42-19 Evaluating Radon-222 Baseline Occurrence and Variability in Central Texas Springs

Abstract:

Central Texas aquifers supply millions of residents and support fragile spring ecosystems. Hydrogeologic research is essential to preserve these water resources amid threats from development and climate variability. Radon-222 (Rn-222) is a radioactive decay product of Uranium-238 which can be used as a groundwater tracer. Rn-222 activity in spring water is a function of the aquifer lithology, residence time, recharge dynamics, and groundwater velocity, providing information about flow paths, permeability, and surface water-groundwater interactions. However, baseline Rn-222 data for springs in Central Texas is limited. We establish initial measurements of Rn-222 activity in Central Texas springs, and compare results across the region. 52 samples were taken from 5 spring complexes, providing the first Rn-222 activity measurements in these springs. 250mL grab samples were collected during baseflow conditions from May-August 2026 and analyzed using the RAD8 (Durridge Co.) following recommended protocols of the manufacturer. We analyzed 38 samples from 4 springs in the karstic Edwards Aquifer Barton Springs complex (Old Mill, Eliza, Barton Main; TWDB: 5842914) that averaged 46 +/- 10 pCi/L and ranged 30-81 pCi/L. Old Mill Spring had the highest average Rn-222 activity (65 pCi/L, n=3), potentially due to influence from the saline portion of the Edwards Aquifer. Eliza and Barton Main had lower Rn-222 activity, inferred to be a result of similar conduit flow paths, consistent with previous studies. We analyzed 4 samples from the Krause Springs complex (TWDB: 5739302) within a coupled Cretaceous clastic and Paleozoic carbonate aquifer, that averaged 72 +/-19 pCi/L and ranged 38-118 pCi/L. A variety of Quaternary alluvial springs within the Colorado River watershed averaged 83 +/-21 pCi/L (n=9) and ranged 52-201 pCi/L. Springs of the Hamilton Pool complex (Middle Trinity Aquifer), a stacked carbonate clastic aquifer, averaged 287 +/-18 pCi/L and ranged 87-685 pCi/L (n=6). Edge Spring (TWDB: 5747318) had the highest Rn-222 activity. Results varied within and across spring complexes, remaining lowest in the karstic Barton Springs complex and higher in porous clastic and mixed aquifers likely due to lithological and permeability differences. This study provides baseline Rn-222 data, laying the groundwork for future hydrogeologic research in Central Texas. 

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.