GROUNDWATER FLOW MODEL OF THE PRESIDIO-REDFORD BOLSONS AQUIFER

The community of Presidio, in Far West Texas, relies on the Presidio-Redford Bolsons Aquifer for its water supply. Population growth will place a greater demand on the groundwater supply in the future. As part of the Texas Water Development Board’s Groundwater Availability Modeling (GAM) program we have developed a draft groundwater flow model of the bolsons aquifer to provide a groundwater management tool for the region.

The Presidio-Redford Bolsons Aquifer is located along the Rio Grande reach of Presidio County, Texas, within sedimentary basin fill deposits formed by Tertiary basin and range block faulting. Data suggest that the bolson deposits are up to 5,000 feet thick near the geographic center of the aquifer. The bolson deposits are overlain mostly by younger Quaternary river alluvium and underlain by Tertiary volcanic rocks and Cretaceous marine sediments. Groundwater recharge occurs through both diffuse infiltration and focused infiltration along stream channels. Groundwater discharges through springs, evapotranspiration, river baseflow, and pumping.

We are modeling groundwater flow in the Presidio-Redford Bolsons using the U.S. Geological Survey code MODFLOW-2000. The draft model includes three layers representing three units (from top to bottom): (1) river alluvium, (2) bolson deposits, and (3) underlying older rocks. Recharge to the aquifer is modeled using the MODFLOW recharge package as a percentage of rainfall with a cutoff minimum rainfall and a dampening factor to account for travel time in the unsaturated zone. Interaction with the Rio Grande, Rio Conchos and riparian evapotranspiration are modeled using the MODFLOW river package. Spring discharge is simulated using the MODFLOW drain package. Groundwater pumping is simulated using the well package. Most of the model boundary is assumed to be a no-flow boundary representing possible groundwater divides. We used a general head boundary at two locations to simulate interaction with regional groundwater flow.

During calibration, parameters for recharge, hydraulic properties, and boundary conditions were adjusted to match over 500 water level targets collected from 1948 to 2008. Calibration was assisted using parameter estimation code (PEST), an industry-standard inverse modeling software package.