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Offsite Well Survey Report
Section 4 - Local Aquifer Characteristics
General inferences regarding aquifer properties and characteristics are presented in this section. More detailed information is provided in the Soils and Geology and Groundwater Resources sections of the Background Information Report (Volume 1-1 of the CSSA Environmental Encyclopedia). It is beyond the scope of this document to provide estimations of hydraulic character for the individual formational members that make up the Middle Trinity aquifer, but generalized data regarding regional trends can be assessed. The bulk of these data originates from those records currently available from the TWDB or TNRCC, and includes those data reported by the drilling firm and results of water quality data. Specific data regarding groundwater and meteorological data have been collected and collated on a quarterly basis as part of CSSA�s groundwater monitoring program (Volume 5, Groundwater Monitoring).
For those wells outside of CSSA, information regarding groundwater levels were derived from data submitted with the standard Water Well Reports or Well Schedules provided to the TNRCC or TWDB by drilling firms. This data may include static water levels measured in wells after completion, and occasionally discharge and drawdown information collected during bailer tests. The data presented here represent spatially varied information that is impacted by regional location, topographical effects, cross-connected water-producing intervals, and a timeframe that spans over three decades. Based upon the data set, only general conclusions can be offered that represent a wide range of regional conditions.
Most wells in the region are either domestic or production wells and therefore have been completed in multiple water zones in order to maximize groundwater yield. Due to the nature of open hole completions, information is limited with respect to individual units that contribute water to the borehole. Therefore, conclusions regarding the properties of specific formational members could not be ascertained. According to data available in the state records, water levels within the survey area have ranged from 50 feet to 550 feet bgs at the time of well installation, with an average regional level of 280 feet below ground surface. In terms of elevation, water levels have ranged between 820 feet to 1,185 feet MSL, with an average elevation of 1,003 feet MSL. Both the minimum and maximum levels were reported at municipal wells at Fair Oaks. During September 1999, water levels at CSSA ranged in depth from 194 feet to 388 feet bgs (between 943 feet and 1035 feet MSL and 983 feet MSL average). A decade of historical water level data at CSSA shows that typical groundwater flow gradient is towards the south, with directional variations ranging from the southwest to the southeast, depending on the level of recharge. During extended periods of drought, the flow direction reflects a greater westerly component of flow. Figure 4.1 exemplifies groundwater flow during periods of reduced recharge and is the only instance in which data from CSSA and Fairco have been compiled together. Five potentiometric maps (representing five monitoring events) are presented in the September 1999 Quarterly Groundwater Report (Volume 5, CSSA Environmental Encyclopedia).
To aid in determining the influence of local precipitation upon water table response using site-specific data, a meteorological station and a downhole level transducer were installed at location CS-16 in August 1995. The level logger was then replaced with more sophisticated equipment in January 1999. The synchronicity of these two instruments has demonstrated the response of the aquifer to precipitation (Figure 4.2). Between October and November 1998, the CSSA vicinity received approximately 18 inches of precipitation, with an aquifer recharge response of approximately 100 feet of increased head. This included a single, 12-inch precipitation event that was responsible for area-wide flooding. Since that time, the aquifer has lost nearly 180 feet of elevation head as of March 2000 due to regional drought and local withdrawal. Historical data has shown that precipitation as little as three inches may invoke an aquifer response as much as 30 feet, usually within 5 days of the recharge event. The recent drought cycle through March 2000 has shown a continued gradual decrease in water level such that wells completed or pumping from the Lower Glen Rose no longer yield water (locations CS-4 and CS-D).
It can only be assumed that precipitation and recharge relationships are similar over the extent of the study area, and that large swings in water table elevations are common throughout the watershed area. Undoubtedly, the range of reported water levels presented in this section have also been affected by such extremes in the hydrologic cycle during the past three decades. Regionally, both the Lower Glen Rose and Cow Creek members are typically completed in a well to dampen the trend developed during dry periods, and to continue to yield sufficient quantities of acceptable groundwater.
Most information regarding aquifer yield for wells in the study area were determined from specific capacity tests performed by the drilling contractor. The intention of these tests was to assist in the determination of the potential yield of the well so that an appropriately-sized pump can be selected. These tests are primarily conducted as either pump, bailer, or jetting tests with durations between 1 hour and 2 hours. During these tests the rate of water evacuated from the borehole is estimated, and the resultant drawdown is measured. Because of the short duration of the tests, the estimated yields are likely biased high.
