ARCHIVED REPORTS XR0006644 - Laserfiche WebLink

Home Browse Search

ug/L, 17,000 ug/L and 20,000 ug/L. Averaging these three concentrations gives an estimated concentration of 37,667 ug/L TPHg at the source that would have arrived at MW8 on or near December 12, 2000 This shows an 81% reduction in the TPHg contaminant plume as it arrives at MW8 The 81 ug/L TPHg found at MW12 during the December 12, 2000 sampling can be back calculated to free product at MW3 and MW10 during the December 1991 through June 1992 sample events, equaling a better than a 99 9%reduction in TPHg as it arrive at MW12 The 1300 ug/L TPHg found at MW2 during the December 12, 2000 sampling is considered dtffusion of the residual TPHg off of the clays and silts located at MW2 The above estimates of degradation indicate that this 1300 ug/L will degrade to approximately 247 ug/L before it reaches MW8 and to approximately 13 ug/L as it reaches MW12 Figure 6 is the lateral extent of Benzene using sample results obtained during the December 2000 monitoring event and the samples obtained from SB8 (April 26, 2001) Figure 6 shows residual Benzene at low groundwater velocity well MW2 This well would be depleted during well purging for sampling and would take sometime to recover before a sample could be obtained. Benzene concentrations found in this well are not surprising and are isolated. Complete degradation is occurring at approximately 30 to 40 feet downgradient of MW2 Figure 7 is the lateral extent of MTBE using sample results obtained during the December 2000 monitoring event and the samples obtained from S138 (April 26, 2001). As indicated in earlier reports, MTBE groundwater sampling results indicate three distinct plumes and different sources exist Earliest evidence of MTBE was found at MW11 (8260 Method = 20 ug/L) dung the March 1997 sampling event All other wells tested below laboratory lower detection limits for MTBE using method 8020, with the exception of MW4 which showed 7 ug/L but was confirmed (8250 Method) to below laboratory lower detection limits (LLDL) of 5 ug/L and MW3 which showed 6 ug/L using method 8020 MWl1 continued to have elevated MTBE (8260 Method) ranging between 19 and 38 ug/L through the last sampling (December 12, 2000). MW 10 and MW 13 were confirmed to contain MTBE during the September 1998 sampling of 12 and 29 ug/L respectively The first MTBE occurrence at MW12 was confirmed during the November 1998 sampling at 33 ug/L, but is now below LLDL of 0.5 ug/L The first occurrence at MW2 was confirmed during the latest sampling December 2000 of 18 ug/L S138 (CPT I) contained MTBE at the 50 foot depth of 18 ug/L and at the 85 foot depth of 12 ug/L Samples obtained from the 98 and 120 foot depths were below LLDL of 0.5 ug/L. Currently the California Public Health Goal (PHG) for MTBE is 13 ug/L If the MTBE found at S138 is associated with 8603, the 18 ug/L found at SB8 is close to this goal and will continue to degrade with tune 5 R-603 May 2001