Manley Hot Springs Gravel Pit
Past Work on Site
In September 1989, the Alaska Department of Environmental Conservation (ADEC) sampled three orphan drums found near the Manley gravel pit. The drums contained rusty water and residues of the herbicides 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 2,4-dichlorophenoxyacetic acid (2,4-D). A lack of vegetation surrounding the drums suggested the contents of one drum had been released at this location. An August 1990 interim remedial action included the removal of an approximately 0.5-foot-thick layer of soil in the area devoid of vegetation. Clean fill was placed in the excavated area, and the contaminated soils and product drums were shipped to a hazardous waste treatment facility in Kansas. In July and August of 1993, Ecology & Environment, Inc. (E&E) and the ADEC performed an assessment to characterize the extent of contamination. Sample results indicated the highest 2,4,5-T and 2,4-D herbicide concentrations remained in the soil just beyond the limits of the 1990 excavation. Herbicides containing the compound 2,4,5-T are known to be contaminated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), a product of the reaction of 2,4,5-T derivatives during herbicide manufacture. Only two of the eleven soil borings drilled at the site were sampled for determination of 2,3,7,8-TCDD and 2,3,7,8-tetrachlorodibenzofuran (2,3,7,8-TCDF); four soil samples were submitted for analysis from one of the borings, and one sample was submitted from the other. Four of the five samples submitted for TCDD and TCDF analysis contained detectable levels of 2,3,7,8-TCDD ranging from 418 to 24,600 picograms per gram dry soil (pg/g); one of those samples was collected from a depth of 4.5 feet below the ground surface (bgs). The fifth dioxin-analysis sample, collected from 6 to 6.5 feet bgs, did not contain dioxin or furan analytes above the laboratory practical quantitation limit (PQL) of 72 pg/g. Following their site-characterization efforts, E&E covered the spill area with a 20-foot by 10-foot sheet of 20-mil high-density polyethylene liner. In 1994 the ADEC had a chain-link fence installed around the perimeter of the spill. In May 2006, Shannon & Wilson drilled five soil borings within the fenced area and collected four surface samples outside the fenced area. A map showing the sample locations and results is provide in Appendix A. Ten samples (including the set of duplicate samples) exceeded the dioxin action level of 39 pg/g. These included three of the four surface samples collected outside the fenced area. Each of the five soil borings (B-1 through B-5) also yielded samples above the dioxin action level; the dioxin TEQ was above the limit in each of the samples collected to a depth of 0.5 foot bgs, and the sample from 2.5 to 3.0 feet bgs in Boring B-3 also exceeded the maximum allowable dioxin TEQ. Boring B-3 also yielded the sample with the highest dioxin TEQ measured to date (29,200 pg/g); this location was coincident with E&E’s highest detected concentration. Based on these sample results we recommended additional sampling to determine the extent of the dioxin contamination prior to developing a corrective action plan. In March 2007, S&W completed a report evaluating the remedial alternatives for the site and recommended capping the soil in place after delineating the full extent of the soil contamination.