BEDROCK GEOLOGY AS THE PRIMARY SELECTION CRITERION IN DETERMINING A SAMPLING STRATEGY FOR WAKE COUNTY, NC INDOOR-AIR AND GROUNDWATER RADON STUDY

Since the primary sources of radon in the home are geology, soils, and construction materials, is it possible to correlate certain types of geology to indoor-air and groundwater radon levels? This paper details the sampling strategy developed for an indoor-air and groundwater radon study conducted by Wake County Environmental Services and the North Carolina Radiation Protection Section.

Recent 1:24,000-scale geologic mapping by the NCGS, under the USGS NCGMP program, has resulted in a refined geologic map of Wake County. Geologic map units were grouped into ten different geologic categories: unmetamorphosed granite plutons, metamorphosed granitic intrusive rocks of the Carolina and Spring Hope terranes, metavolcanic rocks of the Carolina terrane, Crabtree terrane, Falls Lake terrane, Falls leucogneiss, Raleigh gneiss, metavolcanic rocks of the Spring Hope terrane, sedimentary rocks of the Triassic basin, and diabase dikes. These were assigned a percentage of the total sample size according to their respective areal proportions of the county, with a slight bias toward smaller geologic categories, in order to ensure a minimum number of samples in each. Using ArcView GIS and a real estate database, parcels with domestic wells were identified throughout the county and overlaid with the geologic map. Parcels within 50 meters of a contact were excluded due to uncertainty inherent in the mapping.

Within each geologic map category (in statistics called a stratum), parcels were assigned a number and selected, regardless of their area. Out of a list of consecutive parcel numbers for each stratum, a quantity of random parcels was selected, corresponding to the percentage of total project sample size represented by the map unit. Since the project is subject to an individual owner's willingness to participate, additional parcels were randomly selected for invitation to participate in the study. A record was kept of the assigned priority numbering system and the parcel list. During sampling, additional information pertaining to the individual wells and home construction details will be collected. It is hoped that this approach will allow meaningful statistical analysis of the data upon conclusion of the study, and demonstrate whether bedrock geology may be considered the primary determinant of indoor air radon in this region.