The fourth 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 December 2011 monitoring of ground water of the 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 toluene, ethylbenzene or xylenes on site. A slight benzene recording was detected in monitoring well no. 4, which is consistent with results from the past two years.
“The detected groundwater hydrocarbon concentration continues to decline,” Manning wrote in the report. “The detected benzene in on-site wells (1 and 4) was slightly above drinking water MCL [maximum contaminant level] for benzene (1.0 micrograms per liter (µg/l)).”
For the 10th consecutive quarterly sample, no methyl-tertiary-butyl ethane (MTBE) was detected in monitoring well no. 1, directly on the site. In December, MTBE was only detected in off-site wells.
MTBE was detected from the Chevron station south and southwest to monitoring wells no. 5, no. 6, no. 7, no. 8, no. 10 and no. 15. Consistent with the past several years, the greatest MTBE concentration — 2,220 μg/l — was found in the water from well no. 10. This is half of the October reading. The MTBE results consistently fell this quarter and were the lowest readings since 2000 for most wells.
While the total petroleum hydrocarbon, benzene and toluene readings for wells no. 1 and no. 4 were higher than the October 2011 results, they were lower than other quarterly readings and in some cases significantly lower.