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Groundwater Investigation and Associated Source Characterization - Appendix G

Groundwater Monitoring Summary and Associated Actions

Quarterly Groundwater Monitoring Summary

CSSA Well Information Table

Analytical Results for CSSA Monitoring Well Samples, 1994-1995

Groundwater Monitoring Analytical Results, 1991-1992

Pumping Volumes Supplied by CSSA for Dates Prior to Quarterly Groundwater Monitoring

Well 16 Packer Test

Well 16 Transducer Installation

Well 16 Weather Station Installation

Quarterly Groundwater Monitoring

May 1994

September 1994

December 1994

April 1995

June 1995

August 1995

December 1995

February 1996

CSSA does not pump from their wells two days prior to quarterly monitoring unless their needs dictate otherwise. It is unclear as to whether groundwater equilibrium is established within the aquifer 48 hours after CSSA pumps from their water supply wells. An attempt was made to establish equilibrium during December 1994 but the results were inconclusive.

Monitoring Well Installation and Sampling

Well 16 Packer Test

Groundwater Sampling

The packer was set for each test so as to isolate the designated groundwater production zone from other zones (Appendix Figure Well 16 Diagram for Packer Tests). The packer was first set in the Bexar Shale just above its interface with the underlying Cow Creek Limestone, thus isolating the Cow Creek Limestone from the Bexar Shale and Glen Rose. For the second test, the packer was set in the Bexar Shale just below its interface with the overlying Glen Rose Formation to isolate the Glen Rose from the Bexar Shale and Cow Creek Limestone. Once set at the desired location, that section of the well in the groundwater zone of interest was purged of three well volumes by continuous pumping. After pumping, the isolated well section was allowed to recharge, and then a sample was collected for chemical analysis.

Test Results

One of two possible aquifer responses to well 16 packer tests were anticipated for the Glen Rose water levels: If the Bexar shale was leaky and did not act as a confining layer, the Glen Rose and Cow Creek would both respond with a drop in water level as water was pumped from the Cow Creek. If the Bexar Shale was leaky and the Cow Creek was a confined water zone, then water levels in the Cow Creek would drop during pumping and Glen Rose water levels would remain unaffected. To test these assumptions, Glen Rose water levels were measured in wells D and 4 prior to, and during, pumping of the Cow Creek. An attempt was made to measure Cow Creek water levels with a transducer at well 16.

Water levels in all measured wells rose significantly over the two days prior to the packer tests, and continued to fluctuate during the tests. The observed baseline rise was attributed to infiltration and recharge resulting from a recent heavy rainfall at CSSA.

Water levels dropped, rose, and dropped again in well D during the lower Glen Rose packer test (Figure 8.2-10). Fluctuations in well 4 were not observed due to the infrequency of measurements. During the test the initial water level decrease was attributed to pumping, but the subsequent rise and fall reflect the affects of both pumping and precipitation recharge. During the Cow Creek packer test Glen Rose water levels measured in wells 4 and D increased slightly (Figure 8.2-11). This may represent the lingering effect of groundwater entering the local recharge region of these wells while the test was being run. At well 16, a transducer failure prevented aquifer drawdown and recovery measurements from being recorded for the Cow Creek and Glen Rose groundwater zones. Therefore, conclusions as to the ability of the Bexar Shale to act as a confining layer and the response of the Cow Creek and Glen Rose to pumping were not definitive.

Analytical Results

The contaminants of concern were detected in groundwater samples from both the lower Glen Rose and the Cow Creek. Concentrations of PCE, TCE, and cis-1,2-DCE were exactly the same for PCE and TCE, and were +/- 0.01 mg/L for cis-1,2-DCE concentration in these samples. A duplicate sample was collected during the Glen Rose packer test. The following table presents the analytical results for the tests.

Analytical Results of Packer Tests

Summary of Results

The following summarizes the main points from the tests:

Well 16 Transducer Installation

The In-Situ LTM 3000 Well Sentinel datalogger and PXD-260 pressure transducer were installed on 22 August 1995. The pressure transducer cable was run through a 1-1/8" hole cut in the well head cap. This allowed cable passage while protecting the well from surface debris and animals. The cable was run through the wall into an adjacent shed which housed the datalogger. The accompanying appendix figure Well 16 Diagram Transducer Placement illustrates the placement of the transducer under different water level conditions.

Data Summary and Evaluation

April 1995 through February 1996 quarterly water level measurements for well 16 indicate an overall drop of approximately 97 feet. Water levels within the month of December 1994 fluctuated up to seven feet.

