National Water Summary -- Groundwater Resources, Maryland and the District of Columbia
Water Supply Paper 2275
- Groundwater facts for Maryland and the District of Columbia
- Aquifer and well characteristics in Maryland and the District of Columbia
- Principal aquifers in Maryland and the District of Columbia
- Areal distribution of major Groundwater withdrawals and graphs of annual greatest depth to water in selected wells in Maryland and the District of Columbia
Groundwater is an abundant natural resource in Maryland. Although it constitutes only 13 percent of total water used in the State, it is of substantial cultural and economic significance. The area east of Chesapeake Bay is dependent almost entirely on Groundwater for freshwater supplies. Maryland's aquifers provide water for nearly 1.3 million people (about 30 percent of the State's population) and for industry, irrigation, and other uses. In contrast, the District of Columbia depends mostly on surface-water supplies, although nearly 1 million gallons per day (Mgal/d) of Groundwater is used for industry. Groundwater also is relied on for emergency backup for some hospitals, Government facilities, and embassies. Groundwater was very important to the District of Columbia during its early years and was the sole source of water until the city began to use surface water in 1859 (Johnston, 1964, p. 42, 46). Groundwater withdrawals in Maryland and the District of Columbia in 1980 for various uses are given in table 1.
Average annual precipitation, based on the 30-year period of record (1951-80), ranges from about 37 to 47 inches (in.) in western Maryland and from about 42 to 47 in. in eastern Maryland. In the District of Columbia, average annual precipitation is about 43 in. Recharge rates vary, but, generally, about one-fourth to one-third of precipitation reaches the water table. A very small part of this Groundwater moves into the deeper aquifers; most discharges to nearby streams and provides about 50 to 70 percent of the flow of Maryland's streams.
Differing geologic features and landforms of the several physiographic provinces of Maryland and the District of Columbia cause significant differences in Groundwater conditions from one part of the area to another. Physiographic provinces of Maryland are the Coastal Plain, Piedmont, Blue Ridge, Valley and Ridge, and Appalachian Plateaus (fig. 1). Physiographic provinces of the District of Columbia are the Coastal Plain and Piedmont. The Coastal Plain is underlain by gently dipping, unconsolidated strata. The Piedmont and Blue Ridge are underlain by crystalline rock and consolidated sedimentary units. Intensely folded and faulted consolidated sedimentary strata form the Valley and Ridge. These same strata are folded more gently in the Appalachian Plateaus.
Aquifers in Maryland and the District of Columbia generally are of two distinct types — unconsolidated aquifers of the Coastal Plain and consolidated sedimentary and crystalline aquifers of the other physiographic provinces (termed non-Coastal Plain aquifers). Principal aquifers and aquifer groups are described below and in table 2; their areal distribution is shown in figure 1. The aquifer groups include aquifers and interbedded confining beds; the confining beds are not delineated in figure 1.
Coastal Plain Aquifers
The unconsolidated deposits underlying the Coastal Plain form a southeastwardly thickening sequence that consists of sand-and-gravel aquifers interlayered with silt and clay confining beds. These deposits are underlain by consolidated rock similar to that of the Piedmont, at depths ranging from zero at the Fall Line to about 8,000 feet at Ocean City. With the exception of the Columbia aquifer, the Coastal Plain aquifers generally are confined except where exposed or where overlain only by permeable surficial sediments.
The Columbia aquifer, which is the uppermost hydrogeologic unit of the Coastal Plain in most of Maryland east of Chesapeake Bay, is used as a principal water supply throughout that area. The approximate western limit of the aquifer is shown on the map in figure 1, and the relation of the aquifer to other Coastal Plain aquifers is indicated on the cross section. The aquifer generally is unconfined, but deeper zones locally are confined by clay layers. Thin surficial alluvium and terrace gravels are present elsewhere in Maryland, but these are not commonly used for water supply and, thus, are not shown in figure 1.
The aquifers in the Chesapeake Group are used mostly east of the Chesapeake Bay. These include the Cheswold, Federalsburg, and Frederica aquifers, which are used from Dorchester to Queen Annes Counties, and the Manokin, Ocean City, and Pocomoke aquifers, which are used in Somerset, Worcester, and Wicomico Counties. The Piney Point aquifer, which does not crop out, is tapped by wells in an area about 40 miles (mi) wide between Caroline and St. Marys Counties. The Aquia aquifer supplies water to an area about 50 mi wide between Kent and Queen Annes Counties in the northeast and Charles and St. Marys Counties in the southwest. The Magothy aquifer is used in a triangular area with corners in Cecil, Charles, and Dorchester Counties. Aquifers in the Potomac Group are used for water supply primarily north and west of Chesapeake Bay from Cecil to Charles Counties. From Baltimore County to Charles County, the group includes the Patuxent and Patapsco aquifers. In Cecil and Harford Counties, the aquifers are not differentiated and are called the Potomac aquifer. The Patuxent and Patapsco aquifers are the only Coastal Plain aquifers used for water supply in the District of Columbia.
