or for feedback to recalibrate the SESOIL model.

9. The value used for intrinsic permeability ofthe basalt flow, critical to correct calibration of the SESOIL model, v-.-as detemiinca by laboratory tesfing of drill core samples. The measured pemieability of these samples is extremely low and the measurements may be accurate. However, the scale of the pemieability measurements and the scale of the site processes are ver>' different. The measured permeability cannot reasonably be assumed to represent the large scale (site-scale) permeability of the basalt flow. Basalt flows are notorious for large scale fractures and joints. Columnar joints are characteristic and usually extend completely through flow units. For example, a surface reser\'oir constmcted atop a similar, 20 ft-thick basalt flow could not be expected to hold water well. One ofthe core samples contains a small scale (2 mm-wide) fracture which appears to be almost completely filled with clayey calcite, but this sniall fracture should not betaken as representative of the large scale jointing characteristic ofthe rock type. Also, the basalt flow is thought to extend less than 100 feet beyond the site boundary. The basalt layer cannot reasonably be regarded as a completely impemieable, laterally infinite, proteclivelayer, as it was in development of the current perfomianee standards.

10. The water table beneath the site is declining, due to pumping by the groundwater remediation system. This facilitates the transfer of contaminants to groundwater. A declining (or fluctuating) water table enhances transfer of dissolved contaminants by increasing solution and subsequent infiltration of VOCs from the draining capillar>' ftnnge. The SESOIL modeling does not acco'jnt for a fluctuating water table. This additional variable can be regarded as a component of infiltration and could be estimated by manually adjusting the infiltration parameter used by SESOIL to account for this mechanism of contaminant transfer.

11. Pumping records from the groundwater remediation system (GRS) show that about 25 million gallons of water have been extracted and treated from the unit A aquifer since pumping began in 1994. This volume of water is roughly equal to the total volume of pore water contained in the unit A aquifer beneath the area ofthe hazardous waste area ofthe landfill that the GRS is designed to contain (approximately 10 acres). Thus, the groundwater ofthe unit A aquifer presumably has been completely replaced by clean water from outside the area of concem over the last seven years. Despite this continual replacement of water beneath the site, the extent and concentrations of VOCs in groundwater pumped by the GRS has increased during this time period. The rate of VOC migration from

the contaminated vadose zone into groundwater of unit A is continuing to accelerate, 21 years after contaminants were placed at the site.

12. VOCs are, by far, the most mobile and difficult to contain ofthe contaminants present at the site. Of particular tlireat to groundwater are the chlorinated solvents whose vapors are denser than air and which have ver}' low MCLs. Such VOCs are abundant at the site. In significant concentration, denser-than-air vapors, (and unrecognized DNAPL, if present) will migrate downward and accumulate at the base of the vadose zone or above a tmly impemieable layer. Over time, concentrations will be highest just above the water table or impemieable layer. SESOIL does not consider density-driven migration of such compounds, or density-stratification within a model layer.

13. The fact that regulatory agencies and responsible parties agreed to use ofthe SESOIL model in 1995 and its revisions in 2030 does not dictate confinuing adherence to earlier, model-derived perfonnance standards. Use of any model, especially for determination of a single perfomianee standard variable, is appropriate only when the model results accurately reflect real-worid conditions. Since the soil vapor performance standards derived using the SESOIL model have been shown by the unit A groundwater data not to be protective, the model is not accurately representing actual conditions ofthe site.

Conclusions and Recommendations:

1. RIHU recommends that operation ofthe SVTS resume immediately. Clearly, without the SVTS, contamination sources in the vadose zone are not being contained, hitent ofthe SVTS, as set forth in the ROD, is to contain and remove contamination in the vadose zone to reduce the threat to groundwater.

2. O & M problems with the SVTS should be addressed so that the system will perform as intended. The SVTS apparently was effective at removing vapor phase VOCs, and is stated to have removed nearly half of the total VOCs in the vadose zone in just the 25 months that it did operate (see estimates presented in EPA's response to comment number 33). The large amount of VOCs removed by the SVTS that short time period is two orders of magnitude greater than tlie amount removed by the GRS over seven years of operation. The SVTS is far more cost-effective, on a per-pound basis, at removing VOC mass than the GRS. All data indicate that operation ofthe SVTS is beneficial in retarding the movement of VOCs into the unit A aquifer.

3. Concems about the possible creation of dioxins from incomplete combustion of VOCs during thermal oxidation treatment should be investigated, and not used without verification as justification to cease operation ofthe SVTS. Other treatment for extracted vapors, such as GAC adsorption, may be required.

4. Efficient operation ofthe existing vapor extraction wells may obviate modifications necessary for the proposed passive soil venting system just beyond the north edge ofthe impemieable membrane cap. Methane gas being generated in the adjacent municipal landfill is tending to displace VOC vapors northward from beneath the impemieable

membrane cap. Passive venting has been proposed to meet the existing, flawed vapor perfomianee standards. When revised, these standards are likely to be much more stringent, and the contemplated passive venting system alone would not achieve the required concentration values.

5. VOC vapor concentrations used for demonstration of compliance with any revised vapor perfomianee standards must be collected from beneath the basalt flow and just above the water table. Vapor concentrations at the air/water interface are what matter in VOC migration to groundwater - not concentrations at points well above the interface and above the "impermeable" basalt layer.

6. The soil vapor performance standards determined from SESOIL, and met without operation ofthe SVTS, are faulty. If such performance standards are to continue to be used, they must undergo major revision using as yet uncollected data on vapor concentrations just above the water table - beneath the basalt flow.

7. Input parameters used for the SESOIL model should be reexamined and adjusted to recalibrate the model so that results compare favorably with actual site conditions. Another, more simplistic vadose zone leaching model, VLEACH, also should be mn and considered in light of the VOC concentration values in groundwater and as yet unobtained vapor concentrations from beneath the basalt. This will assess, by an independent model, the threat posed to groundwater beneath the site by residual VOCs.

11/26/01

700

HJR

Total VOC Concentration In GRS Influent

Hassayampa Landfill

f r j t ^ h - . r ^ f ^ f ^ t ^ C O C O O O C O O O C O O C D a i C D O C D O O O O O O T - ' -

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TABLE 4. SUMMARY OF PROJECTED PEAK CONCENTRATIONS JN GROUNDWATER FOR CONTifXMINANTS OF POTENTIAL CONCERN (COPC) RESULTING FROM MAXIMUM COPC CONCENTRATIONS 0 E T ; E C T E D IN

SOIL VAPOR SAMPLES OBTAINED IN FEBRUARY AND APRIL 2000 HASSAYAMPA LANDFILL EPA SUPERFUND SITE

MAXIMUM CONCENTRATION DETECTED IN SOIL VAPOR SAMPLES

(February and April 2000)

POLYGON

CONTAMINANT OF POTENTIAL

CONCERN*

COARSE­GRAINED

ZONE

.(W/L)° FINE-

GRAINED ZONE

SPECIAL PITS POLYGON

PIT 1 CAPPED POLYGON

DCM DMK

1,1-DCE TCE

. P £ £ .

DCM DMK

1,1-DCE 1.1.1-TCA 1.2-DCP

TCE PCE

96 1,100

680 1.400

540.

2.200 510

2,700 32,000

450 610

2,100

290 59

790 1.300

250

16.000 6,100

19,000 180.000

1.400 4.100 2,300

DCM ~ Dichloromethane (Methyleno chloride) DMK = DImethytketone (Acetone) 1,1-DCE = 1.1-Dlchloroalhene TCE = TricWoroolhone PCE = Telrachkiroethene 1,1,1-TCA = 1.1,1-Trichloroelhane 12-DCP = 1,2-Dlchloropropane

" pii/L = micrograms per liter

* Peak COPC concentrations In groundwater are based on model projections thai simulate Uie effects of an FML cap over Ihe modeled area,

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EKROL L. MONTGOMERY A ASSOCIATES, WC.

