Retrospective Study of Closed Leaking Underground Storage Tank (UST) Sites in Wisconsin

Wisconsin state regulators now routinely close sites contaminated by petroleum from leaking USTs. Groundwater may still be adversely affected but natural attenuation (NA) is expected to improve groundwater quality to applicable standards. We looked at the protocol for NA closures retrospectively by analyzing information collected from 133 closed sites. At 10 of them, we collected groundwater data 5 to 8 years after their closures. We report on the results that may reshape the current protocol.

Analysis of the database of 133 sites indicated generally good characterization of horizontal flow directions. However, relatively few (26%) of the sites had piezometers (screens submerged below the water-table), so vertical flow characterization was generally poor. At sites where horizontal flow direction was determined at least twice, half had variations of 33^o or more. For 38 sites where no active remediation occurred after UST removal, we found a median of just over 1 year for the time between the observations of their historical-maximum benzene and their closure-maximum benzene. Concentration trend analysis – intended to document “plume stability” – is problematic, not only because of insufficient data, but also because the nonparametric statistical tests we applied on the benzene levels from near-source wells yielded inconclusive results for nearly half of the sites with enough monitoring rounds for the tests.

From 2004 to 2006, we monitored the groundwater quality at 10 sites where we installed temporary wells at locations where benzene had been previously detected. At 7 sites, including all 4 former retail stations among the 10 sites, the post-closure BTEX plume lengths were as long as or longer than previous estimates at closure. While benzene was detected in very few of the temporary wells, naphthalene was detected more frequently, and detected in wells further downgradient than the benzene plumes outlined at closure.