Direct Detection of Residual Nonaqueous Phase Liquid in the Saturated Zone Using SF6 as a Partitioning Tracer

Abstract

Sulfur hexafluoride (SF 6 ) was investigated as a potential detector of residual dense nonaqueous phase liquids (DNAPL) below the water table. In NAPL-water batch partitioning tests, SF 6 had a partitioning coefficient (K NW =concentration in NAPL÷ concentration in water) of 3211.33 to TCE, 2411.27 to DCM, and 45±1.89 to o-DCB. Tracer tests with SF 6 and bromide in packed sand columns containing residual DNAPL show that SF 6 breakthrough is retarded relative to bromide. The SF 6 retardation factor was 2.2 relative to bromide in a column containing 3.69% residual TCE and 2.6 in a column with o-DCB at 3.52% residual saturation. A relatively simple relationship based on equilibrium partitioning and retardation theory was developed that allows very accurate estimates of DNAPL residual content encountered by SF 6 during transport. Using observed retardation factors from the column tests and the experimentally determined K ow values, this simple calculation was found to predict DNAPL residual saturations to within 2.5% of that actually emplaced