ATTACHMENT 2

HSC RESPONSE TO ADEQ COMMENTS

E R R O L L. M O N T C O M t . R V & A.SSOC.IAl ES, INC:. CONSULTANTS IN HYDROGEOLOGY

/ M 4 9 [ A S I -i.COMA D R K t . SL'I'-r 1(10

scorrsn-vLt. ARI^ONA ss^bti .^noi <:)AS-'.'A7

FAX: ( 4 8 0 1 S J S - S r i ?

w w w . e l m o n i gornei v . c o m

E-MAIL, i n to ' f emdphv ! .oi i i

ERROLL MONT GCMERr-. P.G. WILLIAM R VICTOR. P G. RONALD H DEWITT, P G.

MARK M. CROSS. P.G. DENNIS G. HiLL. P.G.

TODD icEAY. P G. JAMES S DAVIS. P.G.

MICHAEL J ROSKO. D.G. CHARLES F. BARTER I : 937 . l 99g )

DANIEL S. WE3EK. P.G LESLIE T. P<;AT^. r c.

January 11, 2002

Kathleen Salyer (SFD-8-2) Remedial Project Manager U.S. ENVIRONMENTAL PROTECTION AGENCY 75 Hawthorne Street San Francisco, California 94105

RE: RESPONSES TO ADEQ COMMENTS DATED NOVEMBER 21 , 2001 HASSAYAMPA LANDFILL EPA SUPERFUND SITE

Dear Ms. Salyer:

On behalf of the Hassayampa Steering Committee (HSC), Errol L. Montgom­ery & Associates, Inc. has prepared the enclosed responses to the November 21, 2001 comments by Arizona Department of Environmental Quality (ADEQ) on soil vapor issues at the Hassayampa Landfill EPA Superfund Site (the "Site"). As you will see in our responses, we believe ADEQ's comments are without technical merit, are not based on objective scientific evidence, and are misleading with regard to in­terpretation of Site conditions, data obtained from monitoring at the Site, purpose and results of SESOIL modeling, and content and intent ofthe Record of Decision, Consent Decree, and other regulatory documents that guide the determined remedy for the Site.

ADEQ's comments are based on a selective interpretation of a very small subset of the Site data and guided by a belief that contaminant mass should be re­moved from the vadose zone without regard to actual or projected impacts to groundwater, or to the regulatory and project framework that led to the establish-

"ment"of-"soiLvapor-i3erformanee"Standards---=ADE-Qis=approachHs=distwbing=in-tlnat=it= has disregarded an extensive accumulation of sound scientific work for the Site, which has been repeatedly presented, explained, and justified to ADEQ, and until recently, has generally been approved by ADEQ. HSC acknowledges that monitor­ing of volatile organic compounds in soil vapor and groundwater must continue at the Site to verify long-term compliance with Performance Standards and to maintain the effectiveness of the groundwater remedy. We urge EPA to accept the soil vapor Performance Standards as protective for the Site and provide HSC the necessary

TUCSON • PHOENIX • FLAGSTAFF • SANTIAGO de CHILE

ERROL L. MONTGOMERY & ASSOCIATES. INC.

approval to proceed with a passive venting system for the remaining area of concern north from the capped area.

If you have any questions or require further discussion prior to our meeting scheduled for February in Phoenix, please contact me.

Very truly yours,

ERROL L. MONTGOMERY & ASSOCIATES, INC.

CO. R. iPuZ,^ /h&H

William R. Victor Project Coordinator

Enclosure (2 copies of responses)

CC" REGULATORY AGENCIES (sent via U.S Priority Mail w/enclosures) Mason Bolitho, Arizona Department of Water Resources (1) Nancy Lou Minkler, Arizona Department of Environmental Quality (2) Kelly Ryan, U.S. Army Corps of Engineers (1)

HASSAYAMPA STEERING COMMITTEE (sent via U.S. Mail w/enclosures) James G. Derouin (1) Carl Meier (1) Keith Bowers (1) Jeffrey J. Meyer (1) Ash Madhok (1) Stephen M Quigley (2)

HASSAYAMPA STEERING COMMITTEE (sent via U.S. Mail w/o enclosures, enclosures available on request) Kenneth R. Arnold Frank C. Brown Tom Cooper Joseph T. Chikowski Barbara Curtis Scott Davis David England

Roger K. Ferland, Esq. Frank R. Fossati Marty Gahn Sanford W. Harvey. Esq. T.W Kearns, Esq. Chris Keller, Esq Ms. Lee Manning

Todd Rallison Peter J McGrath, Jr, Esq. William R. Moore Stephen B. Paige, Esq. David L. Wallis, Esq.

SENT VIA FEDERAL EXPRESS

665\50\HSC.Response.ADEQ Jan2002.translet doc/11 Jan2002

ERROL L. MONTGOMERY & ASSOCIATES. INC.

January 11, 2002

HSC RESPONSES TO NOVEMBER 21, 2001 ADEQ COMMENTS

ON VADOSE ZONE REMEDY AT HASSAYAMPA LANDFILL EPA SUPERFUND SITE

On behalf of the Hassayampa Steering Committee (HSC), Errol L. Montgom­ery & Associates, Inc. (M&A) has prepared the following responses to the Arizona Department of Environmental Quality (ADEQ) comments, dated November 21, 2001, on the vadose zone remedy at the Hassayampa Landfill EPA Superi'und Site (the "Site"). The responses are identified with the corresponding ADEQ comment.

ADEQ COVER LETTER

Paragraph 1, Sentences 3 and 4: "ADEQ (Joes not agree with some of the as­sumptions and conclusions presented in the report, many of which are rather ambiguous or ambivalent. ADEQ reiterates its. earlier position on the use of SESOIL, as communicated to you in May 2000, July 2000 and September 2001.

Response: All assumptions were provided to the U.S. Environmental Protec­tion Agency (EPA) and ADEQ for comment prior to modeling. ADEQ has not been specific about which assumptions it deems ambiguous or ambivalent. ADEQ's position on specific aspects of the modeling was not clear in its previ­ous communications, and the November 21, 2001 ADEQ document does not provide clarification.

=Paraqraph'2:"-""7' e"SE50/L~ir770c:/e7 agreed VponWl996'is'no I'on'ger'adequaTe foF reaching a major compliance decision under the current circumstances. As­sumptions about the site have changed since the agreement, and subsequent revelations about the site have demonstrated that the SESOIL model is no longer appropriate. One of our assumptions was that the soil vapor treatment system (SVTS) will operate continuously over a reasonable period of time until asymptotic levels are attained, and SESOIL could then be applied to evaluate hsk from residual contamination. To the contrary, the "anaemic" operation of the SVTS lasted for only two years, and the system had been shut down since

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1998, and "decommissioning" has commenced, n violation of the Record of Decision that requires the SVTS extract contaminated soil vapor in the vadose zone."

Response: As is documented in several of the following responses, nothing has occurred since the initial SESOIL modeling in 1994 to render the project vadose zone model inadequate. Except for the presence of the cap over the Former Hazardous Waste Disposal Area (FHWDA), none of the substantive assumptions about the Site have changed and there have clearly been no sub­sequent "revelations" that demonstrate inappropriateness of the model for use at this Site. Further, neither the agreements reached about SESOIL modeling nor the Record of Decision (ROD) included any assumptions about asymptotic trends in concentrations of Compounds of Potential Concern (COPCs) in soil vapor at the Site. However, it was agreed that SESOIL would be used to es­tablish quantitative soil vapor Performance Standards for the COPCs, and HSC moved forward in good faith with that understanding.

It has been estimated that a large fraction of the VOCs present in 1993 have been removed and treated. VOC concentrations at soil vapor wells were documented to have decreased substantially after the Soil Venting and Treat­ment System (SVTS) operation.

