Groundwater Model Sensitivity Analysis to Support Design of Pump-and-Treat System at Former Landfill

Numerous contaminants found in groundwater beneath a former landfill in Southern California pose human health risks and may impact the underlying aquifer.  The remedial alternative for the site involved installation of groundwater extraction and treatment (GET) system along the 5000-foot-long downgradient side of the landfill.  To support the GET system design a 3D numerical groundwater flow model was developed using MODFLOW with MODPATH.  Modeling approach included steady-state and transient model applications with extensive sensitivity analyses.  The layer-specific distribution of hydraulic conductivities was obtained through calibration of the unstressed steady-state flow model.  The transient model was used to simulate observed groundwater level rise over time and evaluate its effects on the GET system operation.  Groundwater capture scenarios were assessed with 3D particle tracking pathlines from the steady-state and transient flow simulations.  Model sensitivity analysis evaluated influence of changing parameter values on ability of the system to maintain effective capture.  

The target capture zone for the planned GET system included the whole landfill area. In cross section the system aimed at controlling groundwater contaminants beneath the landfill without letting them escape through the underlying units.  The model base case for effective capture included 29 wells spaced 100 to 500 feet from each other and pumping cumulatively at 15 gpm.  Hydraulic conductivity, regional gradient (both the magnitude and direction), and recharge were identified as main controlling parameters.  A parameter value changed from the base case would result in particles bypassing the extraction wells.  The influence of a parameter on overall performance of the GET system was then quantified in terms of combined pumping rate, number of wells, and well spacing required for effective capture.  Through extensive model sensitivity analyses the optimal system design was selected that ensured effective hydraulic containment of contaminated groundwater with minimal number of extraction wells and low pumping rates.