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THOMAS M. GAU <br />DEPUTY DIRECTOR <br />MANUEL SOLORK) <br />DEPUTY DIRECTOR <br />STEVEN WINKLER <br />DEPUTY DIRECTOR <br />ROGERJANES <br />BUSINESS ADMINISTRATOR <br />THOMAS R. FLINN <br />DIRECTOR <br />November 16, 2007 <br />Mr. Todd Del Frate <br />California Regional Water Quality Control Board <br />Central Valley Region <br />11020 Sun Center Drive, #200 <br />Rancho Cordova, California 95670-6114 <br />BOX 1810 -1810 E. HAZELTON AVENUE <br />STOCKTON, CAUFORNIA 96201 <br />(209) 468-3000 FAX (209) 468-2999 <br />W-Sig-org <br />ENVIR,;1A T HEAL <br />PEMAT/SERVICES <br />SUBJECT: ADJUSTMENT OF EVALUATION MONITORING PLAN AT CORRAL <br />HOLLOW SANITARY LANDFILL <br />Dear Mr. Del Frate: <br />This letter proposes adjusting the evaluation monitoring plan at the Corral Hollow Sanitary <br />Landfill (CHSL) in consideration of the recent discovery of refuse in the adjacent property. <br />As you know, refuse was found at several locations immediately adjacent to the CHSL <br />property line between upgradient groundwater monitoring well SB -1 and downgradient well <br />GW -5 (Figures 1 and 2). The refuse was covered with a minimum seven feet of apparently <br />compacted clayey soil, and extended beyond the 18 -foot reach of the excavator used for <br />this investigation. Such a situation is ideal for the introduction of VOCs into the shallow <br />aquifer, such as are found at GW -5, and can explain why the collection of landfill gas <br />(LFG) from CHSL has not caused appreciable improvement of the groundwater quality at <br />that well. Note that.the groundwater impact is low, with no VOC exceeding 10 ppb. <br />Also, since proposing the off-site investigation of groundwater, it has become known that <br />two of the proposed "hydropunch" wells (HP -5 and HP -6) are located in an area containing <br />refuse burn ash. The potential for impact by the refuse burn ash residue to the <br />groundwater directly below cannot easily be disregarded, limiting the usefulness of any <br />data collected there regarding the impact by the more distant refuse in or adjacent to <br />CHSL. <br />In light of this new information, the County therefore proposes to adjust the work at CHSL <br />as follows: <br />• As previously proposed, install two additional groundwater monitoring wells into the <br />shallow aquifer. Note that the proposed locations are preliminary, showing an <br />optimistic assumption for the westerly extent of the Corcoran formation aquifer. We <br />will investigate subsurface conditions prior to installing the permanent wells to ensure <br />that the wells are installed in the shallow aquifer. These wells should allow more <br />confident assessment of the direction of groundwater flow and groundwater quality in <br />the shallow aquifer. <br />