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

Concentrations of Rn-222 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 Rn-222 concentration measured in domestic water wells completed in fractured Pikes Peak Granite, assuming that all other factors influencing dissolved Rn-222 concentrations are constant. Concentrations of dissolved Rn-222 range from 124 to 840 kBq m(-3) [3,360 to 22,700 picocuries per liter (pCi L(-1))] within a 2.5 km(2) well field, Aquifer tests show that transmissivities range from 0.072 to 160 m(2) day(-1) within the well field, Acoustic televiewer and heat-pulse now 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 Ra-226 along fracture walls and long residence time of water relative to Rn-222 decay, the ratio of fracture apertures should equal the inverse ratio of Rn-222 concentration in each well, Assuming 50% error in hydraulic aperture estimation and 10% analytical uncertainty in Rn-222 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 Rn-222 concentrations between well pairs, Allowing for measurement error, a cubic meter of rock transfers from 1.3 to 20 times more Rn-222 to ground water in the first pair of wells than in the second pair, Nonuniform distribution of Ra-226 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.