Controls on 222Rn variations in a fractured crystalline rock aquifer evaluated using aquifer tests and geophysical logging

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/m3 [3,360 to 22,700 picocuries/liter] within a 2.5 km2 well field. Aquifer tests show that transmissivities range from 0.072 to 160 m2/day 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. 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. Differences in 222Rn concentration between wells in the hydraulically connected pairs can be attributed solely to differences in hydraulic aperture