There has been no "violation of the Record of Decision". In fact, the ROD is not the document that controls remediation at the Site. The Consent Decree controls remediation activities. Under the Consent Decree, EPA approved the partial decommissioning of the SVTS. EPA initiated the partial decommission­ing after it determined that concerns about potential emissions of dioxins needed to be resolved and that the SVTS would be shut down for an extended period. Partial decommissioning consisted of removal and storage of several parts that would deteriorate in the harsh desert environment. In addition, the above-ground pipelines were disconnected from the soil vapor extraction wells to facilitate sampling and a short section of the pipeline was removed and set aside to provide better vehicular access to the soil vapor well field. These ac­tions are reversible and do not constitute any violation of the ROD. During the period of partial decommissioning, investigations were_conducted to determine

'if a passive venting system'would be appropriate to'address the small non-capped area in the northeast part of the Site where soil vapor Performance Standards have not been met. These investigations indicate that passive venting for this area is feasible for achieving the soil vapor Performance Stan­dards for the Site, which are described in the ROD.

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Paragraph 3: "ADEQ disagrees with the conclusion of HSC In their report Re­sponses to Supplemental Issues, Hassayampa Landfill, EPA Superfund Site, July 25, 2000, that "groundwater recharge may be the most important vapor transport process for VOCs in the vadose zone", while admitting in the same paragraph that "the magnitude of groundwater recharge at the site is very small, and would result in negligible transport of VOCs in the vadose zone. Currently, contaminated soil vapors are migrating in the vadose zone, and ADEQ contends that density-driven vapor transport (rather than recharge) may be dominant under prevailing conditions at the site. The SESOIL model can not simulate gravity-induced migration that we believe is critical to the site evaluation."

Response: It is precisely the fact that recharge is likely to be the most impor­tant process at this time for transport of VOCs into groundwater and recharge is small that the potential threat to groundwater from residual VOCs in soil vapor is projected to meet the soil vapor Performance Standards in the capped areas of the Site. This fact is not a "revelation", but is simply a consequence of re­medial activities (including construction of a cap), natural subsurface proc­esses, and climate at the Site. As is shown in the following responses, ADEQ has not provided adequate technical support for its contention that density-driven vapor transport may be dominant in the vadose zone at the Site. By contrast, in the Responses to Supplemental Issues, HSC provided technical support, based on Site data and empirical information given in published arti­cles provided by ADEQ, that density-driven vapor flow is not a transport proc­ess of concern at the Site. ADEQ's contention ignores several important lines of evidence, which are described below. ADEQ also ignores the fact that, like SESOIL, the modeling alternative it promotes (VLEACH) can not simulate den­sity-driven vapor phase transport, nor can many vadose zone models.

Paragraph 4: "The suggestion by HSC that the Hassayampa site Is so complex that only SESOIL can best simulate site conditions lacks credibility. This site is not in any way unique in terms of stratigraphy or hydrology other than the presence of a basalt flow in the vadose zone. Very little data have been presented on

^-^^^^soih-vapor concentrations-within^and^beneath Jhe'basalt;--and-^its interaction-with the capillary fringe. The basalt layer is discontinuous, fractured and may be very porous, as indicated by the easy ivigration of fluid and vapor-phase con­taminants into groundwater despite the basalt 'barrier'."

Response: Several aspects of this comment require clarification and/or cor­rection.

First, HSC has never stated that "only SESOIL" can best simulate Site condi­tions; in fact, HSC has acknowledged in numerous meetings with EPA and

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ADEQ that more complex models are available. What HSC has clearly stated is that based on the vadose zone heterogeneity and transport processes that SESOIL does simulate, and based on the conditions and transport processes that are critical at the Site, SESOIL is an appropriate model for the Site. Among the models reviewed in 1993, it was concurred by EPA, ADEQ, and HSC that SESOIL provides the best combination of capabilities for representing Site conditions without being excessively complex. Further, SESOIL would not be expected to provide less accurate results than the more complex models that require many more input parameters, most of which would be based on assumptions or very incomplete data.

Second, it is precisely the occurrence of the basaltic lava flow unit at the base of the vadose zone that makes the Site unique relative to most contaminated sites in Arizona. The importance of this unit must not be underestimated.

Contrary to ADEQ's comment, the basaltic lava-flow has been demonstrated to be continuous across the FHWDA and, in fact, has only been found to be dis­continuous in a small area northeast from the capped part of the Site. Labora­tory tests conducted by a highly accredited laboratory on cores of the basalt in­dicate that its vertical hydraulic conductivity is very small, even where fractured. Inspection ofthe cores indicates that fractures in the basalt are healed by filling with fine-grained sediments from the overlying fine-grained zone (FGZ) and calcium carbonate. The FGZ is also continuous across the Site. As the FGZ was deposited on top of the basalt in the floodplain of a nearby surface water drainage, the water-borne silt and clay moved down into the open fractures in the basalt and, together with the precipitation of calcium carbonate, healed most or all of the fractures. Considering the mode of deposition of the FGZ, it is difficult to conceive how the fractures in the basalt would not be filled and healed. Core descriptions demonstrate that effective filling and healing occurs in fractures throughout the entire thickness of the basaltic lava-flow unit. These data indicate that fracture permeability may have been significant in past geo­logic time periods; however, the precipitation of carbonate and the deposition of silt and clay from the overlying floodplain sediments filled and healed the frac­tures and decreased fracture permeability to very small levels.

Third; througho'u'tthe"years"'oflh"e"Remedial"Investigation and subsequent ad­ditional investigations, all parties, including EPA and ADEQ, agreed that it was not in the best interests of the remediation to expend additional resources to monitor soil vapor in the basaltic-lava unit. In lieu of such monitoring data, the very conservative assumption was made for the SESOIL modeling that VOC concentrations in the basaltic lava-flow unit were as large as those detected in the FGZ, and that these concentrations were directly overlying the water table. It is important to note that, in the absence of "flushing," the capillary fringe is on one hand difficult to remediate, but on the other hand, is a major deterrent to

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the transport of VOCs from the vadose zone into groundwater due to the very slow rate of diffusion of VOCs in groundwater away from the air-water interface.

Fourth and finally, ADEQ's conclusion that the fractured and porous nature of the basalt layer is indicated by "the easy migration of fluid and vapor-phase contaminants into groundwater" is not only an unfounded evaluation of Site conditions, but is clearly unsupported by current and historic groundwater data obtained at the Site, and represents a substantial exaggeration of the magni­tude, extent, and distribution of VOCs in groundwater. If VOCs had been and were continuing to "migrate easily" through the basaltic lava-flow unit, VOC concentrations detected in the groundwater monitor well network should have been and should continue to be much larger than they are. On the contrary, given the Site history, the disposal volumes, and the groundwater data, it is plainly evident that fluid and vapor phase contaminants have been significantly restricted, if not mostly prevented, from migrating to groundwater. Obviously, the basaltic lava-flow unit and overlying FGZ impede the movement of VOC concentrations from the FGZ to groundwater.

Paragraph 5: " Currently, the residual soil vapor concentrations exceed the minimum groundwater protection levels (GPLs). Under the current circum­stances, any site-specific performance standards or GPLs established using the SESOIL will be erroneous and misleading. ADEQ agrees that the SESOIL is a robust analytic model that could potentially be used for establishing screening levels, if used approphately, or If the approphate parameters were incorporated into this model. Should we make a major compliance decision using a model surrounded by so much doubts about its approphateness? ADEQ recommends to the EPA that other vadose zone models, including VLEACH, be used to cross-check the SESOIL results. The VLEACH is the most widely used and accepted model in Arizona to evaluate potential ground­water impacts, and its flexibility can accommodate the so-called "complexity" at the Hassayampa Landfill site. ADEQ recommends that the HSC apply the VLEACH model using the data available from this site for comparison, to be re­viewed either by ADEQ or the EPA Center for Subsurface Modeling Support (CSMoS) at Ada, Oklahoma."