The transducer data and area rainfall data for September 1995-March 1996 are shown graphically on Figure 8.2-12. An initial decrease in water level was followed by a sharp rise. The sharp increase in water level for early November map indicate rapid local recharge. The much smaller increase which followed in mid-November may represent regional recharge. In both cases water table fluctuations followed precipitation events measured at the San Antonio International airport. The inverted relationship in magnitude observed between water table response at CSSA and rainfall at San Antonio was most likely the result of variation in rainfall over distance. A gradual decrease in the water table was observable up until data collection March 6, 1996. The two water level "dips" on 8 January and 16 January 1996 were the result of transducer disturbance during execution of other groundwater investigative tasks.

Two quarterly groundwater measurements were taken by electric line during the period in which the transducer has been in operation. These two measurements were taken on 11 December 1995 and 27 February 1996 and revealed a drop in water level of 6.37 feet. Transducer measurements recorded a drop of 8.3 to 8.7 feet (represents daily average of measurements taken every half hour). The raw data was sent to In-Situ for evaluation. No evidence of transducer error could be found but In-Situ offered to perform a post-calibration check to confirm the transducer accuracy. Initial reference values assigned to =the transducer were off but easily corrected by a fixed numeric adjustment. None of the absolute value corrections altered the relative change observed in the data. Based on the above the data is valid for comparison with changes in precipitation and contaminant concentration.

Well 16 Weather Station Installation

The Texas Weather Instruments (TWI) WPS-32 weather station was installed approximately 50 feet east of the well 16 pump house on 22 August 1995. Sensor cables were run underground by PVC conduit to a small shed adjacent the pump house. Sensor and cable configurations are presented in the accompanying Appendix Figure WPS-32 Weather Station, Well 16. The weather station central processor and modem are located in the shed along with the well 16 datalogger. After initial difficulties calibrating the rain gauge the WPS-32 appeared to function normally. A direct download was performed on 26 September 1995. Precipitation values were checked in the field and appeared incorrect based on rainfall observed at CSSA. The downloaded data was taken to Parsons ES for more detailed analysis but upon attempting conversion it was discovered that the file could not be converted into usable text. Correspondence with TWI indicated the software was corrupt and a new software version was provided.

On 6 October 1995 an on-site download was again performed. The new software was successful in converting the data to text, however rain gauge calibration was unsuccessful after extensive testing. Monthly minimum and maximum wind speed values were unusually high with respect to daily values and were suspect. Precipitation values were also questionable based on observation and calibration problems. TWI was consulted and offered to review the data. In January 1996 TWI was again contacted regarding the CSSA weather station and indicated they had discovered problems with their software and the central processing unit (CPU) computer chip. A new software version and chip were then sent.

The new chip was installed and tested on 4 March 1996. Weather station response was intermittent and rain gauge calibration was not achieved. The unit and rainfall gauge were disconnected and shipped back to TWI for testing and repair. The CPU board tested as faulty and was replaced. The rain gauge functioned properly. During testing of the CPU and software at Parsons ES, Austin, a minor "glitch" was observed. The cause of the problem was not located but may lie in the configuration of the computers to which this difficulty occurred. The weather station software and hardware were extensively tested at TWO and Parsons ES. This minor "glitch" appears to have no impact upon data collection and retrieval based on in-house and field tests.

The repaired unit and rain gauge were re-installed 16 April 1996. The station performed well and all sensors were successfully calibrated. All station calibrations and testing were performed by direct link with a lap top computer. Testing of weather station response via modem revealed signal interference from a phone line. Contact with the weather station was successfully established via modem on 6 May 1996 after phone line noise had been reduced. Meteorological data has been successfully downloaded by modem for 15 April 1996 through 9 May 1996 and is presented in this appendix.

Data Summary and Evaluation

After replacement of the weather station CPU board, TWI was consulted as to the retrieval and use of data collected prior to the CPU repair. Parsons ES has concluded that due to the nature of the malfunction it is not possible to separate corrupt data from pristine data and therefore none of the information collected prior to 16 April 1996 is usable. Precipitation data for the San Antonio International Airport has been obtained from the National Climatic Data Center for the time period in which the transducer has been operating at well 16. Though less site specific, this information will aid interpretation of the transducer data.

Two quarterly groundwater measurements were taken by electric line during the period in which the transducer has been in operation. These two measurements were taken on 11 December 1995 and 27 February 1996 and revealed a drop in water level of 6.37 feet. Transducer measurements recorded a drop of 8.3 to 8.7 feet (represents daily average of measurements taken every half hour). the raw data was sent to In-Situ for evaluation. No evidence of transducer error could be found but In-Situ offered to perform a post-calibration check to confirm the transducer accuracy. Initial reference values assigned to the transducer were off but easily corrected by a fixed numeric adjustment. None of the absolute value corrections altered the relative change in observed in the data. Based on the above the data is valid for comparison with changes in precipitation and contaminant concentration.