The Appalachian Plateaus Physiographic Province in Segment 11 extends over most of West Virginia, more than one-half of Pennsylvania, and small parts of westernmost Virginia and Maryland (fig. 88). The province is bounded on the east and southeast by the Valley and Ridge Province and by a narrow strip of the Central Lowland Province in Erie County, Pa. The Appalachian Plateaus Province extends into Segments 6, 10, and 12 of this Atlas but is most extensive in Segment 11. In most places, the eastern boundary of the Appalachian Plateaus is marked by an escarpment called the Cumberland Escarpment in Virginia and the Allegheny Front in West Virginia, Maryland, and Pennsylvania. A northward-facing erosional escarpment forms the boundary between the Appalachian Plateaus and the Central Lowland Provinces. The altitude of the Appalachian Plateaus Province is higher than that of the Valley and Ridge Province, as well as the Central Lowland Province.
Aquifers in consolidated sedimentary rocks in the Appalachian Plateaus Province are divided into the following categories-Mississippian aquifers and Permian and Pennsylvanian aquifers (fig. 88). Most of the water-yielding rocks are sandstones (fig. 89), but carbonate rocks of Mississippian age locally yield water in parts of Virginia and West Virginia. Coal beds and seams yield water because they commonly are fractured along joint systems (cleat) that store and transmit water. Devonian siltstone, shale, and thin-bedded sandstone in Pennsylvania, Maryland, and West Virginia locally yield sufficient water for domestic and commercial supplies, especially where the rocks are fractured, but are not considered to be principal aquifers in this report. Unconsolidated glacial and alluvial deposits, discussed in the Surficial aquifer system section of this chapter, are productive aquifers in large areas in Pennsylvania and smaller areas in West Virginia.
The consolidated rocks of the Appalachian Plateaus Province are almost flat-lying to gently folded; the regional dip of the beds rarely exceeds 25 feet per mile. Elongate, gentle folds form alternating anticlines and synclines in Pennsylvania and West Virginia. These gentle warps are surface expressions of small displacements along deep-seated faults in many cases. Deformation of strata in the Appalachian Plateaus is extremely subtle compared to the folding in the Valley and Ridge Province. The Central Lowland Province is similar to the Appalachian Plateaus in the nearly flat-lying attitude of its rock layers and in the geologic formations present, but the formations of Pennsylvanian and Mississippian age have been eroded away in the Central Lowland.
Consolidated sedimentary rocks that compose the Appalachian Plateaus aquifers in Segment 11 range in age from Devonian to Permian (fig. 89). Not all the geologic formations recognized in each of the States are shown in the figure. Some formations are thin or are of local extent; as a result, they are not important in the hydrogeologic framework and are omitted. Most of the productive aquifers consist of sandstone or conglomerate, but limestone formations locally yield significant volumes of water. The water-yielding character of the geologic units shown as aquifers in figure 89 varies because the units change in lithology and thickness from place to place.
In southwestern Pennsylvania, the consolidated rocks nearest the surface are mostly Pennsylvanian in age. Pennsylvanian rocks are the principal coal-bearing formations and consist of cyclic sequences of sandstone, shale, conglomerate, clay, coal, and minor limestone. The sandstones are the most productive aquifers, although coal beds and limestones also yield water; the limestones, however, are thin compared to those of the Valley and Ridge Province. Rocks of Permian age cover most of Greene and Washington Counties in the southwestern corner of the State. Permian rocks are similar in lithology and water-yielding characteristics to the Pennsylvanian rocks, but were not deposited in cycles; they contain only thin coal beds and consist of more shale and less sandstone and conglomerate than the Pennsylvanian strata. Mississippian rocks consist mostly of shale, sandstone, and siltstone with minor conglomerate and limestone and are exposed north and east of the Pennsylvanian rocks where the beds of Pennsylvanian age have been removed by erosion. The principal water-yielding geologic units are sandstones of the Permian and Pennsylvanian Dunkard Group through the Mississippian and Devonian Pocono Formation (fig. 89). Reported typical yields of wells completed in all these units range from 30 to 300 gallons per minute, but some wells yield as much as 600 gallons per minute. Rocks of Devonian age are exposed north of the Mississippian strata in the Appalachian Plateaus Province and in the small part of the Central Lowlands Province in Segment 11. Devonian strata consist mostly of fine grained sandstone, siltstone, and shale, and are not considered to be principal aquifers, although these beds locally yield as much as 200 gallons per minute where they are fractured.
