March 2000 On-Post GW Report Section 3

Water level measurements are collected at CSSA as part of the regular sampling protocol. Water well upgrades were performed on CSSA Wells 1 and 11 between May 7 and 13, 1997, and on Well 9 between November 17 and 20, 1997. The upgrades included the installation of a PVC e-line measuring tube in both wells. Prior to the upgrades, Well 11 water levels were measured by the air-line method.

As mentioned previously, Well 10 was the only water-supply well being pumped during water level measurements. The water table elevations at Wells 9 and 11 therefore reflect the localized influence of the pumping at Well 10. Pumping at water-supply Wells 1 and 9 had been discontinued 48 hours before water levels were measured in these wells. Well I is also pumped occasionally as it is equipped with a windmill and sucker rods. The amount of time required for full water table recovery following pumping is not known. Historically high rainfalls and flooding, which affected recharge and aquifer water levels, preceded the November 1998 sampling event.

The average groundwater elevation decrease for the January 1998 and November 1998 monitoring events do not include groundwater level data for Wells 1, 9, 10, or I since the pumps at these wells run often. The average elevation decrease for the March 2000 event does not include a groundwater level datum from Well 4, which was dry. Regular pumping at Well 11 has been discontinued due to the presence of coliform in this well. The total average rise in groundwater elevations from January 1997 to November 1998 was 175.5 feet. The rise is predominantly the result of extended rainfall in the latter parts of 1997 and 1998. The sharp decline observed in the groundwater elevation average in September 1999 and the continued declines in December 1999 and March 2000 are likely due to drought conditions which occurred throughout 1999 and persist into 2000. Historical groundwater elevations from October 1992 through March 2000 are shown in Table 2.

Several groups of privately owned domestic wells and a municipal water district are located along the western and southwestern boundaries of CSSA. The domestic wells have been grouped geographically and include the Jackson Woods subdivision wells, Ralph Fair Road wells, Leon Springs Villas wells, Hidden Springs Estates wells, and the I-10/Old Fredericksburg Road wells. These domestic/municipal wells are primarily completed in the Lower Glen Rose formation and yield less than 10 gal/min. The Jackson Woods subdivision group has 31 wells and is the largest group of domestic wells. The Fair Oaks Water Company (Fairco) operates 36 large capacity municipal wells that are located just to the northwest of the CSSA boundary. Most of these wells are completed in the Lower Glen Rose, Bexar Shale, and Cow Creek Formations, similar to the majority of CSSA wells.

The impact of pumping by adjacent municipal and subdivision water wells upon the water table at CSSA is unknown. However, in September 1999, Parsons ES initiated a well survey in the vicinity of CSSA. From the data currently available, it appears that Fairco conducts the most significant pumping in the area. In August 1999, Fairco pumped approximately 1.8 million gallons per day (mgpd) for residential use. Additional information about the Fairco wells, and other wells near CSSA, is included in the Draft Offsite Well Survey Report (Parsons ES, 2000). Environmental Encyclopedia Volume 5 will contain the Final Offsite Well Survey Report behind tab �Groundwater Investigation�.

Groundwater potentiometric elevations using e-line and air-line measurements from January 1997, October 1997, January 1998, November 1998, September 1999, December 1999, and March 2000 are depicted in Figures 1 through 7, respectively. Figure 5 includes the weekly average water level (1036.80 feet MSL) measured by Fairco for Fairco Well 20 during the week of September 8, 1999.

Groundwater gradients and flow directions as shown on the potentiometric surface maps are approximate and general in nature. Drawing concrete conclusions from these maps regarding the natural groundwater gradient at CSSA is not appropriate for several reasons. Groundwater elevation data is collected from different types of wells, such as production wells, stock wells, and a few monitoring wells. These wells are screened in different groundwater zones or through multiple zones. The historical groundwater elevation data being analyzed has been measured using a mixture of air and electrical lines. Air-lines are not as accurate as e-lines which makes a direct correlation between the two tenuous. When groundwater levels are collected, surrounding production wells are often pumping; thus, not showing the true static water level condition at CSSA. Furthermore, significant local area pumping occurs near CSSA. Local pumping likely influences the groundwater gradient and flow directions at CSSA and may be the reason that groundwater flow directions have fluctuated greatly from southeast to southwest over time. To assume, then, that CSSA�s groundwater flow direction is naturally to the south or southwest may not be correct. The natural general groundwater flow direction for the area, however, is towards the southeast. Groundwater gradients, approximate flow directions, and average changes in groundwater elevation during each sampling event from October 1997 through June 2000 are shown in Table 3.

Pumping in most water-supply wells was halted at least 48 hours prior to water level measurement. In Figure 3, however, water level measurements from Wells 10 and 11 were not used due to active pumping. The water level measurement for Well 9 was also not used in Figure 3 because the measurement represents the localized influence of this pumping. Likewise, water level measurements from Well 10 were also excluded from Figures 4 through 6 due to active pumping. The water levels measured in Wells 9 and 11 during this sampling event were possibly being influenced by the pumping occurring in Well 10 (see Figure 7). Water level measurements taken from Wells 10 and 11 during October 1997 appear to be flawed and were therefore not included in Figure 2. The groundwater elevations measured in Wells 10 and 11 during October 1997 significantly exceeded those in all other wells at that time and Wells 10 and 11 were pumping in the days prior to the measurements.

The addition of the Fairco well water level in September 1997 resulted in a slightly different interpretation of groundwater flow direction, as shown in Figure 5. The additional data point may provide an indication that groundwater flow at CSSA is being influenced by offsite pumping. The approximate groundwater gradients, flow directions, and average change in the groundwater elevation for each of the monitoring events are summarized in Table 3.

Date of Monitoring Event	Flow Direction	Gradient (ft/ft)	Average Change in Water Level (ft)*
October 1997	South	0.002	-105.4
January 1998	South-southeast	0.003	43.4
November 1998	South-southeast	0.013	-113.5
September 1999	Southwest	0.007	188.4
December 1999	Southwest	0.004	4.9
March 2000	South-southeast	0.009	9.3
Note: Negative values represent increases* in the groundwater elevation.

To aid in determining the influence of local precipitation upon water table response using site-specific data, a meteorological station was installed just east of Well 16 and a pressure transducer was installed downhole in Well 16 in August 1995.

Precipitation data between October 5, 1998 and March 12, 2000 collected from the new meteorological station is shown in Figure 8.

A new transducer was installed in Well 16 and water table data collection began on January 11, 1999. As measured by e-line, a 176.8 foot decrease in water level occurred in Well 16 between November 9, 1998 and March 20, 2000. This decline coincided with an extended period of little precipitation as measured at the meteorological station adjacent to Well 16. Only five rainfall events with greater than 1 inch of precipitation were recorded at the meteorological station at CSSA between November 30, 1998 and March 12, 2000.