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5'TAJE7r CALIFORNIA • PETE WILSON. Governor ' <br /> CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD— <br /> CENTRAL VALLEY REGION / <br /> '; a;1 R ROAD, SUITE A <br /> SP�I IN I.°i I] �{_ s'•�f` .a <br /> ACRAMENTAMENT O. 95827-3096 <br /> PHONE 19161 36161-5600 <br /> FAX 19161 3615666 <br /> .Illl � � 1991 <br /> 19 July 1991 ENvurc.',d;,��rvl,v lt <br /> PERMIT/SERWIALTH <br /> -L <br /> Mr. Abel Haines <br /> Director of Environmental Program Office <br /> Defense Distribution Region West, Sharpe Site <br /> Lathrop, CA 95331 <br /> REVIEW OF THE FINAL REMEDIAL INVESTIGATION REPORT, VOLUME V, DEFENSE DISTRIBUTION <br /> REGION WEST (DDRW), SHARPE SITE, SAN JOAQUIN COUNTY <br /> We have reviewed the Remedial Investigation (RI) Report, Volume V for the DDRW, <br /> Sharpe Site. The Report was submitted on 21 June 1991 by the U. S. Army Toxic and <br /> Hazardous Materials Agency (USATHAMA) and was prepared by Environmental Science and <br /> Engineering, Inc. We have limited our review to Sharpe's responses to our comments <br /> on the November 1990 RI Report. <br /> Sharpe has adequately investigated the site through the years and has attempted to <br /> address the regulatory concerns with each revision of the RI Report. Considerable <br /> data collection and interpretation has been performed by Sharpe since the submittal <br /> of the RI Report in August 1988. We can now approve the RI Report with the <br /> expectation that our major areas of concern, regarding the site conceptual model <br /> and plume definition, will be resolved prior to the selection of the final remedial <br /> action. <br /> In Volume V of the RI Report, Sharpe addressed many of the technical concerns of <br /> the regulatory agencies discussed in the Environmental Protection Agency's 23 April <br /> 1991 letter. These concerns were addressed by preparing a regional cross-section, <br /> revising the existing cross-sections to include the D zone monitor wells and <br /> indicating a relative horizontal scale, performing an extensive geochemical <br /> analysis of the data collected for the arsenic study which included trilinear <br /> diagrams and by preparing a table of aquifer parameters for the different zones. <br /> We are pleased that Sharpe is now preparing ground water level contour maps on a <br /> quarterly basis. This quarterly analysis will help to assess potential seasonal <br /> impacts on the hydraulic system and to assess and optimize the effectiveness of the <br /> extraction system. We are also pleased that Sharpe has committed to an evaluation <br /> of the benefits of using a three-dimensional (3-D) model for the design of the <br /> extraction network in the Central Area. <br /> However, we believe that the conceptual model presented in the RI Report continues <br /> to be confusing and contradictory. Sharpe maintains that the majority of the <br /> subsurface is unsaturated and that for materials below the water table, if the <br /> material is not saturated, it is not transmissive. We do not concur with this <br /> interpretation. If the majority of the subsurface is unsaturated, then the <br /> different water bearing zones cannot be hydraulically connected and be one aquifer, <br /> as Sharpe contends. Furthermore, the presence of unsaturated sediments between the <br /> water bearing zones would imply that the upper water bearing zone is perched. We <br />