ABSTRACT
ABSTRACT: Alternative Concentration Limits (ACLs) are a viable option for groundwater compliance at mining properties. The numerical groundwater flow and transport mode is a critical tool to establishACLs at a point of compliance (POC) that can be shown to be protective at downgradient points of exposure (POE). A three-dimensional variably-saturated multilayer groundwater model was developed to support an ACL Application for a uranium mill tailings facility. The objective of the model was to simulate and predict seepage from the Panna Maria Tailings Impoundment and the long term net effect this seepage has on the groundwater in the various aquifers in the vicinity of the site and ultimately on the San Antonio River. The model domain incorporated regional and local groundwater divides and simulated unsaturated and saturated flow and contaminant transport. It was developed using MODFLOW-SURFACT to simulate flow and transport through variably saturated clay units that lie between the tailings and underlying near surface and deeper, regional sand units. The resulting calibrated model demonstrates the concentrations to be defined at POC monitoring wells that will be defined as ACLs that are protective of human health and the environment at downgradient POE locations. The ACLs will be used to establish groundwater protection criteria that are protective of human health and the environment from potential releases from the facility. The proper calibration and application of this model will provide a reliable and defensible model and a pathway for prompt regulatory approval of the ACLs.
