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' 1 ` Mr. Harlin Knoll . San Joaquin Count Environmental Health-Division � � Y , 4P R I' N 1 May 2001 Page 6 ., ' In EPA's Drinking Water Advisory: Consumer Acceptability Advice and-Health Effects Analysis on MTBE (EPA-822-F-97-009 December 1997), they state regarding MTBE "that keeping concentrations in the range-,of 20 to 40 micrograms per.. liter (µg/1) of water or . below will likely. avert. un ` unpleasant taste and odor effects. . Additionally, the State of p Y, California has proposed primary drinking water,standards as a. public health goal. for MTBE;of 13 µg/l.. Although these value's are:proposedlas goals and considered low by the industry; the concentrations of MTBE'found in the 'Shallow,- unused groundwater at the site are below these aggressive objectives.. Revised Mass Calculation The RWQCB has requested a revised hydrocarbon mass calculation due to concern with one data point collected from an angle boring; PB-1, performed by Foothill Engineering (FE) on 10 April 1997.;:.'From FE's soil'samples collected at 5-foot intervals, the sample at 20 feet (approximately 18 feet.below surface' and: below Tank No. 3) showed a 'TPHg concentration rof 16,200 mg/kg. Samples directly above and below this point were non- detect and 108. mg/kg,.respectively: A second probe, PB-2, approximately 15 feet to the northwest had a;TPH' concentration at 20 foot depth of.1;160 mg/kg and non-detect' above'and below this sample. . QST Environmental sampled.soiis from two auger borings- in oringsin 1998 and 'did not detect TPH: in . four samples from the two. borings located L; approximately 20 feet away from the original PB4 location. During the installation of three groundwater;monitoring wells by Orion in January 2000, one soil sample'from each boring was analyzed and reported as..ion-detect for TPHg: The historical soil sampI 'data intricate that the voluriie'of'ihr pacted soil is limited to, the material found just below the closed-in=place'Tank�'No. '3. The depth to water from the groundwater monitoring wells nearby, is 1.9 ;to 20 feet below. grade. The tank bottom was estimated 'at approxi mately 112. feet below the-surface. For this calculation, Orion assumed that the impacted soil: volumeis` a 10-foot-diameter cylinder with a vertical .height of 5 feet.. Therefore,`the'estim'ated volNume'is 392:7 cubic feet. Because the TPHg concentration goes from the 16',200-.mg/kg Jo, non-detect rapidly. in both vertical: and horizontal .directions, a:conservative assumption- is an average of 6,000 mg/kg of TPHg within this impacted volume. *Usingsthis basis, the mass of.hydrocarbon at the site is estimated to be'259pounds or less than 40 gallons of weathered gasoline. Because of the shallow, damp setting, it,is 'very :likely, that natural attenuation is ' occurring at this site.: The Lawrence Livermre. National Laboratory (LLNL) and; Texas Natural Resource. Conservation Commission .(TNRCC) have strongly suggested that hydrocarbon plurr�es` diminish with .time. ' In Geological ,Circular 97-1, "Extent, Mass, and Duration ofv.Hydrocarbon .Plumes from: Leaking; Underground Storage Tanks in Texas", the median time for site concentrations of gasoline constituents to decease by a [[ factor of 10 is 500 days. However, there is less published data available for natural fSABMSTOCKTON\LTR\LTR008.DOC.5/1/01 G