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Final September 2002 On-Post Quarterly Groundwater Monitoring Report
Section 2 - Basewide Flow Direction and Gradient
A groundwater potentiometric surface map generated from the September 2002 groundwater elevations is shown in Figure 2-1. Among the cluster wells CS-MW6-LGR, CS-MW7-LGR, CS-MW8-LGR, CS-MW9-LGR, and CS-MW10-LGR, only the water level measurements from the Lower Glen Rose were used in creating the potentiometric surface map. The measurement from well CS-H was not included due to a blockage within the well discovered during the well upgrade. The September 2002 potentiometric surface map indicates varying flow directions. The overall calculated groundwater gradient is south-southeast at 0.017 feet/feet. Groundwater flow directions and gradients during past monitoring events are provided in Table 3-1, for comparison.
The September 2002 potentiometric surface map for LGR-screened wells (Figure 2-1) exhibited a wide range of groundwater elevations, from a minimum of 1064.28 at CS-1 to a maximum of 1161.54 feet above mean sea level (MSL) at CS-MW4-LGR (disregarding CS-H). The groundwater elevations are higher in the central and northwestern portions of CSSA. The groundwater elevations are lower to the south with well CS-1 having the lowest groundwater elevation of all measured wells.
There were two exceptions to the general south-southeast direction of flow for groundwater. Well CS-MW4-LGR in the central portion of CSSA and Well CS-H located near the northwest corner of CSSA had two of the highest groundwater elevations measured in September 2002 (Figure 2-1). CS-MW4-LGR had a water level of 1161.54 feet above MSL, which is approximately 109.8 feet higher than the June 2002 water level, and approximately 26.46 feet higher than the nearest comparable well (CS-MW2-LGR) to the north. In the northwestern corner, Well CS-H had a groundwater elevation measured at 1186.20 feet MSL, which is approximately 138.3 feet higher than the nearest well to the north (FO-20) and virtually unchanged since June 2002. However, Well CS-H data may not accurately reflect the potentiometric surface at CS-H due to a recently discovered blockage within the well casing. During September 2002, a new well CS-H-LGR was installed and may be completed by the December 2002 event. In these localized areas, the groundwater gradient is to the north-northwest. These two areas consistently report a different groundwater gradient direction from the overall south-southeast decline in water elevations.
The groundwater gradient presented in Figure 2-1 incorporates measured groundwater elevations from the Lower Glen Rose wells and those wells with open borehole completions (CS-1, CS-2, CS-9, CS-10, and CS-11). In the area near Building 90, in the southwest corner of CSSA, two potentiometric surface maps were created using September 2002 groundwater elevations from wells screened in the Lower Glen Rose and Cow Creek units (Figure 2-2 and Figure 2-3, respectively). The Lower Glen Rose potentiometric surface map for September 2002 indicates a groundwater flow direction to the south-southwest near Building 90. This direction is a minor change from the June 2002 groundwater elevations that indicated flow to the south in the Lower Glen Rose. The Cow Creek potentiometric surface map indicates that September 2002 groundwater flow is to the north. This is opposite of the southern direction recorded in the two prior (June and March 2002) events. The September and December 2001 events showed a northern groundwater flow in the Cow Creek formation.
From June 2002 through October 2002, groundwater level measurements were recorded on various dates to track the impact of rainfall events occurring at CSSA. Groundwater elevations at the well clusters located at CS-MW6, CS-MW7, CS-MW8 and CS-MW10 were closely monitored during these major rain events. Figures 2-4, 2-5, 2-6 and 2-7 indicate groundwater elevations compared to time in each of the well clusters near Building 90. Between June 27, 2002 and July 4, 2002 CSSA received 20.67 inches of rain. These figures illustrate the changes in groundwater elevations over time after the significant July rain event and illustrate groundwater recharge occurring in the different formations of each well cluster. Figure 2-8 illustrates variations in the Lower Glen Rose potentiometric surface flow direction near Building 90 from July through September 2002.
As shown in Figure 2-1, water levels at CSSA vary greatly. This variability is likely associated with various factors:
Differences in well completion depths and formations penetrated;
Differences in recharge rates due to increased secondary porosity associated with the Salado Creek floodplain;
Differences in recharge rates due to increased secondary porosity associated with the fault zone;
Unknown pumping rates from public and private water supply wells located off-post but near the CSSA boundary; and
Locations of major faults or fractures.
With the exception of Figure 2-2, Figure 2-3, and Figure 2-8, most potentiometric surface maps prepared for CSSA are based on water levels from wells with different completion depths. Additional information concerning this issue is included in the Introduction to the Quarterly Groundwater Monitoring Program (Volume 5, Groundwater). Because several wells depicted on Figure 2-1 are open to multiple water-bearing zones, potentiometric surface maps should be considered qualitatively. The differences in water levels across CSSA may stem from differences in the various wells� completions. Wells CS-2 and CS-4 are open-hole completions in the Lower Glen Rose, or the Lower Glen Rose and the upper portion of the Bexar Shale. Also, water level measurements obtained from some wells (i.e., CS-1 and CS-9) represent water levels measured from up to three different formations. Wells completed in the Lower Glen Rose only represent the water level for that formation. Interpretation of the data for the overall potentiometric surface map is complicated by these well completion differences. As more wells are completed in the different formations, use of well data from wells screened through multiple formations can be reduced or eliminated in future water level and gradient determinations.