Presentation on theme: "Aquifers of the U.S. Fractured RockAquifers of the U.S."— Presentation transcript:
Aquifers of the U.S. Fractured RockAquifers of the U.S.
Fractured Systems Crystalline rock; Piedmont, Shield, Batholiths Carbonates; FL, Valley and Ridge, Mid- continent, MT, NV, TX Basalts; Columbia River Plateau, Snake River Plateau, Hawaii Felsic volcanics; Basin and Range Sandstone; Mid-continent, CO Plateau, VandR Till (confining unit); mid-continent
Important Information Background: Location, Lithology, Tectonic history Fracture and matrix characteristics Storage and transmission properties Flow system Distribution of major fluxes (Recharge, Baseflow, ET, Precip, Stormflow, Runoff) Water Quality
Piedmont and Blue Ridge Crystalline rocks with glacial cover
Paleozoic history of southern Piedmont From Hatcher Penobscottian Taconic Acadian Alleghanian
Metamorphic and igneous rock Sandstone and diabase in rift basins Limestone Folded and faulted sedimentary rock
Fault zone, tectonic Thrust fault, tectonic Sheet joints, young Fabric=bedding + tectonic Includes complex folds Faults and joints; tectonic + young? Topography and weathering Fabric and fractures
Transition Fractured metamorphic and igneous rock Saprolite
Gneiss, schist bedrock Granite or other intrusive bedrock Distinct transition Gradational transition
Storage and transmission properties High porosity S y ? porosity Low storage 10 -7 10 -6 m/s
Saprolite sampled at different depths. Measure density and chemical composition. S.G. decreases from 2.1 in rock to 1.6 in shallow saprolite. S.G. decreases as minerals are altered and mass is removed. 0.5 gm/cc removed during weathering. Chemical concentrations indicate S.G. change largely due to weathering of feldspar to kaolinite as a two-stage reaction.
Red clay, mica, quartz. Local lenses of quartz and mica. Relic foliation absent (bioturbation?). to 1-3 m depth. Original fractures healed with clay infilling, new frx possible—dessication, tree roots, burrows. High lateral stress from surface processes. B-horizon Clay, quartz, mica. Relic fabric expressed =saprolite Pervasive alteration of feldspar and FeMgs. Volume decrease due to removal of material during weathering Major fractures filled with clay, Fe+Mn oxides Chemical alteration penetrating wall rock of hydrologically active fractures. Fresh rock common. Fractures in fresh rock. Some minerals deposited in fractures, minor alteration of fracture walls. Feldspar, quartz, mica, amphibole as gneiss and schist. Max. compression horizontal--deep origin of rock. Reactivity of pore water decreases Most feldspar and FeMg altered Volume change. High lateral stress from parent rock relaxes. Max compression vertical? Apparent fracture toughness depends on degree of saturation and clay content (increases above water table). Importance of water table position in weathering? Moderate alteration, some feldspar and FeMgs present. Open fractures common. Horizontal fractures common. Dipping frx present Most fractures, least weathering = highest K. 5-15 m to w.t w.t. +/- 0.5-2 m seasonal 15-30 m to intact rock, based on drilling
Saprolite thickness Typically 50 ft, but variable 10-100 ft w.t. 20-60 ft bgs Frx zones in valleys Flat-lying frx
Saprolite thickness Greatest over valley or highland? Could be either
Exception to the Conceptual Model
What about other exceptions? Hydrogeological conceptual model = guide, not gospel
Specific capacity Characterizing the effects of lineaments on well performance How to characterize performance?
Effects of lineaments on well yields in the Piedmont Other effects of location on well yield Factor of 2 to 7 difference in yield Factors from 3 to 25 between valley and upland
Well yields in different rock types and regions in the Piedmont 15-30 gpm typical Roughly 20 gpm
Effects of well depth and diameter on yield in the Piedmont. Based on Daniels
Crystalline Rock Triassic Rift Basins Fractured sedimentary and volcanic rock Carbonate Aquifers Major Ion Content
Glaciated crystalline rocks in northern Appalachians