ARCHIVED REPORTS XR0010088 - Laserfiche WebLink

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r� Discussion Groundwater movement is northerly at a low flow gradient which is consistent with historically observed trends Contaminants were observed in the Wells MW-2, MW-3, MW-6 and MW-9 Concentrations of TPHG increased in Wells MW-2, MW-3, and MW-9, and increased very slightly in MW-6 MTBE and oxygenates were not observed in any well Fluctuations of observed residual contaminant concentrations are attributed to aquifer recharge and changes in water levels which allow more desorption of residuals These trends are typical of old and highly degraded plumes where local groundwater conditions and desorption phenomena cause slight variations in concentrations The concentrations increase and decrease are within the historically observed trends of previous site data, with an overall site trend of decreasing concentrations of all contaminants The continuing occurrence of contaminants in MW-6 is, in Wright's opinion, currently attributed to an off- site source (presumably somewhere south across East Eleventh Street) since MW-6 is upgradient of the known on-site contami+ant source This off-site plume continues to migrate onto the site at 595 11th Street and has been observed since August, 1997 This trend strongly indicates that an off-site upgradient source is present and contributing similar contaminants to the site Conclusions and Recommendations This round of quarterly monitoring was performed at the request of the PHS EHD representative, who recently reviewed a request for site closure • Due to an increase in contaminants at Well MW-9, PHS EHD has requested additional information and continued groundwater monitoring The information requested in the PHS EHD letter of July 27,1999 will be obtained by researching previous assessment and monitoring data It should be noted that some of the earliest historic information may be fragmentary based upon previous file reviews performed by Wright This information, including the possible encroachment to the irrigation line will be presented in a later submittal Limitations This report has been prepared specifically for the Mr Ed's Muffler Site at 595 E 11th Street in Tracy, and was done according to the current State and local agency suggested guidance documents for these investigations The interpretations, conclusions and recommendations made herein are based on the data and analysis for the soil and water samples collected on-site and should be reviewed in the context of this report and other site reports Please note that reports of contamination must be submitted to the agencies in a timely manner Wright Environmental Services, Inc is not responsible for errors in laboratory analysis and reporting, nor for information not available during the course of the study, and no warranty or guarantee is expressed or implied therein Page 7