Hazard Waste

8/3/2019 Hazard Waste

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Hazardous W aste S iteInvestigationsTow ard B etter D ecis ions

E dited b yR ich ard B. G am m ageBarry A . HervenHealth and Sa fe ty Research D iv is io nO ak R id ge N atio na l L ab ora to ryOa k R id ge , T en ne ss ee

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120 HAZARDOUS WAST E S IT E IN VES T IGA TION S

characterization and early w arning d etection, and provid ing ad equate d ata forrem edial design or remed ial performance assessment is prob lematic. Thedesigner of monitoring networks is caught in a conundrum : the need for alarge num ber of w ells to satisfy the d em and s of com plex ground w afer sys-ferns, w ith the reality of high cost per w ell of len lim iting w ells in the netw orkto an inadequate num ber relative to the complexity of the prob lem .

M ackay ' provided a vivid description of the problem facing the designerof ground w ater m onitoring netw orks:E n vis io n a n e xt rem ely c omp le x. maze in which are lo st a v arie ty o f c he mic als

- som e concentrated and localized, a nd so me d ilute a nd sp read out. Imaginef ur th e r t ha i the c he mic als a re a ll m ov in g a t d iffe re nt ra te s and d irec tion s a s are su lt o f g ra vily a nd /o r th e n ow o f a ir a nd w ale r th ro ug h th e m az e. Then imagineth a.t th e in le ma l w alls of th e maze an: porous, like a hedge, and Ihat thechemicals, ai r or waler can mov e in lO a nd e ve n I.h ro ug h t he m a l ra te s th aI varythroughoul th e maze. La st ly , imag in e thai you m ust rind all of t he c h em i ca lsbu t c an no t e nte r th e maze to do so.In the first part of th is chapter, the author considers the implications for

ground water m onitoring of som e recent stud ies pertaining to contam inantbehavior in ground w ater. The overall em phasis is on organic chemicals . Thed iscussion focuses on three facets of site hyd rogeology : transverse dispersionin heterogeneous sandy aquifers. fractures in aquitards. an d heavier-than-w ater im miscible ind ustrial liquid s. T hese ar e the m ain areas of the author'ssite-investigationexperience in the past decade .T he term "conventional m onitoring w ell" is used frequently in t hi s c h ap te r.T his refers 10 a single well in a single borehole . The well has a screen ofm od erate o r sh ort len gth at the bottom , a sand or gravel pack is placed aroundthe screen, and the entire portion of the borehole annulus above the sand packis sealed to surface with an impervious m aterial such as cement grout orb e nt on it e ' sl ur ry .

H Y DR O D YN A MIC D IS PE R SIO NDispersion has rece ived much attention from the ground \!fater research

com munity in the past three decades. Lehr' arg ued th at ex ce ssive atte ntio nhas been paid to this topic and that il is no longer worthy of priority inresearch. Regard less of the issue of priority, research fmdings of the pastdecade 00 d is pe rs io n h ave im me nse im plic atio ns fo r g ro un d w ater m on ito rin g.T he follow ing consid eration of the topic pertains 10 contam inant transport inporous media such as sand and gravel rather than fractured rock.

Plumes of d issolved-phase contam ination generally travel more or lesshorizontally through sand or gravel aquifers because these aquifers, as broadg eo lo gic d epo sits, are ty pic ally clos e to h orizo ntal. T he re fore. it is co nv en ie ntto consider dispersion in term s of the follow ing three principle d irections:

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122 HAZARDOUS WASTE SITE INVESTIGATIONS

LANOFILL

F IGURE 1. Schematic diagram showing a n arro w contaminant plume from a landfillleak not being detected by a ro w 01down gradienl mon:itor wells becauseof a weak transverse dispefsion (solid outline); dashed line represents strongtransverse dispersion.

