Indoor 222Rn concentrations were measured in 37 houses with alpha track detectors placed in water-use rooms near water sources (bathrooms, laundry rooms, and kitchens) and in non-water-use living rooms, dining rooms, and bedrooms away from water sources. Results show that relative contributions of 222Rn to indoor air from water use are insignificant when soil-gas concentrations are high but become increasingly important as the ratio of 222Rn-in-water: 222Rn-in-soil gas increases. High soil-gas 222Rn concentrations may mask 222Rn contributions from water even when waterborne 222Rn concentrations are as high as 750 kBq m-3. Ground water in Precambrian Pikes Peak granite averages 340 kBq m-3 222Rn, vs. 170 kBq m-3 in Precambrian migmatite, but average 222Rn concentrations in soil gas are also lower in migmatite. Because the ratio of 222Rn-in-water: 222Rn-in-soil gas may be consistently higher for houses in migmatite than in Pikes Peak granite, indoor air in houses built on migmatite may have a greater relative contribution from water use even though average 222Rn concentrations in the water are lower. Continuous monitoring of 222Rn concentrations in air on 15-min intervals also indicates that additions to indoor concentrations from water use are significant and measurable only when soil-gas concentrations are low and concentrations in water are high. When soil-gas concentrations were mitigated to less than 150 Bq m-3 in one house, water contributes 20-40% of the annual indoor 222Rn concentration in the laundry room (222Rn concentration in water of 670 kBq m-3). Conversely, when the mitigation system is inactive, diurnal fluctuations and other variations in the soil-gas 222Rn contribution swamp the variability due to water use in the house. Measurable variations in indoor concentrations from water use were not detected in one house despite a low soil-gas contribution of approximately 150 Bq m-3 because waterborne 222Rn concentrations also are low (80 kBq m-3). This result suggests that 222Rn concentrations in water near the recommended EPA limit in drinking water of 11 kBq m-3 may not contribute measurable amounts of 222Rn to indoor air in most houses.

Concentrations of 222Rn in ground water may vary considerably within megascopically homogeneous rocks over relatively short distances. Calculations indicate that different hydraulic apertures of water‐bearing fractures may account for variations in dissolved 222Rn concentration measured in domestic water wells completed in fractured Pikes Peak Granite, assuming that all other factors influencing dissolved 222Rn concentrations are constant. Concentrations of dissolved 222Rn range from 124 to 840 kBq m‐3 [3,360 to 22,700 picocuries per liter (pCi L‐1)] within a 2.5 km2 well field. Aquifer tests show that transmissivities range from 0.072 to 160 m2 day‐1 within the well field. Acoustic televiewer and heat‐pulse flow meter logging of four wells reveals that, despite tens to hundreds of fractures that intersect each well, a single fracture supplies all the flow to three wells, and one fracture provides 65% of the flow to the fourth well. Aquifer tests indicate that two pairs of the four wells are hydraulically connected. Type‐curve interpretation of early‐time data from aquifer tests reveals classic half‐slope behavior on log‐log plots of drawdown versus time for two wells, suggesting linear flow to a single fracture. Drawdown versus time for the other two wells indicates radial or pseudo‐radial flow, which suggests a higher degree of fracture interconnectivity near those boreholes. Hydraulic apertures calculated using the cubic law are 0.024 and 0.038 cm for producing fractures in the first hydraulically connected well pair and 0.011 and 0.020 cm for flowing fractures in the second well pair. Assuming uniform distribution of 226Ra along fracture walls and long residence time of water relative to 222Rn decay, the ratio of fracture apertures should equal the inverse ratio of 222Rn concentration in each well. Assuming 50% error in hydraulic aperture estimation and 10% analytical uncertainty in 222Rn measurement, differences in 222Rn concentration between wells in the hydraulically connected pairs can be attributed solely to differences in hydraulic aperture. Different hydraulic apertures, however, do not explain different 222Rn concentrations between well pairs. Allowing for measurement error, a cubic meter of rock transfers from 1.3 to 20 times more 222Rn to ground water in the first pair of wells than in the second pair. Nonuniform distribution of 226Ra along fracture walls, heterogeneous emanating power in the rock‐water system, or short ground‐water residence time along the transmissive fracture network may account for the difference between well pairs.

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