Formatted Title
Simulation of Intermittent Groundwater Seep Discharges Upwelling through a Landfill Using Only Precipitation Inputs
Background/Objectives
An intermittent groundwater seep upwelling through a landfill cell, situated on the lower slopes of a mountain at the Anniston PCB Site in Anniston, Alabama, was observed subsequent to installation of a high-density polyethylene (HDPE) liner and cover system intended to prevent storm water infiltration into the landfill cell. Following installation of a subgrade-collection, conveyance, and treatment system, seep discharges up to 35 gallons per minute were observed within days of heavy precipitation events during the winter and spring months, periodically containing polychlorinated biphenyls (PCBs) and 1,4-dichlorobenzene in low part-per-billion concentrations. Seep discharges were minimal to nonexistent during the summer and fall months, even following heavy precipitation events. The objectives of this modeling study were to (1) estimate peak seep discharges in near-real time to manage and prepare for peak discharge events, and (2) estimate maximum expected seep discharges based on design storms to design and evaluate future seep mitigation alternatives.
Approach/Activities
We developed a transient, lumped-parameter hydrologic model based on a concurrently-developed conceptual model of the seep. The hydrologic model, which predicts seep discharges based solely on precipitation inputs, was calibrated to daily observations of seep discharges, precipitation rates, and nearby groundwater levels. The model has been subsequently adjusted and re-calibrated following additional observations of seep discharges. During the winter and spring months, the model is regularly updated with forecasted precipitation rates to estimate upcoming peak flows. The model has also been used to design future upgrades to the seep collection, conveyance, and treatment systems by estimating expected peak seep discharges using 10, 100, and 1000-year design storms of varying lengths as the precipitation inputs.
Results/Lessons Learned
The model has served as a useful indicator tool to provide up to 7 days of advance notice to site personnel for extreme discharge events, which often require a significant, around-the-clock operations and maintenance presence. The correlation coefficient (R2) between the modeled and observed daily seep discharges is 0.64. The model overpredicts low discharge events but predicts peak discharge events within approximately 60% of the observed peak discharges. Subsequent model adjustments have even improved the seep conceptual model, highlighting the importance of transpiration processes as a useful tool for seep mitigation.