some plumes. the difference between detecting or missing a concentrationzone orders of magnitudeabove a regulatory limit is the difference in posi-tioning in depth of the critical well by only a meter or two.At a landfill where the monitoring network is used for early warning ofcontamination. a likely prospect for contamination to enter ground water

would be a hole in the liner. The width of the contaminant entry-area toground water beneath the landfill would tend to be narrow. Due to weaktransverse dispersion. the spacing between monitoring wells down-gradientwould need to be equally narrow to provide much probability that the networkwould eventually detect the leak. Consideration of spacing between moni-toring wells then becomes almost entirely dependent on estimates of the widthof the contaminant entry-area at the source. From this. it follows that at manywaste disposal or industrial sites. the spacing of wells in both the vertical andhorizontal directions is too large to detect the main impacts of the type ofleakages or spills most likely 10 cause ground water contamination (Figure1). If transverse dispersion were to have as strong a spreading effect as waspreviously believed. this monitoring problem would be much less severebecause plumes would spread out rapidly in the transverse directions as theplume front advances. thereby being at least detectable (but not necessarilymappable) by widely spaced wells.

DENSE NON-AQUEOUS PHASE UaUIDSDense non-aqueous phase liquids (DNAPLs) are now recognized as one of

the most common and complex causes of ground water contamination in


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The problems referred for DNAPL site monitoring pertain to the spatialand temporal resolution of concentration distributions. However, an even moredifficult DNAPL monitoring problem is the installation of monitor wells inareas of DNAPL without causing lenses or pools of DNAPL to drain Iiquid-DNAPL down the borehole or well to deeper levels in the aquifer. ManyDNAPLs have a specific gravity between 1.2 and 1.6, low viscosity and lowinterfacial tension. This gives them strong propensity to move downwardthrough small openings created by drilling or well installation. In hydrogeo-logic settings where no major aquitard exists into which casing can be keyedat the bottom of a DNAPL pool. there is no proven technology for drillingthrough such DNAPL zones without draining liquid DNAPL deeper in theaquifer or otherwise inducing artificial complexities in the monitoring data.The worst circumstances are generaJly found in fractured rock. Extreme

precautions can be taken which may minimize these problems in some cases.However. without detailed prior knowledge of the locations of the lenses orpools of DNAPL, the precautions can be futile. Such prior knowledge mustderive from drilling. a. Catch-22 situation. Considering the inadequacy ofpresent drilling technology in this context. It is often inadvisable to drill inareas of known or suspected DNAPL. Unconventional strategies for siteinvestigation should be pursued. Specialized equipment suitable for this chal-lenge is needed.

FRACTURED AQUITARDSIn the simplest conceptualization. hydrogeologica.1 systems are comprised

of aquifers and aquitards. Aquifers are normally the focus of attention becausethey are the source of water supply and are the zones where large contaminantplumes develop. At waste disposal or industrial sites. however, aquitards areoften as important as the aquifers; The problems of monitoring aquitards aremuch different than those of aquifers.Lnsedimentary terrain. aquitards typically are strata comprised of silt. clay.

siltstone, or shale in which nearly all ground water flews through fractures(the term fracture is used here for all types of secondary openings such asjoints. fissures, and bedding planes). In many aquitards, vertical fracturesexist and in situations where the fractures extend from top to bottom, thesefractures ofIen provide pathways for contaminants 10 move through the aqui-lard into underlying aquifers. One of the reasons why ground water contam-ination is now so common in industrial regions is the leakiness of aquitardsdue to fractures, primarily vertical fractures.Using numerical simulations, Sudicky and Mclaren 16 and Harrison et al.17

showed major impacts of dissolved contaminants, such as chloride and trich-loroethylene. on an underlying aquifer due to movement through small verticalfractures in an overlying clay aquitard. Vertical fractures as small as 10 or


