[Home] [Master Table of Contents]
Section 2 - Basewide Flow Direction and Gradient
A groundwater potentiometric surface map generated from December 2003 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 December 2003 potentiometric surface map indicates varying flow directions. An overall calculated groundwater gradient from the north-central area near CS‑MW16‑LGR to the CS‑MW10‑LGR well at the southwest corner of CSSA is to the south-southwest at 0.0052 feet/foot. This groundwater gradient will vary in direction and velocity in different areas of CSSA. Groundwater gradients calculated with different well elevations vary from 0.004 feet/foot to 0.018 feet/foot. General groundwater flow directions and average gradients during past monitoring events are provided in Section 3.0 for comparison.
The December 2003 potentiometric surface map for LGR-screened wells (Figure 2-1) exhibited a wide range of groundwater elevations, from a minimum of 978.65 feet MSL at CS‑MW10‑LGR to a maximum 1,066.16 feet MSL at CS‑MWG‑LGR. Groundwater elevations are generally higher in the northern and central portions of CSSA, and decrease to the southwest and southeast, with well CS‑MW10‑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 December 2003, well CS‑10 also had one of the lowest water levels measured, at 965.43 feet MSL. CS‑10 is completed with screen in all three zones.
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 second-highest groundwater elevation (1,036.89 feet MSL) of LGR screened wells measured in December 2003 (Figure 2-1). This groundwater elevation was 20.26 feet higher than the nearest comparable well (CS-MW2-LGR, 1,700 feet to the north). The CS-MW4-LGR well consistently reports a higher groundwater elevation than other nearby 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 December 2003 groundwater elevations from wells screened in the LGR and CC (Figure 2-4 and Figure 2-5, respectively). The LGR potentiometric surface map for December 2003 indicates a groundwater flow direction to the south near Building 90. This direction is consistent with the December 2002, March 2003, and June 2003 groundwater elevations. The CC potentiometric surface map indicates that December 2003 groundwater flow is to the south. The flow direction within the CC reverses direction dependent upon precipitation events affecting groundwater elevations. The September and December 2001 events showed a groundwater flow to the north in the CC, which reversed to a flow direction to the south for March 2002 and June 2002. The flow direction reversed to the north in September 2002 and
From June 2002 through December 2003, 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 set-up. Figure 2‑6 illustrates the
As shown in Figure 2-1 through Figure 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 and wells completed in the CC only are used for Figure 2‑2. As more wells are completed in the different formations, use of data from wells screened through the CC and BS can also be evaluated.