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December 2004 On-Post Quarterly Groundwater Monitoring Report

Section 2 - Basewide Flow Direction and Gradient

The groundwater potentiometric surface map generated from December 2004 groundwater elevations are 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 0.0012 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 0.0004 ft/ft to 0.015 ft/ft. An average gradient calculated with elevations in wells CS‑G‑LGR, CS‑1, CS‑MW10‑LGR, CS‑MW6‑LGR, and CS‑MW16‑LGR, gives a gradient of 0.0045 ft/ft averaged across the entire base. General groundwater flow directions and average gradients during past monitoring events are provided in Section 3.0 for comparison.

The December 2004 potentiometric surface map for LGR-screened wells (Figure 2‑1) exhibited a wide range of groundwater elevations, from a minimum of 1148.46 feet MSL at CS‑MWG‑LGR to a maximum 1189.53 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‑MWG‑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 2004, well CS‑1 also had one of the lowest water levels measured, at 1103.80 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 (1189.53 feet MSL) of LGR screened wells measured in December 2004 (Figure 2‑1). This elevation was 15 to 20 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 to the north, east, and south at 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 2004 groundwater elevations from wells screened in the LGR and CC (Figure 2-4 and Figure 2-5, respectively).

The BS potentiometric surface map indicates groundwater flow direction to the east with slight variations to the south, similar to the CC map where flow direction is to the east. The LGR potentiometric surface map for December 2004 indicates a groundwater flow direction to the south and east near Building 90. The CC potentiometric surface map indicates that December 2004 groundwater flow is to the east near Building 90 and towards well CS-MW-16CC, near the center of the post. Flow towards CS-MW-16CC reflects the on-going pump and treat pilot test at this well.

From June 2002 through December 2004, groundwater level measurements were recorded to track the impact of rainfall events occurring at CSSA. Groundwater elevations have been recorded quarterly through December 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. The wells presented in this figure are equipped with transducers set to record water level measurements on a daily basis with increased monitoring during significant rain events. Between November 13 and November 23, 2004, 9.12 cumulative inches of rain was recorded by WS-N. 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:

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.

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