Response: Soil vapor monitoring and SESOIL modeling have demonstrated that VOC concentrations in the vadose zone exceed the soil vapor Perform­ance Standards only for 1,1-dichloroethene (1,1-DCE) and dichloromethane (DCM), and only in a small non-capped area north from Pit 1, When the meth­odology for developing soil vapor Performance Standards was evaluated and agreed upon in 1993 between EPA, ADEQ, and HSC, it was understood through a review of available models that SESOIL was able to simulate Site conditions and, specifically, that VLEACH could not simulate these conditions

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well. Nothing has changed since that time to warrant a change in the modeling approach, except the ability to simulate the Site cap, which is equipped with a flexible membrane liner (FML), As is described in the following responses, it is not appropriate to use VLEACH to "cross-check" results from SESOIL because VLEACH simulates the system differently and requires subjective assumptions for parameters that are simulated objectively in SESOIL. It should be empha­sized that use of VLEACH would result in projections that would be enormously variable depending on the input values used, many of which would be as­sumed, or would have to be "manipulated" or "averaged" in some subjective manner to force-fit a very heterogeneous vadose zone into a simulated homo­geneous model profile. Due to the fundamental differences in how these mod­els allow simulation of vadose zone conditions, any agreement between VLEACH and SESOIL model simulations would either be coincidental or the result of trial and error. In addition, the EPA modeling staff in Ada, Oklahoma have already reviewed the selection of SESOIL (when SESOIL was selected) over other models, including VLEACH, and concurred with the selection.

The results of SESOIL model simulations, as would be true of any model simulations, are for the purpose of evaluating the "general" threat to ground­water from present VOC concentrations in the vadose zone; the models cannot simulate the impact of "a few" preferential flow paths that may have been re­sponsible for the majority of VOC loading, such as the path provided by well HS-1, particularly closer in time to when the liquid solvents were disposed in large quantities. The occurrence of such flow paths must be evaluated through empirical groundwater data over a long period of time. Ultimately, VOC con­centrations detected in groundwater can not be directly compared to SESOIL projections of impact because the detected concentrations are a result of initial transport and impact, whereas SESOIL projects potential impacts based on current VOC concentrations in the soil vapor. However, results of SESOIL model simulations for the Site have been compared to VOC concentrations measured at monitor wells across the Site as a "reality check" of the general accuracy and effectiveness of the model. Obviously, there are wells at which concentrations of selected VOCs have always exceeded the simulated threat to groundwater, which is a result of initial transport and impact of the VOCs, Con-

-.versely,-r,for^some.^wells.at-_the^Site,-^-measured--VOC^conGentrations-..are^either-non-detect or smaller than the simulated threat. Of particular consequence is DCM, which is one of two VOCs that are simulated to have an impact to groundwater in excess ofthe groundwater Performance Standards (the other is 1,1-DCE). DCM has not been detected and confirmed in groundwater samples collected from any of the monitor wells at the Site,

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ADEQ HYDROLOGY UNIT COMMENTS

Introductory Paragraph: " However, the recently accelerating deterioration of water quality beneath the site despite the standards reportedly having been met, suggest that the standards are inappropriate to meet the groundwater protectiveness objectives set forth in the Record of Decision, or that the com­pliance vapor concentration sampling data are inaccurate."

Response: As shown in the following responses, the basis for this ADEQ statement is false, depends on selective and misleading interpretation of Site data, and completely ignores long-term data obtained from the groundwater monitor well network at the Site that does not corroborate ADEQ's interpreta­tion.

Comment 1: "Vapor phase VOC concentrations in the vadose zone can be as­sumed to be proportional to the total mass of VOC contaminants present in all phases at equilibrium. Specific vapor concentrations (performance standards) can be derived from conceptual models to serve as a proxy indicator of risk of the movement of VOC contaminants from the vadose zone into groundwater The intent of establishing vapor performance standards using the SESOIL model at the Hassayampa Landfill was to provide a provisional, predictive, and immediately measurable, remediation goal in the absence of more direct knowledge of the potential for groundwater contamination beneath the site. Which particular vadose zone model was chosen to predict protective soil va­por concentrations is not as important as the outcome of the predictions. As with any predictive attempt, new data acquired over time either confirm or re­fute the prediction."

Response: The soil vapor Performance Standards were established using SESOIL because it was determined to be appropriate for simulating Site condi­tions and processes and allowed objective determination of key hydraulic pa­rameters. EPA, ADEQ, and HSC were directly involved in the process that se-

^__^,lected_-,SESOIL-a_nd:r-the,-input.-,parametersJor..:-the^Site;-.therefore, .all .parties should be accepting of the results of that process, as long as there is no new information that would justify a change. As yet, there has been no new infor­mation that would justify a change, except that an FML cap was installed at the Site and the newer version of SESOIL can now simulate the presence of such a cap.

HSC is also confused and concerned about ADEQ's statement that the "par­ticular vadose zone model chosen is not as important as the outcome of the predictions " because it cleady implies that the model and results are

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separate, and that a pre-conceived outcome is used to determine the accuracy of the model. With the exception of cases where model projections are obvi­ously contradictory to all possible outcomes, this concept is clearly inconsistent with scientific method. The accuracy and applicability of model results are only as reliable as the model and modeling approach, and it is therefore crucial to select the particular model that can properly simulate the Site conditions.

Comment 2: "To be acceptable, model results must closely mimic real-world condi­tions and be consistent with all independently derived data. This is especially the case when specific parameters are used to define critical performance standards. Any reliable model must accommodate all pertinent measured con­ditions. Model results, and standards derived from any model should be re­viewed and revised as necessary as new data from ongoing monitoring be­come available."

Response: Please refer to the response to Comment 1. It is well known and accepted that no model can simulate nature perfectly; however, models are routinely used to provide acceptable approximations for practical use. Reliable models are routinely constructed that do not accommodate all pertinent meas­ured conditions. A primary advantage of SESOIL is precisely its flexibility and robustness in representing the vadose zone conditions and in using vadose zone parameters and meteorological data to determine crucial hydraulic pa­rameters.

This is a rather interesting comment, given the preference of ADEQ to use VLEACH, a model that could never satisfy the characterization given in this comment.

Comment 3: "The purpose of the recently completed five-year review of remedial efforts at the Hassayampa Landfill was to evaluate, in the light of data unavail­able in the past, the intenm results of those efforts. This includes assessing the accuracy of predicted effects and the degree of success achieved by the vari­ous actions implemented to protect the environment (including groundwater)

- =r ,. g^(;j-i-,ij-rnaiT"healthr"~Jmplicit Wthe iacVthat "pefiodicTeviews are an'accepted' part ofthe remedial process Is acknowledgement that predicted results may not occur The five-year review also provides an opportunity to recognize and cor­rect deficiencies in achieving the ohginal objectives."

Response: No response necessary; however, previous responses address the issues embedded in this comment.

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Comment 4: "ADEQ's comments on issues of protectiveness in the 5-year review at Hassayampa Landfill are with regard to long term threat to the environment, phmarlly to groundwater, and not to any immediate threat to human health. Substantive comments by ADEQ voice concern that remedial actions at the site are proving inadequate in containing hazardous substances to the vadose zone and are not protecting the environment, especially the groundwater beneath the site (see attached graph). Groundwater degradation continues at an Increasing rate and current actions (groundwater containment only) are inadequate. Fore­seeable, long'term, increased demands on the underlying B unit aquifer by de­velopment in the area will increase the potential for downward migration and lateral spread of contamination."

Response: ADEQ's comments tend to be general in nature and, therefore, it is difficult to respond to the comments. Clearly, a major contention of ADEQ is that groundwater at the Site is degrading due to continuing loading of VOCs from the vadose zone. HSC has provided detailed responses on several occa­sions to address ADEQ's concerns, but has not been given specific rebuttal. In lieu of specific rebuttal, HSC reiterates the following in response to this com­ment:

1. The hydraulic containment provided by the Groundwater Remediation Sys­tem (GRS) has been documented repeatedly to be more than adequate to contain and capture the area of present groundwater contamination and the area over which vadose zone contaminants could potentially impact groundwater.

