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Section 4 - 1994-95 Groundwater Monitoring and Results
In April 1995, well conditions remain unchanged from those stated in the December 1994 quarterly groundwater report with one exception. The U.S. Department of Agriculture center at Kerrville repaired the pump at well H to permit sampling on April 28, 1995. Lubrication grease for the sucker rod was observed in the well H casing at the surface. The well pump failed immediately after sampling.
4.2 - Water Level Measurements and Groundwater Gradient
May 1994, First Quarter
Water level depths for May 1994 varied from a low at 108.02 feet below top of casing (BTOC) (calculated 1,059.78 ft above mean sea level; feet MSL) at well 1 to a high of 218.58 feet BTOC (calculated at 1,095.51 feet MSL) at well 9. The groundwater gradient was southerly at 0.004 foot per foot.
September 1994, Second Quarter
Water level depths for September 1994 varied from a low at 390.64 feet BTOC (calculated 940.48 ft MSL) at well 10 to a high of 303.40 feet BTOC (calculated at 1,022.46 feet MSL) at well G. The groundwater gradient ranged from 0.01 foot per foot to 0.005 foot per foot. Flow direction was to the south and southwest.
December 1994, Third Quarter
Water level depths for December 1994 varied from a low at 171.5 feet BTOC (calculated 996.3 ft MSL) at well 1 to a high of 283.84 feet BTOC (calculated at 1,042.02 feet MSL) at well G. The groundwater gradient ranged from 0.003 foot per foot to 0.1 foot per foot. Flow directions varied from south and southeast.
April 1995, Fourth Quarter
Water level measurements for the fourth quarter were made on April 3, 1995. Water level depths ranged from 168.48 feet BTOC to 372.16 feet BTOC for wells 1 and 10, respectively. Calculated elevations varied from 958.96 to 1040.99 feet MSL. Groundwater gradient is 0.1 foot per foot. Water level elevations for all monitoring events to date are listed in Table 2.
In May 1994 the groundwater flow direction was to the southeast (Figure 2) and most closely resembles that observed in April 1995 (Figure 7). The September 1994 flow gradient was to the south near wells D and 16, and to the southwest near wells 9, 10, and 11 (Figure 3). The flow direction observed on December 5, 12, and 19, 1994 was to the south and southwest (Figure 4, Figure 5, and Figure 6). December monitoring was performed three weeks in a row to determine static water levels. Because it is unclear as to whether static water levels were ever achieved. As indicated by the potentiometric surface map for April 3, 1995 (Figure 7), groundwater flow direction was to the south at wells 2, 3, 4, and 16 and to the northeast, around wells 1, 9, 10, and 11. The northeast gradient may reflect recent pumping at these active water supply wells. This flow pattern is unique compared to the other 1994 flow patterns.
The groundwater potentiometric map for April 1995 was generated using the software contouring program Surfer. All previous potentiometric maps were developed through geometric contouring. The computer generated contours for April 1995 were a close match to geometrically derived contours. As geometric contouring has been successfully utilized in the past, it will continue to serve as a control for any computer generated potentiometric maps.
March 1994-April 1995 precipitation data for San Antonio, Texas was obtained from the National Oceanic and Atmospheric Administration's National Climatic Data Center in Asheville, North Carolina. Total precipitation between sampling events was used in comparing rainfall to groundwater fluctuations over time. Precipitation may vary significantly between that measured at San Antonio and that which actually falls at CSSA. Despite this shortcoming, rainfall and fluctuation in groundwater levels at CSSA correlate. Figure 8 shows the total precipitation measured between selected days for May 25, 1994 through April 6, 1995. A transparent overlay of this precipitation data is provided in front of Figure 9.
Groundwater level measurements taken from May 1994 through April 1995 are graphed in Figure 9. Water levels were at their lowest in September 1994 and increased from December 1994 to April 1995. The highest water levels were observed in May 1994 and April 1995. Water levels in the middle Trinity aquifer appear to lag rainfall events by one week to a month. For example, increased precipitation in mid-September 1994 was followed by a distinct groundwater level rise within wells 16 and D in late September 1994 (Figure 9). Heavy precipitation in October 1994 was also followed by an increase in water level through late November 1994.
