Laserfiche WebLink
URS RECIEVEMD <br />�! <br /> . MAY 0 4 2004 <br /> ENVIRONMENT HEALTH <br /> May 3, 2004 <br /> PERMIT/SERVICES <br /> Mr Mike Infuma 6020 <br /> San Joaquin County Environmental Health Department 06 a3 <br /> 304 East Weber Avenue,Third Floor <br /> Stockton, California 95202-2708 <br /> Reference: Groundwater Monitoring Well Purging Requirements <br /> ARCO Station 6020 <br /> 1711 East Yosemite Avenue,Manteca, California <br /> LDear Mr Infurna <br /> URS Corporation Americas (URS) has prepared this letter on behalf of Atlantic Richfield Company <br /> (ARCO—a BP-affiliated Company) in response to your March 2004 telephone communication, in which <br /> you expressed concern that the low-yield wells at ARCO Station 6020 were not being purged of three <br /> well-casing volumes of water prior to sampling <br /> Background <br />�. On February 16, 2004, Blaine Tech Services, Inc (Blaine) conducted the first quarter 2004 groundwater <br /> sampling event at the site Sample results from two upgradient wells showed the presence of ethanol <br /> (MW-9 at 510 micrograms per liter [µg/Ll and MW-10 at 500 t.g/L) On March 11, 2004, under your <br /> supervision, Blaine resampled the wells to confirm the presence of ethanol Ethanol was not detected in <br /> the analysis of the samples collected During your oversight of the resampling, you observed that each <br /> of two wells were purged to dryness following the removal of approximately one casing volume of <br /> water Monitoring well data sheets are included in Attachment I <br /> Groundwater at the site is present at approximately 21 feet below the ground surface Rapid dewatering <br /> during well purging has been consistent at these and other site wells during past quarterly monitoring <br /> events This reflects the low-yielding silts, sandy silts, and dense sandy silts that make up the upper <br /> water-bearing zone at the site Monitoring wells MW-9 and MW-10 are both 4 inches in diameter Well <br /> sampling in low recovery wells is addressed in various groundwater sampling guidance documents, as <br /> described hereafter <br /> Cal/EPA and EPA Well Sampling Guidelines <br /> The California Environmental Protection Agency (Cal/EPA) and the U S EPA agree that low yielding <br /> wells may restrict the sampler from purging three or more casing volumes prior to sampling Excerpts <br /> from documents prepared by each of these two agencies providing direction on sampling wells that <br /> purge to dryness before the recommended well volume is removed, are cited hereafter ARCO has <br /> followed these guidelines in collecting representative groundwater samples from low-yielding wells at <br /> the subject site Copies of these documents are included in Attachment 11 <br /> URS Corporation <br /> Crown Corporate Center <br /> 2870 Gateway Oaks Drive Suite 300 <br /> Sacramento CA 95833 <br /> Tel 916 679 2000 <br /> Fax 916 679 2900 <br />