Tracer Leak Testing of the Controlled-Release Test Cell at the Groundwater Remediation Field Laboratory, Dover Air Force Base, Delaware

A two-stage dual-tracer leak test was performed on the test cell at the Groundwater Remediation Field Laboratory at Dover Air Force Base, Delaware to quantify the leakage across the cell and determine the exchange rate between system gas and the atmosphere. Ambient air containing helium and sulfur hexafluoride SF6 at known concentrations was injected into the cell at a flow rate of 1.4 scfm. Vapor was extracted from the cell at the same flow rate while monitored for helium and SF6 concentrations until steady state was achieved. The data showed 53.3 and 64.9 percent leakage for the helium and SF6 tracers, respectively. Tracer data from soil gas from the 45 soil-gas monitoring probes in the cell, and 4 additional soil-gas probes inserted between the two cell walls showed that the cell wall was leaking on the north end. An average leakage or dilution factor of 40.4 percent was calculated for the south plenum. Results from Stage 2, a tracer bleed-off test, showed a first order decrease in the concentrations of both tracers. The exchange rate was determined to be once in approximately 10.2 days.