|Bedrock Regional Aquifer Systematics Study
Loudoun County, Virginia is one of the fastest-growing counties in the nation. Much of this growth occurs in the western part of the county, an area within the Blue Ridge geologic province that is underlain by Mesoproterozoic crystalline bedrock. This region has no major sources of surface water available and therefore must rely on wells tapped into ground water for its domestic water needs. There is considerable concern in the county that ground water in the Blue Ridge fractured-bedrock aquifer will not remain a sustainable resource in the face of such rapid growth. The county has an extensive digital database that includes a geologic map, digital topography, and geospatially-located wells.
Lawrence Drew, David Sutphin and Scott Southworth are assessing the influence of bedrock geologic structures on the magnitude and spatial distribution of domestic well yields in western Loudoun County, through statistical analysis. The statistical analysis is assessing temporal trends in water well yields and the relationship of lithology to mean well yield, following the same analytical procedures used in the report by Sutphin and others (see below). Another analysis is using exploratory variography to look for preferred directional trends in wells with similar yields. When combined with geologic map data, such directional trends can demonstrate the influence on ground water behavior of an underlying geologic fabric such as foliation trend, joint set, or dike swarm.
Two holes to 1000 ft depth are being drilled to investigate the distribution of fractures and ground water ages as a function of depth, through coring, geophysical logging, and sampling, in the crystalline bedrock underlying western Loudoun County, Virginia. The positions of the two wells are designed to compare and contrast subsurface hydrologic conditions between an area that is near a zone of recharge (high topographic ridge) and one that is not; combined analysis of the two holes should yield important insights into the hydrogeologic profile of the Blue Ridge. The two holes will also serve as monitoring wells to gauge the long-term effect of development on the ground water resources of the region.
Culpeper basin The goal of this task is twofold: a detailed and accurate characterization of the hydrogeologic framework of the early Mesozoic Culpeper basin, and the development of a regional ground-water flow model that incorporates this framework as completely as possible. The basin consists of gently- to moderately- west-dipping clastic sedimentary rocks that have been intruded by dikes and sills of diabase. The sediments and diabase have markedly different fracture patterns and hydrogeologic characteristics, with the diabase being markedly less hydraulically transmissive and effectively acting as barriers to regional ground-water flow. To produce an accurate flow model of the basin, the three-dimensional geometry of the diabase intrusions must therefore be understood as completely as possible, as well as the fracture characteristics of both the diabase and sedimentary rocks. Because the Early Mesozoic basins along the Eastern Seaboard have many geologic features in common, it is expected that the ground-water flow model developed for the Culpeper basin will have a high transfer value.
Michael Ryan is conducting field studies of the fractures, fracture mechanics, and the structure of Culpeper basin rocks, including measurements of fracture spacing, apertures, jointing interconnections, fracture secondary mineralization and crack orientations. These studies are being incorporated into FracMan analyses of rock bulk permeability and fracture statistics in paleoflow and contemporary flow studies.
Audiomagnetotelluric surveys and borehole geophysical surveys have been conducted by Herbert Pierce and Michael Ryan. These surveys are defining the geophysical structure of the ground water-bearing compartments of the Basin and are characterizing the geophysical and hydrogeologic attributes of the major rock types within the Basin. David Daniels is compiling aeromagnetic, ground magnetic surveys, gravity, and topographic data for the Culpeper basin, which provides key information on the broad-scale structure of the Basin, the sub-surface distribution of diabase, and the relationships between major drainages, lithologic contacts, and Basin-bounding fault systems.
An updated, digital geologic map of the Culpeper basin at 125,000-scale is being completed by Joseph Smoot and David Sutphin. The map incorporates new mapping in northern portions of the Basin and a greater definition of lacustrine units within the basin. On-line publications: Audio-magnetotelluric Data from the Culpeper Basin
Shenandoah Valley The northern Shenandoah Valley and adjacent Blue Ridge is underlain by regolith developed on fractured rock aquifers that are increasingly being relied upon to supply water to local communities and individual residences. This is an area with an expanding economy and a growing population, and, to meet future water needs, these aquifers are likely to be developed to supplement current withdrawals. The drought of 2002 has focused attention in this region of Virginia and West Virginia on the quantity and sustainability of the ground-water resources. An improved understanding of these complex aquifer systems is required to effectively develop and manage them as a sustainable source of water.
The hydrogeologic framework and water budget of this region will be studied as part of a long-term cooperative hydrogeologic project with the EESPT Karst Project, the Virginia and West Virginia Districts of the Water Resources Discipline, and the WRD National Research Program, with funding support coming from the USGS Eastern Region Integrated-Science Initiative, the USGS Ground-Water Resources Program, the national Cooperative Geologic Mapping Program, and several federal-local cooperative projects. The study area includes the USGS Leetown Science Center. The bedrock spans a broad range of lithologies from Mesoproterozoic basement gneisses of the Blue Ridge to folded unmetamorphosed siliciclastic and carbonate sedimentary rocks, and structures range from high-grade metamorphic foliation to bedding-plane parting, cross joints, and thrust faults. Although much of the basic mapping has been done, there is little information on the relationship of potentially water-transmitting fractures to the mapped lithologies and structures, and detailed geologic mapping is lacking in some areas. Regolith in this area includes residuum and colluvial slope deposits that constitute important recharge zones. BRASS is beginning the hydrogeologic characterization of the non-carbonate rocks through field investigations, gathering of fracture data, and determination of those areas that require more detailed geologic mapping. The geologic data will be integrated with the hydrologic data collected by WRD, which will include streamflow and water-level data from stream gages and monitoring wells, and domestic well yields, and these will provide input into a comprehensive regional ground-water flow model, allowing an unprecedented look at the influence of a highly diverse geologic framework on ground water flow and storage in this region.