APPLYING DYE TRACING TO CHARACTERIZE SURFACE-WATER AND GROUNDWATER INTERACTIONS IN THE TRINITY AQUIFERS, CENTRAL TEXAS

Increasing reliance on the karstic Trinity aquifers of central Texas for water supplies has led to decreasing recharge, springflow, and streamflow. Recent studies of these aquifers and the streams that flow over them have shown how interconnected they are. Streams in the area are largely dependent on springs for base flows, and many springs are dependent on recharge to the aquifers from the streams. Streams of interest to this study are the Blanco River, Cypress Creek, and Onion Creek. Both the Blanco River and Onion Creek flow across Trinity geologic units and then over the Edwards geologic units where significant recharge to the karstic Edwards Aquifer takes place. Cypress Creek is a tributary to the Blanco River and flows across the Trinity.

In December 2017, two dyes, fluorescein and Rhodamine WT, were injected into two recharging swallets in the upper watershed of Onion Creek that was previously assumed to be a gaining stream with no recharge. Concerns had been made about plans for discharge of treated wastewater into Onion Creek and potential impacts on the underlying Trinity aquifers. Both dyes were detected in nearby down-gradient water-supply wells and shallow springs. In one well about 1.3 miles downgradient of the injection point, dye was detected visually within 24 hours of the injection.

In March 2018, fluorescein dye was injected into Saunder’s Swallet on the Blanco River. This swallet is about a half mile upstream of Park Spring that is also on the Blanco River. Within a day, visible dye was present in Park Spring. Dye from this injection was detected in Pleasant Valley Spring that is also situated on the Blanco River, but about 4 miles downgradient of Saunder’s Swallet.

Also in March 2018, Rhodamine WT dye was injected into Raccoon Cave, about ¾ mile upgradient of Jacob’s Well Spring on Cypress Creek. In this case, no visible dye was seen in Jacob’s Well, and the detection of dye in Cypress Creek downstream of Jacob’s Well was at such low levels as to be considered equivocal.

Results of these dye trace studies support findings of the earlier hydrogeologic studies that illustrate the complex and interconnected relationships between surface water and the Trinity aquifers. It is clear that some of the water recharging the Edwards Aquifer from these streams has been surface water and Trinity aquifer water several times over, or more.