As previously mentioned, most wells in the vicinity of CSSA are completed within multiple zones to maximize the yield of potable water. For this reason, it is difficult to quantify the difference in specific yields between the formational units, except that the Cow Creek is widely recognized as the greatest producing interval of the Middle Trinity aquifer. The average estimated yield for wells included in this survey is 43 gpm, and ranged from 5 gpm to 115 gpm. This estimate included the higher-yielding municipal and/or public supply wells operated by Fairco, Bexar Met, and CSSA that typically produce rates of 75 gpm or more. The domestic wells that are more typical for the Jackson Woods, Ralph Fair Road, and IH-10/Old Fredericksburg Road geographic areas typically yield between 21 and 27 gpm from the Lower Glen Rose and Cow Creek members. Wells that are exclusively withdrawing water from the Lower Glen Rose and/or Bexar Shale (JW-3, JW-4, RFR-4) appear to yield less than 15 gpm.
Available chemistry data from the TNRCC and TWDB has been collated to evaluate natural water quality parameters from the CSSA vicinity. This comparison included common cations/anions, total dissolved solids (TDS), pH, and water hardness (as CaCO3). This type of data could only be obtained for those municipal wells in which periodic testing is required as a public supply water source. While this record is incomplete, it does typify the water source from the Middle Trinity aquifer. Since data are only available from wells that are completed across multiple water-bearing units, no conclusions regarding differing character between the Middle Trinity formation members could be reached.
Chemistry data obtained during the well research are included with their respective source wells in Appendix B. Additional data obtained from the TNRCC and TWDB are also included in Appendix E. Data for those wells included in this survey have been compiled in Table 4.1. Data from 17 public supply wells located either at CSSA, Fair Oaks Ranch, Leon Springs Villa, or Hidden Springs Estates, in addition to composite samples collected from the Fair Oaks and CSSA distribution systems are available through the state records. Many of the water sources have multiple samples, and have been included in this table. Statistics have been generated for each geographic region, as well as the area-wide general statistics. It is important to note that location FO-3, while listed on the table, is not included in the actual statistical calculation because it is not completed in the Middle Trinity aquifer. This well is completed in the Hosston Sand of the Lower Trinity aquifer, and produces highly mineralized water.
As seen in Table 4.1, the results of the analyses show that the water quality is very similar for all three geographic regions. As expected, the samples show that groundwater from the limestone aquifer is predominately mineralized with calcium, magnesium, and bicarbonate, with appreciable amounts of chlorides and sulfates. The average range of TDS is less than 500 mg/l, which is considered a freshwater source in the State of Texas. However, the mineralization of groundwater from the limestone matrix has resulted with a hardness generally greater than 300 mg/l, which is considered high. The groundwater is slightly alkaline, with an average pH of 7.5.
The data in Table 4.1 was used to generate the Piper diagram in Figure 4.3. Piper diagrams consist of three components: two trilinear diagrams along the bottom and one diamond-shaped diagram in the middle. The trilinear diagrams illustrate the relative concentrations of cations (left) and anions (right) in each sample. For both trilinear diagrams, the chemical data is normalized with only the components shown on that trilinear diagram. Each corner represents 100% of the labeled component, with the percentage of that component decreasing away from the corner. Also, any data that plots inside the diagram contains all three components, whereas data that lie along the lines of the triangle contain two components, but none of the third component.
The diamond field is designed to show both anion and cation groups. For each sample, a line is projected from its point in the cation and anion trilinear diagrams into the diamond region and the datapoint is plotted where the lines intersect, as demonstrated by the dashed lines for FO-3 in Figure 4.2. For clarity, only the most recent chemical analyses from wells with multiple data sets were plotted in this diagram. The diagram clearly shows that the water source is primarily of the calcium-magnesium bicarbonate type, and that most of the data plots very close together. One exception is FO-3, which produces significantly elevated concentrations of sulfate, calcium, sodium, chlorides, and magnesium in comparison to the rest of Middle Trinity aquifer wells. The distinct character of FO-3 is evident on the Piper diagram since it plots well beyond the clustering of the remainder of the data set. The close grouping of the remaining data indicate that the groundwater from the other wells is very similar in character and quality.
One possible trend that is evident in both Table 4.1 and Figure 4.2 is that the groundwater becomes more mineralized as it travels downgradient towards Leon Springs Villa and Hidden Springs Estates. This is a common phenomenon that occurs when meteoric waters pass through highly mineralized aquifer materials. As seen on the cation triangle, this appears to be especially true for the increased magnesium content at locations LS-3, LS4, HS-2, and HS-4. However, any such trends are less apparent on the anion trilateral graph. Currently, no data are available to independently characterize the individual formational members of the Middle Trinity aquifer.