Hydrology
Northeastern Aquifer Systems: New York and New England
Aquifers in the northeastern region of the United States are developed in three principle
hydrogeologic environments.
Surficial Aquifers: These are the most productive and widely used aquifers, with
commonly achieved yields in the range from 10 to 1000 gpm. Surficial aquifers are
developed in sands and gravels deposited by glacial ice during the last retreat of glacial
ice from 21,000 to 12,000 years ago during the Late Pleistocene Wisconsin glaciation.
Valley fill glacial aquifers - most surficial aquifers are developed in valley fill.
1. Outwash deposits - stratified drift deposited away from the ice by flowing meltwater.
2. Ice-contact deposits - coarse, poorly stratified drift deposited along the margin of the
ice sheet as sediments accumulated in crevasses or along the face of the ice.
Valley fill aquifers can be both confined and unconfined. Confining units are formed
from lacustrine and marine clays deposited in marine embayments or ice margin lakes.
Marine deposits formed inland of the coast and down the St. Lawrence seaway during the
waning stage of glaciation as rising sea levels flooded crust that had not yet rebounded
from the weight of glacial ice.
In valleys where meltwater streams drained away from glacial ice, deposits of glacial
sediments formed in a consistant sequence.
1. Glacial ice flows downvalley, carving the valley into a wider and deeper "U" shape.
Tills are deposited along the base and sides of the valley as the glacier flows and then
stagnates.
2. Stagnating ice continues to melt. Sand and gravel collect on the surface of the
wasting ice in crevasses. These sediments are deposited along the margins of the
melting glacier (ice contact deposits). Meltwater carries sediments away from the
ice. Coarse material is deposited near the ice margin, finer stratified drift is deposited
downstream from the ice. Generally, the valley will fill with coarse ice contact
deposits along its margins and with finer stratified drift in the interior of the valley, as
well as above older ice margin deposits. As the ice continues to retreat, terminal and
recessional moraines can create meltwater dams forming small proglacial lakes where
fine sands and silts are deposited.
3. After the ice has melted away, streams flowing through the valley erode and redeposit
the uppermost glacial sediments as alluvium.
Surficial valley aquifers are recharged in several ways. Some recharge is from direct
precipitation on the aquifer, however, studies have shown that a significant source of
recharge (as much as 60%) is from direct unchanneled runoff coming down the relatively
impermeable slopes of the surrounding hills. Tributary streams flowing down into the
valley over impermeable bedrock also contribute recharge as they flow out over the
aquifer sediments, temporarily becoming losing streams. Pumping of the aquifer can also
cause baseflow recharge if the water table is lowered below stream level in the valley.
Cape Cod glacial aquifer - Terminal moraine, outwash, ice contact, and glacial lake
deposits - unconfined.
The aquifer system of Cape Cod is similar in many ways to Long Island's aquifer system.
The Cape Cod glacial aquifer is surrounded on all sides by saltwater and is recharged
solely by precipitation. Groundwater flow is radial from six distinct recharge areas in the
elevated central part of the cape. Each recharge area drives flow through the aquifer until
it discharges to the surface in the ocean or at ocean inlets called narrows. Flow within
each cell is independent of and isolated from flow in the other cells. Like Long Island,
Cape Cod is prone to saltwater intrusion from both passive and active encroachment.
Upper glacial aquifer, Long Island - Morainal deposits of till and stratified drift, plus
extensive outwash deposits - unconfined.
Coastal Plain Aquifers: These are found only beneath Long Island in the northeast.
Correlative sedimentary layers beneath Cape Cod are saturated with salt water and are
not useable as aquifers.
4. Magothy aquifer - mostly unconfined, in hydraulic connection with the upper glacial.
5. Lloyd aquifer - confined
Consolidated Bedrock Aquifers: These aquifers are developed in bedrock where most
porosity is due to fractures in the rock. Yields from bedrock aquifers are considerably
less than from surficial aquifers, with average amounts from 10 to 100 gpm.
Carbonate rock aquifers - Developed in limestones, dolostones, and marbles, these
aquifers have the highest yields due to the enhanced porosity from dissolution of
carbonate rock by acidic groundwater. Mostly unconfined in upper 200 ft, may be
confined at depth.
Sandstone aquifers - Developed in sandstones. Mostly unconfined in upper 200 ft, may
be confined at depth.
Crystalline bedrock aquifers - Developed in the joints, fractures, and cleavages of the
metamorphic and igneous bedrock of New England and the Adirondacks. Yields are
generally low at from 2 to 10 gpm, but the widespread extent of these aquifers makes
them important water providers for domestic use in rural areas. Mostly unconfined,
locally confined at depth.