Two packer tests were performed on well 16 from April 3-10, 1995 to observe aquifer characteristics and to determine the effectiveness of the Bexar Shale as a confining layer between the lower Glen Rose Formation and Cow Creek limestone of the middle Trinity aquifer. Results of the packer test are discussed in a July 1995 Parsons ES technical memorandum. For purposes of groundwater monitoring, it is notable that heavy rainfall may have a much more immediate impact at CSSA than interpretation of the San Antonio data would imply. As observed by the Parsons ES field team during the packer test April 3 and April 6, water levels rose as much as 79 feet following a rain event on April 3-4, 1995.
Sampling methodology for 1994-1995 groundwater monitoring is discussed in Appendix A. Chain-of-custody records and data validation for 1994-1995 are in Appendix B. Twelve water samples (including the QA/QC samples) taken for fourth quarter analyses in March and April 1995, were sent to the Parsons ES subcontracted laboratory, Chemron, Inc., for analysis by EPA method SW8260. The analytical method employed prior to the fourth quarter sampling (SW8010), did not discriminate between cis- and trans-1,2-DCE. The fourth quarter laboratory reports are in Appendix C.
The highest concentrations of PCE, TCE, and DCE in well 16 for the first three quarters of monitoring were 196 ug/L, 509 ug/L, and 150 ug/L, respectively, and 8.9 ug/L, 15 ug/L, and 43 ug/L in well D (Figure 10 and Figure 11). Results for the first three quarters are discussed in more detail in the May, September, and December 1994 Parsons ES quarterly monitoring reports. Monitoring results for all four quarters of 1994-1995 are summarized in Table 3. A table of analytical results prior to May 1994 is in Appendix D.
HVOs were detected in April 1995 samples from CSSA wells 2, 3, 4, 16, and D. However, only samples from wells 16 and D contained contaminants (PCE, TCE, and cis-1,2-DCE) above their respective MCLs. PCE concentrations were 170 ug/L in the well 16 sample and 110 ug/L in the well D sample. TCE concentrations ranged from 130 ug/L to 170 ug/L and cis-1,2-DCE concentrations were 240 ug/L to 190 ug/L. HVOs were not detected in wells 1, 9, 10, 11, G, H, and I. The duplicate samples taken at wells 16 and D contained concentrations similar to the regular samples.
Wells 16 and D
Wells 16 and D have exhibited HVO contamination in the past. They are located within 500 feet of each other just south of burn area 2 and north (upgradient) of burn areas B-3 and the oxidation pond. No wells exist immediately upgradient or downgradient of these wells; therefore, the horizontal extent of contamination is not known. Graphs of groundwater HVO concentrations over time depict contamination trends in CSSA wells 16 and D (Figure 10 and Figure 11).
Figure 10 is a graph of PCE, TCE, and DCE concentrations in well 16 since August 1991. Figure 11 is a graph of the same chemical constituents in well D since December 1991 (well D was not sampled in August 1991). DCE concentrations for August 1991 were reported as both cis- and trans-1,2-DCE. In December 1991, November 1992, and September and December 1994 analyses cis-1,2-DCE was not a target constituent.
Transparent overlays of May 1994-April 1995 precipitation data are provided in front of Figure 10 and Figure 11. The precipitation overlay for Figure 10 reveals an increase in PCE and TCE following heavy rainfall in September, October, and December 1994. The same correlation exists for HVO concentrations in well D.
Other Wells
PCE and TCE have been detected intermittently in wells 2 and 4 since November 4, 1992. PCE was detected in well 3 in November 1992, May 1994, and September 1994 and in well 6 in May 1994. PCE has not been detected in well 3 since September 1994. All contaminants detected were below their respective MCLs. HVOs have not been detected in any of the other wells (1, 9, 10, 11, G, H, and I).