Three-Dimensional Capture Zone Evaluation of the Perchlorate Remediation System at the Black Mountain Industrial Complex

The Black Mountain Industrial Complex in Henderson, Nevada is the site of multiple industrial companies and has been used for a variety of manufacturing activities that has resulted in contamination of the soil and groundwater. Perchlorate is the main constituent of concern in the groundwater, and pump and treat groundwater remediation at the Site has been ongoing for over 20 years. We developed a three-dimensional groundwater flow and contaminant transport model of the Site to evaluate the capture zone and efficiency of the pump and treat systems.

The model spans an area of over ten thousand acres and covers the extent of the perchlorate plume. Vertically, the model extends downward from the ground surface through the shallow Quaternary alluvium and several hundred feet into the Upper Muddy Creek formation. One of the challenges of modeling this Site is that the Quaternary alluvium layer is unsaturated in much of the model domain, and the standard MODFLOW solvers have difficulties converging in these drying/re-wetting situations. MODFLOW-NWT, the new Newton formulation of MODFLOW-2005, was successfully used to model this complex hydrogeologic system. The steady-state flow model was auto-calibrated to over 260 observed groundwater head data. Particle tracking was conducted to perform the capture zone analysis of the three well fields of interest. A mass transport model was also developed to quantitatively evaluate the well field capture efficiency, which is defined as the mass flux of perchlorate captured divided by the total perchlorate mass flux.

Based on the simulations, the majority of the perchlorate plume mass is being captured by the three well fields. In addition, the capture efficiency goal of 95% was met in two of the three well fields. We used the model to evaluate and recommend capture enhancements for the well field that did not meet the capture goal.