The shallow subsurface in the Guanyinyan study area, China, is characterised by extensive fractures which are oriented NE, NW and EW. These fractures have lengths of about 200 to 300 m, and are spaced at about 1 to 7 m from each other. The bedrock is sandstone and mudstone overlain by a thin veneer of weathered rock and soil. These fractures are important from a hydrological perspective because the building of a dam is planned at this locality. In an effort to quantify the density and openness

more »

... f bedrock fractures in the Guanyinyan study area, RnA (daughter of Rn) concentrations within the soil cover were measured at 232 test sites. The expectation was that RnA concentrations within the soil would be anomalously high above and immediately adjacent to the fractures and that RnA concentrations could be directly correlated to the density and openness of the bedrock fractures. On the basis of a statistical analysis of the acquired radiometric data and field observations, bedrock was classified into low openings (under 100 pulses of RnA), intermediate openings (100 to 200 pulses) and high openings (greater than 200 pulses). Low openings correspond to old fractures that have been filled, and intermediate and high openings to fractures that have been partly filled; this was confirmed in tunnels in the area. This work has positive implications for the location of groundwater resources in fractured-rock aquifers such as in South Africa, where most aquifers are fractured rock.