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' ARCO Products Compam <br /> i Canterpo e Driv <br /> LPalma. Cai forma 90623-1066 <br /> e "' iepnone 7t1 070 1300 <br /> Mailing Address. Box 5077 <br /> Buena Park. California 90622-5077 <br /> December 11, L996 <br /> N[r. Gordon Boos <br /> California Regional Water Quality Control Board <br /> Central Valley Region <br /> 3443 Routier Road <br /> Sacramento, California 95827-3098 <br /> r_ <br /> Subject: Purging of Ground Water Monitoring Wells <br /> Dear Mr. Boggs: _ <br /> h? <br /> Begit.rmg with the first quarter of 1997, :ARCO Products Company (:ARCO) will discoti nue <br /> purging ground water monitoring wells prior to obtaining ground water samples at <br /> underground storage tank (UST) sites within the Central Valley Region that have experienced <br /> a release of petroleum hydrocarbons. This applies only for monitoring wells completed in <br /> unconfined ground water aquifers. <br /> This request is supported by two recent publications discussing the statistical differences <br /> between no-purge samples (i.e., samples obtained without removal of water from the well <br /> casing prior to sampling) and post-purge samples (i.e., samples obtained after the removal of <br /> water prior to sampling). The first publication was prepared for The Western States <br /> Petroleum Association (WSPA) by SECOR International Incorporated (SECOR), titled "Final <br /> Report, The California Ground Water Purging Study for Petroleum Hydrocarbons," dated <br /> October 28, 1996. The study analyzed data collected at 101 sites including a total of 1,065 <br /> monitoring wells over a sampling period of two quarters. The conclusions of this study are as <br /> follows: <br /> The results of analyzing the entire data set indicate that there is a systematic <br /> tendency toward slightly higher petroleum hydrocarbon concentrations in the <br /> no-purge sample than in the post-purge sample with the median value of the <br /> relative percent difference between no-purge and post-purge samples being <br /> approximately 9.5 percent. <br /> The degree of variability introduced into the sampling process by the absence <br /> Of purging is no larger than, and in many cases much smaller than, the <br /> variability introduced by the choice of a purging method. In simpler terms, <br /> the decision to select a no-purge sampling methodology will not effect the <br /> overall variability in chemical data, and will provide a comparable, and in <br />