Hydrogeology of the carbonate rocks, Frederick and Hagerstown Valleys, Maryland
1973, Nutter, L.J.
Report of Investigations 19
The Frederick and Hagerstown Valleys in western Maryland are underlain predominantly by Cambrian and Ordovician limestones and dolomites and contain many karst features (sinkholes, large springs, subsurface drainage, and closed depressions) characteristic of carbonate-rock terranes.
Analysis of the water budget of a principal basin in the Hagerstown Valley indicates that precipitation is about 35 inches, total runoff about 12 inches, and evapotranspiration about 23 inches. Hydrograph separation at three gaging stations in the Hagerstown Valley for 1967 showed that base flow (ground-water discharge) is about 80 percent of the total runoff.
Ground water in the carbonate rocks is recharged by precipitation percolating through the soil and residuum, through alluvial sediments along stream channels, and by direct flow into sinkholes. The mountain wash, composed of a mixture of boulders, cobbles, pebbles, sand, silt, and clay, is found along the mountain areas bordering the Hagerstown Valley. The mountain wash stores considerable quantities of water that slowly recharge the underlying carbonate rocks. Streams that flow off South Mountain on the east side of the Hagerstown Valley are important sources of recharge. Many of these streams lose a substantial part of their flow to the underlying carbonate rocks, and a few flow directly into sinkholes. Hydrographs of observation wells show that recharge can occur even during the growing season in the study area.
Nearly all the major springs in Maryland are in the Frederick and Hagerstown Valleys. Most of the first-order streams can be traced upstream to one or more discrete springs. Comparison of the hydrographs of a stream draining carbonate rocks and one draining noncarbonated metamorphic rocks indicates that for the carbonate-rock basin the discharge peaks are not as sharp, the recession curve after a peak is flatter, and the base flow during the growing season is substantially higher than in the basin draining metamorphic rocks.
One of the most prominent features of most carbonate-rock terranes is the low density of streams in comparison with areas underlain by most other rock types. In the study area many valleys shown as having perennial streams on quadrangle maps contain streams that flow only during the early spring or that are underdrained and carry only surface runoff after periods of heavy rain.
Several problems in carbonate-rock terranes relate to the hydrogeology including instability of the ground, unsuitable conditions for disposal of wastes, poor environment for construction of surface reservoirs, and scarcity of perennial streams.
Factors relating to the occurrence and availability of ground water in carbonate-rock terranes include the following: geologic structure, solution-cavity development, topography, lithology and texture, and thickness of residuum. The geologic structure is extremely important in governing ground-water occurrence because the joints, fractures, and faults provide the framework for the gross secondary permeability. The principal reason that carbonate rocks are more permeable than other crystalline rocks is that their soluble nature permits development of solution cavities along joints, faults, bedding planes, and certain beds. Topography is controlled largely by joints, faults, and bedding and is very useful for selecting well sites where high-yielding wells are likely. Water wells in valleys have a median specific capacity of 1.0 gpm per ft (gallons per minute per foot) compared with 0.09 gpm per ft for wells located on hilltops and upland areas. Lithology is important in development of solution cavities because beds of nearly chemically pure limestone are more soluble than adjacent less pure beds. Well yields in general increase with increasing thickness of residuum, although the relationship is somewhat obscure.
The hydraulic properties of carbonate rocks differ greatly from place to place. The transmissivity, as determined from aquifer tests, ranges from less than 10 to 31,000 square feet per day in the study area. However, the results of aquifer tests in carbonate-rock aquifers are useful only as an approximation of the transmissivity in the vicinity of the wells tested.
The chemical quality of the ground water in the study area is influenced by the mineralogy of the carbonate-rock aquifers. The water is almost always hard or very hard, and the use of water softeners is fairly common. Nitrate concentration exceeded the limit recommended by the U.S. Public Health Service of 45 mg/l (milligrams per liter) in 37 of the 139 samples analyzed. High nitrate content is often an indication of organic pollution, and most of the samples that exceeded the recommended limit for nitrate were from dug wells, which are generally more susceptible to pollution than drilled wells.