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June 1995 Quarterly Groundwater Monitoring Report
Quarterly groundwater monitoring was performed the week of June 12, 1995 at Camp Stanley Storage Activity (CSSA). This work was performed under contract F33615-89-D-4003, Order 67, Mod 2.
Groundwater elevations increased between April 1995 and June 1995. Groundwater flow direction was to the southeast during this period. Cumulative groundwater elevations from October 1992 through June 1995 are shown in Table 1.
All wells were measured with an electric water level indicator (e-line) except wells 10 and 11 which were measured by the air-line method. Well 9 was measured using both the air-line and e-line.
The air-line and e-line data yielded different results for depth to water at well 9. The air-line depth to water was 378.3 feet while the e-line depth to water was 261.75 feet. The discrepancy between the depths measured by air-line and e-line has been prevalent throughout quarterly groundwater monitoring at CSSA. The air-line method is generally less accurate than electrical or acoustic sounders (Driscoll, 1986). However, the air-line method is the only option available at wells 10 and 11 due to physical constraints at these wells.
Groundwater potentiometric elevations using e-line measurements from June 1995 are graphically depicted on Figure 1. The potentiometric indicate that the groundwater gradient of 0.0075 is to the southeast which is consistent with previous monitoring results.
Groundwater potentiometric elevations using air-line and e-line measurements from June 1995 are depicted on Figure 2. Figure 2 shows a large cone of depression centered around well 10. the pump at well 10 was turned off on Friday, June 9, 1995. Groundwater measurements were made on Monday, June 12, 1995. The pump at well 10 was not reactivated until all groundwater measurements had been completed. The cone of depression indicates that the aquifer had not recovered from pumping at well 10 when water level measurements were made.
The average increase in groundwater elevations from April to June 1995 was 25 feet for both air-line and e-line measurements. The average increase in groundwater elevations from April to June 1995 was 37 feet for e-line measurements only. Regardless of the value used, the increase is greater than expected for a three-month time span. The increase is most likely related to a precipitation event which occurred the day previous to water level measurements. The National Climatic Data Center reported 0.67 inches precipitation for Sunday, June 11, 1995 at the San Antonio International Airport.
The 1995 analytical results for groundwater samples collected in March and April and June are shown in Table 2. Cumulative plots of analytical results for wells 16 and D are presented on Figure 3 and Figure 4.
Three compounds were detected in wells 16 and D: trichloroethene (TCE), tetrachloroethene (PCE), and cis-1,2-dichloroethene (DCE). The concentration of these compounds decreased from April to June 1995. Figure 3 and Figure 4 show the fluctuation in contaminant concentrations over time.
TCE was detected in well 2 and PCE was detected at wells 2 and 4 in April 1995. Detection limits for TCE and PCE were 1 ug/L and 0.4 ug/L in April 1995. No compounds were detected at wells 2 or 4 in June 1995. However, higher detection limits of 2 ug/L and 5 ug/L were used for the samples analyzed in June 1995. The detection levels achieved in the analyses for EPA method 8260 in April 1995 were five times lower than routinely reported for these parameters. The analytical laboratory was using a low detection calibration at the time the April samples were analyzed.
Water levels were an average of 25 feet higher in June 1995 than measured in April 1995. This change is consistent with historical rises and falls observed during CSSA groundwater monitoring.
A discrepancy was noted in water levels measured by electric line versus air line at well 9.
Using air-line measured water levels, the June 1995 potentiometric map shows the influence of pumping on groundwater gradient around wells 9, 10, and 11.
Both potentiometric maps indicate a southeasterly groundwater flow direction.
PCE, TCE, and cis-1,2-DCE concentrations decreased between April 1995 and June 1995.
TCE and PCE concentrations decreased in well 2 between April 1995 and June 1995 and PCE concentrations decreased in well 4.