Occurrence and geochemistry of radium in water from principal drinking-water aquifer systems of the United States

Abstract

► 226 Ra plus 228 Ra exceed 0.185 Bq/L in seven eastern and central USA principal aquifers. ► Radium concentration was highest in low pH and anoxic waters. ► Geochemical environments and low aquifer sorption capacity controlled Ra occurrence. ► Alpha recoil affected the presence of 224 Ra in western USA principal aquifers. ► Differences in Ra isotope ratios depended upon geology of the principal aquifer. A total of 1270 raw-water samples (before treatment) were collected from 15 principal and other major aquifer systems (PAs) used for drinking water in 45 states in all major physiographic provinces of the USA and analyzed for concentrations of the Ra isotopes 224 Ra, 226 Ra and 228 Ra establishing the framework for evaluating Ra occurrence. The US Environmental Protection Agency Maximum Contaminant Level (MCL) of 0.185 Bq/L (5 pCi/L) for combined Ra ( 226 Ra plus 228 Ra) for drinking water was exceeded in 4.02% (39 of 971) of samples for which both 226 Ra and 228 Ra were determined, or in 3.15% (40 of 1266) of the samples in which at least one isotope concentration ( 226 Ra or 228 Ra) was determined. The maximum concentration of combined Ra was 0.755 Bq/L (20.4 pCi/L) in water from the North Atlantic Coastal Plain quartzose sand aquifer system. All the exceedences of the MCL for combined Ra occurred in water samples from the following 7 PAs (in order of decreasing relative frequency of occurrence): the Midcontinent and Ozark Plateau Cambro-Ordovician dolomites and sandstones, the North Atlantic Coastal Plain, the Floridan, the crystalline rocks (granitic, metamorphic) of New England, the Mesozoic basins of the Appalachian Piedmont, the Gulf Coastal Plain, and the glacial sands and gravels (highest concentrations in New England). The concentration of Ra was consistently controlled by geochemical properties of the aquifer systems, with the highest concentrations most likely to be present where, as a consequence of the geochemical environment, adsorption of the Ra was slightly decreased. The result is a slight relative increase in Ra mobility, especially notable in aquifers with poor sorptive capacity (Fe-oxide-poor quartzose sands and carbonates), even if Ra is not abundant in the aquifer solids. The most common occurrence of elevated Ra throughout the USA occurred in anoxic water (low dissolved-O 2 ) with high concentrations of Fe or Mn, and in places, high concentrations of the competing ions Ca, Mg, Ba and Sr, and occasionally of dissolved solids, K, SO 4 and HCO 3 . The other water type to frequently contain elevated concentrations of the Ra radioisotopes was acidic (low pH), and had in places, high concentrations of NO 3 and other acid anions, and on occasion, of the competing divalent cations, Mn and Al. One or the other of these broad water types was commonly present in each of the PAs in which elevated concentrations of combined Ra occurred. Concentrations of 226 Ra or 228 Ra or combined Ra correlated significantly with those of the above listed water-quality constituents (on the basis of the non-parametric Spearman correlation technique) and loaded on principal components describing the above water types from the entire data set and for samples from the PAs with the highest combined Ra concentrations. Concentrations of 224 Ra and 226 Ra were significantly correlated to those of 228 Ra (Spearman’s rank correlation coefficient, +0.236 and +0.326, respectively). Activity ratios of 224 Ra/ 228 Ra in the water samples were mostly near 1 when concentrations of both isotopes were greater than or equal to 0.037 Bq/L (1 pCi/L), the level above which analytical results were most reliable. Co-occurrence among these highest concentrations of the Ra radionuclides was most likely in those PAs where chemical conditions are most conducive to Ra mobility (e.g. acidic North Atlantic Coastal Plain). The concentrations of 224 Ra were occasionally greater than 0.037 Bq/L and the ratios of 224 Ra/ 228 Ra were generally highest in the PAs composed of alluvial sands and Cretaceous/Tertiary sandstones from the western USA, likely because concentrations of 224 Ra are enhanced in solution relative to those of 228 Ra by alpha recoil from the aquifer matrix. Rapid adsorption of the two Ra isotopes (controlled by the alkaline and oxic aquifer geochemistry) combined with preferential faster recoil of 224 Ra generates a 224 Ra/ 228 Ra ratio much greater than 1. The 228 Ra/ 226 Ra activity ratio was locally variable, and was generally lower than 1 ( 226 Ra rich) in samples from PAs with carbonate bedrock, but was typically greater than 1 ( 228 Ra rich) in PAs composed of unconsolidated sand.