Improving Estimation of Inter-basin Groundwater Flowinto Northern Yucca Flat, Nevada National Security Site,
Using Multi-models and Multi-kinds of Observations
Ming Ye1, Liying Wang1, and Karl F. Pohlmann2
1Department of Scientific Computing, Florida State University
2Division of Hydrologic Sciences, Desert Research Institute
Supported by DOE contract DE-AC52-06NA26383
andNSF Grant NSF-EAR 0911074
Nevada National Security Site and Yucca Flat
Nevada National Security Site• Located 65 miles north of Las Vegas• 928 nuclear tests from 1951 to 1992
Yucca Flat• Located at NNSS northeast corner• 739 nuclear tests were conducted
Photo courtesy of National Nuclear Security Administration / Nevada Site Office
As part of the Corrective Action Investigation, models of groundwater flow and radionuclide transport are being developed.
This study arises from DOE’senvironmental restoration program on the NNSS, specifically for Yucca Flat.
Inter-basin Groundwater Flow (Qy) from Northern Yucca Flat (Climax Mine) to Yucca Flat
• The Climax Mine area is located at the northern end of the Yucca Flat-Climax Mine CAU.
• Three tests were conducted in the very different hydrogeologic setting of the Climax Mine granite stock.
• Modeling objective: Provide simulated inter-basin groundwater flows (Qy) from the northern Yucca Flat with consideration of model uncertainty.
Estimates of Inter-basin Groundwater FlowPrevious studies show different estimates:• 1,180 m3/d (Winograd and Thordarson, 1975), but too small according to
Farnham et al. (2006)• 25,3000 m3/d (IT Corporation, 1996)• 40,829 m3/d (Belcher et al., 2004), consistent with the isotope study of
Carroll et al. (2008)• 44,407 ~ 106,622 m3/d (Pohlmann et al., 2007; Ye et al., 2010)
Estimates are inherently uncertain due to• uncertainty in the estimation methods (e.g., mass balance, isotopes, and
modeling), • uncertainty between different models, and• uncertainty in model parameters.
• Choose the estimate of 25,000 m3/d as the likely target.• Improve the estimate using multi-models and multi-kinds of
observations.
Numerical Modeling of the Death Valley Regional Flow System (DVRFS)
• U.S. Geological Survey (Belcher et al., 2004)
• MODFLOW 2000• Provides the framework
for sensitivity analysis and model calibration
• The entire DVRFS model is calibrated, as calibration data in northern Yucca Flat are limited.
• Convert from transient state to steady state
DVRFSmodelboundary
NorthernYucca Flat
Modified from Figure F-10, Belcher (2004)
Six Alternative Groundwater Flow Models• Pohlmann et al. (2007) and Ye et
al. (2010) considered a total of 25 models.
• The six most plausible ones are considered in this study.
• Two recharge models (R2 and R5)• Three geological models (G1 – G3)• Developed based on the DVRFS
framework
R2: Net infiltration model (NIM1) R5: Chloride mass balance model (CMB2)
Six Alternative Groundwater Flow Models• Pohlmann et al. (2007) and Ye et
al. (2010) considered a total of 25 models.
• The six most plausible ones are considered in this study.
• Two recharge models (R2 and R5)• Three geological models (G1 – G3)• Developed based on the DVRFS
framework
G2: Base Model G3: CP Thrust Alternative
Sensitivity Analysis and Model Calibration
• Select about 50 parameters to calibrate for each model.
• Calibrate all the six models using MODFLOW 2000.• Estimated inter-basin flow is significantly larger than
the target of 25,000 m3/day.
11,37510,76710,14010,88610,3849,883SSWR
39,50957,25227,57960,15250,44550,289Qy (m3/d)
495849584951Number of calibrated parametersG3R5G3R2G2R5G2R2G1R5G1R2Model
Reduction of Inter-basin Flow
• Small change in hydraulic conductivity may result in large change in Qy (red).
• SSWR (sum of squared weighted residual, blue) is quadratic about hydraulic conductivity.
• Reducing Qy estimate is possible by varying model parameters, but at (tolerable) sacrifice of model goodness-of-fit.K11_ICU
Qy
(m3 /d
)
SS
WR
0.002 0.004 0.006 0.00840000
45000
50000
55000
60000
65000
70000
10000
11000
12000
13000
14000
15000
(b)
Centered parameter study (implemented in DAKOTA) shows that
Modeling Procedure• Calibrate each model. • Conduct Morris runs for each model to determine the
critical parameters to which the Qy estimate is most sensitive.
• Conduct MC simulations for the critical parameters to evaluate uncertainty in Qy and SSWR.
• For each model, select the optimum Qy (close to 25,000 m3/d) having a reasonable SSWR value.
The method is general when the target Qy changes.
11,37510,76710,14010,88610,3849,883SSWR
39,50957,25227,57960,15250,44550,289Qy (m3/d)
495849584951Number of calibrated parameters
G3R5G3R2G2R5G2R2G1R5G1R2Model
Morris AnalysisSelect 9 out of 58 parameters for model G3R2, marked in red and green
0 20000 400000
10000
20000
30000
40000
50000(a)
The method selects the parameters:
• Adjacent to northern Yucca Flat
• Upgradient and downgradient of the area
Uncertainty in Qy Estimates
• For individual models, the Qy estimate is uncertain due to parametric uncertainty.
• Different models give significantly different Qy estimates, indicative of model uncertainty.
• Examining the multimodels provides an opportunity to improving the estimate of inter-basin groundwater flow.
