CONTROLS ON RN-222 VARIATIONS IN A FRACTURED CRYSTALLINE ROCK AQUIFEREVALUATED USING AQUIFER TESTS AND GEOPHYSICAL LOGGING

Concentrations of Rn-222 in ground water may vary considerably within megascopically homogeneous rocks over relatively short distances. Calc ulations 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 Gr anite, assuming that all other factors influencing dissolved Rn-222 co ncentrations are constant. Concentrations of dissolved Rn-222 range fr om 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 transmissi vities range from 0.072 to 160 m(2) day(-1) within the well field, Aco ustic televiewer and heat-pulse now meter logging of four wells reveal s that, despite tens to hundreds of fractures that intersect each well , a single fracture supplies all the flow to three wells, and one frac ture provides 65% of the flow to the fourth well, Aquifer tests indica te that two pairs of the four wells are hydraulically connected, Type- curve interpretation of early-time data from aquifer tests reveals cla ssic half-slope behavior on log-log plots of drawdown versus time for two wells, suggesting linear flow to a single fracture, Drawdown versu s time for the other two wells indicates radial or pseudo-radial flow, which suggests a higher degree of fracture interconnectivity near tho se boreholes. Hydraulic apertures calculated using the cubic law are 0 .024 and 0.038 cm for producing fractures in the first hydraulically c onnected well pair and 0.011 and 0.020 cm for flowing fractures in the second well pair, Assuming uniform distribution of Ra-226 along fract ure walls and long residence time of water relative to Rn-222 decay, t he ratio of fracture apertures should equal the inverse ratio of Rn-22 2 concentration in each well, Assuming 50% error in hydraulic aperture estimation and 10% analytical uncertainty in Rn-222 measurement, diff erences in 222Rn concentration between wells in the hydraulically conn ected pairs can be attributed solely to differences in hydraulic apert ure. Different hydraulic apertures, however, do not explain different Rn-222 concentrations between well pairs, Allowing for measurement err or, a cubic meter of rock transfers from 1.3 to 20 times more Rn-222 t o ground water in the first pair of wells than in the second pair, Non uniform distribution of Ra-226 along fracture walls, heterogeneous ema nating power in the rock-water system, or short ground-water residence time along the transmissive fracture network may account for the diff erence between well pairs.