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t <br />Mr. William Barber 0 - 2 - • 21 January 2003 <br />Because it is a sentry well for the deep zone and may be near a secondary source area, it shall continue <br />to be monitored, but on an annual basis, as long as monitoring well MW -10 has stable or decreasing <br />amounts of VOCs. <br />4. MW -10: The Request proposes changing the sampling frequency of this well from semi-annual to <br />annual. MW -10 is an intermediate zone well, monitors for downward migration of pollutants, and has <br />low levels of 1,1-DCE. MW -10 must continue to be monitored semi-annually. <br />5. MW -13: The Request proposes changing the sampling frequency of this well from semi-annual to <br />annual. MW -13 is the southern -most "sentry" well for pollution leaving the eastern border of the <br />property, and is potentially downgradient of pollution near MW -5R and MW -6. MW -13 must continue <br />to be monitored semi-annually. <br />Contingency Plan <br />We are concerned that the data show the seasonal presence of trichloroethylene (TCE) near monitoring <br />well MW -5R, outside the capture zone of the groundwater extraction system. To address possible <br />VOC pollution in wells MW -5R and MW -6, on 15 September 2000 Board staff approved the following <br />contingency plan. <br />1. Maintain the GWETS and optimize pumping to maximize the capture area. <br />2. Continue quarterly monitoring of the southern extent of the capture area for at least two years. <br />If concentrations do not show a decreasing trend after two years, conduct soil gas sampling to <br />identify potential source areas near MWs 5R and 6. <br />4. Plan remedial action depending on the outcome of the soil gas survey. <br />The data submitted in the Request show that well MW -5R contained 26 µg/l, 40 µg/1, 36 µg/l, and <br />37 µg/l, of TCE during November 1998, October 1999, November 2001 and November 2002, <br />respectively. We believe the data show that there may be a secondary source area in the vicinity of <br />MW -5R and MW -6. Therefore, the contingency plan should be implemented at this time. <br />By 14 February 2003, please submit your comments on the draft MRP and a schedule to submit a <br />work plan for a soil gas survey, as required by the contingency plan. If you have any questions, you <br />may call me at (916) 255-3077 or e-mail at djuthg@rb5s.swrcb.ca.gov. <br />Z;L�2�� <br />GERALD J. DJUTH, P.E., C.E.G. <br />Associate Engineering Geologist <br />cc: Ms. Margaret Lagorio, San Joaquin County Public Health Services, Stockton <br />Mr. Fred Kintzer, Parsons Engineering Science, Inc, Walnut Creek <br />