Reports
Availability of ground water for urban and industrial development in upper Montgomery County, Maryland
1963, Johnston, P.M. and Otton, E.G.
Open File Report 63-02-1
Abstract
Montgomery County is underlain, except for about 60 square miles of the New Oxford Formation, by crystalline metamorphic rocks such as schist, phyllite, and quartzite. These have been intruded by light- and dark-colored igneous rocks of granitic and basaltic character. Superficial alluvial deposits of relatively small areal extent lie upon these consolidated rocks in scattered areas.
The report area is divided into eight major hydrologic units of somewhat different geohydrologic character; the units, designated by Roman numerals, are based chiefly on the character of the underlying rocks. Statistical studies of well yields, specific capacities, and depths show that the chances of obtaining adequate ground-water supplies are greater in some hydrologic units than in others.
Because of the relative sizes of the hydrologic units, by far the largest amounts of ground water are pumped from the crystalline rocks. Yields of individual wells in these rocks range from practically nothing to 180 gpm (gallons per minute).
However, moderately large quantities of ground water are available in the New Oxford Formation (defined as unit VI), where wells drilled to depths of 315 to 475 feet south of Poolesville yield as much as 80 gpm. Wells in similar geologic settings in Virginia and Pennsylvania yield 200 to 1,000 gpm at depths of 550 to 1,030 feet. Therefore, there is reason to believe that large supplies of ground water may be available from the New Oxford Formation at comparable depths in Montgomery County. However, only test drilling can answer this question definitely.
No large ground-water supplies are to be expected from most of the alluvium (unit I), although test drilling in the flood plain of the Potomac River may reveal permeable sands and gravels hitherto unknown.
The quality of the ground water in Montgomery County is satisfactory, with some exceptions, for domestic, commercial, industrial, and irrigational purposes. High iron content, corrosiveness, and pollution are troublesome in some places, but these problems cannot be identified with any particular hydrologic unit or group of units. Hardness generally is not a problem except possibly in unit VI, where the hardness of water from 5 wells was as great as 210 ppm (parts per million). However, water at deeper horizons in the unit may be considerably harder. Moderately hard to very hard water, averaging 106 to 260 ppm, is found also in units I, VII, and VIII, but these units are of relatively minor importance, because of their small areal extent.
Development of public ground-water supplies in the areas of Poolesville, Barnesville, Germantown, and Damascus may be feasible, but adoption of any program of ground-water development should depend upon economic factors. In the Poolesville area a public ground-water supply can be developed, but it may be necessary to drill to depths as great as 1,000 feet. At this depth several hundred gpm may be available from each well, but the water may be hard and require softening.
In the Barnesville, Germantown, and Damascus areas the average yield of most wells is considerably less than 50 gpm. It may be necessary to drill to depths as great as 500 to 600 feet to explore the crystalline rocks thoroughly; even then the chances of obtaining as much as 50 gpm per well are uncertain.
Damascus is currently (1961) being supplied from wells of the Damascus Utilities Corporation. As additional ground water is needed it can probably be obtained best by drilling wells farther down the gullies radiating from the central ridge on which the town is located.
In order to develop a public supply in any of these areas, it would seem prudent to start on a small scale, putting down one or more test wells in the most favorable sites and, if successful, adding other wells as the demand increases.
Inasmuch as no deep wells (more than 300 feet) have been drilled in these areas, any well that is to be drilled must be considered a test well until its production can be proved by an adequate pumping test.
Ground-water supplies for domestic use are available in most places, but in some localities it may be advisable to have a test well drilled especially where wells are to be in hydrologic units I (alluvium), III (basic rocks), VII (serpentine), or VIII (diabase), or in areas of relatively high elevation and strong relief in the remaining units.
