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Using Direct‐Push Tools to Map Hydrostratigraphy and Predict MTBE Plume Diving

At a number of sites, a plume of methyl tert‐butyl ether (MTBE) in ground water has dived below the screen of conventional monitoring wells and escaped detection. Techniques are needed to predict the vertical extent of MTBE in an aquifer. Two techniques that are emerging in the site characterization market are electrical conductivity logging and pneumatic slug testing performed in temporary push wells. These techniques were evaluated at a diving plume of MTBE in the aquifer that supplies water to the village of East Alton, Illinois. The plume stayed near the water table for the first 100 m from the potential sources and then dived below conventional monitoring over the next 100 m. At the location where the MTBE plume dived, the depth to water was 9.1 m below land surface. The first 10 m of material below the water table had an electrical conductivity near 100 mS/m, indicating silts and clays. An electrical conductivity near 25 mS/m, indicating sands or gravels, was encountered at a depth of 10.6 m below the water table, and the sands and gravel extended to a depth of at least 15.2 m below the water table. Pneumatic slug tests measured low hydraulic conductivity in the interval of silt and clay (0.34 and 0.012 m/d) and higher hydraulic conductivity in the interval with sands and gravels (12.5, 11.6, and 11.3 m/d). Ground water with the highest concentration of MTBE was produced just below the contact between the silt and clay and the sands and gravel.