Relationships between radium and radon occurrence and hydrochemistry in fresh groundwater from fractured crystalline rocks, North Carolina (USA)

Abstract

a b s t r a c t a r t i c l e i n f oNaturally-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 222 Rn/ 226 Ra 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 224 Ra/ 228 Ra 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 oxygensaturated 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.

Spatial distribution of the 226Ra activity concentrations in well and spring waters in Serbia and their relation to geological formations

“…High radium levels were found in deep geothermal aquifers; e.g., values up to 3.66 Bq L − 1 have been reported in some thermal springs from Spain (Ródenas et al, 2008). Radium mobility may be influenced by chemical parameters, including pH (Szabo et al, 2004), salinity (Sturchio and Zapecza, 1987), redox conditions (Szabo and Zapecza, 1987;Vinson et al, 2009), and major dissolved ions such as chloride, sulfate, and carbonates that can react with radium by forming weak complexes (Onishchenko et al, 2010). The hydrodynamic features of the aquifers may also influence the distribution of radionuclides.…”

“…Radium is known to be readily adsorbed onto clay minerals in exchange for sodium (Appello and Postma, 1994). The adsorption of radium is less effective under high concentrations of alkaline earth metals that compete with radium for adsorption site (Vinson et al, 2009).…”

Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania

“…Analytical precisions for δ 18 O and δ 2 H were estimated as AE0.1‰ and AE1.5‰, respectively. Radium isotope analyses ( 226 Ra and 228 Ra) were measured at the Laboratory for Environmental Analysis of RadioNuclides (LEARN) using a Durridge RAD7 radon-in-air monitor ( 226 Ra) and Canberra DSA2000BEGe gamma detector ( 228 Ra) following methods described in (29) and (48). Strontium isotopes were analyzed by a thermal ionization mass spectrometer on a ThermoFisher Triton.…”

“…Other features that characterize the produced waters from the Marcellus Formation are the high activities of naturally occurring nuclides of 226 Ra and 228 Ra and low 228 Ra∕ 226 Ra ratios (7). 226 Ra and 228 Ra are the disintegration products of 238 U and 232 Th, respectively, and are generated in groundwater from alpha recoil, desorption from sediments, and dissolution of aquifer material (7,29). In most of the shallow groundwater we sampled (Table S1), combined Ra activities were low (<5 pCi∕L).…”

Natural Radioactive Isotopes in Water in Relation with Geology and Hydrological Investigations in the Territory of Luxembourg

“…High levels of radium are associated with low levels of radon and vice versa. Radon activity in groundwater is controlled by the concentration of radium in the aquifer matrix [Vinson et al, 2009]. However, the amount of radon that can enter the groundwater depends on the emanating coefficient of the radium bearing material in the aquifer.…”