PREDICTING RISK FROM RADON IN SOURCE WATERS FROM WATER QUALITY PARAMETERS

Overall, 47 groundwater samples were collected from 45 small CWSs and analyzed for radon and other water quality constituents. In general, groundwaters from unconsolidated deposits and sedimentary rocks had lower average radon levels (ranging from 223 to 284 pCi/L) than metamorphic and igneous rocks (ranging from 268 to 5,820 pCi/L), which is consistent with similar investigations that have found the highest radon levels in groundwater originated from crystalline-rock aquifers which typically contain relatively high amounts of uranium (PA DEP, 2008; DeSimone, 2009).

The MCL of 300 pCi/L and proposed alternative MCL of 4,000 pCi/L for states or water systems that implement a multimedia mitigation plan for radon are used as guidelines to evaluate human health risks of radon in source water supplies. The majority of groundwater samples from unconsolidated deposits contained radon levels below 300 pCi/L (66%); conversely, water samples with radon levels below 300 pCi/L were primarily from unconsolidated deposits (57%). Samples exceeding 300 pCi/L were found in each lithology group, with the exception of low/med/high grade metamorphic rock basalt (with only one sample). Of 47 groundwater samples, only four samples exceeded 4,000 pCi/L, all originating from igneous and metamorphic rocks.

Overall, the statistical analysis indicates that the final predictive model correlates with radon levels greater than 900 pCi/L with a R² value of 98%. At lower radon levels, there is greater scattering about the reference line indicating a decreased predictability of radon in groundwater supplies used for drinking water.

Based on GIS mapping, there is only limited spatial correlation within EPA Region III indicating insufficient data to accurately contour radon levels across the region (most likely due to the limited number of sites and relatively large scale). However, there is a relationship between predicted indoor air screening levels and radon levels in groundwater. More specifically, radon levels in source water ≥4,000 pCi/L are strongly related to predicted indoor air screening levels >4 pCi/L and subsequently geologic faulting in the region.