2. The graph attached to the ADEQ comments shows only the VOC concen­trations in the GRS influent versus time, which represents the concentra­tions reaching the groundwater extraction wells at any particular time. It should be noted that this graph does not include all pertinent data reported by M&A. Based on the abundant long-term data from the monitor well field, it is clear that VOC concentrations are not consistently rising in the ground­water target zone, but have risen and fallen as groundwater with elevated VOC concentrations has passed by the monitor wells on its way to the ex-

=.^.==.,.:^4raction,yvells,^_Again.,=,based.--on4he,abundant-available data,4he elevated.. VOC concentrations were likely established long ago and are not the result of continuing loading from the vadose zone. The rise in concentrations at the extraction wells is the result of the most contaminated groundwater fi­nally reaching the extraction wells from upgradient areas.

This nse in concentrations was anticipated and expected when the GRS was designed. The fact that the concentrations have increased gives greater support to HSC's conceptualization of the Site and the approach to the remedy. Any claim that groundwater quality is degrading at an increas-

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ing rate based on the expected rise of GRS influent concentrations is with­out merit.

3, The handwritten notes on the table attached to ADEQ's comments (Table 4 from the Re-evaluation Modeling Report, M&A, March 2001) imply that SESOIL results for the Special Pits Polygon and the Pit 1 Capped Polygon should be compared to the downgradient groundwater extraction wells in­stead of the groundwater monitor wells located inside the respective poly­gons. This comparison is inappropriate and misleading. First, the extrac­tion wells yield groundwater from the entire Site and should not be com­pared to specific areas of the Site. Second, based on results from the groundwater monitor wells and on fundamental hydrogeologic principles, VOC concentrations detected at the extraction wells were and are most likely the result of loading to groundwater many years ago, not the loading projected by SESOIL for present VOC concentrations in soil vapor.

4, Results of groundwater monitoring indicate that Unit B groundwater has not been impacted by VOCs from Unit A groundwater. Data from continuous monitoring of groundwater levels in Unit B have shown no year-to-year de­clines. Seasonal declines due to increased pumping in the region during summer months have fully recovered in the winter. The Unit B monitor wells at the Site are being monitored for groundwater level and water qual­ity. In accordance with the Section 3,1.6 of the "GROUNDWATER PER­FORMANCE STANDARDS VERIFICATION PLAN" (CRA and M&A, Sep­tember 5, 1995), appropriate actions will be taken if VOCs from Unit A are found to be impacting Unit B,

Comment 5: "Of particular concern to ADEQ is the lack of operation of the Soil Va­por Treatment System (SVTS) system, and the view supported by EPA's re­sponses that it can be permanently decommissioned because soil vapor per­formance standards are said to have met. The rate of VOC migration into unit A groundwater has increased dramatically since operation of the SVTS was discontinued In 1998. Although soil vapor standards determined from the

^.^_^,,.SESOIJ^_.^modelingJin^M9A.JM6,^-^^^^^^ -may,_ have Meen-_, achieved. (above the basalt flow, see below), they are clearly not protective of the groundwater, as was intended in the ROD."

Response: The premise of this comment is that VOC loading to Unit A has "increased dramatically." This premise is not supported by the long-term groundwater monitoring data, as described previously in several responses given herein.

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Comment 6: "The soil vapor performance standards were, according to modeling results, thought to be protective of groundwater of the Unit A aquifer The standards were provisionally accepted as indicating that VOC contamination would not migrate in significant quantity from the vadose zone to groundwater The SESOIL model using the input parameter values that were assumed (i.e. calibration), fails to accurately describe conditions and processes at the site. Groundwater VOC data strongly suggest that the modeling is flawed (see at­tached table). One likely cause of the modeling failure is that it doesn't take density-driven vapor transport into account. Continuing and accelerating VOC contamination of groundwater under conditions predicted by the model to be protective strongly suggest that the model is not calibrated correctly."

Response: Again, the premise of this comment is that VOC loading to Unit A has increased and this increased loading indicates flaws in the model. This premise is not supported by the long-term monitoring data, as described in the responses to Comments 1 and 4. Input parameters for SESOIL were provided prior to modeling in the "VADOSE ZONE ANALYTICAL MODELING INPUT PARAMETER TECHNICAL PROPOSAL" (M&A, December 30, 1993). These input parameters were reviewed by EPA and ADEQ and were approved as adequately representing conditions and processes at the Site, In "RE­SPONSES TO SUPPLEMENTAL ISSUES, HASSAYAMPA LANDFILL EPA SUPERFUND SITE "(M&A, July 25, 2000), HSC provided a detailed discussion and strong technical reasons why it is not likely that density-driven vapor trans­port is an important process presently at the Site. In contrast, ADEQ has pro­vided no support for its belief that this process is important at the Site,

The issue of "model calibration" requires clanfication. Model results for VOC loading to groundwater can not be calibrated in a quantitative manner to results of groundwater monitoring due to two principal reasons: 1) model results given are for peak concentrations that occur many years after start of simulation and assume that VOCs entering the water table (essentially the capillary fringe) can mix with the bulk of groundwater; and 2) for wells that are located in areas where the groundwater has existing concentrations of VOCs, it cannot be de­termined if potential increases in concentrations are due to additional input from the-vadose-zone-or-due-to-Jarger-concentrations-in-upgradient-gr-oundwater-

In general, results of vadose zone transport models for VOC loading to groundwater are difficult to "calibrate" to some easily measured field parameter or VOC concentration in groundwater. Model calibration for SESOIL occurred chiefly in matching results of the hydrologic subroutine to average measured values of soil moisture content. As is indicated in the 1994 Vadose Zone Mod­eling Report, the primary calibration factor in SESOIL, called the "disconnect­edness index" was determined by matching the measured and simulated

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moisture contents, but was then skewed toward the largest simulated recharge rate.

In the absence of "true calibration," it is crucial to use site-specific data to the extent practicable, and then apply the model in a conservative manner. The input parameters to SESOIL and conservative approach used for the Site have been extensively explained and justified in several reports. In addition, as ex­plained in the responses to Paragraph 5 and Comment 1, it is apparent that model results overestimate the impact of some VOCs to groundwater due to historic and present non-detection of these VOCs at the monitor wells at the Site.

Comment 7: "SESOIL cannot stimulate density-driven vapor transport of VOCs, although the process is likely to be operating and important at the Hassayampa site. SESOIL assumes that soil gas concentrations within a model layer are evenly distributed and stable. No evidence of mobile NAPL VOCs (free prod­uct) has been observed at the site and it Is assumed not to be present. Aside from not being able to model density driven vapor transport, which should be considered at the Hassayampa site, the SESOIL model itself is other wise ade­quately robust and well demonstrated when correctly calibrated. An attempt to derive soil vapor performance standards as a proxy intenm benchmark for protectiveness of groundwater using SESOIL was not unreasonable. Beahng the aforementioned points in mind, SESOIL should be able to better approxi­mate vadose conditions at the site than the present performance standards in­dicate. "

Response: Regarding density-driven vapor transport, see response to Com­ment 6. The processes and conditions simulated for the Site using SESOIL were and are appropriate and were concurred with by EPA and ADEQ. ADEQ's present criticism of the manner in which SESOIL was used for the Site lacks technical support and is not corroborated by Site data. Further, in the framework of the ROD and the Consent Decree, the soil vapor Performance Standards are not "proxy interim benchmarks", but are firmly established in the ROD-and-in-the-Consent-Decreeas-^the-quantitativexriteriaJo-use-in-xieterminr^ ing if the remedial action is complete.

