ABSTRACT Basalt flows of the Columbia River Basalt Group (CRBG) host a series of regionally extensive aquifers between western Idaho and the Pacific Ocean that serve as an important source for domestic, municipal, agricultural, and industrial water supply throughout much of this area, and are the sole source for some communities in the Willamette Valley. Rapid growth and increased pumping have resulted in significant water level declines in some locales in the Willamette Valley, forcing some communities to develop other water sources, and/or develop aquifer storage and recovery projects to store water in CRBG aquifers. The CRBG generally consists of multiple concordant, tabular sheet flows. The primary water-bearing horizons within the CRBG are associated the vesicular and/ or brecciated flow top and flow bottom (pillow/hyaloclastite) structures that form the interflow zone between two flows. The interiors of the CRBG flows typically have limited vertical permeability and act as aquitards, creating a series of layered confined aquifers. The dominant groundwater flow pathway in the CRBG aquifer system is along these individual, laterally extensive, interflow zones. Tectonic structures may modify the dominant flow regime in the CRBG by offsetting or otherwise disturbing originally laterally continuous interflow zones. Faults result in a wide spectrum of effects on flow in the CRBG aquifers depending on the nature of the fault. The hydraulic properties inherent to CRBG aquifers, including high degree of confinement, low bulk permeability and limited recharge have led to overdraft conditions in many areas. Conversely, these characteristics create favorable conditions for aquifer storage and recovery system development in the central Willamette Valley and Tualatin Basin.

Abstract Multiple generations of late-diagenetic dolomites and calcites in Upper Knox carbonates from the southern Valley and Ridge, and the greater central Tennessee regions exhibit a complex cathodoluminescent cement stratigraphy. Dolomite and calcite zones can be correlated throughout mineralized and unmineralized regions of each study area based on their luminescence, dissolution contacts, and paragenetic relationship to noncarbonate authigenic minerals. Zonal correlations in each study area are interpreted to have time significance, and to record regionally extensive diagenetic events. Late-diagenetic carbonate cements occur primarily in porous (4 to 16%), impermeable to permeable (0.0 to 1030 md) late-diagenetic replacement dolomites. Late-diagenetic replacement dolomites and associated carbonate cements primarily occur in transgressive cycles of third-order (1 to 5 m.y.) depositional sequences reflecting a strong depositional and early-diagenetic fades control on their distribution. Widespread dissolution of late-diagenetic dolomites and interbedded thin limestone intervals occurred during advanced stages of replacement dolomitization at intermediate burial depths; dissolution produced thin to thick (10s to 100s m) dolomitic intervals characterized by well-developed matrix porosity. In contrast, thick intervals of limestone or early-diagenetic dolomite, which were stratigraphically isolated from intervals of late-diagenetic dolomite, experienced minimal dissolution. In the central Tennessee region, the stratigraphic distribution of late-diagenetic replacement dolomites defines a predictable pattern of laterally continuous intervals of porous dolomite bounded by thick intervals of nonporous carbonate host rock. Late-diagenetic dolomite intervals are interpreted to have served as hydrologically connected, stratabound fluid conduits within the overall Knox regional aquifer. Long-term (> 100 m.y.) recurrent fluid activity in dolomite conduits of the central Tennessee region is suggested by partial to complete occlusion of secondary porosity by zoned carbonate cements, AWT minerals, as well as by the late Paleozoic to Tertiary ages of associated calcite cements and chemical remagnetization. Integration of the CL dolomite stratigraphy of Upper Knox carbonates from the southern Valley and Ridge region with radiometric dates of associated noncarbonate minerals, and with dolomite fluid inclusion and stable isotope data, suggests that late-diagenetic zoned dolomites provide the most complete spatial and temporal record of regional diagenetic events, and of fluid flow systems that developed in the Knox regional aquifer of the southern Appalachian basin. Five generations of zoned dolomite cements bracket main ore-stage sphalerite, and authigenic feldspar and magnetite, which yield Late Devonian to Early Carboniferous ages, and Late Carboniferous to Permian ages, respectively. The precipitation temperatures of zoned dolomites estimated from fluid inclusion homogenization temperatures and systematic trends in their S 18 0 values record a regionally developed prograde-to-retrograde thermal history, which is interpreted to initially record fluid migration in response to the far-removed mid-Paleozoic, Acadian Orogeny, and subsequently larger-scale fluid movement governed by gravity- and compression-driven flow in response to the late Paleozoic Alleghenian Orogeny.

