THE LLANO UPLIFT CRYSTALLINE ROCK AQUIFER

The Llano Uplift is a broad structural dome with Mesoproterozoic granitic and metamorphic rocks that hosts a crystalline rock aquifer. Faulting and fractures, decompressive fractures, and weathering control aquifer properties. Drill log data show that wells in granites have higher median yields and lower total dissolved solids than in metamorphic rocks. The more productive wells are associated with thicker regolith and valleys. Wells are shallow (generally < 100 m). Permeability data imply decreased open fracture intensity with depth, although sample bias is a consideration. The decrease of incremental well yield with depth is greater in the Llano Uplift than in other US areas. Lithologic descriptions and well-yield data indicate the regolith and the fissured or partially weathered zones provide the highest permeability with vertical fractures creating permeability anisotropy. Rock Quality Designation (RQD) was evaluated on a number of drill cores, which allowed inferences on permeability zones.

Core fracture skins were analyzed for porosity and mean pore diameter with Hg-injection porosimetry and with optical and SEM microscopy. EDS spectroscopy detected the presence of trace elements. Some near-surface fractures with significant apertures (> 1mm) are filled and fracture skins can be thick with white weathering rinds with porosities ranging from 6 to < 14% . Iron banding occurs particularly on the subhorizontal fractures; subvertical fractures possess clay, hematite, and pyrolusite coatings. With depth, fracture skins become thinner and contain relatively more trace elements (e.g., cerium and lanthanum) with depth. These skins will affect solute transport. Further study should include: better mapping/definition of the regolith and weathered zones; evaluation of open fracture properties with depth; controls on fracture skin formation and fracture filling; and quantifying the effects of lithology, climate, and geological history on crystalline rock aquifers.