IS THERE SUBSTANTIAL PREFERENTIAL AND RAPID FLOW IN SILICATE-ROCK AQUIFERS?

Flowmeter and tracer test results were compiled to shed light on the frequency of preferential flow in aquifers in silicate rocks. Suitable flowmeter data were found from 63 wells, 59 of which had all the measurable flow from a number of fractures, with typically >90% of the flow coming from just three to five fractures. The wells comprise 9 in basalt, 8 in sandstone, 15 in shale, mudstone or a range of sedimentary siliciclastic rocks, and 27 in crystalline rocks. The remaining four wells have intergranular flow, a combination of intergranular and fracture flow, or flow from closely-spaced fractures that cannot be individually resolved. The tracer test data comprise 49 tests over distances >100 m in crystalline, volcanic, and both coarse-grained and fine-grained siliciclastic sedimentary rocks. Results gave a range of 9 m/d to 1730 m/d, with a geometric mean groundwater velocity of 151 m/d. Preferential flow along fractures is common in silicate rocks, and measured rapid groundwater velocities show that, in some cases, these fractures must be connected over distance of at least thousands of meters. Calculations suggest fracture apertures in the mm-cm range and effective porosity of the fracture network in the 0.0001 to 0.001 range. Physical fracturing processes (e.g. tectonics, unloading) do not provide a good explanation for the permeability differences between the major silicate bedrock lithologies, represented by sandstone, basalt, granite and shale. However it's been found that there is a strong correlation between bedrock permeability and both aquifer solute concentrations and dissolution rates of a rock's constituent minerals (Worthington, Davies, Alexander, Earth-Science Reviews, in press). Consequently, weathering provides a better explanation for the preferential flow revealed by the flowmeter measurements and tracer test results. The results show the utility of these measurements for characterizing flow and transport in silicate bedrock aquifers, and demonstrate that preferential and rapid groundwater flow is common in silicate aquifers