ARCHIVED REPORTS_XR0006046 - Laserfiche WebLink

Home Browse Search

• The static water level is more than seven feet deeper than it was in June. With an average depth of 56 2 feet below grade, the static level is near the top of the Riverbank aquifer and is within the screened interval of GT-5, GT-6, GT-9, and GT-10 The calculated water table elevations were used to construct the groundwater gradient map in Figure 3 A southward slope is apparent, but the gradient is not planar Due to a slightly higher water table in GT-10, the contours curve around this well The depth to groundwater in this well has frequently been somewhat anomalous, causing flexures, depressions, or mounds in the gradient As suggested in previous reports, this could be due to the fact that the screened interval in flus well is primarily within the Modesto Formation, whereas the screened interval in all of the other wells is primarily in the underlying Riverbank Formation Partial hydrologic separation between these two formations could account for the slight difference in water levels 3.2 Analytical Results Hydrocarbon concentrations are below detection limits in all wells except GT-6 and GT-10 (Table 2) It is interesting to note that the TPH-d concentration in GT-6 is essentially identical to what it was in October and December of 1999, when the well was purged before sampling The same is true for the BTEX and MTBE results in GT-10, but the TPH-g concentration is considerably lower than in the past It is also interesting that the TPH-d concentration in GT-10 rose sharply during the third quarter and returned to the level that was first measured in this well in March 1999 However, although the concentration was very similar at that time, the depth to the static water level was 10 • feet less than its present depth, which implies that the TPH-d concentration is unrelated to water depth In contrast, comparison of the water depth and TPH-d concentration in GT-6 suggests that there could be a relation between these parameters in this well The concentration declined fairly steadily from October 1996 to March 1999, as did the water depth (Figure 4) The depth to water began to increase during the second quarter of 1999, and the TPH-d concentration followed suit When the water level stabilized during the late part of the third quarter of 1999 through the end of 1999, the TPH-d concentration also stabilized, and then both declined in the first quarter of 2000 Since then, both have been on the increase, although concentrations are still far below what they were initially Since March of 1999, when groundwater monitoring began on a consistent quarterly basis, it appears that concentrations peak near 100 ppb when the water depth is approximately 55 feet, and decline below detection limits when the water rises to a depth of 45 feet At the latter depth, the static water level is 5 feet above the screened interval, and laboratory results may therefore not be representative of true groundwater concentrations at that depth Hence, the apparent correlation between water depth and TPH-d concentration in GT-6 may not be real 4.0 CONCLUSIONS The groundwater gradient and flow direction continue to fluctuate over a large range of compass directions A northward flow direction has been measured during the first quarter in more than one year, but the flow direction has tended to reverse and become southward or westward later in the 3