Présentations Colloques

Oral Presentation
8.03
Session 8.03: Geometrical structure and hydrogeological properties of Hard-Rock aquifers.
Setlur Neeraja
Crystalline rock aquifers- The Llano Uplift, Central Texas, USA
The crystalline rock aquifers of Llano Uplift are locally vital+ they are hosted in Mesoproterozoic granitic and metamorphic rocks that form the core of a broad structural dome. Late Paleozoic normal faulting and fractures, decompressive fractures, and weathering control aquifer properties. Driller report data (n=1530) show that wells in granites have higher median yields (57 litres per minute) and lower total dissolved solids (420 mg L) than in the metamorphic rocks (30 lpm and 500 mg L). The more productive wells are associated with thicker regolith and valleys, which reflect increased fracture density. Median regolith thicknesses are about twice as thick over granitic rocks (12m) than over metamorphic rocks. Wells are shallow (generally < 100 m depth with a median of 37 m). Permeability data imply decreased open fracture intensity with depth, although sample bias is a consideration. The decrease in incremental well yield with depth is greater in the Llano Uplift than in other areas of the US. Lithologic descriptions and well-yield data indicate the regolith and the fissured or partially weathered zones provide the greatest permeability. Rock Quality Designation (RQD < 50%) on drill cores, where available, can predict transmissive zones. Consistent vertical fracture orientations create permeability anisotropy. Some near-surface fractures with significant apertures (> 1mm) are filled and fracture skins can be thick with white weathering rinds that can have porosities approaching 14% and iron banding, particularly on the subhorizontal fractures+ subvertical fractures commonly have clay, hematite, and pyrolusite coatings. These affect solute transport in this shallow aquifer. **Further needed studies 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.**
United States