Comment 8: "SESOIL considers vapor concentration in contact with the water table surface (capillary fringe) to assess mass movement into solution (Henry's Law / water-air partition coefficient). However, at the site, no vapor concentration data are available for this interface: the water table lies about fifteen feet below the base of a 20 to 30 ft-thick basalt flow. The nearest measured vapor con­centrations are from above the basalt. Meaningful soil vapor concentrations

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have not been, and cannot presently be measured for comparison with the performance standards, or for feedback to recalibrate the SESOIL model."

Response: The water table below the basaltic lava-flow unit (Unit A ground­water) has declined during the entire period of record at the Site due to regional decline in water levels and operation of the GRS. Presently, the water table occurs about 12 feet below the basaltic unit in the area of concern.

An important assumption made in the SESOIL model for the Site overcomes any data gaps related to lack of vapor sampling below the basaltic unit. This conservative assumption is that VOC concentrations in the vadose zone above the water table are as large as those measured for the FGZ, which are the largest measured at the Site, This assumption negates any concern that soil vapor data are not available at the changing groundwater interface.

In addition, the groundwater mixing cell model used in conjunction with SESOIL assumes that all VOCs that reach the bottom of the vadose zone during each time step are fully integrated into the aquifer ("mixed"), which ignores the diffu­sion-limiting transport of VOCs in water through the capillary fringe. In reality, in the absence of flushing, VOC movement across the vadose zone/capillary fringe interface vi/ould be limited by the slow diffusion of VOCs in water through a relatively immobile capillary fringe. However, the modeling approach used for Hassayampa forces VOC loading and complete mixing to occur instantane­ously, at the highest concentration measured in each model polygon, and over the entire polygon. These "assumptions" are clearly conservative and overes­timate the amount of loading to groundwater.

Comment 9: "The value used for inthnsic permeability of the basalt flow, chtical to correct calibration of the SESOIL model, was determined by laboratory testing of drill core samples. The measured permeability of these samples is ex­tremely low and the measurements may be accurate. However, the scale of the permeability measurements and the scale of the site processes are very different. The measured permeability cannot reasonably be assumed to repre-

^^^^^sent the .large _scale.(.site-scale).permeability.-ofJhe basalt flow. ^ notohous for large scale fractures and joints. Columnar joints are charactehstic and usually extend completely through flow units. For example, a surface res­ervoir constructed atop a similar, 20 ft-thick basalt flow could not be expected to hold water well. One of the core samples contains a small scale (2mm-wide) fracture which appears to be almost completely filled with clayey calcite, but this small fracture should not be taken as representative of the large scale jointing charactehstic ofthe rock type. Also, the basalt flow is thought to extend less than 100 feet beyond the site boundary. The basalt layer cannot reasona-

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ERROL L. MONTGOMERY &. ASSOCIATES, INC. 14

biy be regarded as a completely impermeable, laterally infinite, protective layer, as it was in development ofthe current performance standards."

Response: The core samples analyzed for vertical hydraulic conductivity were selected from exploration borehole EX-2 at four depths in the basaltic lava-flow unit from a 23-foot continuous core. Continuous core was also obtained from a 9-foot depth interval of the basalt at exploration borehole EX-4. HSC recog­nizes that these core samples do not necessarily represent every possible fracture pattern or size in the basalt unit, but the samples do demonstrate the effectiveness of fracture "healing" during deposition of the overlying FGZ. It should be reiterated that the FGZ includes well-cemented siltstone/claystone layers, which indicate the amount and extent of cementing agents and proc­esses. Therefore, as described in the response to Paragraph 4 of ADEQ's cover letter, it is very plausible and likely that the mode of deposition of the FGZ over the basalt would have provided an effective mechanism for filling and healing fractures in the basalt, regardless of fracture and joint size.

The example of a reservoir over a basalt flow is not a reasonable analogy to the subsurface conditions at the Site. A reservoir typically would have many feet of hydraulic head over its base - no such conditions exist at the Site. To begin to be analogous, the basalt would need to be repeatedly inundated by muddy flood flows for thousands or tens of thousands of years so that the fractures in the basalt would be filled with the sediments and calcium carbon­ate. Perched groundwater zones that provide reliable water supplies to wells are commonly found above fine-grained soil zones, which overlie and impede groundwater movement into and through the lava flow rocks.

Based on more than 60 borings drilled at the Site, absence ofthe basaltic lava-flow unit at the Site has only been documented for a small area northeast from the capped area of the Site. Groundwater monitor well MW-11 UA was installed in this non-basalt area to monitor groundwater quality. Except for four small concentrations of trichlorotrifluoroethane (TCTFA or Freon 113) far below the maximum contaminant level (MCL), no VOCs have been detected and con­firmed in groundwater samples collected from this well. The area where basalt

,wasjnol,enco.untered-.during. drilling was simulated Jn .the,.SES01L,model for the Site as the Non-basalt polygon, and soil vapor Performance Standards for the non-basalt area were established separately from the areas underlain by ba­salt.

The basaltic lava-flow unit was not assumed to be a "completely impermeable, laterally infinite, protective layer" for the SESOIL model, as ADEQ has unchar-acterized it in its comment. It was assigned a permeability based on appropri­ate laboratory tests, it was assumed to occur where drilling of numerous bor­ings confirmed its occurrence, and the protectiveness of the unit in conjunction

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ERROL L. MONTGOMERY & ASSOCIATES, INC 15

with other pertinent subsurface features was determined through objective modeling for which input parameters had been approved by a team of environ­mental experts from the regulatory agencies.

Comment 10: "The water table beneath the site is declining, due to pumping by the groundwater remediation system. This facilitates the transfer of contaminants to groundwater A declining (or fluctuating) water table enhances transfer of dissolved contaminants by increasing solution and subsequent Infiltration of VOCs from the draining capillary fhnge. The SESOIL modeling does not ac­count for a fluctuating water table. This additional vahable can be regarded as a component of infiltration and could be estimated by manually adjusting the in­filtration parameter used by SESOIL to account for this mechanism of contami­nant transfer"

Response: A declining water table, as opposed to a fluctuating water table, does not increase VOC loading to groundwater from the capillary fringe. The zone above a declining water table drains to specific retention, then functions the same as the zones above it with regard to recharge.

A fluctuating water table can enhance a groundwater remediation by flushing the zone that was temporarily dewatered, thereby removing more residual contaminants that may have sorbed to the aquifer media or remained in the retained soil moisture held in the pores by capillarity. When the GRS is oper­ated intermittently, water table fluctuations at the Site are in the magnitude of 1 foot or less.

The suggestion that resaturation and repeated drainage of an aquifer zone by a fluctuating water table should be incorporated into a vadose zone model as ad­ditional recharge is inappropriate and has no technical justification. In addition, even for sites where a fluctuating water table does result in more flushing of contaminants, it would be essentially impossible to know how to adjust for this contribution using recharge rate. Furthermore, HSC fails to see how this con­dition has anything to do with the soil vapor Performance Standards and the

—-—operation-of4he-SVTS., _ .-_- r.-.-

Comment 11: "Pumping records from the groundwater remediation system (GRS) show that about 25 million gallons of water have been extracted and treated from the unit A aquifer since pumping began in 1994. This volume of water is roughly equal to the total volume of pore water contained in the unit A aquifer beneath the area of the hazardous waste area of the landfill that the GRS is designed to contain (approximately 10 acres). Thus, the groundwater of the unit A aquifer presumably has been completely replaced by clean water from

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ERROL L. MONTGOMERY & ASSOCIATES, INC. 16

outside the area of concern over the last seven years. Despite this continual replacement of water beneath the site, the extent and concentrations of VOCs in groundwater pumped by the GRS has increased during this time period. The rate of VOC migration from the contaminated vadose zone into groundwater of unit A is continuing to accelerate, 21 years after contaminants were placed at the site."