The Appalachian Plateaus Province in Maryland is only in Garrett County and the adjoining western one-third of Allegany County (fig. 88). Rocks of Pennsylvanian age cover most of the Plateaus area, but Mississippian and Devonian rocks are exposed along the crests of northeast-trending anticlines and in some of the deeper valleys. The Pennsylvanian and upper Mississippian geologic formations and their water-yielding characteristics are similar to those of Pennsylvania (fig. 89). Yields of wells completed in Pennsylvanian rocks range from 20 to 430 gallons per minute, but yields of wells completed in Mississippian strata only range from 20 to 180 gallons per minute. Devonian rocks in Maryland locally yield only small quantities of water.
The water-yielding geologic units of West Virginia are similar to those of Pennsylvania (fig. 89), except that the sandstones of the Mauch Chunk Group yield little water, and the Mississippian Greenbrier Limestone locally is a productive aquifer. The Greenbrier is exposed primarily in parts of Tucker, Randolph, Pocohontas, Greenbrier, and Monroe Counties in the southeastern part of the State. Yields of wells completed in Permian and Pennsylvanian sandstones range from 5 to 400 gallons per minute. Yields from the Greenbrier Limestone range from 5 to 100 gallons per minute to wells, but some springs that issue from the Greenbrier discharge 1,000 gallons per minute or more. Yields of wells completed in sandstone of the Pocono Group range from 5 to 120 gallons per minute, but the water in the deeper parts of the Pocono is highly mineralized.
The Appalachian Plateaus Province in Virginia covers Buchanan and Dickenson Counties and parts of four adjoining counties in the southwestern corner of the State. The principal water-yielding geologic units are sandstones of the Harlan, the Wise, and the Lee Formations of Pennsylvanian age and the Mississippian Greenbrier Limestone (fig. 89). Water from these aquifers is used mainly for domestic supply because well yields are generally less than 12 gallons per minute from the Pennsylvanian aquifers and less than 50 gallons per minute from the Greenbrier Limestone. Many of the sandstone beds in the Pennsylvanian rocks are tightly cemented and are less permeable than the coal beds, which tend to be highly fractured and thus yield water. Some deep coal mines in this area are reported to be dry, which suggests that water-bearing fractures in all the rocks extend only a few hundred feet below land surface.
Bedrock aquifers in the unglaciated part of the Appalachian Plateaus Province accept less recharge than those in the Valley and Ridge Province. This is because the unglaciated part of the Appalachian Plateaus is highly dissected, much of the area is sloping, and the slopes are covered with only thin accumulations of regolith. Accordingly, most of the precipitation that falls on the area runs rapidly off the slopes. However, a small part of the precipitation infiltrates the Earth's surface and moves downward through the unsaturated zone to infiltrate weathered bedrock and shallow fractures in unweathered bedrock. The general movement of the water is from areas of high head, usually at high altitude, toward areas of low head, usually in lowlying areas. The water generally moves vertically downward in areas of recharge, then horizontally in the aquifers, and finally upward in discharge areas as it follows paths of least resistance. The movement of the water is steplike because it moves vertically through fractures, then horizontally through sandstone or coal beds, and vertically again when it encounters other fractures. The water will follow permeable beds or a zone of fractures laterally for considerable distances. Saline water or brine is near the surface in much of the area because circulation of fresh ground water generally extends no more than a few hundred feet below the land surface. Most of the ground water moves through local or intermediate-scale flow systems; no regional flow occurs.
Circulation of ground water in the more dissected parts of the Appalachian Plateaus can be envisioned as the drainage of "hydrologic islands" into bounding valleys underlain by older rocks; these "islands" consist of younger rocks at higher altitudes. The edges of two such "islands" with an intervening valley are shown in figure 90. Water moves down tributary valleys toward major rivers, partly as surface water through gaining streams and partly as ground water in alluvial or bedrock aquifers. Saline water or brine at shallow depths is virtually stagnant.
Springs commonly mark the intersection of the water table with a valley wall (fig. 90). Low-permeability rocks, such as shale or siltstone, or ironstone layers, retard the vertical movement of water. The water moves laterally in permeable strata atop the low-permeability rocks until it discharges as springflow. Most of the water that discharges from springs and much that discharges to surface streams is in the aquifers under unconfined conditions.
