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Section 2 - Basewide Flow Direction and Gradient
The groundwater potentiometric surface map generated from September 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 are used to create the potentiometric surface map. 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 0.003 ft/ft. The groundwater gradient varies in direction and velocity in different areas of CSSA. Groundwater gradients calculated with different well elevations and in different locations range from 0.003 ft/ft to 0.016 ft/ft. General groundwater flow directions and average gradients during past monitoring events are provided in Section 3.0 for comparison.
The September 2004 potentiometric surface map for LGR-screened wells (Figure 2‑1) exhibited a wide range of groundwater elevations, from a minimum of 1079.76 feet MSL at CS‑MW11B‑LGR to a maximum 1148.45 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 September 2004, well CS‑1 also had one of the lowest water levels measured, at 1053.87 feet MSL. CS‑1 is completed with screen in all three formations.
There are exceptions across CSSA to the general south-southeast direction for flow of groundwater. Well CS‑MW4‑LGR in the central portion of CSSA had the highest groundwater elevation (1148.45 feet MSL) of LGR screened wells measured in September 2004 (Figure 2‑1). This elevation was 30 to 40 feet higher than the nearest comparable wells (CS‑MW2-LGR and CS-MW5-LGR). 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 September 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 September 2004 indicates a groundwater flow direction to the southeast near Building 90. This is a change from the groundwater flow directions reported previously which varied from south, northeast and southwest. The CC potentiometric surface map indicates that September 2004 groundwater flow radiates out, to the east and south near Building 90.
From June 2002 through September 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 on a weekly basis through July 2003 and then quarterly measurements were recorded through September 2004. Figure 2‑6 presents groundwater elevations from various well clusters compared to daily precipitation values from the northern CSSA weather station (WS-N) 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 WS-N 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.