NGWA Event and Education Recordings Archive

In the Northern Segment of the Edwards Balcones Fault Zone (BFZ) aquifer (Central Texas), investigating groundwater-stream interactions is an important part of managing groundwater for water supply and critical habitat for a federally-listed, threatened aquatic species. However, characterizing groundwater-stream relationships is challenging in a fractured, karsted limestone basin. Using a natural tracer such as radon-222 can be helpful in differentiating reaches of surface versus groundwater-dominated flow. Radon-222, with a half-life of 3.8 days, is the alpha decay product of radium-226 in the uranium-238 series, and its concentration in water is thought to reflect the uranium content of source geology as well as residence time.

Because radon-222 is readily lost to the atmosphere through gas exchange, it is expected that surface water would have low radon concentrations while groundwater would have high values. This conceptual model was tested at Salado Creek, a gaining and losing stream in the Northern Segment of the Edwards BFZ aquifer. Radon emanation of each collected water sample was measured using the RAD7 (Durridge Co., Inc., Billerica, MA), an instrument that quantifies the decay of radon nuclei. Results indicate generally low radon throughout the basin, with noticeably high levels in the middle reach of Salado Creek which flows through the village of Salado. Low radon numbers indicate surface-dominated waters, while high numbers in the middle reach reflect groundwater contribution from the Salado Springs complex. Variations in radon concentration are influenced by small rapids along the creek that increase gas exchange and release of radon into the atmosphere. By analyzing for radon along Salado Creek, groundwater-dominated reaches are easily distinguished from reaches dominated by surface water. Radon can be combined with flow and other water quality data into a suite of monitoring measures, which can in turn be applied to environmental impact studies and identification of critical habitat.