Controls on ²²²Rn Variations in a Fractured Crystalline Rock Aquifer Evaluated Using Aquifer Tests and Geophysical Logging

Abstract

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.

Sequential Analysis of Hydrochemical Data for Watershed Characterization

“…The gneisses exhibit complex structural deformation including a west-northwest folding episode associated with upper amphibolite metamorphism (Taylor 1976) and a subsequent deformational event that produced large-scale north-northeast trending folds. Unconsolidated sand and gravel locally overlay bedrock (especially along the stream channels); however, these deposits are thin, narrow, and discontinuous (Folger et al 1996). Unconsolidated sand and gravel locally overlay bedrock (especially along the stream channels); however, these deposits are thin, narrow, and discontinuous (Folger et al 1996).…”

Development and calibration of a portable radon sampling system for groundwater 222Rn activity concentration measurements

“…Fracture groundwater, however, sometimes exhibits large variations of 222 Rn activity concentrations in the same area and in essentially the same rock types. Folger et al (1996) report 222 Rn activity concentration variations by a factor of seven in groundwater extracted in a small, 2.5 km 2 well field corresponding to the same granitic rock. Factors influencing the radon activity concentration in groundwater drawn from fractures seem to include the 226 Ra concentration and distribution along the fracture wall, the efficiency of 226 Ra transfer from rock to water, the groundwater flow velocity in the fractures and the fracture aperture.…”

Analytical Estimation of Water-Oil Relative Permeabilities through Fractures

“…In order to judge about the assumption of dominance of viscous and gravity forces to capillary force in a single fracture using Figure 13, it is essential to answer one important question: how wide are natural fractures? Several researchers [64][65][66][67][68][69][70][71] reported observation of fracture apertures ranging from about 30 to over 6 000 lm. These researchers stated that the apertures in the range of 50 to 200 lm seem to be very common, while 6 000 lm fracture apertures were rarely observed.…”