DELINEATION OF SPRINGSHEDS USING POTENTIOMETRIC SURFACES, SOUTHWEST TRAVIS COUNTY, CENTRAL TEXAS

The regional karstic Trinity Aquifer of central Texas provides a sole source water supply and baseflows for the region. Increased groundwater demand coupled with droughts has resulted in groundwater mining (storage depletion) and springflow capture from the various subunits, threatening the future of water availability in the region. In western Travis County small watersheds contain perennial and ephemeral springflows that sustain unique and fragile ecosystems within county preserves and parklands adjacent to areas experiencing significant depletion. The recharge area, or springsheds, of these small but critical springflow systems has never been delineated.This study uses a detailed potentiometric surface map to better characterize the regional and subregional groundwater flow paths of the Middle Trinity Aquifer in southwest Travis County. Manual water level measurements from 69 sites occurred from January 2022 to October 2022, generally during drought conditions. Additional data includes historic water level measurements, aquifer geochemistry, and lidar-derived DEMs. ARCG software was used for analysis and contouring. Results show a generalized 13 mi² westward-flowing aquifer system generally coincident with the small watersheds flowing (westward) into the Pedernales River. Springs occur near 790 ft-msl with a distinct ridge of elevated water levels at ~820 ft-msl east of the Pedernales River, before declining to 511 ft-msl further east. The potentiometric ridge defines the extent of the recharge zone and a hydrologic boundary between the small watershed-bound (west flowing) springflow systems and the adjacent regionally depleted (east-flowing) aquifer to the east. The boundary is generally coincident with the exposed Middle Trinity aquifer units. The findings characterize a localized springflow system influenced by local geology. Delineation of the springshed will help understand the local flow, groundwater divides, and capture zones and will be key to developed groundwater budgets. This information will be fundamental to the development of strategies for the aquifer and springflow conservation.