ARCHIVED REPORTS XR0009985 - 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) Periodic groundwater mounding present in the vicinity of MW-3R and MW-9R is attributed to the recent soil excavation and replacement of native soil/sediment with somewhat more porous fill material TPHG concentrations increased slightly and Benzene was not detected in MW-2 Well MW-9R showed that THG was present but volatiles were not detected (see Figures 2, 3 and 4) Coiitatnunants were not detected in well MW-3R 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 any well 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 in these graphs 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 to the vadose zone and upper aquifer in the area around MW-3R and 9R Concentrations appear to continue a stable and/or declining trend with contaminants not detected in MW-3R 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, MW- -9R and MW-2 this quarter 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. In our opinion, the mapped contaminant dissolved plume continues to occur near the center of the site, similar to historic site observations. The TPHG and Benzene levels show a much lower dissolved concentrations in monitoring well MW-9R that is attributed to removal of residual contaminant Contaminants were not detected in well MW-3R this quarter 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