Water that leaks downward across shale or other low-permeability confining units is present under confined conditions. Water in wells that penetrate an artesian aquifer rises above the top of the aquifer and can flow at the land surface. Confined conditions frequently occur in the troughs of the gently warped synclines that characterize parts of the Appalachian Plateaus (fig. 91). The figure shows an aquifer that is overlain and underlain by less-permeable material with recharge areas at a higher altitude than the central, down-folded part of the aquifer; thus, the water is under pressure. The potentiometric surface, which represents this pressure, is defined as the height to which water will rise in tightly cased wells that penetrate the aquifer. The potentiometric surface is drawn down around a flowing well or a well that is being pumped because of the release of part of the pressure.
Fresh ground water generally circulates only to shallow depths in the Appalachian Plateaus Province. In much of the area, saline water or brine is not far below the land surface with only a thin transition zone between the freshwater and saltwater. In the Pittsburgh, Pa., area, wells drilled deeper than 100 feet below the level of the nearest major stream might yield saline water. The discovery of saltwater springs in the 18th century led to a flourishing salt industry in West Virginia. The origin of the brine that feeds the springs is uncertain, but one possible explanation is that salt has been leached from deposits of rock salt and other evaporites. Such deposits are in the Upper Silurian Salina Group, which underlies much of western Pennsylvania, and in the Wills Creek Shale of Maryland, West Virginia, and Virginia. The brine might move upward along deep-seated fractures.
The presence of brine in the Appalachian Plateaus Province compared to its near-absence in the Valley and Ridge Province also might be attributed to the following factors that determine the relative effectiveness of flushing of the brine by freshwater:
° In the Appalachian Plateaus, extensive, almost flat-lying confining units of shale, siltstone, clay, and dense limestone effectively impede the vertical movement of water. This is especially true of the Pennsylvanian rocks, which cover a large part of the area.
° The aquifers and confining units are not as intensely fractured in the Appalachian Plateaus as in the Valley and Ridge. The fractures also decrease in number with depth, and the circulation of water likewise decreases with depth.
° Most of the Appalachian Plateaus lacks the thick, solution-riddled carbonate-rock aquifers of the Valley and Ridge; such aquifers are conduits for the vigorous circulation of water.
The rocks of the Appalachian Plateaus are only mildly deformed compared to those of the Valley and Ridge. The lower part of the geologic section is, therefore, not brought near the land surface by thrust faults or overturned folds and is unable to receive freshwater as recharge or to readily discharge entrapped fluids, such as brine.
In the Central Lowland Province, very saline water is at shallow depths in the consolidated rocks below the glacial drift. The bedrock is generally low-permeability Devonian shale and is an extension of deeply buried formations that contain saline water in the Appalachian Plateaus Province. The younger, more permeable bedrock formations that contain most of the freshwater circulation system in the Appalachian Plateaus Province have been eroded away in the Central Lowland Province.
FACTORS THAT AFFECT GROUND-WATER FLOW
Underground mining of coal disturbs the natural ground-water flow system when the mines are active because artificial drains are constructed to dispose of unwanted water and mining activities can create new fractures and thus increase permeability. The regional water table can be lowered when the drains are effective, and ground-water flow directions can be changed in some cases until flow moves across former ground-water divides into adjoining basins. Ground water tends to flow toward mines, which are usually dewatered by pumping. Adverse effects of mine drainage on well yields are greatest where the mines are not much deeper than the bottoms of the wells and where vertical fractures connect the aquifers and the mines. Abandoned mines can collapse, which causes fracturing of the rocks that overlie the mine and might be accompanied by an appreciable depression on the land surface. These conditions are likely to enhance recharge to the ground-water system and to reduce surface runoff and evapotranspiration.
Uncased boreholes that penetrate several aquifers, which might have different heads, provide artificial interconnections, or "short circuits," between the aquifers. The water that enters the borehole from aquifers with higher heads moves up the borehole and then laterally into aquifers with lower heads so that the composite head in the well is different than that in each of the aquifers. Flow within the borehole will continue, even when the well is not being pumped, until the head in the two aquifers becomes equal. The freshwater flow system of an area can be significantly altered where numerous uncased wells exist and may result in a change in the potentiometric surface.