T he very productive limestone aquifers of Tertiary and Quaternary age in the Coastal Plain of the Southeastern States contain a water-table circulation system where aquifers are at or near the land surface; the Tertiary limestone unit is a homoclinal artesian system confined beneath younger beds in coastal areas. The Tertiary limestone has a total thickness of several thousand feet and ranges in age from Paleocene to Pliocene. The part of the limestone that represents a hydrologic unit, as much as 1000 feet thick, ranges in age from middle Eocene to middle Miocene. That unit is the principal artesian aquifer in Florida and southeastern Georgia. It is also an aquifer, but of lesser importance, in South Carolina, North Carolina, and Alabama. In southeastern Florida the Quaternary limestone forms a shallow, although highly productive, aquifer. In general, the Tertiary formations dip gently toward the coast, except where the regional dip is interrupted by folds such as the Cape Fear arch, the Ocala uplift, and the Chattahoochee anticline, or by faults such as those on the Ocala uplift. A piezometric map showing the head of water in the principal artesian aquifer in Florida and southeastern Georgia reveals chief areas of recharge where limestone crops out in interstream areas and where sinkholes breach overlying beds; it reveals principal discharge areas in certain stream valleys, springs, and offshore. Lateral movement of the water in the limestone is generally controlled by the hydraulic gradient to the nearest discharge area. The movement may be either controlled by or independent of faults and folds; it may be along joints for short distances. Solution by downward movement of water in the zone of aeration to the water table has formed vertical tubular openings or shafts. Lateral movement of the water from areas of recharge to areas of discharge has formed caves and other solution channels, chiefly in the upper part of the zone of saturation. Since they were formed, some caves have been drowned, and others have been exposed in the zone of aeration as the water rose and fell, chiefly in Pleistocene time. The principal aquifer in Florida probably has caves and solution channels comparable in size and extent to those of Mammoth Cave, Kentucky; however, only a small percentage of these are above the water table at the present time. In a large part of the region the most noticeable change in chemical quality of the water in the limestone resulting from circulation of water is the freshening accompanying removal of salty water from the aquifer. Changes in sea level during Pleistocene time have raised and lowered the water table; thus the position of the upper part of the zone of saturation, in which conditions were most favorable for solution and for the formation of cavities, has changed repeatedly. At the lowest stand of the Pleistocene sea, it is conceivable that the water level in part of the limestone in north-central Florida was a few hundred feet lower than at present. The present pattern of solution openings probably was developed in Pleistocene time.

Regional interbasin movement of ground water in highly deformed miogeosynclinal carbonate rocks of the eastern Great Basin has received considerable attention in the literature since 1960. That these regional carbonate aquifer systems — some of which may integrate as many as 13 intermontane basins — are actually compartmentalized by major structural features, has not received adequate emphasis. In south-central Nevada, major wrench, thrust, and normal faults and folds exert marked control on ground-water movement. Deformation of the carbonate rocks results in regions of high transmissibility, but juxtaposition by faulting or folding of thick clastic strata against carbonate aquifiers results in prominent ground-water barriers, some of which are more than ten miles long. The apparent hydraulic gradients across the thick clastic aquitards vary from 150 to 1300 ft per mile; by contrast gradients in the adjacent carbonate aquifers vary from 0.5 to 10 ft per mile. Barriers may also result from gouge developed along the major fault zones. Recognition of the structural barriers as well as the conduits is essential for construction of an initial working flow net of a region. In upland areas the clastic aquitards may exert control on distribution of recharge. Where present in central parts of the flow system, the aquitards act as prominent ground-water dams, and localize minor spring discharge. In discharge areas they localize major spring lines. Major structures with hydrologic reflections include the Las Vegas Valley shear zone and the Tippinip thrust fault.

The presence beneath the central and western San Joaquin Valley of a large body of confined ground water of lower mineral content than overlying unconfined ground water has long been an enigma to hydrogeologists. The confined waters beneath the diatomaceous Corcoran Clay Member of the Tulare Formation occupy a zone 300 to 730 m thick, are of sodium sulfate composition, and are notably uniform in chemical and stable isotope content as compared to the overlying unconfined water. The confined waters clearly were derived neither from the modern streams of the Coast Ranges to the west, which have higher mineral content, nor solely from streams draining the Sierra Nevada to the east, which contain relatively little sulfate. A paleohydrologic model, consistent with the structural and stratigraphic history of the region, attributes the uniform chemical and isotopic composition of the confined water to mixing and recharge that occurred in temporary lakes during extreme flood events. The confined waters appear to be ancient, probably representing recharge in pre-Corcoran time (older than 615,000 to 725,000 yr B.P.). Low structural relief on the west side of the San Joaquin Valley during much of Pleistocene time, together with small differential head across the thick, poorly permeable, lacustrine Corcoran Clay Member, have combined to preserve the confined paleowaters from being flushed. A combined chemical-stable isotope model suggests that waters with a δ 18 O value of about −10 permil from the Sierra Nevada mixed in intermittent lakes with runoff waters from the then lower Coast Ranges of about −6 permil, resulting in blends of about −9 permil and 450 to 500 mg/L sulfate content in the principal confined aquifer system. Shallower ground waters above the Corcoran Clay Member are more variable in chemical and isotopic composition and appear to represent recharge from streams similar to those now crossing the area.