environmental fate and transport modeling of …

ENVIRONMENTAL FATE AND TRANSPORTMODELING OF EXPLOSIVES IN THEUNSATURATED ZONE

Joseph Robb (AMEC)Jay Clausen (AMEC)

Bill Gallagher (MAARNG)

National Defense Industrial Association29th Environmental and Energy Symposium & ExhibitionApril 7-10 2003 Richmond, VA

MODEL OBJECTIVES

• Determine the likelihood that explosives wouldmigrate to the water table

• Determine the appropriate soil action level forexplosives that migrate to the water table.

SEASONAL SOIL COMPARTMENT MODEL (SESOIL)

Precipitation

Volatilization

and DiffusionEquilibrium

Partitioning

Recharge

• One dimensional

vertical transport model

for unsaturated zone

• Simulates water

movement, sediment

transport and pollutant

fate and transport

Downward

transport

Evapotranspiration

SESOIL MODEL DEVELOPMENT FOR MMR

• Model divided into 4 layers with 10 sublayers each

• Meteorological Data from Hatchville, MA Station

• Site-specific soil properties

• Chemical constants

MODEL CALIBRATION

Variables

Calibration Targets

12.3%12.2 to 12.4%Soil Moisture

Calibrated ValueAcceptable RangeParameter

0.1 cm/yr0 cm/yrSurface Runoff

69.8 cm/yr60 to 70 cm/yrRecharge

46.5 cm/yr45 to 55 cm/yrEvapotranspiration

3.8E-09 cm22.0E-09 to 1.0E-08 cm2IntrinsicPermeability

3.93.7 to 4.0DisconnectednessIndex

0.430.25 to 0.45Effective Porosity

SENSITIVITY ANALYSIS – EFFECT ON SOILMOISTURE

11

11.5

12

12.5

13

13.5

14

Porosity D.I. Perm. Precip.

OrignialPlus 1%Plus 10%

Soi

l Moi

stur

e (%

)

SENSITIVITY ANALYSIS – EFFECT ONRECHARGE

6062646668707274767880

Porosity D.I. Perm. Precip.

OrignialPlus 1%Plus 10%

Gro

un

dw

ater

Rec

har

ge

(cm

/yr)

SENSITIVITY ANALYSIS – EFFECT ONEVAPOTRANSPIRATION

4546474849505152535455

Porosity D.I. Perm. Precip.

OrignialPlus 1%Plus 10%

Eva

po

tran

spir

atin

(cm

/yr)

USE OF SESOIL TO DERIVE SOIL CLEAN-UPSTANDARDS

• SESOIL developed for EPA in 1981

• Used by NJDEP, ORDEQ, HIDOH, MADEP, WIDNR

OVERALL APPROACH - RDX

• Utilize SESOIL and Summers Groundwater MixingZone models

• Perform transport calibration so model predictsaverage observed mass flux and average observedgroundwater concentration

• Use calibrated model to calculate soil concentrationthat results in 2 ug/L RDX in groundwater (EPALifetime Health Advisory)

DEMOLITION AREA 1

MODEL SETUP FOR DEMO 1

• Depth of Soil Contamination = 1 ft (0.3 m)

• Depth to water table = 40 ft (12.2m)

• Organic carbon = 0.5% (0-12 ft) and 0.01% (12-40 ft)

• Bulk Density = 1.8 g/ml

• Time = 100 years

MODEL INPUT - CHEMICAL PROPERTIES

1E-04Henry’s Law Constant(m3-atm/mole)

70.8Koc (L/kg)

38.4Solubility (mg/L)

RDX

NO2O2N

NO2

N N

N

SESOIL + SUMMERS MODEL

CGW = [(QPCP) + (QaCa)]

(Qa + Qp)

QPCP

QaCa

Vadose Zone

Saturated Zone

QP = Flow through vadose zone

CP = Soil concentration

Qa = Flow through aquifer

Ca = Groundwater concentration

TRANSPORT CALIBRATION

• Estimate soil concentration (average, area weightedaverage, geometric mean, median)

• Vary source size to match observed mass flux (0.4 –0.5 kg/yr)

• Vary mixing zone size to match average groundwaterconcentration (115 ug/L RDX)

DEMO 1 RDX IN SOIL

020406080

100120140160

Freq

uenc

y

0.1 0.2 1 5 10 100 More

Concentration (mg/kg)

Average (all results) = 80.17 mg/kg

Area weighted avg. = 22.32 mg/kg

Geometric mean = 0.12 mg/kg

Median = 0.06 mg/kg

TRANSPORT CALIBRATION RESULTS

0.35

0.5

0.5

0.5

PredictedMass Flux(kg/yr)

1.6 x 108

(4 acres)

1.1 x 108

(2.7 acres)

6.5 x 105

(700 ft2)

2 x 105

(220 ft2)

SourceSize* (cm2)

Predicted GWConcentration(ug/L)

SoilConcentration(mg/kg)

0.06

0.12

22.32

80.17

2.6Median

49Geometricmean

115Areaweightedaverage

115Average(all results)

PRELIMINARY RDX SOIL CLEANUPSTANDARDS

21.22 x 105 (220 ft2)(based on averagecalibration)

20.46.5 x 105 (700 ft2)(based on area weightedaverage calibration)

Predicted GWConcentration(ug/L)

SourceConcentration(mg/kg)

Source Size (cm2)

COMPARISON OF RESULTS

• AFCEE for CS-19 Site: 5.5 mg/kg

• INEEL: 0.2 to 2.0 mg/kg (currently under review)

• AMEC: 0.4 to 1.2 mg/kg –Preliminary value–Currently under EPA review

SENSITIVITY ANALYSIS

• High Sensitivity– Estimate of initial soil concentration– Estimate of average groundwater concentration

• Moderate Sensitivity– Source thickness– Mixing zone thickness– Mixing zone length

• Low Sensitivity– Number of sublayers per layer– Literature Koc vs. Laboratory Measured Kd values

THANKS

• Impact Area Groundwater Study Program

• US Army Corp of Engineers – New England District

• Air Force Center for Environmental Excellence

POTENTIAL NEXT STEPS

• Install Lysimeters to measure RDX in vadose zonepore water

• Refine SESOIL model based on observations