Response: The amusing proposition that, based on the quantity of ground­water pumped from the extractions wells, one pore volume of groundwater in the target zone should have been removed by now and, therefore, groundwater contamination at the Site should be largely remediated and VOC concentra­tions should be declining, is simplistic and completely contradicts the present state of knowledge regarding flow through heterogeneous formations, diffusion-limiting transfer of solutes from lower-permeability zones, and general retarda­tion processes.

First, it is well documented at sites across the nation that, due to retardation, preferred pathways, and delayed migration from fine-grained layers, the re­moval of one pore volume is a poor predictor for the trend of VOC concentra­tions at extraction wells.

Second, ADEQ's calculation over-simplifies the capture zone concept by ig­noring the fact that a substantial fraction ofthe groundwater extracted from Unit A is drawn from areas outside the target zone, M&A estimated the capture zones for one pore volume in Unit A using the analytical model WINFLOW and an appropriate range of assumptions (see Attachment A to these re­sponses).

Based on published ranges of porosity for sediments similar to those in the FGZ, the range of 25 to 50 percent porosity was selected for modeling opera­tions. Figure A-1 shows the groundwater level contours from the most recent monitoring round, which was conducted in October 2001; inspection of this fig­ure verifies that the GRS continues to maintain hydraulic control for Unit A groundwater in the target area north from the extraction wells. Figure A-2

-shows..simulatedr.flowJines.^and-r.capture.zones .for-a -porosity of-50-percent,^ Figure A-3 shows simulated flow lines and capture zones for a porosity of 25 percent. Other assumptions and features for Figures A-2 and A-3 are de­scribed in the text for Attachment A,

The recent rise in VOC concentrations in GRS influent began in early 2000, which is about 6 years after pumping started. Results of WINFLOW simula­tions indicate that the range of locations for the 6-year one-pore-volume cap­ture zone (Figures A-2 and A-3) is in the area of highest concern (abandoned state well HS-1 and Pit 1),

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ERROL L. MONTGOMERY & ASSOCIATES, INC. 17

Third and perhaps most importantly, ADEQ is again selectively ignoring groundwater monitor well data for the Site, which refutes the claim that VOC concentrations are consistently nsing in the target zone.

Comment 12: "VOCs are, by far, the most mobile and difficult to contain of the contaminants present at the site. Of particular threat to groundwater are the chlorinated solvents whose vapors are denser than air and which have very low MCLs. Such VOCs are abundant at the site. In significant concentration, denser-than-air vapors, (and unrecognized DNAPL, if present) will migrate downward and accumulate at the base of the vadose zone or above a truly im­permeable layer Over time, concentrations will be highest just above the water table or impermeable layer SESOIL does not consider density-driven migra­tion of such compounds, or density-stratification within a model layer"

Response: See several previous responses given herein regarding density-driven vapor flow. HSC does not disagree that this process may have occurred in the vadose zone at the Site and that it may have helped result in the larger concentration of VOCs in the FGZ. Again, ADEQ is urged to refer to and con­sider the technical information given in the RESPONSES TO SUPPLEMENTAL ISSUES regarding density-driven vapor flow and why its present occurrence, particularly as a means of VOC loading to groundwater, is not supported by Site data.

Comment 13: "The fact that regulatory agencies and responsible parties agreed to use of the SESOIL model in 1995 and its revisions in 2000 does not dictate continuing adherence to earlier, model-derived performance standards. Use of any model, especially for determination of a single performance standard vah­able, is approphate only when the model results accurately reflect real-world conditions. Since the soil vapor performance standards derived using the SESOIL model have been shown by the unit A groundwater data not to be protective, the model is not accurately representing actual conditions of the site."

=^^"='Res]gon§er="C1"eaTlyf"the"=sT5il"^apw shown to be inadequate; therefore, this ADEQ comment has no basis.

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CONCLUSIONS AND RECOMMENDATIONS

Conclusion/Recommendation 1: "RIHU recommends that operation of the SVTS resume immediately. Clearly, without the SVTS, contamination sources in the vadose zone are not being contained. Intent of the SVTS, as set forth in the ROD, is to contain and remove contamination In the vadose zone to reduce the threat to groundwater."

Response: The provisions of the ROD allow containment and removal to cease when the soil vapor Performance Standards are demonstrated to have been met. This demonstration has been made for the capped part of the Site and passive venting has been demonstrated to remove VOCs in the small area where soil vapor Performance Standards have not been met. ADEQ has failed to provide technical support for its contention that the soil vapor Performance Standards are inadequate. Therefore, it is not necessary to resume operation of the SVTS.

Conclusion/Recommendation 2: "O&M problems with the SVTS should be ad­dressed so that the system will perform as intended. The SVTS apparently was effective at removing vapor phase VOCs, and is stated to have removed nearly half of the total VOCs in the vadose zone in just the 25 months that it did operate (see estimates presented in EPA's responses to comment number 33). The large amount of VOCs removed by the SVTS that short time period is two orders of magnitude greater than the amount removed by the GRS over seven years of operation. The SVTS is far more cost-effective, on a per-pound basis, at removing VOC mass than the GRS. All data indicate that operation of the SVTS is beneficial in retarding the movement of VOCs into the unit A aquifer"

Response: Removal of VOC mass for an indefinite time period is not, itself, a purpose of the remediation. Rather, VOC mass is to be removed until Per­formance Standards can be achieved at the Site, followed by long-term moni­toring to evaluate effectiveness of the remedy. Soil vapor Performance Stan-_dards_hajy ^ been achieved in the capped area of the Site; therefore, it \s_r\q_ longer necessary to remove resTdual VOC mass from the vadose zone in this area. If results of continued long-term monitoring for both soil vapor and groundwater indicate that the soil vapor Performance Standards are not pro­tective of groundwater, appropnate action would be required.

Conclusion/Recommendation 3: "Concerns about the possible creation of dioxins from incomplete combustion of VOCs duhng thermal oxidation treatment should be investigated, and not used without verification as justification to

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cease operation of the SVTS. Other treatment for extracted vapors, such as GAC absorption, may be required."

Response: ADEQ miscasts the issue. The issue under consideration is whether the soil vapor Performance Standards have been met. Under any cir­cumstances, carbon adsorption is one of the least cost-effective alternatives to treat soil vapor removed by the SVTS.

Conclusion/Recommendation 4: "Efficient operation of the existing vapor extrac­tion wells may obviate modifications necessary for the proposed passive soil venting system just beyond the north edge of the impermeable membrane cap. Methane gas being generated In the adjacent municipal landfill is tending to displace VOC vapors northward from beneath the impermeable membrane cap. Passive venting has been proposed to meet the existing, flawed vapor performance standards. When revised, these standards are likely to be much more sthngent, and the contemplated passive venting system alone would not achieve the required concentration values."

Response: Data obtained from monitoring of VOCs and landfill gasses at the Site during the past year do not provide sufficient evidence to conclude that VOC vapors are moving in any particular direction. It is not necessary to revise the soil vapor Performance Standards, Furthermore, similar to many of the claims or conclusions presented by ADEQ in their comments, there is no basis or technical support given by ADEQ for its statement that a proper passive venting system would not achieve the soil vapor Performance Standards in the area of concern north from the capped area.

Conclusion/Recommendation 5: "VOC vapor concentrations used for demonstra­tion of compliance with any revised vapor performance standards must be col­lected from beneath the basalt flow and just above the water table. Vapor con­centrations at the air/water interface are what matter In VOC migration to groundwater- not concentrations at points well above the interface and above the "impermeable" basalt layer"

Response: The SESOIL model has been shown to provide conservatively large projections for groundwater impact by assuming, among other things, that the largest concentrations measured in the FGZ also occur at the groundwater interface. Much of the "vadose zone" underlying the basalt flow was created by groundwater level decline after startup of the GRS due to regional responses and operation of the GRS. The response to Comment 8 provides other con­servative assumptions that overcome the need for the additional data collec­tion.

