The third quarter groundwater monitoring report of the underground plume of hydrocarbons along Highway 243 southwest of the Shell and Chevron gasoline stations indicates continued declining levels. The October 2011 monitoring of ground water of the Village Market (Shell station) site of a former leaking underground fuel tank (LUFT) again detected virtually no contaminants.
In his summary of the monitoring results, Robert C. Manning, the senior geologist and project manager, detected no significant concentrations of total petrochemicals or touluene, ethylbenzene or xylenes on site. A slight benzene recording was detected in monitoring well (MW) 4, which is consistent with results from the past two years.
For the ninth consecutive quarterly sample, no methyl-tertiary-butyl ethane (MTBE) was detected in MW 1, directly on the site. In the MW 4 sample,18.2 micrograms per liter (μg/l) were detected, a 1.5 μg/l increase above the spring test result.
In the off-site wells, MTBE was detected from the Chevron station south and southwest to MWs 5, 6, 7, 8 and 10. Consistent with the past several years, the greatest MTBE concentration, 4,400 μ/l, was found in the MW 10 water. This is nearly 20 percent lower than the spring reading. While higher than the December level of 500 μ/l, this was in the same range as the 2010 second and third quarter results, and below previous years.
The same pattern occurs in the MW 7 results — 238 μ/l in October compared to 979 μg/l in March 2011, 382 μg/l in June, and the lowest in years.
“It is our opinion, the elevated MTBE concentration in cross-gradient wells MW 10, MW 13, down-gradient well MW 7, and to a lesser extent down-gradient well MW 5 is from a Chevron gasoline station located across Hwy. 243, approximately 100 feet north of the site,” wrote Robert C. Manning, the senior geologist and project manager. “A site characterization investigation is scheduled to be conducted for the above Chevron station upon the issuing of a letter of commitment by the State Underground Cleanup Fund.”