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• Figure 1 • <br /> Converging Lines of Evidence for Evaluating Horizontal Capture <br /> (evaluating vertical capture requires additional analysis) <br /> REMAIN6 CLEAN <br /> BARRIER 115.0 <br /> LIEGEND WALL - - - I <br /> H EXTRACTION WELL // 116.1 113.2 <br /> 0 MONITORING WELL 1152 • \ 0 <br /> 115.2 WATER ELEVATION / $ <br /> (F t HEW-1 v EW4 t <br /> - -TARGET CAPTURE O H I <br /> ZONE ; EA <br /> SORE 11521 y 114.8.8 13.7 I 113.1 DECREASING <br /> ESTIMATEDPLUME p 1156 I CONCENFRATIONS <br /> BOUNDARY H EW-3 <br /> \ Y <br /> GROUNDWATER FLOW \ EW-2 <br /> DIRECTION \ '• <br /> ` 174.0 /' 0 <br /> INWARD FLOW 11.4.9r _ _ _ / 113.4 <br /> 0 150 3001150 <br /> 62 <br /> SCALE IN FEET REIAAIl'J6^LEPN <br /> Data: Background.- <br /> Hydraulic <br /> ackground.Hydraulic conductivity • barrier wall,plus wells EW-1 and EW-2,act to contain the contaminant source <br /> • K = 10 friday • EW-3 and EW4 address the downgradient plume <br /> • relatively homogeneous • target capture zone is a specified concentration contour based on risk assessment, <br /> natural attenuation addresses plume fringe <br /> Pum in • plume delineated by monitoring to the north and south <br /> • EW-1&2=3 gptn each Potential Evidence for Capture: <br /> • EW-3 &4=4 gpm each . ground water flow budget(Exhibit 4)consistent with target capture zone <br /> • fully penetrating wells 0 (Q>4 gpm required based on simple calculation,actual Q=8 gpm) <br /> Aquifer thickness • water levels demonstrate"inward flow"across barrier wall <br /> • B =20 ft • potentiometric surface indicates flow in the direction of EW-3&4,but resolution is <br /> • unconfined aquifer insufficient to confirm capture <br /> • sentinel wells downgradient to the east show decreasing concentrations,provides <br /> Target capture zone width increased confidence that capture is occurring <br /> (north to south) Next Steps: <br /> W=600 ft - delineate on a map the interpreted capture zone and compare it to target capture zone <br /> - consider seasonal variation in ground water flow and plume <br /> Hydraulic gradient • consider additional piezometers in vicinity of EW-3 and EW4 <br /> • i=0.006 fl/foot • consider use of ground water flow model and particle tracking <br /> • continue to monitor sentinel wells(concentrations should decrease to clean up levels) <br /> • ensure vertical capture <br /> potential impacts, sentinel wells would likely be Predictions from models are subject to uncertainty <br /> required in each unit of concern. based on the presence of heterogeneity in natural <br /> systems that can be difficult to characterize and <br /> Particle Tracking in Conjunction with Ground Water represent in the model. Ideally, the numerical model <br /> Modeling. Particle tracking in conjunction with a can be"verified"by reproducing measured drawdown <br /> ground water flow model can indicate if all model cells responses to various pumping scenarios, increasing <br /> within a target capture zone are captured by a simulated confidence in the model's ability to accurately predict <br /> extraction system. A three-dimensional model can be capture. <br /> particularly helpful in evaluating capture at sites where <br /> vertical heterogeneity and/or migration greatly affect Tracer Tests. Demonstrating capture of a tracer <br /> contaminant fate and transport. However, the injected into the contaminant plume can increase the <br /> reliability of this line of evidence for interpreting actual confidence that capture of the plume has been <br /> capture depends on the reliability of the model. achieved. Valuable data can be obtained from <br /> 6 <br />