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March 2003 On-Post Quarterly Groundwater Monitoring Report

Section 2 - Basewide Flow Direction and Gradient

A groundwater potentiometric surface map generated from the April 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 the water level measurements from the LGR were used in creating the potentiometric surface map. The April 2003 potentiometric surface map indicates varying flow directions. The overall calculated groundwater gradient is south-southeast at 0.012 feet/feet. Groundwater flow directions and gradients during past monitoring events are provided in Section 3.0, for comparison.

The April 2003 potentiometric surface map for LGR-screened wells (Figure 2-1) exhibited a wide range of groundwater elevations, from a minimum of 1116.45 at CS-MW10-LGR to a maximum of 1162.16 feet above mean sea level (MSL) at CS-MW4-LGR. The groundwater elevations are generally higher in the central portion of CSSA (CS-MW4-LGR). The groundwater elevations become lower to the southwest and southeast with well CS-MW10-LGR having the lowest groundwater elevation of all LGR screened wells. From all the wells measured, CS-1 has the lowest groundwater elevation (1093.54 feet) and FO-20 has the highest groundwater elevation (1165.45 feet).

There is an exception to the general south-southwest direction of flow for groundwater. Well CS-MW4-LGR in the central portion of CSSA had the highest groundwater elevation (1162.16 ft MSL) of LGR screened wells measured in April 2003 (Figure 2-1). This groundwater elevation is 10.48 feet lower than the December 2002 water level, and approximately 19.10 feet higher than the nearest comparable well (CS-MW2-LGR) to the north. The CS-MW4-LGR well consistently reports a higher groundwater elevation than other nearby wells screened in the same formation and implies groundwater flow is to the northwest in this area.

The groundwater gradient/potentiometric surface map presented in Figure 2-1 incorporates measured groundwater elevations from the LGR screened wells only. In the area near Building 90, in the southwest corner of CSSA, two potentiometric surface maps were created using April 2003 groundwater elevations from wells screened in the LGR and CC units (Figure 2-2 and Figure 2-3, respectively). The LGR potentiometric surface map for April 2003 indicates a groundwater flow direction to the south near Building 90. This direction is consistent with the December 2002 groundwater elevations. The CC potentiometric surface map indicates that April 2003 groundwater flow is to the south. This is consistent with the December 2002 groundwater flow direction to the south, the same as the southern flow direction recorded in the June and March 2002 events. The September 2002 groundwater elevations indicated a groundwater flow to the north in the CC. The September and December 2001 events also showed a groundwater flow to the north. Analysis of precipitation data shows that the groundwater flow direction in the CC reverses to flow to the north in response to increased precipitation events and increased recharge.

From December 2002 through April 2003, 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 present groundwater elevations 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. These figures illustrate the changes in groundwater elevations over time after the significant July rain event and illustrate groundwater recharge to the different formations at each well cluster.

As shown in Figure 2-1, 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 �open hole� 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 differences. Wells completed in the LGR only are used for Figure 2-1. As more wells are completed in the different formations, use of well data from wells screened through the CC and BS Formations can also be evaluated.

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