IS WEATHERING THE MAIN CAUSE OF PERMEABILITY CONTRASTS IN THE FIVE MAJOR LITHOLOGIES? (Invited Presentation)

The permeability of bedrock aquifers varies by more than four orders of magnitude between different lithologies, but the reasons for this large range remain unexplained. Analysis of flowmeter data from 96 bedrock wells showed that fracture flow almost always dominates, with >90% of the flow usually coming from just three to five fractures in all five major lithologies (represented by limestone, sandstone, basalt, granite and shale). There are weak correlations between permeability and physical properties such as tensile strength and compressive strength, but much stronger correlations between permeability and both solute concentrations and dissolution rates. This suggests that weathering may be a major factor in permeability enhancement in bedrock aquifers (Worthington, Davies, Alexander, Earth-Science Reviews, in press). Weathering is widely recognized in limestone aquifers, which are both more soluble and more permeable than the other lithologies. However, this explanation also makes sense for silicate rocks. Mafic igneous rocks such as basalt are composed of more soluble minerals (e.g. olivine, plagioclase) than felsic igneous rocks such as granite (with quartz and K feldspar), and basalt is more permeable than granite. Shale is largely composed of low-solubility minerals such as quartz and clay minerals, and its permeability is consequently much lower than the other lithologies. Weathering appears to play an important role in enhancing permeability in most bedrock aquifers.