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Final March 2004 On-Post Quarterly Groundwater Monitoring Report
Section 2 - Basewide Flow Direction and Gradient
A groundwater potentiometric surface map generated from March 2004 groundwater elevations is shown in Figure 2‑1. Among the cluster wells CS‑MW6‑LGR, CS‑MW7‑LGR, CS‑MW8‑LGR, CS‑MW9‑LGR, CS‑MW10‑LGR, and CS‑MW12‑LGR, only water level measurements from the LGR were used in creating the potentiometric surface map. The March 2004 potentiometric surface map indicates varying flow directions. An overall calculated groundwater gradient from the north-central area near CS‑MW16‑LGR to the CS‑MW6‑LGR well at the southwest corner of CSSA is to the south-southwest at 0.0046 feet/foot. This groundwater gradient will vary in direction and velocity in different areas of CSSA. Groundwater gradients calculated with different well elevations and in different locations vary from 0.0015 feet/foot to 0.016 feet/foot. General groundwater flow directions and average gradients during past monitoring events are provided in Section 3.0 for comparison.
The March 2004 potentiometric surface map for LGR-screened wells (Figure 2‑1) exhibited a wide range of groundwater elevations, from a minimum of 1001.82 feet MSL at CS‑MW11B‑LGR to a maximum 1099.43 feet MSL at CS‑MW4‑LGR. Groundwater elevations are generally higher in the northern and central portions of CSSA, and decrease to the southwest and southeast, with well CS‑MW11B‑LGR having the lowest groundwater elevation of all LGR screened wells. Groundwater in the west-central portion of the inner cantonment shows a drawdown effect from the pumping of drinking water wells CS‑9 and CS‑10. In March 2004, well CS‑10 had one of the lowest water levels measured, at 956.94 feet MSL. CS‑10 is completed with screen in all three formations and was pumping at the time of measurement.
There are exceptions across CSSA to the general south-southwest direction for flow of groundwater. Well CS‑MW4‑LGR in the central portion of CSSA had the highest groundwater elevation (1099.43 feet MSL) of LGR screened wells measured in March 2004 (Figure 2‑1). This groundwater elevation was 51 to 53 feet higher than the nearest comparable wells (CS‑MW1-LGR and CS-MW2-LGR), approximately 1,750 feet to the north. The CS‑MW4‑LGR well consistently reports a higher groundwater elevation than other wells screened in the same formation. Unlike the general trend at CSSA, groundwater flow appears to radiate outward from CS‑MW4‑LGR.
The groundwater gradient/potentiometric surface map presented in Figure 2‑1 incorporates measured groundwater elevations from the LGR screened wells only; Figure 2‑2 incorporates the BS screened wells only; and Figure 2‑3 incorporates the CC wells only. In the area near Building 90 in the southwest corner of CSSA, two potentiometric surface maps were created using March 2004 groundwater elevations from wells screened in the LGR and CC (Figure 2-4 and Figure 2-5, respectively). The LGR potentiometric surface map for March 2004 indicates a groundwater flow direction to the east near Building 90. This is a change from the groundwater flow directions reported previously which varied from south to north. The CC potentiometric surface map indicates that March 2004 groundwater flow is to the north-northeast near Building 90. Analysis of precipitation data show that the groundwater flow direction near Building 90 in the CC may reverse flow direction in response to variations in precipitation that affect groundwater elevations.
From June 2002 through March 2004, groundwater level measurements were recorded 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. Figure 2‑6 presents groundwater elevations from each well cluster compared to daily precipitation values from the northern CSSA weather station adjacent to well CS‑MW16‑LGR. Between June 27, 2002 and July 4, 2002, CSSA received 20.67 cumulative inches of rain, the most cumulative inches of rain since the weather station was installed. Figure 2‑6 illustrates the changes in groundwater elevations over time and illustrates groundwater recharge to the different formations at each well cluster.
As shown in Figures 2-1 through 2-6, water levels at CSSA vary greatly. This variability is associated with several factors:
Differences in well completion depths and formations screened;
Differences in recharge rates due to increased secondary porosity associated with the Salado Creek area;
Differences in recharge rates due to increased secondary porosity associated with local fault zones;
Pumping from public and private water supply wells located on- and off-post; and
Locations of major faults or fractures.
Until June 2001 when cluster wells were first installed and monitored at CSSA, most potentiometric surface maps were 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). Interpretation of past data for the overall potentiometric surface map is complicated by these well completion depth differences. Wells completed in the LGR only are used for Figure 2-1, wells completed in the CC only are used for Figure 2-2, and wells completed in the CC only are used for Figure 2-3.