USING CPT AND MIP TOOLS TO CHARACTERIZE MTBE TRANSPORT IN FINE GLACIAL SEDIMENTS, WESTERN, MT

In April of 1994, a leaking 60,480L gasoline underground storage tank was removed from George's Conoco in Ronan, Montana. Investigations discovered a free product plume extending under Highway 93, with dissolved phase contamination (including MTBE) extending 460 m west to Spring Creek. Though geochemical sampling has established the general plume extent, the influence of aquifer heterogeneities on plume position and transport behavior is poorly described. The purpose of this work is to characterize the physical controls on plume migration. In addition to standard well installation and geochemical sampling, geotechnical tools including cone penetration testing (CPT) and Membrane Interface Probe (MIP), were used to examine subtle changes in sand, silt and clay. These tests were supported by site coring, grain size analyses, and lab and field hydraulic conductivity testing. CPT results revealed glacial diamict sediments, dominated by silt and fine sand with a surface deposit of clay. It appears the plume preferentially travels in sequences of sand and silt in the water table aquifer between depths of 3 to 10 m. These physical interpretations were supported by MIP vertical profiles. Both CPT and MIP tools proved useful in interpreting the physical conditions at the Ronan Site.