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Conclusion/Recommendation 6: "The soil vapor performance standards deter­mined from SESOIL, and met without operation ofthe SVTS, are faulty. If such performance standards are to continue to be used, they must undergo major revision using as yet uncollected data on vapor concentrations just above the water table - beneath the basalt flow."

Response: As emphasized in our other responses, ADEQ has no technical basis for this comment. Please see the response to Comment 8.

Conclusion/Recommendation 7: "Input parameters used for the SESOIL model should be reexamined and adjusted to recalibrate the model so that results compare favorably with actual site conditions. Another, more simplistic vadose zone leaching model, VLEACH, also should be run and considered in light of the VOC concentration values in groundwater and as yet unobtained vapor concentrations from beneath the basalt. This will assess, by an independent model, the threat posed to groundwater beneath the site by residual VOCs."

Response: Again, as described in our other responses, the input parameters for SESOIL were reviewed and approved by all parties involved and there are no conditions that have changed since then to warrant re-evaluation of the in­put parameters, except the presence of the FML cap, which was simulated in the Re-evaluation Modeling Report. Results from VLEACH can not be com­pared to results from SESOIL and it would be inappropriate and of no benefit to do so. VLEACH would require substantial and subjective manipulation and as­sumptions to simulate the heterogeneous profile at the Site and would provide widely varying results based on the range of assumptions used. Based on the model review conducted and approved by the regulatory agencies during the effort to establish soil vapor Performance Standards, VLEACH would not be an appropriate model for this Site.

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ERROL L. MONTGOMERY & ASSOCIATES, INC. A-1

ATTACHMENT A

CAPTURE ZONE MODELING

Modeling was conducted to compare simulated capture zones with obser­vations made at the Site during the period of operation of the Groundwater Re­mediation System (GRS). The modeling approach used was similar to the ap­proach used for previous hydraulic containment modeling conducted by Mont­gomery & Associates for the Site (M&A and CRA, 1992 and 1995). The 1992 modeling operations were the basis for placement ofthe GRS extraction wells.

The modeling approach used for this evaluation of hydraulic capture in­cluded simulations using the computer program WINFLOW (Rumbaugh and Rumbaugh, 1995) to simulate hydraulic capture zones in Unit A for the average pumping regimen during the period of operation ofthe GRS. Hydraulic capture zones are defined as the aquifer zone from which groundwater would be with­drawn by an extraction well field after pumping for a specific duration. After the given period of pumping, the volume of groundwater withdrawn would comprise one pore volume of the part of the aquifer in the capture zone.

The WINFLOW program simulates a steady-state, two-dimensional flow field for a homogeneous, isotropic aquifer. The resultant steady-state flow field combines the effects resulting from a planer initial hydraulic gradient and with­drawal of water from the extraction wells. WINFLOW simulated unconfined con­ditions for these modeling operations.

INPUT PARAMETERS

Aquifer parameters for Unit A used as input for the WINFLOW program include:

1) pumping rates in gallons per minute (gpm) for extraction wells of EW-lUA 1,43 gpm EW-2UA 0.91 gpm EW-3UA 1.09 gpm EW-4UA 2.95 gpm;

2) hydraulic conductivity of 2.8 feet per day; 3) base of Unit A at 810 feet above mean sea level (ft msl); 4) top boundary of Unit A at 844 ft msl;

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5) hydraulic gradient of 0.0035 (dimensionless); 6) constant head of 877 ft msl at a point 10,000 feet north from well EW-1 UA; 7) porosity of 25 and 50 percent; 8) recharge of zero; and 9) leakage of zero.

Pumping rates used for the extraction wells are equal to the average rates during the period of operation of the GRS.

A hydraulic conductivity for Unit A of 2.8 feet per day, or 20.94 gallons per day per square foot (gpd/ft^), was used because results for drawdown simula­tions with this hydraulic conductivity closely approximate measured water levels in the area of groundwater transport between the area of concern and the extrac­tion wells,

A hydraulic gradient of 0,0035 (dimensionless) was used and corresponds to the measured gradient before GRS operations began. For steady-state simu­lations, the program requires a constant head, called a "reference head," point. A point 10,000 feet up-gradient (north) from extraction well EW-1 UA was se­lected as the reference point to minimize the effect of the constant head condi­tion. The reference head was 877 ft msl; this value was calculated by multiplying the hydraulic gradient by the distance and adding the pre-GRS water level for extraction well EW-lUA.

Unit A comprises unconsolidated sediments including chiefly sandy silt and clay with sand layers. Values of porosity reported for similar unconsolidated strata (Freeze & Cherry, 1980) range from about: 25 to 50 percent for sand; 35 to 50 percent for silt; and 40 to 70 percent for clay. WINFLOW was used to simulate conditions for both 50 and 25 percent porosity.

RESULTS OF SIMULATIONS

Figures A-2 and A-3 show results of steady-state WINFLOW simulations using different values for aquifer porosity. Figure A-2 shows simulated flow lines

=and capture zones using an^aquifer porosityof'50 percent? Figure A-3 show's'"'" simulated flow lines and capture zones using an aquifer porosity of 25 percent. The flow field is identical for both simulations. The simulation indicates that all groundwater in the target zone north from the extraction wells is captured.

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ERROL L. MONTGOMERY & ASSOCIATES, INC. A-3

REFERENCES CITED

Montgomery, Errol L., & Associates, Inc., and Conestoga-Rovers & Associates, 1992. Results of preliminary hydraulic capture zone simulations and recommendations for groundwater extraction and injection for the Former Hazardous Waste Disposal Area at the Hassayampa Landfill, Maricopa County, Arizona. May 8, 1992.

, 1995. Hydraulic containment evaluation report, Hassayampa Land­fdl Superfund Site, Maricopa County, Arizona. January 17, 1995.

Rumbaugh, Jim, and Rumbaugh, Doug, 1995. WinFlow Version 1.07. Envi­ronmental Simulations, Inc. 1995.

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665.507\HASA10-01 WL\07Jan2001

FIGURE A-1. GROUNDWATER LEVEL CONTOURS FOR UNIT A, HASSAYAMPA LANDFILL EPA SUPERFUND SITE, OCTOBER 8, 2001

ERROL L. MONTGOMERY A ASSOCIATES, INC.

665,507\CAPTURE50\07JAN2002

® MW-11UA

Fence . ^

MW-5UA

F O R M E R

H A Z A R D O U S

X W A S T E

D ISPOSAL

A R E A

MW-3UA

® MW-8UA

.® MW-9UA

E X P L A N A T I O N

1^ MW-1 UA UNIT A GROUNDWATER WELL AND IDENTIFIER

^=—^—^=TIMIT- "OF^CAPTURE"ZONE'^ - - " -AFTER 4 YEARS OF PUMPING

FLOW LINE AND DIRECTION OF GROUNDWATER MOVEMENT

250 500

(§) MW-10UA

FEET

FIGURE A-2. SIMULATED FLOW LINES AND CAPTURE ZONES USING AQUIFER POROSITY OF 50 PERCENT, HASSAYAMPA LANDFILL EPA SUPERFUND SITE

ERROL L. MONTGOMERY & ASSOCIATES, INC.

665,507\CAPTURE25\07JAN2002

FORMER HAZARDOUS WASTE DISPOSAL AREA

® MW-8UA

.® MW-9UA

EXPLANATION

(g|MW-lUA UNIT A GROUNDWATER WELL AND IDENTIFIER

4 LIMIT OF CAPTURE ZONE AFTER 4 YEARS OF PUMPING

FLOW LINE AND DIRECTION OF GROUNDWATER MOVEMENT

250

FEET

500

® MW-10UA

FIGURE A-3. SIMULATED FLOW LINES AND CAPTURE ZONES USING AQUIFER POROSITY OF 25 PERCENT, HASSAYAMPA LANDFILL EPA SUPERFUND SITE

ERROL L. MONTGOMERY A ASSOCIATES, INC,