ARCHIVED REPORTS XR0009739 - Laserfiche WebLink

Home Browse Search

' Bank of Stockton Cutter Lumber Site Page 12 Groundwater Monitoring Well Construction Project No 102 2 ' April 24, 1998 ' 4.0 Conclusions and Recommendations ' 4.1 Conclusion - Distribution of Petroleum Hydrocarbons ' 4.1.1 Distribution of Hydrocarbons in Soil The laboratory analytical results from the soil samples collected while drilling ' these groundwater monitoring wells are generally consistent with those from earlier soil borings as documented in the report of April 8, 1997 The soils of the vadose zone generally consist of dense, impermeable, sticky, plastic clay and silt ' Several lenses of sand and silty sand are present, especially in semi-continuous bed at about 20 and 40 feet below grade (see Figure 8) ' Soils in the vicinity of the point of release are heavily impacted with gasoline range hydrocarbons and, to a much lesser extent, with diesel range hydrocarbons Based on the laboratory results from soil samples, detectable ' levels of gasoline are present at depths extending to at least 50 feet below the surface The impacted soil of the vadose zone (Figure 3) take the rough form of liean inverted cone which has a varying total volume depending on the level of groundwater Based on the results of this and earlier investigations, it appears the base of the cone extends about 80 feet in the East West direction and about ' 50 feet in the North-South direction Based on the results of the work described herein as well as previous work, it ' appears that the level of the surface of groundwater has varied between about 24 feet below grade as observed in this present field event, to depths of at least 43 feet below grade This broad range of fluctuation has resulted in the ' contaminant hydrocarbons being "smeared" vertically through 20 or more feet Due to the propensity of hydrocarbons to adhere to clay minerals which are abundant at the site, the "smeared out" gasoline does not quickly rise to the ' groundwater surface as the level of groundwater rises A good example of this effect is found in the soil samples from MW-6 where the highest levels of ' contamination occur at a depths some 16 feet below the surface of groundwater 4..1.2 Distribution of Hydrocarbons in Groundwater ' Groundwater Gradient and Bearing The level of groundwater has risen a full 10 feet since the most recent ' monitoring event in early 1997 As shown on the map in Figure 4, the direction of groundwater flow is to the east, but the data suggest that localized influences may alter the direction on a small scale basis ' 2937 Veneman Ave ,#B240 Geo-Phase Environmental Inc Phone (209) 569-0293 Modesto, CA 95356 Fax (209) 569-0293