ORIGINAL RESEARCH article
Front. Water
Sec. Environmental Water Quality
An application of U/Th-series radiotracers to study groundwater transport processes in surficial and Upper Floridan Aquifer System in central Florida, USA
Provisionally accepted
- 1Wayne State University, Department of Environmental Science and Geology, Detroit, United States
- 2Wayne State University Department of Environmental Science and Geology, Detroit, United States
- 3Florida Department of Environmental Protection, Tallahassee, Florida, Tallahassee, Florida, United States
- 4US Geological Survey Pacific Coastal and Marine Science Center, Santa Cruz, United States
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Word count: 320 U-Th series radionuclides are elegant, naturally occurring tracers to study groundwater transport mechanisms. Constituent dissolution and sorption/desorption processes that involve Fe-Mn oxyhydroxides play a major role in the mobility of select trace elements and radionuclides. For example, as radium is strongly bound to manganese oxides in most natural waters, the behavior of Mn in groundwater can be expected to also affect Ra adsorption-desorption rates. For this study, the redox state of an aquifer in central Florida was studied to examine the groundwater mobility of Ra and other trace elements. From a suite of thirteen groundwater samples and complimentary s oil core samples, U/Th parent – daughter disequilibria were utilized to obtain sorption-desorption rate constants for radium. The large observed variability in 222Rn and 224Ra activities can be attributed to a change in fine scale, phosphorite-bearing lithologies. The range of 223Ra/226Ra activity ratios is quite large (0.003 to 0.156) relative to the theoretical ratio of aged host rock, 0.046, indicating faster replenishment of 223Ra as compared to 226Ra. The adsorption rate ranged from 0.03 to 2.4 min-1, corresponding to residence time of 0.4 to 33 min for Ra in groundwater. In contrast, the Ra desorption rate is slower, from 0.5 to 21 x 10-4 min-1, which corresponds to a desorption time scale of 0.3 to 14 days. The retardation factor varied between (0.12±0.03 and 7.0±2.7) x 103. Within these samples, there was no significant correlation between the adsorption/desorption rate constants and the concentrations of redox-sensitive trace elements (i.e., Fe, Mn, or As), suggesting that redox processes alone do not control the adsorption-desorption constants. Furthermore, there is no apparent relationship between alkalinity and Ra adsorption-desorption rate constants, which suggests that weathering processes alone do not control Ra sorption. The distribution coefficient varied between (0.16 and 9.3) x 103 L/Kg (mean: 4.1±4.0 x 103). This study provides insight into the adsorption-desorption kinetics of Ra within an aquifer controlled by fine-scale lithogenic and redox processes.
Keywords: Adsorption-desorption rate constant, Distribution coefficient (Kd), Groundwater transport of U-Th series radionuclides, Major ion chemistry in Florida groundwater, Residence time of Ra in groundwater
Received: 08 Aug 2025; Accepted: 22 Dec 2025.
Copyright: © 2025 Baskaran, Jweda, Newton and Swarzenski. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Mark Baskaran
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