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and data collected by Weston. In a letter dated May 30, 1990, Weston responded that they cannot comment on the sample collection method used by the RWQCB staff since this information was not provided. In addition, Westion commented that the RWQCB's samples were simply grab samples collected from surface strata and not collected in an attempt to provide site characterization. Upon review of Weston's response, the RWQCB recinded C&A Order 87-734 and 89-705 on October 9, 1990 and issued C&A Order 90-812 requiring the owners of Trinkle & Boys to initiate a quarterly groundwater .� monitoring program for one year. Following one year of monitoring, the RWQCB would issue a new monitoring and reporting program, if necessary (3,6,7). As of August 4, 1992, the RWQCB has not acted on the Trinkle & Boys site. In complying with C&A Order No. 90-812, Trinkle & Boys initiated a groundwater monitoring program and submitted the first quarter report to RWQCB on February 26, 1991. Analysis of groundwater from the first and third quarter (September 19, 1991) — reports revealed the presence of carbamate pesticides consisting of Bromacil at 5.3 microgram per liter (ug/1)), carbaryl at 0.4 ug/l, diuron at 6.0 ug/l, methiocarb at 1.0 ug/l, and carbofuran at 2.0 ug/I. The reference dose screen concentration for the following contaminants are: diuron at 70 ug/l, carbofuran at 180 ug/l, and carbaryl at 3,500 ug/l (8,9). The RWQCB indicated that the groundwater monitoring program was completed upon submittal of the fourth quarter groundwater monitoring report on September 19, 1991. RWQCB has considered initiating a new groundwater monitoring program; however, no action has been taken as of August 4, 1992 (10). 4.0 HRS Considerations Based upon regional geologic data, the Tracy area is located on an alluvial plain with soils consisting predominantly of silty clay and clayey silty sand interbedded with lenses of gravel and gravelly sand. These soils are usually found within the first 80 feet below ground surface. The primary water bearing strata is the Tulare Formation which is generally located below the alluvial deposits. Separating the Tulare Formation into upper i and lower units is the Corcoran Clay Member of the Tulare Formation. Thickness of the Tulare Formation and the Corcoran Clay member are thought to be up to 1,000 feet and between 100 and 200 feet, respectively. Well logs for irrigation wells located within one mile of the site indicate that the subsurface soils include altemating layers of clay, sandy clay, sand, and gravel to a depth of 180 to 200 feet with the Corcoran Clay member present at depths from 250 an 350 feet bgs The upper aquifer is found above the i Corcoran Clay Member and is under unconfined, semi-confined, and confined conditions due to the variations in the stratigraphy. Groundwater beneath the site occurs at 10 feet bgs and is unconfined to a depth of approximately 200 to 250 feet (2). Groundwater in the area is used for drinking with the nearest drinking water well to the Trinkle & Boys site less than 150 feet away. The RWQCB shallow groundwater sample at 18 inches below ground surface shows elevated levels of Endosulfan I at 36 ppb. The reference dose screen concentration for Endosulfan 1 is 1.8 ppb. Water samples collected from Weston quarterly monitoring wells installed on site were analyzed according to EPA method 632 and showed contamination with diuron at 6.0 ppb, carbaryl at 0.4 ppb, carbofuran at 2.0 ppb, and Bromacil at 5.3 ppb. Contaminants detected in the shallow groundwater can potentially migrate into deeper portions of the unconfined aquifer. One drinking water well provides drinking water to Trinkle & Boys and a house located less than 200 feet north of the site. Approximately 2,734 people use groundwater for drinking within four miles of the Trinkle & Boys site (2,3,11,14,15). Groundwater is the only source of drinking water at the Trinkle & Boys site. URS Consultants, Inc. Page 5