INTRODUCTION
Sedimentary rocks that consist primarily of sandstone form aquifers in several areas of New York, Massachusetts, and Connecticut (fig. 102). The Potsdam Sandstone of Cambrian age in parts of northern New York together with the overlying Theresa Formation, also of Cambrian age, is an aquifer that yields small to moderate quantities of water to wells. Small quantities of water also are obtained from sandstones of the Silurian Medina Group in the Mohawk River Valley and along the south shore of Lake Ontario. In the Connecticut River Valley of Connecticut and Massachusetts, sandstones in the Newark Supergroup of early Mesozoic age form a significant aquifer. These rocks also are present in an early Mesozoic basin in New Jersey that extends into southeastern New York. Although sandstone aquifers generally are limited in areal extent and yield small to moderate quantities of water, they are significant sources of water for rural, domestic, industrial, and small-community supplies in their area of occurrence where the surficial aquifer system is not present.
GEOLOGY
The Cambrian Potsdam Sandstone forms a discontinuous fringe around the northern and western borders of the Adirondack Mountains in New York. The Potsdam generally consists of tan to grayish-white quartz sandstone with siliceous and cal-careous cement. Locally, some of the basal sandstone beds are either red from hematitic cementation or green from chloritic cementation. Basal beds of coarse conglomerate are present in some places.
The Potsdam Sandstone overlies granite and hornblende-syenite gneiss of Precambrian age (fig. 103); its thickness ranges from a featheredge to about 200 feet. The Potsdam might have formed originally as a beach deposit. It grades upward into the Theresa Formation of marine origin, which consists of hard, bluish-gray, thinly bedded, sandy dolomite with calcareous sandstone layers in the basal part. The upper beds of the Theresa Formation range from calcareous and dolomitic sandstones to sandy dolomite, which is characterized by numerous fossils. The thickness of the Theresa Formation ranges from a featheredge to about 70 feet. Dolomite and limestone of the Black River Group of Ordovician age overlie Cambrian rocks.
The sedimentary rocks of central Connecticut and Massachusetts have been assigned to the Newark Supergroup of early Mesozoic age (fig. 102). They are lithologically similar to the Newark Supergroup, which consists of red sandstone, shale, and conglomerate and fills a number of fault-block basins that extend from New Jersey to South Carolina. These basins are discussed in Chapter L of this Atlas. In Connecticut and Massachusetts, these sedimentary rocks uncon-formably overlie pre-Mesozoic igneous and metamorphic rocks in the lowland trough of the Connecticut River Valley. The sedimentary rocks are bounded on the east by a major fault with several thousand feet of vertical displacement (fig. 104). The fault formed an eastward-tilting trough that became filled with sandstone and interbedded lava flows, which are in contact with pre-Mesozoic crystalline rocks beneath and across the fault. The rocks in the trough dip eastward at about 15 degrees. They feather out to the west to expose the underlying pre-Mesozoic bedrock. Glacial deposits overlie the early Mesozoic rocks.
The Newark Supergroup consists of continental deposits of reddish arkosic sandstone (fig. 105), feldspathic sandstone, conglomerate, and shale, with some limestone and siltstone. The sandstone is interbedded with at least three thick basaltic lava flows (fig. 106) that also dip eastward at about 15 degrees. Because they are hard and resistant to erosion, the lava flows typically form topographic ridges. The softer sandstone, which is easily eroded, typically forms lowlands and rarely crops out.
The Newark Supergroup has been subdivided into three formations: the New Haven Arkose, the Meriden Formation, and the Portland Arkose. The New Haven Arkose is the lowermost formation and consists of all sedimentary rocks below the lowermost lava flow. The Meriden Formation consists of the three lava flows and interbedded sandstone. The Portland Arkose is the uppermost formation and includes all sedimentary rocks above the uppermost lava flow. Maximum thickness of the Meriden Formation is about 800 feet. The maximum thickness of the Newark Supergroup is estimated to be about 4,000 feet.
GROUND-WATER HYDROLOGY
The sandstone aquifer in northern New York (fig. 102) generally is hydraulically connected with the aquifer in overlying carbonate rocks and, for the most part, the two aquifers are confined by overlying glacial deposits. Hydraulic heads in the glacial deposits generally are from 30 to 40 feet higher than the potentiometric surface of the two aquifers; therefore, recharge to the sandstone aquifer is by downward percolation of water through the overlying glacial deposits and carbonate rocks. Where the sandstone aquifer is overlain by glacial deposits, recharge to the aquifer is by downward percolation of water through the overlying deposits. After the water enters the sandstone aquifer, it moves horizontally from recharge areas in the interstream highlands toward the streams and Lake Ontario. At these surface-water bodies, the water moves upward through the carbonate rocks or the glacial deposits or both into the surface-water bodies.
The intergranular porosity of the sandstone aquifer generally ranges from about 4 to 30 percent with an average of only about 10 percent because most of the intergranular space is filled with siliceous or calcareous cement. Therefore, ground-water movement in the aquifer is primarily through second- ary openings, such as joints, fractures, and bedding-plane openings. Data for 12 domestic wells completed in the sandstone aquifer in northern New York indicate that the wells penetrated from 16 to 55 feet of sandstone (average 32.5 feet) and yielded from 3 to about 30 gallons per minute. Wells with the largest yields were completed only in the Theresa Formation, thus indicating that it might be more fractured than the Potsdam Sandstone.
The sandstone of the Newark Supergroup also is well cemented by carbonate cement that has filled many of the intergranular openings; porosity is estimated to be about 7 percent. Thus, water in the aquifer in the lower Mesozoic sandstone primarily is contained in and moves through secondary openings, such as joints, fractures, and bedding planes. Bedding-plane openings probably transfer the greatest quantity of water.
