GROUNDWATER AVAILABILITY OF THE BARTON SPRINGS SEGMENT OF THE EDWARDS AQUIFER , TEXAS : NUMERICAL SIMULATIONS THROUGH 2050 by

A two-dimensional, numerical groundwater-flow model was developed for the Barton Springs segment of the Edwards aquifer to evaluate groundwater availability and predict water levels and spring flow in response to increased pumpage and droughts during the period 2001 through 2050. A steady-state model was developed on the basis of average recharge for a 20-yr period (1979 through 1998) and pumpage values for 1989. Transient simulations were conducted using monthly recharge and pumping data for a 10-yr period (1989 through 1998) that includes periods of low and high water levels. Values of hydraulic conductivity were estimated by calibrating the steady-state model using trial and error and automated inverse methods. Good agreement was found between measured and simulated flow at Barton Springs (root mean square error [RMS error, average of squared differences in measured and simulated discharges] 12 cfs), between measured and simulated water levels at different times and between measured and simulated water levels in many of the monitoring wells. To assess the impact of future pumpage and potential future droughts on groundwater availability, transient simulations were conducted using extrapolated pumpage for 10-yr periods (2001 through 2050) and average recharge for a 3yr period and recharge from the 1950’s drought for the remaining 7 yr. Results of these simulations were compared with those using average recharge and future pumpage. Predicted water-level declines in response to future pumpage under average recharge conditions are small ( 35 ft), whereas water-level declines under future drought conditions were much greater ( 270 ft). Simulated spring discharge in response to future pumpage under average recharge decreased proportionally to future pumpage (2 cfs per decade), whereas spring discharge decreased to 0 cfs in response to future pumpage under drought-of-record conditions. Management of water resources under potential future drought conditions should consider enhanced recharge and conservation measures. INTRODUCTION This modeling study focuses on a segment of the Edwards aquifer within and adjacent to Austin, Texas, that discharges into Barton Springs and Cold Springs and is hydrologically distinct from the rest of the Edwards aquifer. This region, referred to as the Barton Springs segment of the Edwards aquifer, constitutes the sole source of water to about 45,000 residents. Barton Springs pool was created when a dam was installed immediately downstream of the 1 spring and it also serves as a municipal swimming pool in Zilker Park, downtown Austin. The Barton Springs salamander, listed as an endangered species, is restricted to the region immediately surrounding the spring. Increased population growth and recent droughts (1996) have focused attention on groundwater resources and sustainability of spring flow. A combination of increased pumpage and severe drought could severely impact future water resources. A numerical groundwater flow model is a tool that can help in assessing the impacts of current and future pumpage on groundwater resources and spring discharge. A groundwater flow model numerically represents the aquifer using a computer. Information about the aquifer, such as water levels, recharge, and spring discharge, provides input to the model and helps us evaluate the reliability of the model. A calibrated groundwater model can provide a valuable tool for evaluating the impact of pumping and drought on an aquifer. The objective of this study was to evaluate long-term groundwater availability in response to future pumpage and potential future droughts. To meet this objective, it was necessary to develop a two-dimensional numerical, finite-difference groundwater model of the Barton Springs segment of the Edwards aquifer. This model will provide (1) a management tool to the Barton Springs Edwards Aquifer Conservation District (BSEACD) and to the Regional Water Planning Group and (2) a tool for evaluating groundwater availability under drought-ofrecord conditions. This report describes the construction and calibration of the numerical model and the results of predictive simulations of water levels and spring discharge for the next 50 yr based on projected demands from the Regional Water Planning Group and the BSEACD. The various components of the modeling study included (1) developing a conceptual model that included our current understanding of the geology, (2) quantifying groundwater recharge from stream-gage records, (3) calibrating a steady-state model using average recharge for a 20-yr period (1979–1998) and trial and error and automated inverse methods, (4) running a transient model for a 10-yr period (1989–1998), (5) conducting sensitivity analyses to determine the primary controls on the simulations, and (6) running predictive simulations through 2050. This report describes (1) the study area, previous work, and hydrogeologic setting used to develop the conceptual model; (2) the code, grid, and recharge assigned during model construction; (3) calibration of the steady-state model to estimate the hydraulic conductivity distribution; (4) the transient model for the 10-yr period; (5) sensitivity analysis for the steadystate and transient model; and (6) predictions of water-level changes and spring discharge under 2 future pumpage and drought-of-record conditions; (7) the limitations of the current model; and (8) suggestions for improvements. The model developed in this study differs from the previous two-dimensional, finitedifference model developed by Slade and others (1985) in the grid resolution (minimum 500 ft versus a minimum of 1,500 ft) in explicitly representing the aquifer thickness in the simulation, in simulating transient flow for a long time (10 yr versus 5 mo), and in predicting groundwater availability under increased pumpage and potential future droughts for the period through 2050. The spatially distributed model developed in this study allows the effect of pumpage in different regions of the model area to be assessed, which is not possible with the lumped parameter model developed by Barrett and Charbeneau (1996). More details on these other models are provided in the Previous Work section. STUDY AREA The Barton Springs segment of the Edwards aquifer constitutes the study area and includes parts of Travis and Hays Counties (fig. 1). The study region is within the Lower Colorado Region (Region K) water-planning group and includes the Barton Springs/Edwards Aquifer Conservation District (fig. 2). The model boundaries are all hydrologic boundaries and include the Mount Bonnell fault to the west, which acts as a no-flow boundary (Senger and Kreitler, 1984); a groundwater divide to the south along Onion Creek (Guyton and Associates, 1958); the “bad-water” line to the east; and the Colorado River (Town Lake) to the north. Groundwater circulation in the Edwards aquifer decreases to the east and total dissolved solids (TDS) increase. The bad-water line marks the zone where TDS exceeds 1,000 mg/L, which generally coincides with Interstate 35. The groundwater divide in the south separates the Barton Springs segment from the San Antonio segment of the Edwards aquifer, which discharges into Comal and San Marcos Springs. Physiography and Climate Physiographically the aquifer lies on the transition between the Edwards Plateau to the west and the Blackland Prairie to the east. The topography of the area is that of the Rolling