Year of Award

2007

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

William W. Woessner

Keywords

fractured sediment, hydrocarbon contamination, MTBE, Ronan

Abstract

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 lenses of clay varying in thickness. It appears the plume preferentially travels in sequences of sand and silt in the water table aquifer between depths of 3 to 10 m, though some contamination has been discovered at greater depth and is believed to be passing underneath Spring Creek and traveling west, potentially impacting water users in the Flathead Valley. Through field and laboratory analysis, the governing hydrogeologic controls on the ground water flow system were described. A model was developed to simulate the contaminant transport and compared to observed plume movement, and a heat transport model was developed for comparison with both the modeled ground water flow and the field-observed ground water flow characteristics. It became evident that under the documented hydraulic conductivities for sediments found at this site, the velocity of plume movement was considerably more rapid than expected. This suggests that either an interconnected fracture network is responsible for the transport of hydrocarbons throughout the site, or the known date of contaminant release from George’s Conoco is unreliable.

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© Copyright 2007 Mary Kathryn Sutherland