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California Regi aer ality rol Board Central Valley Region Steven T.Butler,Chair Winston H. Hickox Gray Davis Secreta?yfor Sacramento Main Office Governor Environmental Internet Address: http://wm�v.swTcb.ca.gov/-,rwgcb5 Protection 3443 Routier Road,Suite A,Sacramento,California 95827-3003 Phone(916)255-3000•FAX(916)255-3015 8 March 2000 « F l" 'ED Mr. Wes Johnson MAR 0 9 2000 County of San Joaquin Department of Public Works ENVIRONMENTAL �� P.O. Box 1810 PERMIT ! SERVICES Stockton, CA 95201 WORKPLAN TO EVALUATE EXTENT OF VOC IMPACTS TO SHALLOW GROUNDWATER, CORRAL HOLLOWSANIT.4RYLANDFILL, SANJOAQUIN COUNTY (CASE NO. 1896) Staff has completed review of the Workplan dated 6 December 1999 and submitted on 15 December 1999. The Workplan indicates that the county proposes to investigate the lateral extent of volatile organic compounds (VOCs) in shallow groundwater along the northwestern property boundary of the Corral Hollow Sanitary Landfill in San Joaquin County. The county suspects that the VOCs detected in groundwater from MW-5 are a result of landfill gas dissolving into the shallow groundwater. EMCON, consulting for the county, has demonstrated through use of gas partitioning coefficients that it is realistic to assume that the concentrations of VOCs in groundwater at MW-5 are a result of the VOCs found in the landfill gas. The highest concentrations of landfill gas at the Corral Hollow landfill are documented along the northwestern property boundary adjacent to Corral Hollow Road. The county has proposed evaluating the extent of the clayey layer in which the shallow groundwater is perched, and to grab samples of the shallow groundwater where it is encountered. The county proposed to drill three borings to determine the depth to groundwater and the clayey layer. The county will analyze the groundwater for VOCs and other parameters normally included in the quarterly sampling. The county will then properly abandon the borings. The proposed investigation should indicate if greater VOC concentrations in groundwater are present along the property boundary other than those found at MW-5. Depending on the results of the field investigation, additional evaluation may or may not be needed, however the proposed work is an appropriate initial step to determining the extent of the VOC impacts. Regional Board staff approves the Workplan and the county should proceed,with the following modifications: 1. A field geologist who has adequate experience in drilling and describing soils shall log proposed borings 1, 2, and 3 using the Unified Classification System. 2. The proposed borings shall be logged and sampled at five-foot intervals until approximately 2 to five feet prior to the anticipated depth of the clayey zone and shallow groundwater. Upon encountering the clayey zone,borings shall be continuously cored and described to the total depth of the boring. California Environmental Protection Agency �a Recvcled Paper