Publication Summary

Groundwater samples for BTEX, TPH-G, and VOCs were collected in October 2006 and March 2007 from 25 monitoring wells in downtown Montesano to determine the lateral extent of contamination in the surficial aquifer.

High concentrations of gasoline-range petroleum hydrocarbons were detected in wells from three source areas: Tony′s Short Stop/Grays Harbor Grange, Key Bank (Sterling)/Whitney′s Inc., and Brumfield-Twidwell.

Benzene exceeded the MTCA Method A cleanup level of 5 ug/L in ten wells, with concentrations ranging from 14 to 28,000 ug/L. TPH-G exceeded the cleanup level of 800 ug/L in eight wells (1200 to 490,000 ug/L). Tetrachloroethene exceeded the cleanup level of 5 ug/L in one Key Bank well.

Groundwater beneath downtown Montesano is contaminated with petroleum products. The contamination is largely the result of releases from leaking underground storage tanks. To characterize the lateral extent of contamination, the Washington State Department of Ecology (Ecology) collected groundwater samples from 25 monitoring wells during October 2006 and March 2007. Samples were analyzed for benzene, toluene, ethylbenzene, and xylene (BTEX), total petroleum hydrocarbons as gasoline (TPH-G), and volatile organic compounds (VOCs).

The highest concentrations of gasoline-range petroleum hydrocarbons were detected in monitoring wells at or near three previously identified source areas: Tony′s Short Stop/Grays Harbor Grange, Whitney′s Inc./Key Bank (Sterling), and Brumfield-Twidwell (GeoEngineers, 2005). BTEX and TPH-G concentrations in samples collected from these wells were above (exceeded) the Model Toxic Control Act (MTCA) cleanup levels for groundwater. Benzene concentrations across the site ranged from non-detect to 28,000 ug/L. TPH-G concentrations ranged from non-detect to 490,000 ug/L. Free-phase petroleum product was also observed in wells at the three source areas, which continue to serve as sources of dissolved phase contamination.

Chlorinated solvents have also been detected in some wells at the three source areas. Most VOC concentrations were near or below the laboratory practical quantitation limits. However, tetrachloroethene exceeded the MTCA Method A cleanup level for groundwater (5 ug/L) in well KBMW-1. The occurrence of chlorinated solvents in wells at Whitney′s Inc./Key Bank (Sterling) suggests the presence of a VOC contaminant plume in this portion of the project area.

Ecology also collected water samples from the City′s storm drain and abandoned sanitary sewer to determine if these underground utilities were providing another contaminant migration pathway. Petroleum related contaminants were detected in some of the samples.

Because of the high concentrations of groundwater contamination and the potential for contaminants to continue to migrate from the three source areas, additional investigations are being conducted to better define the nature and extent of the contamination.

Link to EIM data for User Study ID PMART004