Relationships between radium and radon occurrence and hydrochemistry in fresh groundwater from fractured crystalline rocks, North Carolina (USA)
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Naturally-occurring radionuclides (uranium, radium, and radon), major dissolved constituents, and trace elements were investigated in fresh groundwater in 117 wells in fractured crystalline rocks from the Piedmont region (North Carolina, USA). Chemical variations show a general transition between two water types: (1) slightly acidic (pH 5.0-6.0), oxic, low-total dissolved solids (TDS) waters, and (2) near neutral, oxic to anoxic, higher-TDS waters. The uranium, radium, and radon levels in groundwater associated with granite (Rolesville Granite) are systematically higher than other rock types (gneiss, metasedimentary, and metavolcanic rocks). Water chemistry plays a secondary role on radium and radon distributions as the 222Rn/226Ra activity ratio is correlated with redox-sensitive solutes such as dissolved oxygen and Mn concentrations, as well as overall dissolved solids content including major divalent cations and Ba. Since 224Ra/228Ra activity ratios in groundwater are close to 1, we suggest that mobilization of Ra and Rn is controlled by alpha recoil processes from parent nuclides on fracture surfaces, ruling out Ra sources from mineral dissolution or significant long-distance Ra transport. Alpha recoil is balanced by Ra adsorption that is influenced by redox conditions and/or ion concentrations, resulting in an approximately one order of magnitude decrease (~ 20,000 to ~ 2000) in the apparent Ra distribution coefficient between oxygen-saturated and anoxic conditions and also across the range of dissolved ion concentrations (up to ~ 7 mM). Thus, the U and Th content of rocks is the primary control on observed Ra and Rn activities in groundwater in fractured crystalline rocks, and in addition, linked dissolved solids concentrations and redox conditions impart a secondary control. © 2008 Elsevier B.V. All rights reserved.
Published Version (Please cite this version)10.1016/j.chemgeo.2008.10.022
Publication InfoDwyer, Gary; Hirschfeld, D; Vengosh, Avner; & Vinson, DS (2009). Relationships between radium and radon occurrence and hydrochemistry in fresh groundwater from fractured crystalline rocks, North Carolina (USA). Chemical Geology, 260(3-4). pp. 159-171. 10.1016/j.chemgeo.2008.10.022. Retrieved from https://hdl.handle.net/10161/6927.
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Sr. Research Scientist and Instructor in Earth and Ocean Sciences
Dwyer's experience lies in the development of tracers and indicators of environmental change, and their application to modern and ancient environmental systems. Research areas include paleoceanography, paleoclimatology, carbonate sedimentology, marine geology and environmental geochemistry.
Professor of Earth and Ocean Sciences
My research aims to link environmental geochemistry and isotope hydrology in order to trace the sources and mechanisms of water contamination and relationships with human health. Current research includes global changes of the chemical and isotopic compositions of water resources due to human intervention and contamination, salinization of water resources in the Middle East and Northern Africa, naturally occurring contaminants (arsenic, fluoride, boron) and radioactivity in water resources, the
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