210Po in drinking water, its potential health effects, and inadequacy of the gross alpha activity MCL

Abstract

Polonium-210 (210Po) is a naturally-occurring, carcinogenic member of the 238U decay series and the granddaughter of 210Pb. It has a half life of 138.4days and is rarely found in drinking water at levels exceeding 5mBq/L because it strongly binds to aquifer sediment. When the current US Maximum Contaminant Level (MCL) covering 210Po was promulgated in December 2000, very little was known about its occurrence and the processes responsible for mobilizing it. More is now known about the processes that mobilize 210Po from sediments and a review of recent occurrence data show that it may not be as rare in the US as the US Environmental Protection Agency (USEPA) thought in 2000. Worldwide, only about 2200 analyses for 210Po in drinking water were identified, with activities exceeding 500mBq/L being found only in Finland, India, Sweden, and the US. The median of 400 210Po analyses from the US is 4.75mBq/L and >10% of the samples exceed 500mBq/L. Current compliance-monitoring regulations in the US essentially guarantee that 210Po contamination will not be detected except in very contaminated wells. Major problems with the US Gross Alpha Activity MCL include the volatility of 210Po and extended holding times and sample-compositing methods that can allow the majority of 210Po in a sample bottle to decay before analysis. In light of new information, the radionuclide rule should be changed and direct measurements of 210Po should be made in all public-water supply wells to rule out its presence. Much of the important biological and toxicological research on 210Po is more than four decades old and new laboratory research using modern tools is needed. Biological and epidemiological investigations of known contaminated areas are needed to assess the effect 210Po exposure is having on animals and humans consuming the water.