environmental fate and transport modeling of …
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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