Yields from 688 wells completed in the sandstone aquifer of the Newark Supergroup ranged from 0.5 to 578 gallons per minute; the average yield was 34 gallons per minute. The depths of these wells range from 40 to 973 feet with an average depth of 203 feet.
Yields from the interbedded basaltic lava flows, which have only fracture permeability, generally are smaller than those from the sandstone aquifer. The average yield of 53 wells completed in the basalt ranged from 1.5 to about 50 gallons per minute with an average yield of 13 gallons per minute. The depths of these wells range from 60 to 500 feet.
Yields to wells completed in either the sandstone aquifer or the basalt tend to be larger in areas where these aquifers are overlain by the surficial aquifer system rather than in areas where they are overlain by till. This indicates that the surficial aquifer system provides storage for water that subsequently moves downward into the aquifers as water is withdrawn from them.
Ground-water movement through the sandstone aquifer of the Newark Supergroup is not well defined; however, the presence of freshwater at depths of several hundred feet in the aquifer indicates that regional ground-water movement occurs. Recharge occurs in areas where the aquifer crops out and in upland areas where the sandstone aquifer is overlain by permeable sand and gravel of the surficial aquifer system. Water in the sandstone aquifer mostly moves down a slight hydraulic gradient in local flow systems to areas of discharge at small streams where it moves upward to the stream through the surficial aquifer system and becomes part of the streamflow.
There also is a regional flow system in which the water moves deeply into the aquifer and toward the Connecticut River where the water is discharged through the surficial aquifer system to the river. Some water also discharges directly to the ocean in the coastal areas of the Connecticut River Basin.
Few wells penetrate more than about 500 feet into the sandstone aquifer, and little hydrologic information is available beyond that depth. Because the number of joints, fractures, and bedding-plane openings in all types of rock typically decrease with depth, permeability of the sandstone aquifer should similarly decrease until ground-water movement ceases at some depth. Decreased circulation of ground water results in an increase of dissolved minerals in the water. Thus, the boundary of freshwater is lower in the sandstone aquifer, probably within about 1,000 feet of the land surface, below which use of the aquifer is limited by a decrease in permeability and by a deterioration in water quality.
GROUND-WATER QUALITY
The chemical quality of water in sandstone aquifers of the Newark Supergroup and in Lower Paleozoic rocks generally is suitable for drinking, as well as most other uses. A summary of water-quality data for water from the aquifers in the Newark Supergroup in Connecticut and southeastern New York and from the Potsdam Sandstone and sandstone of the Medina Group in north-central New York is shown in table 14. Median, maximum, and minimum values of each chemical constituent for about 100 chemical analyses are listed.
Median values of calcium, sulfate, hardness, and dissolved solids shown in table 14 indicate that excessive concen-trations of dissolved minerals generally are not a problem. Locally, large maximum values indicate that highly mineralized water exists, particularly at depth in the sandstone aquifers of the Newark Supergroup and in sandstone of the Medina Group. Large sulfate concentrations may result from dissolution of gypsum that is disseminated in places in the sandstone of the Newark Supergroup. The calcium from the gypsum contributes to excessive hardness and large concentrations of calcium. Both ions contribute to increased concentrations of dissolved solids.
Large chloride concentrations in water from aquifers in the Newark Supergroup of Rockland County, N.Y. (table 14), may indicate local contamination from road-deicing chemicals or some other source of chloride on the land surface. Locally, large chloride and calcium concentrations in water from sandstone of the Medina Group in Wayne County, N.Y., contribute to dissolved-solids concentrations in excess of 8,000 milligrams per liter. In general, water from sandstone of the Medina Group appears to be more mineralized than that in the sandstone aquifers in the Newark Supergroup.
FRESH GROUND-WATER WITHDRAWALS
Aquifers in the Cambrian sandstone of New York and the lower Mesozoic sandstone of Connecticut and Massachusetts generally yield only small quantities of water to wells. Therefore, they are used primarily as a source of supply for households, commercial establishments, and small communities and industries that require only modest quantities of water. About 28 percent, or 29 million gallons per day, of the total water withdrawn from the sandstone aquifers in Segment 12 during 1985 was withdrawn for domestic and commercial use (table 15). In Connecticut and Massachusetts, 74 percent (fig. 107), or about 16 million gallons per day, of the water withdrawn from these aquifers was used for domestic and commercial purposes.
Water from sandstone aquifers also is used for industrial and mining purposes, especially in New York. Withdrawals for these use categories accounted for about 38 percent, or about 40 million gallons per day, of total withdrawals from the sandstone aquifers in the three States during 1985 but accounted for about 46 percent, or about 39 million gallons per day, of the total water withdrawn in New York. Water was withdrawn for industrial and mining uses, primarily for inplant domestic uses, gravel pit or quarry dewatering, boiler makeup water, and gravel washing rather than for use as industrial process water.
Agricultural use accounted for only about 1 percent, or about 1.3 million gallons per day, of total withdrawals from the sandstone aquifers during 1985. This water was used largely for stock watering and milk processing.
In New York and Connecticut, about 33 percent, or about 35 million gallons per day, of the total water withdrawn from the sandstone aquifers was used for public supply (table 15). This use, however, generally was in small communities or as a supplemental supply to surface water in larger communities because of the generally limited yields of wells completed in the sandstone aquifers.
Total withdrawals from the sandstone aquifers in the three States were about 106 million gallons per day during 1985 (fig. 107). The largest withdrawals, which were about 85 million gallons per day, were in New York. Connecticut and Massachusetts withdrew about 21 million gallons per day.
