ARCHIVED REPORTS_XR0011909 - Laserfiche WebLink

Home Browse Search

• eight of the samples were analyzed for TPH-diesel, six of these were also analyzed for TPH- gasoline by EPA method 602 and for BTEX and MTBE by method 8020 3.0 RESULTS 3.1 Groundwater Gradient and Flow Direction Depth-to-groundwater measurements and calculated groundwater elevations are presented in Table 1 Results from previous monitoring events are included for comparison The static water level is approximately 2 feet shallower than it was in October of 1999, and now averages approximately 53 feet in depth Hence, it has now risen almost to the contact between the Modesto Aquifer and the Modesto Agwtard In some of the borings, the highest concentrations of contaminants were detected at this contact (e g GT-3, GT-7, GT-10, GT-11) The present water elevation (relative to sea level) is almost minus 26 feet The calculated water table elevations were used to construct the groundwater gradient map in Figure 3 The gradient has again changed, and is considerably steeper than it was in October In the middle of 1999, the flow direction was to the northwest or north, but by October, the gradient had flattened and revealed no apparent flow direction By December, an eastward-trending trough had appeared in the gradient, and the predominant flow direction was to the east, toward the domestic well on the neighboring property 3.2 Laboratory Results In most wells, contaminant concentrations decreased during the fourth quarter (Table 2) Neither gasoline nor diesel were detected in GT-1 and GT-2, which had seen sharp increases during the previous quarter The TPH-d concentration also decreased slightly in GT-3 and in GT-5, although slight increases were detected to BTEX compounds in GT-3 Concentrations remained constant in GT-6, except that gasoline and MTBE were not detected No hydrocarbons were detected in the sample that was collected from the domestic well tank Diesel was detected in GT-9 for the first time since October of 1996 Because this well was not purged, the presence of diesel could not be attributed to cross-contamination from the purge pump and laboratory error appears unlikely Perhaps it reflects dissolution of adsorbed hydrocarbons that may have been"smeared"downward during previous groundwater depth fluctuations The gasoline concentration decreased by at least 50% from its previous level in GT-10, although BTEX concentrations remained relatively stable Diesel, however, increased sharply, to its highest concentration since the well was first sampled in March of 1999 An isocontour map of the extent of diesel has been constructed from the December data (Figure 4) The map does not conform well with the interpreted groundwater gradient map, and the configuration of the plume is unusual and difficult to explain It does not appear to be elongate 3