SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

I S. Soil and groundwater contamination levels encountered during phase II (borings 10-15) failed to confirm those encountered during phase I (borings 1-9). The highest concentration of hydrocarbons in soil was found to be 170,000 ppb for TPH/gasoline at five-foot sample in boring 6. This level is well below the 1,000,000 ppb action level set by the California Department of Health Services for soil excavation (State Water Resources Control Board, 1987, p.84). The highest groundwater contamination was found in the sample withdrawn from boring 6, where groundwater was encountered at about 13 feet, just after drilling into a peat material. Groundwater taken from this boring was found to contain 1.2 ug/l (equivalent to ppb) of benzene and 3800 ug/1 of gasoline. Action levels for benzene in groundwater are currently 0.7 ppb (per conversations with Robert Evans and David Brent, Central Valley Regional Water Quality Control Board, September 19, 1988). Although the benzene content of the water sample taken from boring 6 exceeds acceptable levels for drinking water we believe that the down-hole variation in measured contaminant intensity, in combination with the drilling procedures used, warrant further discussion. Boring 6 was made on the levee. During the boring we encountered 2 feet gravelly fill over about 11 feet of silty clay. At about 12.5 feet we drilled from silty clay into a peat material and encountered groundwater almost immediately. Drilling was done using hollow-stem augers, and sampling was done through the augers using a hammer-driven split spoon sampler. In accordance with standard practice the sampling devices were steam-cleaned prior to each sample drive and the auger was steam-cleaned between borings but continuously advanced between drives within any single boring. The contamination levels of the soil samples taken at 5 feet and 10 feet and the water sample taken upon termination of the boring at 15 feet are shown in Table 2. It is apparent from Table 2 that contamination encountered in boring 6 was greatest, in every measured contaminant, in the soil sample from 5 feet. Contamination in the soil sample from 10 feet was approximately 1/100 of the contamination measured at 5 feet. Contamination in the groundwater sample from 14 feet was found to be intermediate in value between the two soil samples - i.e. , groundwater was less contaminated than the sample at 5 feet, but 3 to 76 times as contaminated as the soil sample taken from above it at 10 feet. Both the analyzed soil samples were taken from an 11 foot thick auger of silty clay- probably a low-permeability material. The groundwater sample was taken from groundwater encountered when we bored into a mixture of peat and clay- probably a more permeable material. We suspect that the augerhead and plug may have contacted and retained contaminated material while the boring was deepened from the more contaminated material (5 feet) to less contaminated material (10 feet and below water at 13 feet). The greater contaminant levels measured in the groundwater sample, when compared to the more shallow sample from 10 feet, may be due to transfer of contaminants from the auger by water, to an aqueous sample.