Well yields of Coastal Plain aquifers depend on thickness and intergranular permeability of the sand and gravel layers and on well construction. Where permeable layers are sufficiently thick, well fields may produce several million gallons per day. Most Coastal Plain aquifers contain saltwater in downdip areas. Natural water quality generally is suitable for most uses; locally, however, excessive concentration of iron [0.3 milligrams per liter (mg/L)] may exist and the water can be hard (120 mg/L as calcium carbonate). The water may also be acidic in some areas with pH values as low as 5. In a few locations, aquifers have been contaminated from surface sources. The presence of saltwater in the Coastal Plain aquifers is discussed by Meisler (1981), Gushing and others (1973), and Hansen (1972).
Non-Coastal Plain Aquifers
Aquifers of the Piedmont, Blue Ridge, Valley and Ridge, and Appalachian Plateaus consist of consolidated sedimentary and crystalline rock. Well yields depend on the presence of open fractures, although a few sandstones have some intergranular permeability. Well yields generally are small but may be as much as several hundred gallons per minute. Fracture openings in carbonate units (limestone, dolomite, and marble) commonly are enlarged by solution, and some wells that intercept enlarged openings have large yields. Aquifers in the Newark Group, the Appalachian sedimentary aquifers, and the carbonate aquifers generally are unconfined to partly confined in the upper hundred feet or so but may be confined at depth. The Piedmont and the Blue Ridge crystalline aquifers generally are unconfined to partly confined.
Natural water quality generally is suitable for most uses. The most common problems are iron and manganese concentrations, which sometimes exceed national drinking-water regulations (U.S. Environmental Protection Agency, 1982a,b); in some units, water hardness is in excess of 120 mg/L as calcium carbonate, and pH is less than 5. Brine underlies freshwater in the Appalachian sedimentary units but generally is at depths deeper than common drilling for Groundwater wells. Locally, pollutants from surface sources have contaminated the Groundwater.
Groundwater Withdrawals and Water-Level Trends
Major areas of Groundwater withdrawals and trends of Groundwater levels near selected pumping centers are shown in figure 2. All centers that produce more than 1 Mgal/d are in the Coastal Plain, with the exception of a quarry north of Baltimore (location 10, fig. 2). The largest concentration of pumping is near Baltimore and Annapolis. Pumping centers that produce from O.1 to 1 Mgal/d are distributed throughout the State.
Water levels generally decline in response to increases in pumping and recover as pumping is reduced. The hydrographs shown in figure 2 reflect the response of aquifers to pumping at selected withdrawal centers in the Maryland Coastal Plain. Increased pumping for private and public water supplies, powerplants, and military facilities has caused water levels to decline in the Aquia aquifer (location 10, fig. 2) and in the Magothy aquifer (locations, fig. 2). Water levels in the Patuxent aquifer in the Glen Burnie area (location 4, fig. 2) have declined steadily since the mid-1950's in response to increasing withdrawals, principally for public supplies, from the Patuxent and Patapsco aquifers.
Water levels in the Piney Point aquifer near Cambridge (location 15, fig. 2) have recovered in response to reduced pumping. Withdrawals from the Piney Point aquifer were reduced partly because new wells were drilled to tap other aquifers and partly because water use declined. Water levels in the Patapsco aquifer in the Baltimore area (location 3, fig. 2) also show recovery. There, pumping induced movement of brackish water from the Chesapeake Bay to the Patapsco aquifer. This caused wells in the Patapsco aquifer to be abandoned in favor of the deeper Patuxent aquifer.
By contrast, little change in water level is noted in the hydrograph of a well in the Columbia aquifer near the major pumping center at Salisbury, Md. (location 16, fig. 2). The aquifer is unconfined, and the cone of depression caused by pumping diverts water from local streams; the diversion helps maintain Groundwater levels, although streamflow may decline as a result.
The District of Columbia relies mainly on surface water and has no specific legislation directed at Groundwater management. In Maryland, however, Groundwater management and planning legislation are extensive. Two State-level organizations implement most of the regulatory, planning, and research programs.
The Maryland Department of Health and Mental Hygiene, through its Office of Environmental Programs, is responsible primarily for regulatory and operational programs with regard to water-quality aspects of Groundwater management. As part of its responsibilities, the Office of Environmental Programs issues well-construction permits (Code of Maryland Regulation 10.17.13, implemented in 1945), requires well-completion reports from licensed well drillers, and regulates the disposal of water to the Groundwater system (Health-Environmental Article, 9-3222).
