ARCHIVED REPORTS XR0012691 - Laserfiche WebLink

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lF• _ _ _ 3 San Joaquin County Flood Control and Water Conservation District published a ground water contour map in their 1986 semi annual ground water report (Figure 6). This map shows that unconfined and semiconfined ground water flows to the north In the area around Tracy. However, the shallow ground water on this property does not conform to the direction of the regional flow, As shown on the site map, the gradient is to the south. Measurement of ground water level was taken from the three monitoring wells Installed during the field work. A surveyors level was used in taking these measurements. Artificial ground water recharge along the leaking irrigation pipe, which is located on the north side of the property, probably accounts for the southward trending During the winter months when the irrigation pipeline does not carry water,,gthisfent. recharge does not occur and the gradient may return to its northerly trend. Soil and Ground Water Contamination F �f Low levels of both soil and ground water contamination exists along the western half of the property. In this area the contamination is concentrated in a concentric pattern around the former underground tank location (Plate 2). The distribution of ground water contamination reflects the distribution of soil contamination, but is more wide x i spread. The good correlation between the distribution of soil and ground water contamination Is interpreted to mean that soil contamination resulted from gasoline #`a spreading out frorn the tank on top of the ground water or capillary fringe and being i. wicked up through the sediments by capillary action. j Soil samples were tested in five locations (borings #2, 3, 6, 7 and 8). These samples were collected from 12 feet below the ground surface, which is approximates 6 •' inches above the perched water tablwas shorts a to i2 distance above the wafer table and because ba ing cuttings Indicate that en because ialzone of sediments 1 to 3 feet thick above the ground water has been contaminated. Evidence for this is two fold: 1) the sediments in this zone are discolored to hues of blue, gray, or t green, and 2) the boundary between natural colored and discolored sediments correlates well with the Interface between uncontaminated and contaminated soil. I As can be seen by the contamination distribution contour ma Plat concentrations of gasoline in the soil are centered around boring #3 and have been t distributed in a plume that is oriented NW-SE. Soil was not tested in bore#1, located at the.center of the tank excavation because the tank excavation went to a depth of about 11 to 12 feet. However, field observation indicates that this area has also been impacted by gasoline which has migrated back into the fill dirt from the surrounding undisturbed soil. Analytical data, given in Table 1, Indicates that soil contamination is fairly low. The highest level of gasoline (3.13 ppm) and toluene (5500 ppb) contamination is in boring w #3. .Benzene is present only in boring #3, and then only at 51 ppb. Xylene has the highest level of contamination (6200 ppb) in boring #2. One soil sample from boring #6 was analyzed for total lead. The level of lead contamination is not high enough to cause concern6.3 ( ppm). It total lead is above 13 ppm, sail aeration is not_aslowed,— The!ow levizls of soil contamination are probably due to microbial degradation of the - various components of gasoline. Because of the shallow occurrence of the pollutants, ' biodegradation will gradually reduce the levels of contamination and over.time will become non detectable. r