Reports
Geology and ground-water resources of the Baltimore area
1952, Bennett, R.R. and Meyer, R.R.
Bulletin 4
Abstract
The Baltimore area comprises the city of Baltimore, and most of the area from the Susquehanna River south to Laurel, essentially between the Piedmont Plateau and the Chesapeake Bay. Most of the large ground-water developments are in the industrial districts in and near Baltimore; consequently that part of the area was investigated in greater detail. With the exception of the northern part of Baltimore, which is in the Piedmont Plateau, the area is chiefly in the Coastal Plain.
The Piedmont Plateau is underlain by pre-Cambrian crystalline rocks consisting mostly of gabbro, schist, granite, and gneiss. Owing to their greater resistance to erosion the land surface of the plateau is higher and more rugged than the Coastal Plain, which is underlain by soft unconsolidated sediments of Lower and Upper Cretaceous and Pleistocene ages. The land surface of the Coastal Plain slopes gently southeastward toward Chesapeake Bay. In some places estuaries, which are tributaries of Chesapeake Bay, extend northwestward across the Coastal Plain to the Piedmont Plateau.
The Coastal Plain sediments were deposited on the southeastward-sloping surface of the crystalline rocks. They form a wedge-shaped mass that thickens progressively from west to east. The strike of the formations of Cretaceous age is approximately parallel to the boundary between the Piedmont Plateau and the Coastal Plain (Fall Line). As these formations dip gently to the southeast they crop out as bands of irregular width trending northeast. The Pleistocene deposits are essentially flat lying and were deposited on the eroded surface of the pre-Cambrian and Cretaceous rocks.
In most of the area the sediments of Lower and Upper Cretaceous age, which consist essentially of irregular and lenticular beds of sand, gravel, and clay of continental origin, may be divided into three formations: the Patuxent formation of Lower Cretaceous age, and the Arundel clay and Patapsco formation of Upper Cretaceous age. Their combined thickness ranges from about 475 to 750 feet. The Pleistocene deposits, which consist chiefly of irregular beds of sand, gravel, and clay of continental and estuarine origin, are divided, in this report, into an upland and lowland unit. The combined thickness of these units ranges from nearly nothing to 175 feet.
Owing to the great difference in water-bearing properties of the crystalline rocks in the Piedmont Plateau and the unconsolidated sediments in the Coastal Plain, ground water occurs under two widely different sets of conditions. In the crystalline rocks the water is contained chiefly in joints and other fractures which are not uniform in size and gradually disappear with depth, consequently the water-bearing zones are very irregular and inhomogeneous. The sand and gravel in the Coastal Plain sediments are considerably more porous and permeable and form relatively uniform and widespread aquifers. In general water-table conditions occur in the outcrop areas of the crystalline rocks and the Coastal Plain sediments, but down dip from the outcrops the ground water occurs under artesian conditions. Because of their low permeability, where they underlie a substantial thickness of unconsolidated sediments the crystalline rocks are not considered to be an aquifer in most of the Coastal Plain area.
The reported yields from 106 industrial wells ending in the crystalline rocks in Baltimore show an average yield of 50 gallons a minute, and a median yield of 35; the mode, or most typical value, is 10 gallons a minute. The reported yields range from 0 to 350 gallons a minute.
Industrial wells in the Patuxent formation in and near its outcrop have yields of about 200 to 300 gallons a minute, whereas wells in this formation down dip in the southeastern part of the area have yields of about 500 to 900 gallons a minute. Pumping tests and flow net analysis show that the coefficient of transmissibility of the Patuxent formation, the principal water-bearing formation in the area, averages about 20,000 gallons a day per foot in the industrial districts near the outcrop, and about 50,000 in the industrial districts in the southeastern part of the area. The coefficient of storage of the Patuxent formation, under artesian conditions, is about 0.00026; under water-table conditions in the outcrop area, it is estimated to be 0.15 to 0.20.
The Patapsco formation is an important water-bearing formation in the southeastern part of the industrial area where it is separated by clay into a lower and upper aquifer. The lower aquifer yields as much as 500 to 750 gallons a minute to industrial wells. Its coefficient of transmissibility averages about 25,000 gallons a day per foot. The upper aquifer, which yields as much as 500 to 800 gallons a minute to wells, has a greater thickness than the lower aquifer, so that its coefficient of transmissibility probably is more than 25,000.
