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*MEMORANDUM CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD - CENTRAL VALLEY REGION 3443 Routier Road, Suite A Phone: (916) 255-3000 Sacramento, CA 95827-3098 j CALNET: 8-494-3000 TO: Antonia K. J. VorsterA wv FROM: Camilla Williams Senior WRC Engineer ____1 Associate Engineering Geologist � James D. Taylor --AE—L Associate Engineering Geologist / ) DATE: 3 August 1993 SIGNATURE: uCClli �� Y SUBJECT: FIRST QUARTER 1993 GROUND WATER MONITORING REPORT, DEFENSE DISTRIBUTION REGION WEST (DDRW), SHARPE, SAN JOAQUIN COUNTY This memorandum is prepared to summarize three areas of concern found during my review of the First Quarter 1993 Ground Water Monitoring Report for DDRW, Sharpe. I reviewed the Quarterly Report in preparation of the case study on Sharpe for the response to the Environmental Protection Agency (EPA) Administrator' s decision on Mather in May 1993. I have provided a brief discussion on each of these items below: 1 . The ground water sample from MW-5058 had 9.7 micrograms per liter (ug/1 ) of trichloroethylene (TCE) which exceeds the Primary Maximum Contaminant Level (MCL) of 5 ug/1 . This well serves as a early warning detection well for the Stuart's Nursery supply well which was just installed in early 1991 . Stuart's supply well should be sampled to determine the concentration of volatile organic constituents (VOCs) . If the well is used for drinking water purposes and if TCE or other VOCs exceed the Primary MCL, then Sharpe should supply an alternative drinking water source to this location. The area down gradient of MW-505 should be canvassed to determine if there are any other private supply wells that may be threatened by this plume (Plume 7&8) . Because of the elevated VOC concentrations in MW-505B, technically the leading edge of the plume has not been defined. If the concentrations in this well continue to increase, Sharpe should install a new monitor well (s) to define the leading edge of the plume. 2. . The arsenic concentration in MW-407A was 1030 ug/l which far exceeds the Primary MCL of 50 ug/1 . This concentration represents a new high for arsenic at this site as the historical high for arsenic was about 700 ug/1 . This well has also historically had the highest bromacil concentrations, however bromacil is not monitored on a regular basis in this well . This dramatic increase in arsenic concentration may be due to the extreme wet season that occurred during this quarter and to the dry wells (which are located along the western border of the site in this area) which are used to manage runoff. There is a strong correlation between the dry wells and the arsenic and bromacil concentrations found in this well . The dry wells should be abandoned, as I believe that these contribute to the high contaminant concentrations in this area. Some monitoring of the runoff in the area of the dry wells is warranted to determine if the increase in contaminant concentrations in MW-407A are related to runoff. This may require a modification of the storm water National Pollutant Discharge Elimination System (NPDES) permit (No. 92-184) . Bromacil should also be monitored on a regular