Although bedrock formations in the Appalachian Plateaus Province can be traced over many miles, the distribution of lo-cal aquifers within these formations depends, in most cases, on the distribution of fracture permeability. Erosion is one factor that controls the distribution of fractures. Local aquifers, in some cases, are in valleys (fig. 92). Near-vertical tensile fractures and horizontal fractures are associated with slumping that takes place along valley walls, and recharge tends to be concentrated by the fractures. Under the valley floors, the most significant fractures are parallel to bedding, or nearly horizontal. Relief of compressional stress results when the weight of the rock that overlies a valley is reduced, because part of the rock is removed by erosion. This causes the remaining rock to separate along bedding planes and also results in the formation of vertical fractures. Fractures that underlie the valley are interconnected with those along the valley walls, and the interconnected fracture set enhances the permeability of the rock. Away from the valley walls, fractures are scarce and less likely to be interconnected; accordingly, wells in these areas will tend to have lower yields than those in the valleys. Furthermore, water in the shallow fractures on hilltops tends to drain in dry seasons, and yields of some wells might accordingly decline. In general, well yields are directly proportional to the number of interconnected fractures.
The chemical quality of water in the freshwater parts of the bedrock aquifers of the Appalachian Plateaus Province is somewhat variable but generally is satisfactory for municipal supplies and other purposes. Most of the water in the upper parts of the aquifers is not greatly mineralized and is suitable, or can be treated and made suitable, for most uses. Saline water commonly is in the aquifers at depths of only a few hundred feet below the land surface.
The undifferentiated sedimentary-rock aquifers consist principally of sandstone and fractured shale and coal. Most of the minerals that compose these aquifers do not readily dissolve, and the water is a calcium sodium bicarbonate type (fig. 93A). Dissolved-solids concentrations are small and aver-age only about 230 milligrams per liter. Hardness averages about 95 milligrams per liter, which is considered to be moder-ately hard. Water from predominately shale aquifers in Pennsylvania is reported to be hard, whereas that from predominately sandstone aquifers is reported to be soft. The median hydrogen ion concentration, which is measured in pH units, is 7.3. The median iron concentration is about 0.1 milligram per liter, but concentrations as large as 38 milligrams per liter have been reported.
Carbonate-rock aquifers consist mostly of calcium and magnesium carbonate minerals, which are readily soluble and affect the chemical composition of the ground water. Limited data show that dissolved-solids concentrations in water from carbonate-rock aquifers in the Appalachian Plateaus Province average about 180 milligrams per liter, and hardness averages about 170 milligrams per liter, which is considered to be hard. The median hydrogen ion concentration, which is measured in pH units, is 7.5 (slightly basic). The median iron concentration is near zero. Water in these aquifers is mostly a calcium bicarbonate type (fig. 93B).
Contamination of ground water by the improper construction or plugging of oil and gas wells is a common problem in the Appalachian Plateaus Province. The area is the cradle of the oil industry in the United States; drilling for oil began in the 1860's, and drilling for brine began even earlier. Natural brines are associated with accumulations of oil and gas and are at shallow depths in many places. Wells that penetrate aquifers that contain brine, if not properly cased and cemented, can provide conduits for the brine to enter shallower freshwater aquifers. It was once a common practice for brine produced with oil and gas to be discharged into open pits from which it seeped downward to contaminate fresh ground-water bodies. Such practices are generally prohibited now, but effects of the past remain.
In coal-mining areas, which in the Appalachian Plateaus Province are generally within the limits of Pennsylvanian rocks (fig. 88), ground water commonly includes water that has been in contact with mine workings or that has infiltrated and leached mine spoil piles (fig. 94). Water affected by coal-mining operations is usually acidic. Sulfur-bearing minerals, such as pyrite, that are present in the coal are exposed to air in mines and spoil piles, and the oxidized sulfur combines with water to form sulfuric acid. The acid water commonly contains large concentrations of iron, manganese, sulfate, and dissolved solids and is highly colored (fig. 95). An exception is in the southern coal fields of West Virginia where the coal is low in sulfur, mine drainage tends to be alkaline, and water from working or abandoned mines is commonly used for public supply.
FRESH GROUND-WATER WITHDRAWALS
Total freshwater withdrawals from consolidated sedimentary-rock aquifers in the Appalachian Plateaus and the Central Lowland Provinces were estimated to be 282 million gallons per day during 1985. About 47 percent of this amount, or about 133 million gallons per day, was withdrawn for domestic and commercial supplies (fig. 96). About 116 million gallons per day, or about 41 percent of the total withdrawals, was pumped for industrial, mining, and thermoelectric power purposes; most of this water was used in coal mining operations.