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 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 m to w.t w.t. +/ m seasonal m to intact rock, based on drilling

Saprolite thickness Typically 50 ft, but variable ft w.t ft bgs Frx zones in valleys Flat-lying frx

Saprolite thickness Greatest over valley or highland? Could be either

Groundwatersheds

Exception to the Conceptual Model

What about other exceptions? Hydrogeological conceptual model = guide, not gospel

Lineaments

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 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

Other Hydrogeologic Settings in the Piedmont

Stockton Fm. Lockatong Fm. Passaic Fm. Orange Mt. Basalt Feltville Fm. Preakness Basalt Towaco Fm. Hook Mt. Basalt Boonton Fm. Conglomerates Palisade sill

Newark Basin, NJ Mesozoic Basins Sandstone aquifers Shale confining units Redbeds in the Hartford Basin, Conn Conglomerate Basalt Arkose

Carbonate rock aquifers

Water quality in Crystalline Rock aquifers

Crystalline rock Triassic Basin Carbonate rock meq/L

SC, GA, Al =100 mgpd