The Maryland Department of Natural Resources, through its agencies the Water Resources Administration and the Maryland Geological Survey, has a major role in Groundwater-resource planning and management. The Water Resources Administration provides direction in the development, management, and conservation of the water of the State and regulates Groundwater use through an appropriation-permit program (Natural Resources Article, 8-802, enacted in 1933). This program requires a permit to appropriate ground or surface water and requires water-use reports for withdrawals of 10,000 gallons per day or more. Domestic and farm users (including irrigation use) are exempt from these requirements. The Maryland Geological Survey is responsible for the maintenance of a statewide water-data network and the investigation of the State's water resources; these responsibilities are accomplished in cooperation with the U.S. Geological Survey. The research, data collection, and analyses provided by this cooperative program form an information base upon which Groundwater management decisions are made by the Water Resources Administration.
In addition to reports listed below, hydrologic and geologic information was derived from the series of Bulletins, Water Resources Basic-Data Reports, and Reports of Investigations prepared cooperatively by the U.S. Geological Survey and the Maryland Geological Survey, and published by the Maryland Geological Survey.
Cleaves, E. T., Edwards, Jonathan, Jr., and Glaser, J. D., 1968, Geologic map of Maryland: Maryland Geological Survey Map.
Gushing, E. M., Kantrowitz, 1. H., and Taylor, K. R., 1973, Water resources of the Delmarva Peninsula: U.S. Geological Survey Professional Paper 822,58 p.
Edwards, Jonathan, Jr., 1981, A brief description of the geology of Maryland: Maryland Geological Survey, Miscellaneous Publication, 1 p.
Froelich, A. J., Hack, J. T., and Otton, E. G., 1980, Geologic and hydrologic map reports for land-use planning in the Baltimore-Washington urban area: U.S. Geological Survey Circular 806, 26 p.
Hansen, H. J., 1972, A user's guide for the artesian aquifers of the Maryland Coastal Plain, Part 2, Aquifer characteristics: Maryland Geological Survey, Miscellaneous Publication, 123 p.
Herring, J. R., 1983, Maryland water withdrawal and use report 1980: Maryland Department of Natural Resources, Miscellaneous Publication, 50 p.
Johnston, P. M., 1964, Geology and Groundwater resources of Washington, D.C., and vicinity: U.S. Geological Survey Water-Supply Paper 1776,97 p.
Maryland Department of Natural Resources, 1982, The quantity and natural quality of Groundwater in Maryland: Maryland Department of Natural Resources, Water Resources Administration, Water Supply Division, 150 p.
Maryland Department of Natural Resources and Maryland Department of Health and Mental Hygiene, 1983a. The water supplies of Maryland, Vol. 1, Water supply management and conservation — A report to the General Assembly of Maryland in response to Joint Resolution No. 19 Laws of Maryland, 1981: Maryland Department of Natural Resources, 169 p.
___1983b. Water supplies of Maryland, Vol. IV, The status of water supply development and potential water supply problems in Maryland — A report to the General Assembly of Maryland in response to Joint Resolution No. 19 Laws of Maryland, 1981: Maryland Department of Natural Resources, 96 p.
Maryland Geological Survey, 1967, Generalized geologic map of Maryland: Maryland Geological Survey Map, 1 sheet.
Maryland State Planning Department, 1969, Groundwater aquifers and mineral commodities of Maryland: Maryland State Planning Department Publication No. 152,36 p.
Meisler, Harold, 1981, Preliminary delineation of salty Groundwater in the northern Atlantic Coastal Plain: U.S. Geological Survey Open-File Report 81-71,4 pi.
Nutter, L. J., 1974, Well yields in the bedrock aquifers of Maryland: Maryland Geological Survey Information Circular 16,24 p.
Otton, E. G., and Richardson, C. A., 1958, Limestone aquifers of Maryland: Economic Geology, v. 53, p. 722-736.
Raisz, Erwin, 1954, Physiographic diagram, p. 59, in U.S. Geological Survey, 1970, National atlas of the United States: Washington, D.C., U.S. Geological Survey, 417 p.
Solley, W. B., Chase, E. B., Mann, W. B., IV, 1983, Estimated use of water in the United States in 1980: U.S. Geological Survey Circular 1001,56 p.
Truitt, P. G., 1984, Maryland air and water quality atlas: Maryland Department of Health and Mental Hygiene, Office of Environmental Programs, 55 p.
U.S. Environmental Protection Agency, 1982a. Maximum contaminant levels (subpart B of part 141, National interim primary drinking-water regulations): U.S. Code of Federal Regulations, Title 40, Parts 100-149, revised as of July 1, 1982, p. 315-318.
___1982b. Secondary maximum contaminant levels (section 143.3 of part 143, National secondary drinking-water regulations): U.S. Code of Federal Regulations, Title 40, Parts 100-149, revised as of July 1, 1982, p. 374.
Walker, P. N., 1970, Water in Maryland — A review of the Free State's liquid assets: Maryland Geological Survey, 52 p.
Prepared by Laurence J. McGreevy and Judith C. Wheeler
For further information contact
U.S. Geological Survey
8987 Yellow Brick Road
Baltimore, MD 21237