29 APR 09

Subsurface Mass Computation

Hadnot Point and Vicinity, Camp Lejeune, North Carolina

Barbara Anderson, P.E., MSEnvEATSDR

The findings and conclusions in this presentation are those of the author and do not necessarily represent the views of the Agency for Toxic Substances and Disease Registry or the U.S. Department of Health and Human Services

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Overview

• Site locations, contaminant statistics

• Mass computation – purpose, scope, methods

• Illustration of mass computation for TCE

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Site locations within the Study Area

Landfill Area

Hadnot PointIndustrial Area

Site 88

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Groundwater Contaminant Data

1984 – 2004 ….. Timeframe of available contaminant data

868 ….. Wells, boreholes, and hydropunch locations2,420 ….. Groundwater samples analyzed for PCE, TCE, DCE, Vinyl Chloride

2,611 ….. Groundwater samples analyzed for BTEX

Maximum detected concentrations in groundwater, in µg/L170,000 ….. PCE180,000 ….. TCE

36,000 ….. Benzene

* Based on contaminant data that ATSDR had received and reviewed through February 2009.

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Computation of Contaminant Mass at Hadnot Point and Vicinity

Purpose

• Calibration of fate and transport model (provides starting point and lower limit for mass loading)

• Assessment of plume stability over time• Comparison to other, similar sites

Scope

• PCE, TCE, and Benzene• Dissolved phase contaminant mass (some unsaturated zone and

free product areas considered)• Multiple areas across the study site

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General Methodology

(1)

Select and prepare contaminant data sets (point data)

(2)

Develop two-dimensional (horizontal) concentration grids/distributions using interpolation techniques

(3)

Calculate average contaminant concentration across two-dimensional horizontal plume

(4)

Contaminant mass = Ave. conc. across horiz. plume x

Planar area of plume x Aquifer thickness x

Aquifer porosity x

Conversion factors

Aquifer thickness

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Data Preparation and Interpolation

Data preparation• Select data sets by considering

– Horizontal distribution of contaminants (identify sites within the study area)– Vertical distribution (sample altitudes)– Temporal distribution (sample collection dates)

• Multiple detections at same location: using average vs. maximum values

• Nondetects and censored nondetects: set to zero

Interpolation• Ordinary kriging using standard, default assumptions within Surfer software

• 10 foot x 10 foot grid cell size

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• Landfill Area

• Hadnot Point Industrial Area 1 (HPIA 1)

• Hadnot Point Industrial Area 2 (HPIA 2)

• Site 88

1984 1988 1992 1996 2000 2004Time, in years

0

40

80

120

Num

ber

of T

CE g

roun

dwat

er a

naly

ses

58

139

16

28

60

45

101

73

59 6166

37

72

Number of availableTCE groundwater analyses per year for the Landfill Area

Extraction well startup(October 1996)

Illustration: TCE Mass Computation

LandfillArea

HPIA 1

HPIA 2

Site 88

Data range: 1984–1993

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2500000 2501000 2502000 2503000 2504000 2505000

Easting coordinates, in feet (State Plane Coordinate System for North Carolina, NAD 1983 datum)

-250

-200

-150

-100

-50

0

50

Sam

ple

altit

ude,

in fe

et re

fere

nced

to th

e N

atio

nal G

eode

tic V

ertic

al D

atum

(NG

VD

)

-250

-200

-150

-100

-50

0

50

ExplanationBelow detection limitTCE detected in groundwater Symbol size is proportional to concentration Min = 0.5 µg/L, Max = 58,000 0.5 µg/L

West East

Minimum0.5 µg/L

Maximum58,000 µg/L

Brewster Boulevard Aquifer System

Tarawa Terrace Aquifer System

Castle Hayne Aquifer System

Vertical distribution of TCE, Landfill Area, 1984–1993

Upper Castle Hayne Aquifer – River Bend unit (UCHRBU)

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2500000 2501000 2502000 2503000 2504000 2505000 2506000 Easting coordinates, in feet (North Carolina State Plane Coordinate System, NAD 1983 datum)

342000

343000

344000

345000

346000

347000

348000

349000

350000

351000

352000

Nor

thin

g co

ordi

nate

s, in

feet

(N

orth

Car

olin

a S

tate

Pla

ne C

oord

inat

e S

yste

m, N

AD

198

3 da

tum

)

0

60

6.4

37

0.9

54000

1.65

1.2

0

0

6350

20000

03.1

0

4.05

7949

34

0

100

500

1000

5000

10000

20000

TCE Concentrations in the Lower Aquifer (UCHRBU) Landfill Area, 1984–1993

• Use Surfer’s grid volume report utility to obtain:– Grid “volume”, in µg/L - ft2 (essentially the sum of

the area-weighted concentration for each grid cell)– Planar area of the plume, in ft2

• Average TCE concentration for the plume, in µg /L = Grid “volume" / Planar area of the plume (Ricker 2008)

Ricker JA. A Practical Method to Evaluate Ground Water Contaminant Plume Stability. Ground Water Monitoring & Remediation. 2008; 28(40):85–94. *** D R A F T - SUBJECT TO CHANGE ***

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TCE mass [grams] = TCE average concentration [µg/L] x Planar area [ft2] x Aquifer thickness [ft] x Aquifer porosity x 28.31685 [L/ft3] x 0.000001 [g/µg]

Planar area of the plume 11,862,377 ft2 Calculated using Surfer utility

Average TCEconcentration

4,821 µg/L Calculated as “volume” of concentration grid divided by planar area of plume (both values obtained using Surfer utility)

Aquifer thickness 22 ft2 Average estimated thickness of Upper Castle Hayne –River Bend Unit (UCHRBU) in the Landfill Area

Aquifer effective porosityAquifer total porosity

0.200.40

Estimated porosities for UCHRBU from site-specific data

Conversion factors 28.31685 Liters per cubic foot

1 x 10-6 Grams per microgram

Dissolved phase TCE 7,100,000 grams14,000,000 grams

1,300 gallons (using effective porosity)

2,600 gallons (using total porosity)

TCE Concentrations in the Lower Aquifer (UCHRBU)Landfill Area, 1984–1993

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Questions?

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