Crystalline-rock aquifer system of the Llano Uplift, Central Texas, USA

The crystalline-rock aquifer system of the Llano Uplift (Central Texas, USA) hosts an important local water resource that has been relatively little studied. The Uplift is a structural dome of Precambrian granitic and metamorphic rocks. Late Paleozoic normal faulting and fractures, decompressive fractures, weathering, lithology, and rock fabric control the aquifer properties. Data from driller reports (over 2,000 wells) show that wells in granites have higher median yields than wells in metamorphic rocks. There is a weak correlation of well yield with regolith thickness, and median regolith thickness is greater over granites than over metamorphic rocks. Fracture permeability, which is very heterogeneous, is the major control. Wells are shallow (generally <100 m depth), but more recent wells have been drilled more deeply. Permeability data imply decreased open-fracture density and aperture with depth, although sample bias is a consideration. Diamond drill cores show that many near-surface fractures with significant aperture are filled by rock fragments from weathered surrounding rocks and that fracture skins are thinner and contain iron oxides in a more reduced state with depth. Fracture skins can be porous, with porosity ranging to over 10%. There is a need to compare crystalline-rock aquifer systems to assess weathering, tectonics, fractures, and mineralogy/petrology to assess the characteristics of these systems, which are critical water resources in large areas of the world.