Smoothed histogram of Qy estimate for the individual models
Optimum Qy Estimate Select the realization with• Qy close to 25,000 m3/d and• Smallest SSWR
Original Qy and SSWR from Model Calibration39,50957,25227,57960,15250,44550,289Qy (m3/d)
11,17011,45510,67811,56012,38012,491SSWR
G3R5G3R2G2R5G2R2G1R5G1R2Model
Selected Qy and Corresponding SSWR
11,37510,76710,14010,88610,3849,883SSWR
21,74722,98819,17322,67225,74324,444Qy (m3/d)
Qy (m3/d)
SS
WR
10000 20000 30000 40000 50000 60000 70000
10000
12000
14000
16000
18000
20000(b)
0
Model G3R2
Measures of Goodness-of-FitThree kinds of observations• Head• Discharge• Constant-head
boundary flow
Simulated hydraulic head
Obs
erve
dhy
drau
liche
ad
0 200 400 600 800 1000 1200 1400 16000
200
400
600
800
1000
1200
1400
1600(a)
Simulated boundary flow and discharge
Obs
erve
dbo
unda
ryflo
wan
ddi
scha
rge
-20 -10 0 10-20
-15
-10
-5
0
5
10(b)
3.88-2.152.3611,455760Total
1.57-1.290.953715Constant-head boundary flow
3.51-2.653.4955445Discharge
3.94-2.112.3610,864700Hydraulic head
[SSWR/NOBs]1/2
Average negative weighted residual
Average positive weighted residualSSWRNumber of
observationsType of
observations
Sources of Reduction• Inflow is reduced at the northern boundary of
the northern Yucca Flat area.• This is caused by reduction of flow through
several segments along the constant-head boundary.
-34,26422,98857,252Qy at cross section J=101 (m3/d)
-33,92725,63559,562Qy at cross section J=137
65834769Qx at cross section I=216
-33,86226,46960,331Total outflow (m3/d)
-33,49719,75053,247Qy at cross section J=63
-2893,9914,280Qx at cross section I=90
03,4923,492Recharge
-33,78627,23361,019Total inflow (m3/d)
DifferenceRealization 1768Calibrated model
The estimates of constant-head boundary flow are subject to large uncertainty.
Why Not Choose Model G2R5?
11,17011,45510,67811,56012,38012,491SSWR
G3R5G3R2G2R5G2R2G1R5G1R2Model Selected Qy and Corresponding SSWR
21,74722,98819,17322,67225,74324,444Qy (m3/d)
Model G2R5 has the smallest Qy estimate and best goodness-of-fit (SSWR).
• ds
42.12
82
4,474
23,289
938
1,381
72,061
645
-7,245
-14
2,299
4,221
-5,113
-22,419
-2,281
-2,165
G2R2
37.19205.78SSWR
343-2,400500C_SILU0100
4,5592,9411,682C_OWLS1203
23,7716,90615,000C_PANA1100
49116,41315,100C_EURS0900
1,639-4,275667C_CLAY0800
64,116-17,03912,476C_STNC0700
653-9,0642,334C_GRDN0603
-7,149-13,393-2,521C_PAHR0505
-341-999-2,346C_PAHR0502
2,0475,4401,827C_PAHR0501
3,63214,2255,927C_SHPR0404
-4,665-11,459-4,959C_SHPR0403
-20,134-42,755-15,305C_SHPR0402
-2,001-3,709-4,410C_SHPR0401
-2,081-7,319-3,633C_LASV0303
G3R2G2R5ObservationObservation
• For model G2R5, simulated boundary flows have opposite directionsfrom the observed.
• This also happens for model G3R5, but not for any other models.
Conclusions• Estimates of inter-basin groundwater flow are
inherently uncertain due to variation between estimation methods and parametric and model uncertainty in numerical estimation.
• Using multiple models provides an opportunity to better improving the estimates.
• Our approach combining Morris (screening) sensitivity analysis and Monte Carlo simulation is general, and the results can be used together with any target estimate of inter-basin flow.
• The best model cannot be selected simply based on the overall SSWR, but on examination of different kinds of observations.
Hydrostratigraphic Models 1 and 2
Legend
2. Yucca Flat-Climax Mine Base
1. DVRFS Hydrogeologic Model
• North-south cross-section west of the Climax stock outcrop.• The Cenozoic volcanic units tend to be thin in the DVRFS
model.• Structural features are more complicated in the base model.
Hydrostratigraphic Models 3, 4, and 5
Figures modified from Bechtel-Nevada (2006)
West-East cross sections about5 km south of Climax stock outcrop
West-East cross sections throughClimax stock outcrop
Model 5 is a combination of Models 3 and 4
Base Model 3. CP Thrust
Base Model 4. Hydrologic Barrier
0 20000 400000
10000
20000
30000
40000
50000(a)
0 2000 4000 6000 8000 100000
5000
10000
15000
20000(b)
Contours of Qy in northern Yucca Flat for the calibrated G3R2 model at (a) cross section J=63 and (b) cross section J=101, andfor realization 1768 at (c) cross section J=63 and (d) cross section J =101.
Contours of Qy across the entire model domain for the calibrated G3R2 model at (a) cross section J=63 and (b) cross section J=101, and for realization 1768 at (c) cross section J=63 and (d) cross section J =101. The northern Yucca Flat area is marked with the black square.
The relationship between the Qyestimate and summed discharges based on 2,000 realizations of model G3R2.
The relationships between three kinds of SSWRs and Qy.