Laserfiche WebLink
v <br /> Mr. Carl Navarra Page 2 of 4 <br /> 7500 W. Eleventh Street February 23, 2010 <br /> Tracy, CA <br /> the potential for vapor intrusion issue, therefore ITSJCI developed site-specific ESLs to <br /> make the evaluation based on contaminant species and concentrations in soil and <br /> groundwater, assuming groundwater was not a drinking water source. TSJCI determined <br /> that vapor intrusion was not a significant issue for the subject site or the property at 7501 <br /> W. Eleventh Street, Tracy. <br /> These factors favor site closure consideration, however, the EHD notes the following: <br /> • Plotting the trend of MTBE concentrations over time on a semi-log scale does not <br /> convincingly demonstrate significant degradation of MTBE in the plume core area, <br /> especially if the initial value from monitoring well MW-3 is omitted. That concentration, <br /> not confirmed by any subsequent sample analyses, dropped nearly an order of <br /> magnitude by the second monitoring event, to a concentration that is in the normal range <br /> of concentration variation over time. This leads the EHD to consider the initial <br /> concentration to not be representative of normal conditions in shallow groundwater at <br /> that location, and may be an artifact from the well installation. <br /> • The site-specific Tier II evaluations of contaminant concentrations in soil and <br /> groundwater utilized the "groundwater not a source of drinking water" scenario for <br /> justifying leaving impacted soil and groundwater in place and for evaluating the potential <br /> for vapor intrusion. The Central Valley Regional Water Quality Control Board reminded <br /> the EHD that all groundwater in the region is considered a potential drinking water <br /> source and that the Basin Plan must be modified to exclude groundwater from that <br /> designation in any area. <br /> The following plots demonstrate contaminant concentration trends in the source area and in <br /> selected down-gradient wells: <br /> MTBE Concentration Trends <br /> 10,000 <br /> 1,000 <br /> a <br /> CL �-MW-3 MTBE <br /> a <br /> vi �-MW-4 MTBE <br /> c <br /> c MW-6 MTBE <br /> a 100 <br /> - -MW-13 MTBE <br /> C <br /> MW-18 MTBE <br /> O <br /> U <br /> 10 <br /> 1 <br /> 5P <br /> Ip <br /> Sampling Dates <br /> Site Closure Request Response Letter 0210 <br />