BWRC and LEADERS Seminar Series – Steve Usher

Sophie Felleiter

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Typically, clean up of old leaking gas stations are a “dig and dump” affair.  The level of science of assessment is limited to drilling and soil sampling and testing for hydrocarbons, including water samples.  In the majority of cases this is an effective and economic solution as the contamination is usually shallow.  In this case study a lacustrine clay aquitard appeared to be of substantial thickness, and one confirmatory borehole into the confined aquifer below yielded startling results, with LNAPL trapped under the aquitard.  150 downgradient private water wells existed starting about 100 m from the site.  Initial efforts focused on the dig and dump process, but engineers wisely chose not excavate too much of the aquitard, but an understanding of WHY it was in the confined aquifer was not attained.  A proper geologic model was constructed by experienced hydrogeologists, including review of the prehistoric glacial conditions which explained the source of aquifer penetration.  Successful remedial efforts then focused on a pump and treat with hydraulic containment, turning this high risk site into one of ongoing maintenance and progressive cleanup.