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be on hand. There is now much standardization in ground water monitoringand. with this, much improvement in the quality of data provided by individualmonitoring wells. With these improvements in well design, well materials,sampling pumps, and field and laboratory protocols. has come a large increasein the cost of each chemical data point. A major lmpllcanonin site investi-gations of the three factors previously described (dispersion, DNAPL, andfractured aquitards) is recognition of the need for much more detailed three-dimensional hydraulic and chemical data. The recent advances in monitoringtechnologies and protocols have done little 10 fulfill this need. To meet thisneed, the cost per chemical data point must decline rather than continue toincrease.At some sites, the cost of a single nest of monitoring wells (several wells

in the nest or an equivalent modular multilevel device in a single borehole)with one full chemical data set from the nest exceeds $5 0 thousand or even$100 thousand. This includes the cost of handling and disposing of contam-inated borehole cuttings and water. If the site is a DNAPL site or is a siteon fractured rock, one nest is usually an insigni ficant step towards achievementof the level of understanding necessary for a reliable risk assessment or remedyselection. Many tens of nests per site or many more may be necessary toachieve these practical goals. II is entirely feasible at many sites (such asSUPERFUND or RCRA sites) 10 spend millions of dollars on a monitoringnetwork without producing the site data necessary for the goals [0 be accom-plished. This is no t to say that the achievements in monitoring of the pastdecade have been ill-advised; rather, it is that they have been primarily un-idirectional. leaving an urgent need for advances of a much different naturein the 199Os.

SOME DIRECTIONS FOR DEVELOPMENT OF NEWTECHNOLOGY

This consideration of needs in monitoring technology and strategy focuseson three-dimensional complexities in ground water systems caused by weakdispersion, fractures, DNAPL, or combinations thereof. The challenge is todevelop efficient means for achieving adequate spatial and temporal distri-butions of data so that the level of understanding of the ground water systemis commensurate with the needs of site risk assessment or remedy selection!design. The practical goal should be to reduce the cost per data point and toincrease the probability of acquiring data points from the important or essentiallocations over the most relevant time scale .

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Detailed spatial snapshots of hydraulic head and ground water concentrationscan often provide an adequate data base for risk assessment or remedy selectionbecause ground water now is typically so slow that spatial dlstributions changelittle over months or even years. It can be argued mat sampling over monthsis necessary to establish credible analyses following the formation disturbancecaused by drilling. The need, however. is for drilling techniques that minimizeformation water disturbance so that immediate sampling has validity.Technically advanced versions of the piezo-cone and the screened hollow-

stem auger are examples of recent technologies for obtaining head and chem-ical data at many depths in boreholes as drilling proceeds. The screened augeris used in permeable sand or gravel to depths generally nOI greater than 40or 50 m. The piezo-eone functions best in soft silty or clay deposits in whichthe cone can be pushed under heavy load applied at surface. Because thesetechniques perfonn well only in a few types of overburden deposits andbecause of depth limitations, they can only be used in some regions. Theyoffer possibilities for drilling in DNAPL zones because frequent samplingand analysis is done as the hole advances. This provides for dril ling 10 ceasewhen concentrations indicative of DNAPL are encountered. However, thesemethods. like other drilling methods, do not prevent DNAPL from drainingdeeper in the hole if the hole penetrates below a DNAPL zone. The piezo-cone and the screened auger have yet to be connected 10 field analysis equip-ment to produce laboratory-grade analyses on-site as drilling proceeds.The technically advanced piezo-cone has sensors positioned near the tip of

the drive-point penetrating the geologic material. The advantages of these lipsensors are many. There is a need for other drill ing methods to have chemicaland pressure sensors on or near the drill bit.Use of Conventional Monitoring Wells for ContaminationScanningSampling of conventional wells using the standard protocol involves purg-

ing of several well-casing volumes before sampling. The purging is intendedto produce samples of formation water free of artifacts related 10 reactionswith well materials or gas transfer at the top of the water column. Normally,only one sampling is done after purging. The objective of this standard pro-tocol is to produce a sample representative of the formation water in theimmediate vicinity of the wel1 screen. This type of sample is referred to hereas a point sample. If the monitoring is being done to locate plumes thai couldbe narrow and therefore difficult to locate or to locate local high-concentrationzones within large complex plumes, point sampling has minimal probabilityof accomplishing these objectives.An alternative use of the conventional monitoring well is to sample the

well at the well screen using a down-hole point sampler (e.g . canister orcartridge sampler) or deep pump with a packer to avoid well casing and gas-


