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WORKPLAN FOR DELINEATION D OF GROUNDWATER CONTAMINATION IN THE RIVERBANK AQUIFER Gillies Trucking Co. 3931 Newton Road Stockton, California 1.0 INTRODUCTION In a letter dated June 17, 2004, San Joaquin County Environmental Health Department (EHD)requested a work plan to complete the vertical and lateral delineation of groundwater contamination at 3931 Newton Road in Stockton, California. This request. derived from Staff review of the Fourth Quarter 2004 Report, which was prepared by Upgradient Environmental and submitted in December 2003. EHD concurred that the geologic model for the Modesto Formation and upper two sand bodies of the Riverbank Formation is consistent with the site-generated data, but correctly pointed out that there is insufficient data to fully evaluate the third Riverbank sand. Only two borings have been drilled deep enough to penetrate this unit, so neither its geometry nor its groundwater chemistry is known. EHD further observed that questions remain regarding the full extent of groundwater contamination within both the first and second Riverbank sand bodies, and that due to poor hydrogeologic communication between these units, the . potentiometric maps that have been generated in the past from water-depth measurements are not definitive. The purpose of this work plan is to remedy these shortcomings in the database by drilling additional wells in areas where data are currently lacking. 2.0 THIRD RIVERBANK AQUIFER As noted above, three hydrologically separate sand bodies have been identified by past drilling below the Modesto Formation at the site, and the deepest of these is the least well-known. It was penetrated in boring GT-11'in 1999,and in boring CPT-1 in 2003, at a depth of approximately 110 feet. This sand body did not appear to be contaminated by diesel fuel in GT-11, but diesel-range hydrocarbons were detected in a groundwater sample from this sand body in CPT-1, implying that there has been some degree of . impact to this aquifer. It appears to be less than 10 feet thick in CPT-1, but it is.clearly more than10 feet thick nearby in GT-11; although its base was not reached. In other borings,than, sand bodies 1 and 2 range from less than 5 to more than 20 feet thick, so it is reasonable to assume that the thickness of sand body 3 also exceeds 20 feet, at least locally. Inasmuch as groundwater flow and contaminant migration rates often tend to be greatest along the thickness axis of fluvial sand bodies, additional drill holes through the 3rd Riverbank sand body are needed to identify and map the axis and determine the lateral extent of diesel contamination.