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Section 1 - Introduction
The Middle Trinity aquifer is comprised of the Lower Glen Rose Limestone, Bexar Shale, and the Cow Creek Limestone members of the Travis Peak Formation. At Camp Stanley Storage Activity (CSSA) and throughout the surrounding area, the Middle Trinity aquifer is used as a principal source of potable water. With respect to both time and cost restraints, two aquifer tests were conducted utilizing the existing well network. No additional wells or piezometers were installed as pumping or observation points. Groundwater pumping tests were conducted with respect to areas associated with CS-10 and CS-16 for this project. Figure 1 presents the location map for those wells used during the aquifer testing.
CS-10 was an ideal choice to perform an aquifer pumping test because of its proximity to two observation wells (CS-9 and CS-11) within 800 feet of the pumping well, and is already equipped with pumping and plumbing equipment. The economics and ease of implementation made CS-10 an ideal selection. The data collected served to evaluate parameters associated with the Middle Trinity aquifer in the extreme western portion of CSSA, which may be representative of aquifer parameters at off-post public water supply well fields located to the west of CSSA.
The second pumping test was conducted in the CS-16 area located in the central portion of CSSA. Groundwater at CS-16 is impacted by halogenated hydrocarbons. Analytical testing results from groundwater samples collected from various wells in the CS-16 area identified the presence of various volatile organic compound (VOC) constituents at concentrations exceeding Federal Safe Drinking Water Act (SDWA) maximum contaminant levels (MCL). The CS-16 test was designed to provide aquifer characteristics within the area of the groundwater plume using the existing monitoring well network.
The principal aquifer characteristics of interest for the project included an analysis to evaluate aquifer specific capacity, transmissivity, hydraulic conductivity, and storativity. Specific capacity is defined as the well yield per unit of drawdown and is typically expressed in terms of gallons per minute per foot (gpm/ft) of drawdown. Transmissivity (T) is an aquifer property that allows engineers and hydrologists a mechanism for calculating the amount of water an aquifer system is capable of transmitting across the entire thickness of the aquifer. Transmissivity is defined as the flow rate achievable through a 1-foot section of the aquifer system extending the full thickness of the aquifer under a hydraulic gradient equal to 1.
Hydraulic conductivity (K), like transmissivity, is a measure of the capacity of an aquifer to transmit water. The difference between hydraulic conductivity and transmissivity is that hydraulic conductivity focuses on a smaller section of the aquifer (typically a 1-foot by 1-foot area). Hydraulic conductivity is useful in calculating the velocity of flow (distance per unit time) within a porous media.
Storativity (S) is a measure of the amount of water released from an aquifer per unit area, per unit change in head. In other words, storativity is an indication of the amount of water that can be removed from an aquifer by pumping or draining. Storativity values are indicators of whether an aquifer is functioning under a confining pressure (confined condition) or water table (unconfined condition). Storativity values commonly associated with unconfined aquifer systems generally range from 0.01 to 0.3. Storativity values for confined aquifers generally range between 0.001 to 0.00001. Storativity values between 0.001 and 0.01 typically represent semi-confined aquifer systems.
CSSA intends to develop a groundwater compliance monitoring system to evaluate the extent of groundwater impact. This information will be used to support the CSSA conceptual site model (CSM). Pumping tests were performed to identify key aquifer parameters required for planning the groundwater detection monitoring system and for developing a comprehensive model. The parameters identified through the performance of groundwater pumping tests, when combined within a hydrogeologic model, are useful in predicting the:
| Amount of drawdown associated with the aquifer at various distances away from the pumping well; | |
| Amount of drawdown associated with a well at any time following the initiation of pumping; | |
| Lateral extent of the radius of influence associated with pumping wells under varying pumping rate scenarios; | |
| Amount of drawdown associated with a well at any time using any pumping rate; | |
| Relationships between multiple pumping wells and aquifer response within localized areas; | |
| Well efficiency; | |
| Well Design; | |
| Well pump selection; and | |
| Groundwater �pump and treat� remediation system capacity requirements. |
The following sections detail the activities and results of this preliminary evaluation of the aquifer parameters. Section 2.0 provides an overview of selected published information with bearing on this project. The remainder of the document details the activities associated with the field investigation program and presents an evaluation of the collected data together with an analysis of the project results and conclusions. References cited within the report are presented within Section 2.0 of this report.