Formatted Title

Establishing Compliance Points in Spring-Fed Surface Water for Effective Remediation and Management of Contaminated Groundwater in a Karst Aquifer

Background/Objectives

Well-developed karst features and presence of dense non-aqueous phase liquids to depths up to 450 feet below ground surface complicate site characterization and remediation on a 230-acre formerly used defense site in Pennsylvania (PA). An interim pump-and-treat (P&T) system has been operating at 220 gallons per minute since 1994, and its effectiveness in reducing concentrations of the contaminants has reached asymptotic levels in the source area. Application of multiple dye-tracer studies and infrared thermography surveys identified three submerged springs in a nearby creek to the west, and data collected during shutdown and restart-up tests of the extraction wells confirmed that these springs discharge contaminated groundwater from the site into the surface water. Because of site groundwater entering the creek, monitoring points and water quality criteria need to be established in the creek to verify compliance with surface water quality criteria.

Approach/Activities

Under Comprehensive Environmental Response, Compensation, and Liability Act or Resource Conservation Recovery Act corrective action programs, the point of compliance in surface water is the point at which releases enter the surface water. However, several states including PA allow establishment of compliance points through modeling. The PENTOXSD model, recommended by PA Department of Environmental Protection (PADEP), was used as the mass balance technique to determine the point of application for the water quality criteria in the creek. The model simulates a discharge to surface water and evaluates receiving stream concentrations at specified design flows and Criteria Compliance Times developed using the U.S. Environmental Protection Agency ambient mixing equation. The PENTOXSD model was applied with conservative calculations for all variables to ensure protection of human health and the environment. The modeling results were selected as guidance to determine the compliance distances: (1) 350 feet downstream of each modeled discharge location for the Acute Fish Criteria or Criteria Maximum Concentration, (2) 2,700 feet downstream for the Chronic Fish Criteria or Criteria Continuous Concentration, and (3) 5,900 feet downstream for the human health criteria. Sensitivity analyses showed that the model was not sensitive to reasonable variations in spring discharge rates but was sensitive to changes in the receiving stream geometry. Based on the modeling results and conservative principles for protection of human health and the environment, 12 surface water compliance locations and associated water quality criteria were identified for monitoring in the creek for the three submerged springs.

Results/Lessons Learned

Monthly monitoring of these identified surface water compliance points started in September 2019 when they were approved by PADEP. The first two years of monitoring were conducted when the P&T system was operating, and the results verified compliance with water quality criteria in the creek under pumping conditions. Since September 2021, the monitoring has been conducted with the P&T system shut down, and the results have been showing continuous compliance with water quality criteria in the creek under non-pumping conditions. Data will continue to be collected at these locations under non-pumping conditions to ultimately optimize the P&T system in the future.