Biogeochemical factors affecting the presence of 210Po in groundwater

The discovery of natural Po-210 enrichment at levels exceeding 500 mBq/L in numerous domestic wells in northern Nevada. USA, led to a geochemical investigation of the processes responsible for its mobilization. Po-210 activities in 63 domestic and public-supply wells ranged from below 1 mBq/L to 6590 590 mBq/L, among the highest reported levels in the USA. There is little spatial or depth variability in Pb-210 activity in study-area sediments and mobilization of a few percent of the Po-210 in the sediments would account for all of the Po-210 in water. Stable-isotope measurements indicate SO4 reduction has occurred in all Po-210 contaminated wells. Sulfide species are not accumulating in the groundwater in much of Lahontan Valley, probably because of S cycling involving microbial SO4 reduction, abiotic oxidation of H2S to S-0 by Mn(IV), followed by microbial disproportionation of S-0 to H2S and SO4. The high pH, Ca depletion, MnCO3 saturation, and presence of S in Lahontan Valley groundwater may be consequences of the anaerobic S cycling. Consistent with data from naturally-enriched wells in Florida, Po-210 activities begin to decrease when aqueous sulfide species begin to accumulate. This may be due to formation and precipitation of PoS, however, Eh-pH diagrams suggest PoS would not be stable in study-area groundwater. An alternative explanation for the study area is that H2S accumulation begins when anaerobic S cycling stops because Mn oxides are depleted and their reduction is no longer releasing Po-210. Common features of Po-210-enriched groundwater were identified by comparing the radiological and geochemical data from Nevada with data from naturally-enriched wells in Finland, and Florida and Maryland in the USA. Values of pH ranged from <5 in Florida wells to >9 in Nevada wells, indicating that pH is not critical in determining whether Po-210 is present. Where U is present in the sediments, the data suggest Po-210 levels may be elevated in aquifers with (1) SO4-reducing waters with low H2S concentrations, or (2) anoxic or oxic waters with extremely high Rn activities, particularly if the water is turbid. Published by Elsevier Ltd.