ARCHIVED REPORTS XR0002723 - Laserfiche WebLink

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'3 groundwater has been impacted since the last groundwater samples were collected in February 1993 . Groundwater levels have risen by several feet throughout the County since late 1992 , and site- specific data indicate that groundwater has risen more than 5 feet since December 1992 . As a result, groundwater is now present at a depth of less than 50 feet, and has probably contacted '. contaminated soil in the vadose zone. The presence of dissolved hydrocarbons in MW-5, where no soil contamination was detected, s appears to rule out the possibility of contamination during drilling and indicates that groundwater contamination in that well is probably due to down-gradient groundwater flow. At present, the data are too limited to contour contamination values or define the 1 extent of groundwater contamination. GeoAudit recommends that the groundwater monitoring program continue. The next event should take place during the fourth quarter of 1993 . ' 5.4 Soil Remediation Concentrations -of hydrocarbons in soil samples from MW-4 and B-18 �. are relatively low, and no hydrocarbons were detected within 10 feet of the present groundwater interface. The presence of hydrocarbons in groundwater samples from MW-4 and MW-5 indicates that groundwater has been impacted, and the existence of an unexpectedly large volume of contamination within the vadose zone north of the site implies that greater ' concentrations of hydrocarbons may be present in groundwater beneath Harding Way. Remediation of soil contamination north of MW- 4 would greatly reduce the risk of future groundwater contamination, but logistical problems and the fine-grained nature of the sediment limit the remedial alternatives that would likely be effective in this case. Contaminated soil beneath Harding Way would be impossible to excavate without severely disrupting traffic on this busy thoroughfare. In-situ vapor extraction is also not feasible when all of the contamination occurs in silty clay. In- situ bioremediation has been successfully performed in fine-grained ' soil, but limited experience suggests that it is more difficult and expensive than in sandy soil. ' Contaminated soil between MW-4 and B18 could be excavated and treated with conventional on-site ex-situ methods, but the total volume of hydrocarbons in this area is clearly a small fraction of the volume of hydrocarbons north of MW-4. Remediation south of 14W-4 would therefore probably do relatively little to reduce the risk of future groundwater contamination. In view of 1) the absence of benzene in any of the soil samples collected from MW-4 and B18, 2) ' the low concentrations of TPH (< 100 ppm) , 3) the dense, fine- grained nature of the soil, and 4) the apparent lower limit of soil contamination between 35 and 40 feet, it may be possible to leave this contamination in place. ' 16