Natural Attenuation of Diesel-Range Hydrocarbons in Soil

1 Natural Attenuation of Diesel-Range Hydrocarbons in Soil 

The Washrack/Treatment Area (WTA) Superfund site at McChord Air Force Base is located south of Tacoma in western Washington State. The portion of the site selected for remedial action in the Record of Decision (ROD) is the location of a 

The site is situated on a glacial outwash plain, and surface deposits consist of fill and undisturbed gravel and sand outwash containing minor silt and peat. Underlying the outwash at a depth of 20 or more feet is glacial till. Groundwater occurs under water table conditions within the highly permeable outwash deposits at a depth of 8 to 12 ft across the site. 

Recovery of LNAPL via trenches was specified in the ROD but found to be unsuccessful due to its discontinuous and relatively thin apparent thickness of less than 1 in. A backhoe test pit program revealed the hydrocarbons to be distributed over a distinct 3-ft-thick smear zone. Observation of the hydrocarbons in the smear zone indicated significant weathering may have occurred as evidenced by a silver-gray staining of petroleum on the sand and gravel. 

The dominantly unsaturated nature of the upper smear zone was also distinct in that it had a substantially lower percentage (10%) of total volatile (gasoline-range) vs. total recoverable (diesel-range) hydrocarbons. In contrast, the lower smear zone contained 25% total volatile hydrocarbons. The difference was thought to be due to the dominantly saturated nature of the lower smear zone, which limited the amount of volatilization that could occur. However, the dramatic difference in TPH concentration was thought to be largely attributable to intrinsic bioremediation. 

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Site Conditions Favorable for Natural Attenuation of Diesel-Range Hydrocarbons in Soil, McChord Air Force Base, Washington... 

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