Ground-water geology along the northwest foot of the Blue Ridge between Arnold Valley and Elkton, Virginia
The area discussed in this report lies along the northwest flank of the Blue Ridge in Rockbridge, Augusta, and Rockingham counties, Virginia. It spans the boundary between the Blue Ridge and the Great Valley physiographic provinces.
The southeastern (Blue Ridge) portion of the area is mountainous and underlain by Precambrian crystalline and lower Cambrian elastic rocks. It is a major water catchment area for the eastern edge of the Shenandoah Valley and is chiefly a National Forest preserve. Cambriancarbonate rocks and fine-grained elastics underlie the relatively level areas between the mountains and the major subsequent streams near their base. A thick unconsolidated mantle which slopes gently away from the mountains conceals bedrock over much of the area. Major well fields have been developed on level alluvial floodplains and terraces adjacent to the rivers. Some produce several million gallons of water each day from bedrock aquifers at depth of from 50 to over 700 feet. The average temperature within the area is about 55°F. Average annual precipitation is approximately 41 inches and is greatest in the mountains. About six inches becomes ground-water recharge. Artificial withdrawal by wells would increase the rate of recharge. Evapotranspiration frequently exceeds total precipitation during the summer and early autumn.
Most streams which drain the Blue Ridge are perennial near the headwaters and intermittent northwest of the mountains. They may be influent to bedrock aquifers within or near the base of the mountains and near the confluence with subsequent streams where the clay mantle is deeply eroded, but are effluent in the intervening reaches. The perennial streams are dominantly effluent. The major rivers are sub•sequent and effluent. They receive surface drainage and overflow of ground-water reservoirs from the Blue Ridge and from dominantly carbonate terranes to the west and northwest. Streamflow of the major streams represents approximately one third of the average annual areal precipitation.
The stratigraphic sequence within the area from older to younger is designated as fellows: Precambrian crystalline rocks; Precambrian-Lower Cambrian Catoctin Greenstone and Swift Run Formations; LowerCambrian elastics; Lower Cambrian Tomstown (Shady) Delomite; Lower Cambrian Waynesboro (Rome) Formation; Middle Cambrian Elbrook Formation; and the Upper Cambrian Conococheague Limestone.
The Precambrian igneous and metamorphic rocks, and the lower portion of the Lower Cambrian elastics are normally relatively poor aquifers. Minor production is obtained from fractured zones at locations within the mountains.
The Antietam formation, the upper portion of the Lower Cambrianelastics, is a major potential aquifer. One well in Buena Vista produces over 600 gpm of water of low mineralization. Similar sites abound along the base of the Blue Ridge.
The Tomstown Dolomite is a major aquifer at Waynesboro where the DuPont well field produces over 11,000,000 gpd. The producing characteristics of the Tomstown formation near the mountains at locations remote from the major rivers, where it is commonly concealed by a thick impermeable mantle of clay, are virtually unknown.
Thick beds of limestone and dolomite in the Waynesboro Formationare prolific aquifers at Glasgow and near Elkton. Argillaceous portions of the formation are commonly aquicludes although secondary permeability may be developed by fracturing.
Several wells produce over 1000 gpm from aquifers in the Elbrookand Conococheague formations near Grottoes and south of Elkton. With some exceptions, the water produced from these formations is harder than that produced from older formations to the east.
A mantle of Cenozoic gravel and clay up to 400 feet thick overlies the bedrock over wide areas. The lower portion consists primarily of silty clay which is largely residual. Near the base of the mountains where it is thickest. it consists largely of leached colluvial material derived from the adjacent formations. It is characteristically an aquiclude which inhibits direct downward percolation of water to the underlying bedrock. Water encountered in the bedrock below it is commonly under mild artesian head.
The upper portion of the mantle consists dominantly of alluvial and colluvial gravel with a sandy clay matrix and discontinuous beds of sand or of sandy or silty clay. It lies unconformably over residual clay and bedrock and is probably of Pleistocene age. It grades into talus near the foot of the mountains.
The Cenozoic mantle yields only small amounts of water of variable quality to domestic wells although several large springs issue from it.
The structural geology of the area is complex. Interpretation is complicated by facies changes and poor exposure. Fractures produced by deformation of the brittle rocks provide permeability. The main effect of the structure is its effect on the distribution of potential aquifers. The occurrence of ground water within the area is probably influenced more by topography, distribution of the unconsolidated mantle, and lithologic characteristics of the bedrock than by structure.
Calcium and magnesium bicarbonate is the principal chemical constituents of ground water produced from major wells and springs within the area. Water from the carbonate aquifers is commonly moderately hard (61-120 ppm as CaC03). Nearly all of the hardness is temporary(carbonate) and is approximately equivalent to the alkalinity. Concentrations of deleterious substances are low. Water from the elastic rocks is characterized of low mineralization and pH. It is corrosive to ferrous metals.
The temperature of well waters varies from 12° to 15°c. (54° - 59°F.)with few exceptions.
Quality of ground water can commonly be correlated with the geologic formation from which it is produced. The concentration of total dissolved solids in waters from the bedrock aquifers tends to increase with decreasing age of the aquifer and with distance from the mountains. Water from limestone is commonly more highly mineralized than that from dolomite. Waters from. the same formation tends to be more highly mineralized west of the major rivers than they are to the east.
Mineralization of most of the waters studied is derived from the dissolution of the carbonate aquifers and is controlled by equilibrium relations between dolomite, calcite, and dissolved carbon dioxide. The degree of saturation of waters with respect to solid calcite and dolomite can be determined semiquantitatively by comparison of the equilibrium pH computed from water analyses with the measured pH. Waters from typical dolomite reservoirs are supersaturated with respect to dolomite.
The ratio of the concentration of calcium to the concentration of magnesium of most samples reflects the composition of the reservoir rock. Most samples contain more calcium than magnesium. Relationships between the calcium-magnesium ratio, the total mineralization, and the degree of saturation of water samples with respect to the solid carbonates are useful to relate the geology of the area to its hydrology.
The quality of river water fluctuates widely with meteorologic variations, but that of ground-water produced from bedrock in adjacent wells east of the river remains relatively constant. Recharge to the wells at depth is evidently sufficient to prevent downward percolation of appreciable quantities of surface flow into the good bores although pumping levels are commonly below river level. The quality of the water suggests that recharge is dominantly from the east.
Dolomite aquifers underlying floodplain and terrace deposits east of the major rivers are most favorable for the industrial development of ground-water resources. Prospective areas are outlined. Wells located in minor stream valleys near the boundary between the Blue Ridge and the Valley also offers prospects of production from Antietam or Tomstownaquifers. Test-drilling is warranted.
Drilling of test wells should be the first step of industrial site investigation. The location of wells should be based upon a detailed local surface geological investigation. / Doctor of Philosophy
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/101047 |
| Date | 20 May 1962 |
| Creators | Leonard, Robert B. |
| Contributors | Geology |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | xi, vi, 211 leaves, 20 unnumbered leaves of plates, application/pdf, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 03937274 |
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