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130 HAZARDOUS WASTE S ITE I NVES TIGAT IONS

RADIUS OFCAPTUREFOR CONVENTIONALSAMPLING

DETECTION OFPLUME BY LONGER TERMPUMPING FOR MONITORING

AGURE 4. Schematic maps showing the radius of water withdrawal for a moni tor wellsubjected 10 a standard sampling protocol ( left) and the expanded radiuslor a protocot based on aquiler scanning (r ighl).

Investigation of Fractured AquitardsThe fractures in aquitards that are generally the most important in ground

water contamination are vertical or near-vertical. The usefulness of verticalboreholes/vertical monitoring wells is often very limited because of the lowprobability of detecting the critical fractures. Angle or horizontal boreholesprovide much greater probability of obtaining data from critical fractures. Inclay or silt aquitards. continuous cores from angled holes can be divided intonumerous segments for analysis of extracted pore water or solids. The mainlimitations in this approach are in (be drilling. The methods nonnally availablefor drilling angle holes in clay deposits, such as hollow stem augering, providelittle control at depth on the angle of the lead augers, and therefore poorinfonnation on the subsurface location of the bit. The position of deep coresis poorly known. Technologies solving this problem in the petroleum industryhave not been widely used in hydrogeology. Angled boreholes in clay aqui-tards are rare in hydrogeology even though the need is apparent. Technologytransfer in this area is lagging. Perhaps this is due to the difficulty of installingmonitor wells in angled holes. However. in many cases, few wells are neededif sufficient angle cores are taken for analysis of contaminant concentrationsin core samples.Investigations of contaminant migration in vertical or ncar vertical fractures

in fractured-rock aquitards have much different problems than those in clayaquitards. Continuous cores of fractured rock usually provide little data oncontaminant occurrence on the fracture surfaces. Water or air and water arecirculated in the borehole at all times during the coring operation. Withavailable rock coring methods this circulation flushes contaminants from thefracture surfaces. In low matrix-porosity crystalline rock such as granite, there


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13 2 HAZARDOUS WASTE S ITE INVES T IGAT IONS

obtain concentration vs, time relations for the ports. This provides insight onconcentration patterns in the fracture network.The main limiting factor inthe use of the modular systems is the cost and

turnaround lime of analyses. On-site analytical systems thai do laboratory-grade analyses with fast tum-around are needed to bring modular monitoringsystems into the mainstream of ground water contamination studies. Untilthis happens. investigations of contaminant occurrence and migration in frac-lured rock at many sites will remain hampered by inadequate spatial distri-bution of data points.The modular systems described above can only be installed in boreholesopen from lOP 10bottom at the lime the system is lowered down the borehole.

Therefore. these systems. like conventional wells. do not circumvent thepropensity for DNAPL to run down the borehole if the hole penetrates a zoneof free liquid-phase DNAPL.