The upland unit of the Pleistocene deposits is thin and caps the hills and ridges and is not an important aquifer. In some places, the lowland unit is sufficiently thick and permeable to yield large quantities of water to wells.
The large ground-water supplies in the industrial area have been developed since around 1900. The pumpage increased progressively to a peak of about 47,000,000 gallons a day early in 1942. Late in 1942 the pumpage was decreased by about 13,000,000 gallons a day and in 1945 was 34,000,000 gallons a day. From 1942 to 1945 the pumpage outside the industrial area increased from about 3,000,000 to 5,000,000 gallons a day. The total pumpage in 1945 for the entire area was 39,000,000 gallons a day. The pumpage, in gallons a day, from each water-bearing formation, is approximately: pre-Cambrian crystalline rocks, 1,000,000, Patuxent formation, 30,000,000; Patapsco formation, 6,000,000; and Pleistocene deposits, 3,000,000.
Originally the artesian head in the aquifers generally was within a few feet of the land surface, but with the progressive increase in pumpage during 1940 to 1942 the artesian head in the Patuxent and Patapsco formations, in and near the centers of pumpage, declined respectively to as much as 160 and 190 feet below the land surface. The decrease in pumpage late in 1942 resulted in a rise in the artesian head; and at present, in most of the industrial area, the artesian head in the Patuxent and Patapsco formations ranges, respectively, from about 40 to 100 and 10 to 50 feet below the land surface. Detailed records of water-level fluctuations in observation wells show that during 1943-45 the general trend of water levels in most parts of the area was either slightly upward or essentially horizontal.
The ground water in the Baltimore area normally has a low mineral content, but the lowering of the water table or artesian head has caused salt water, chiefly from the Patapsco River estuary, to enter the aquifers and spread laterally throughout a large part of the industrial area.
Industrial wastes, chiefly sulfuric acid, also have contaminated the ground water in a small part of the industrial area.
The contamination of aquifers through leaking wells is a serious ground-water problem in the industrial area. As the artesian head in the Patuxent formation is lower than it is in the Patapsco formation of Pleistocene deposits, highly mineralized water may enter a well through a defective casing or move downward outside an improperly sealed casing and contaminate the water in the Patuxent formation. Although this problem is now serious, the repair of leaking active wells and effective plugging of abandoned wells would appreciably reduce this contamination in a relatively short time.
Practically all ground water pumped in the Baltimore area is derived from precipitation on the outcrops of the aquifers. The potential rate of recharge to the aquifers in the Coastal Plain exceeds the theoretical maximum quantity of water that can be transmitted through them. The rate of recharge, therefore, does not limit the quantity of water that can be pumped in the artesian part of the area.
The concentration of pumping has caused the water table or artesian head in the Patuxent formation to be so low in and near some of the centers of pumping that very little additional water can be developed from that formation in those parts of the industrial area. The water table or artesian head between the centers of pumping, however, is relatively high and if the wells were spaced at greater distances additional water could be pumped. The heavy pumping from the Patuxent formation, chiefly in and near its outcrop adjacent to the Patapsco River estuary, has caused local encroachment of salt water. However as pumping in and near that part of the outcrop area prevents most of the salt water now in the formation from moving to major well fields in the other industrial districts, it would not be advisable to decrease or discontinue this pumping.
Heavy pumping over a period of many years throughout the industrial area has caused local encroachment of salt water in the Patapsco formation, so that most of the pumping from the formation has been discontinued. The present pumpage is chiefly from the aquifer in the lower part of the formation in the southeastern part of the area where a large part of the water is derived from areas in which the Patapsco formation contains fresh water. Although this pumping is still causing encroachment of salt water, it would not be desirable to discontinue the pumping at least until all leaking wells drilled to the Patuxent formation are repaired or plugged. Even if the pumping were discontinued, many decades would pass before the salt water in the formation would be flushed out. It would not be advisable, however, to develop additional supplies of ground water from this formation in the industrial districts up dip as they are near the main source of contamination.
The economic value of ground water in the industrial area is different for various types of uses and there is little uniformity in the chemical quality of water that can be used. The application of the term safe yield in the sense that it represents a single rate of pumping for the entire industrial area would be unrealistic. It is apparent that owing to the contamination of the water the safe yield has been exceeded for some industries.
