ARCHIVED REPORTS XR0009987 - Laserfiche WebLink

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2.0 Discussion Groundwater flow direction is northerly at a flow gradient that is consistent with historically observed northerly flow trends (see Figure 1) Sinuous groundwater contours present in the vicmity of MW-3R is attributed to the recent soil excavation and placement of somewhat more porous fill TPHG concentrations declined in MW-2, MW-3R and in MW-9R since the fourth quarter 2004 (see Figures 2 and 3) Benzene concentrations rose slightly in Well MW-2 and decreased slightly in MW-9R Well MW-4 showed that contaminants were not detected MTBE and related oxygenates were not detected in any well Separate phase product was not observed in MW-3R and MW-9R during this sampling event. Selected combined Time vs Contaminant Graphs, (combined data of MW-3 and MW-3R, and combined data of MW-9 and MW-9R) were prepared to show overall historic dissolved trends that are attached to this report Wright has added the data from MW-3R and 9R to the same graphs since these wells were installed only about 5 feet from the previous locations It is Wright's opinion that the combining the historic data to the new wells data will enable ease in continued contaminant trend tracking in the areas of historically most elevated TPHG and Benzene plumes Soil excavation activities removed residual contaminant in the vadose zone and upper aquifer in the area around MW-3R and 9R While some residual is known to remain, the dissolved TPHG and Benzene concentrations have decreased The interpreted location of the dissolved plume based on the historic groundwater flow and interpreted contour information continues to generally show very little to no net plume movement beyond the vicinity of Wells MW-3R and MW-9R 3.0 Conclusions and Recommendations Groundwater flow direction was northerly under a low calculated gradient and water levels in monitoring wells had changed slightly since the previous quarter The excavation area near MW- 3R and MW-9R has somewhat more porosity than the native soil/sediment and may cause some localized groundwater interpreted contour sinuosity near Well MW-3R In our opinion, the mapped contaminant dissolved plume continues to occur near the center of the site, similar to historic site observations and previous investigation reports submitted to SJC/EHD The TPHG and Benzene levels show a much lower dissolved concentrations in monitoring wells MW-3R and MW-9R that is attributed to removal of residual contaminant Fluctuations of TPHG and Benzene may occur from desorption since some residual sources under the large utility pipelines could not be removed during the soil excavation given the seasonal rise and fall of groundwater levels The site is scheduled for continued quarterly monitoring at this time Page 3 of 4