DIRECT-DETECTION OF RESIDUAL NONAQUEOUS PHASE LIQUID IN THE SATURATED ZONE USING SF6 AS A PARTITIONING TRACER

Sulfur hexafluoride (SF6) was investigated as a. potential detector of residual dense nonaqueous phase liquids (DNAPL) below the water table. In NAPL-water batch partitioning tests, SF6 had a partitioning coefficient (K-NW = concentration in NAPL divided by concentration in water) of 32 +/- 1.33 to TCE, 24 +/- 1.27 to DCM, and 45 +/- 1.89 to o-DCB. Tracer tests with SF6 and bromide in packed sand columns containing residual DNAPL show that SF6 breakthrough is retarded relative to bromide. The SF6 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 SF6 during transport. Using observed retardation factors from the column tests and the experimentally determined K,, values, this simple calculation was found to predict DNAPL residual saturations to within 2.5% of that actually emplaced.