A method for determining the extent of bulk 210Po and 210Pb adsorption and retardation in aquifers

Abstract

The naturally-occurring radionuclides 210Po and 210Pb are normally found in very low concentrations in the environment. However, under some conditions 210Po contributes significantly to the radioactivity in ground waters and can constitute a health risk. Naturally-occurring 210Pb can be used in predicting the behavior of contaminant Pb. While studies have identified the processes controlling 210Po and 210Pb behavior, the extent of adsorption and rates of removal in an aquifer cannot be determined without quantifying the rates of supply of these isotopes from decay of parent nuclides on and within aquifer solids. This study demonstrates how the isotope systematics of the 238U decay series, which includes 210Po and 210Pb, can be used to quantify the adsorption and retardation factors of 210Po and 210Pb in aquifers. The rates of 210Po and 210Pb supply to ground waters are determined from the concentrations of other radionuclides in the series, especially 222Rn, and so the distribution coefficients between aquifer solids and groundwater can be calculated. The effects of different supply rates due to different distributions of parent radionuclides in the aquifer can then be separated from differences in adsorption, which is controlled by aquifer water chemistry. A review of groundwaters where 210Po, 210Pb, and 222Rn data are available indicates that systematic variations can be found in bulk effective adsorption coefficients and retardation factors. Overall, a method is provided for future studies of 210Po and 210Pb behavior, and greater understanding of the occurrence and migration of these radionuclides.