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ATC Associates Inc <br /> 1117 Lone Palm Avenue, Suite B <br /> (M%TCModesto, California 95351 <br /> 209-579-2221 <br /> A s s 0 c i A s E s r H c fax 209-579-2225 <br /> CONCLUSIONS <br /> The concentration of petroleum hydrocarbons in groundwater samples collected from MWI and <br /> MW2 are within the historical range as established by previous sampling events and have <br /> generally decreased over time Concentrations of petroleum hydrocarbons remained below <br /> laboratory reporting limits in samples collected from MW3, MW4, MW5, MW6, and MW7 <br /> TPHk has not been detected in samples collected from wells MWI, MW2, and MW3 since at <br /> least 2000 TPHd has not been detected in samples collected from wells MWI through MW4 <br /> since at least March 2001 TPHk has never been detected in samples collected from wells MW4 <br /> through MW7 TPHd has never been detected in samples collected from wells MW5 through <br /> MW7 The lateral extent of the petroleum hydrocarbon plume to groundwater appears to be <br /> receding <br /> RECOMMENDATIONS <br /> Based on the results of the fourth quarter 2004 monitoring episode, we recommend the <br /> following <br /> • Conduct the first quarter 2005 groundwater monitoring of wells MWI through MW7 and <br /> analyze samples from only MWI and MW2 for TPHg, BTEX, and MTBE by EPA <br /> method 80I5B / 8260B quarterly No longer submit samples for TPHd and TPHk <br /> analyses Inspect well box conditions and maintain all well boxes in a watertight state <br /> • Sample MW3 through MW7 on an annual basis only, as suggested in the SJCEHD <br /> correspondence dated November 19, 2003 <br /> • ATC additionally suggests the annual sampling be conducted in the second quarter <br /> (April) when the groundwater has seasonally been at its highest elevation <br /> • Implement the Addendum to Corrective Action Plan and Site Conceptual Model as <br /> approved by SJCEHD in their correspondence dated March 17, 2004 <br /> S 16257714QR-2004 doc 4 <br />