Publication Summary

Groundwater samples for BTEX, TPH-G, and VOCs were collected in October 2005 and March 2006 from 20 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 eleven wells, with concentrations ranging from 7.2 to 8400 ug/L. TPH-G exceeded the cleanup level of 800 ug/L in seven wells (990 to 78,000 ug/L). Tetrachloroethene exceeded the cleanup level of 5 ug/L in two wells.

Groundwater beneath downtown Montesano is contaminated with petroleum products. The contamination is likely the result of historic releases from leaking underground storage tanks. To characterize the lateral extent of groundwater contamination in the downtown area, samples were collected from 20 existing monitoring wells during October 2005 and March 2006. Samples were submitted for analysis of benzene, toluene, ethylbenzene, and xylene (BTEX), as well as 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 from three sites which have been identified as source areas. These sites are Tony′s Short Stop/Grays Harbor Grange, Key Bank (Sterling)/Whitney′s Inc., and Brumfield-Twidwell. 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 exceeded the MTCA Method A cleanup level of 5 ug/L in 11 of the sampled wells, with concentrations ranging from 7.2 to 8400 ug/L. TPH-G concentrations exceeded the MTCA Method A cleanup level of 800 ug/L in seven of the sampled wells, with concentrations ranging from 990 to 78,000 ug/L.

In addition to petroleum components, some chlorinated compounds were detected in selected wells sampled at the three source areas. Most concentrations were near or below the practical quantitation limits. However, tetrachloroethene was detected above the MTCA Method A cleanup level for groundwater of 5 ug/L in wells KBMW-1 and KBMW-2.

Results of this project confirm that the surficial aquifer is contaminated with gasoline-range petroleum hydrocarbons, including free-phase petroleum product at the Key Bank (Sterling) and Tony′s Short Stop sites. Groundwater is also contaminated with chlorinated compounds in some locations.

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

Link to EIM data for User Study ID PMART004