CONCLUSIONThe major advances in ground water monitoring at waste disposal and

industria! sites made during the past decade pertain primarily to the design.construction, and installation of conventional monitoring wells and to theequipment and protocols for sampling these wells. Also, advanced protocolsused for routine laboratory analyses, particularly for organic chemicals. haveachieved common use. The importance of these advances notwithstanding.little progress has been made in the development of cost-effective technologiesfor detailed determination of the spatia! distribution of contamination in mosttypes of hydrogeologic systems. This deficiency severely impedes progresstowards reliable risk assessments of ground water contamination. selectionof site remedies. and monitoring of progress of remedial action. Recent studiesof dispersion in sand and gravel aquifers show weak transverse dispersion.This magnifies the difficulty of achieving adequate understanding of thespatial disuibution of contaminants using conventional wells.To decrease the cost of chemical analyses in site investigations and to

provide greater insight on the location and internal character of contaminantplumes. there is an urgent need for new technologies for reliable and accurateon-site chemical analyses. preferably technologies with a high degree ofautomation and ease-or-use. These technologies should fulfill many needssuch as rapid tum-around time so that field decisions can be made duringdrilling. development of concentration-vs.-time relations for monitor wellsand modular multilevel systems, automated analyses for pumping tests andpump-and-treat remediation, and scanning of aquifers for contamination.Many waste or industrial sites within SUPERFUND and RCRA and other

sites such as those on military land have DNAPL in the ground water lone.The monitoring methods used in DNAPL areas are generally poorly suited


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13. M acF arlane. 0 -,5 . . 1 . A . Cherry . R . W . Gillham , and E . A . Sud icky . M igrationof contam inants in groundwater at a landfill: a c as e st ud y . l. Groundwaterflo w a nd p lu me d elin ea tio n. J. Hydrology. 63 :1 -2 9 (1 98 3).

14. Schwille, F. D ense chlorinated solvents in porous and fractured m edia-m odelexperiments. T ranslated b y 1. F. Pankow . (C helsea. M I: Lew is Pub lishers.1988).

15. Huling. S . G. an d 1. W . W eaver. Dense nonaqueous phase liquids. U .S. En-v iro nm en tal P ro te ctio n A g en cy . EPN540/4-91-002. 1991.

16. Sud icky , E . A . and R . G. M cL aren. T he L aplace T ransform G alerkin T echniquefor large-K ale sim ulation of m ass transport in d lscete ly -fracrured porous m e-dia, Waler Resources Res .. i n p re ss .

17. Harrison. B .. E . A . Sudicky. and J. A . Cheny. N um erical an alysis of so lutem igration through fractured clayey deposits into underly ing aquifers. WalerResources Res .. s ub mitte d. A p ril 1991.

18. Kueper, B . H . and D . B . M cWhorter. The behavior of dense non-aqueous'phase liquids in fractured clay and rock. Ground Waler, in press.

19. Bianchi-Mosquera. G. C ., D . M . M ackay , and G. D . Hopkins. M onitoring oforganic tracer tests: recem methods and results. N ational Research and D e-velopm ent C onference on the C on trol o f H azard ous M aterials. F eb ru ary .20-22. 1991. A naheim , CA .. Proc, 502-505.20. Black. W . H . H. R. Sm ith, and F . D. Pauon. Mu lt ip le -le ve l g ro un dw at ermonitoring w ith the M P System , N WWA -AGU Conference on Surface andB orehole G eophysical M ethods and Groundw ater Instrum entation. D enver.C O. O cto be r 15-11. 1986. Proc. 41-61. -21 . Welch, S. J. and D . R . Lee. A m ultip le packer/standpipe system for groundw ate r m on ito rin g in co ns olid ate d m ed ia , Ground Water Monitoring Rev., Sum-m er. 1987.22. Dunnicliff . 1. G eo te ch nic al in stru me nta tio n fo r m on it.o rin g file d p erfo rm an ce .(N e w Y ork .: W ile y ln re rsc ie nce , 1988) pp . 136-139.

23. LeBlanc. D , R ., S . P. Garabedian, K. M. Hess, L . W . Gelhar. R . D . Qoadri.K. G. Stollerw erk, and W . W . W ood. Large-scale natural grad ient tracer testin sand and gravel. C ape Cod . MA , I. E xperim ental design and observedt ra ce r mov em en t, Waler Resource Res., 27(5):895-910 (1991).

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