CHARACTERIZATION AND QUANTIFICATION OF SPRINGFLOW AND DISCRETE RECHARGE IN THE TRINITY AND EDWARDS AQUIFERS ALONG THE BLANCO RIVER, CENTRAL TEXAS, USA

The Blanco River flows for 87 miles through central Texas as it traverses the karstic Trinity and Edwards aquifers. The river has gaining and losing reaches with major springs and swallets (recharge features) indicating a high degree of surface water and groundwater interaction. This river provides a critical water supply to a rapidly growing population and habitat for threatened and endangered species. Characterizing and quantifying spring flows and recharge rates along the Blanco River is essential to develop and implement effective water policies and management strategies.

Over the past 10 years, numerous gain/loss flow studies were conducted during drought and post-flood conditions. Results indicate relatively consistent spatial patterns of recharge and springflow, including the discovery of the largest spring complex in the southern Trinity Aquifer, Pleasant Valley Springs. Specific reaches provide discrete recharge to both aquifers and springflow from the upstream Trinity Aquifer that eventually recharges the Edwards Aquifer. The gain/loss studies provided information to locate and install additional U.S. Geological Survey stream gauging stations, which produce long-term flow records that can be applied to improve monitoring and modeling of the system.

This study defines five separate hydrologic domains along the Blanco River with mean water budgets over the five-year period of 2018-2022. From upstream to downstream these include: 1. Direct stream recharge into the Middle Trinity Aquifer from the upper reaches of the Blanco and Little Blanco Rivers (21,000 ac-ft/yr); 2. Perennial springflow from the semi-confined Middle Trinity Aquifer (30,000 ac-ft/yr); 3. Ephemeral springflow from unconfined Middle Trinity Aquifer (12,000 ac-ft/yr); 4. Ephemeral springflow from Upper Trinity and Edwards Aquifer unconfined springs (7,000 ac-ft/yr); 5. Recharge into the Edwards Aquifer (15,000 ac-ft/yr). These values include both very wet years (2019) and very dry years (2022), and do not reflect specific hydrologic conditions such as droughts and floods. A more detailed temporal analysis of flows in these five domains shows the importance of base flow from Middle Trinity springs to contribute recharge in the Edwards Aquifer.