Physical Geology
Lecture 12 Groundwater
Fresh water falls to the surface of the earth as precipitation. The average annual precipitation on the area of the United States is equivalent to a layer of water covering the country to a depth of 76 cm (about 2.5 feet).
Of the 76 cm, 45 cm returns to the atmosphere as evaporation and transpiration.1 cm infiltrates into the ground to become groundwater. 30 cm flows as runoff over the land in streams and rivers.
The numbers for personal consumption are also high. You need about .75 gallons per day of fresh water to survive. The typical American uses about 50 to 80 gallons of water per day.
Many urban areas, such as New York City, get their water from surface sources such as rivers, lakes, and reservoirs. New York City gets its fresh water from the Catskills, where large reservoirs gather and hold runoff water from streams, which is then shipped to the city by means of large, manmade, underground water tunnels.
Here on Long Island we get our water from beneath the surface - groundwater.
Currently, groundwater geology is the fastest growing branch of the science. This is due to both the increasing demand for fresh water resources, as well as the coming to grips with the unfortunate facts of severe groundwater contamination due to decades of haphazard dumping and toxic waste mismanagment. Hydrogeologists or hydrologists are becoming increasingly important for securing and managing over-exploited supplies of clean, fresh water.
Firsthand knowledge of groundwater comes from digging wells. When a hole is sunk into the ground the well passes first through an upper zone of moist soil.
Next comes a zone of unsaturated regolith or bedrock in which the pores and spaces between the rock are filled with air called the zone of aeration.
Finally, a zone is reached where all of the pores and spaces between the rock are filled with water, the zone of saturation. The upper surface of the zone of saturation is called the water table.
Most of the groundwater within a few hundred meters of the surface is in motion. However, flow velocities are much lower beneath the ground than they are for surface streams. Average velocities for groundwater flow are measured in centimeters per day.
The amount of groundwater that a particular kind of sediment or rock can hold depends on what percentage of the material is porous, i.e. consisting of empty space.
Sedimentary rocks can be very porous, or not, depending on the size and shape of the grains, and on how much cement has crystallized between the grains.
Sands and gravels are notoriously porous, with porosities of up to 20%.
Some clays are very porous, with porosities of up to 50%.
Igneous and metamorphic rocks tend to have low porosities, unless they are extensively jointed and fractured.
The ease of movement of water through sediment or rock is referred to as permeability. Generally, materials that have low porosities also have low permeabilities.
The opposite cannot be said to be always true. One might guess that highly porous clays would have the greatest permeability, but in fact, all clays are highly impermeable.
The reason for this permeability also depends on the size of the pore spaces in the rock or sediment. Water has a high molecular attraction for many materials, causing it to stick to surfaces forming a thin film.
If the pore sizes are too small, as in clays (< .005 mm) then the pores essentially become clogged by films of water and no additional water can pass through.
Layers of sediment or rock below the surface that have high porosity and permeability are called aquifers. The definition of aquifer is economic rather than scientific. If a useful amount of water can be extracted from a layer of rock that layer is considered an aquifer.
Layers of clay or bedrock that are highly impermeable to groundwater are commonly referred to as aquicludes.
Water that is added to the water table from above or outside is called recharge. Regions of land that collect rainwater and pass it downward to the water table are called recharge areas.
Water that is removed or that leaves the water table is called discharge. Valleys and other depressions where groundwater can escape and flow in streams along the surface are called discharge areas.
The amount of time that it takes the groundwater to complete its underground journey from recharge area to discharge area depends on the path it takes and the material it passes through
Groundwater will seep out of the ground wherever the water table intersects the surface of the land. These seepages are called springs.
A well sunk into the ground will fill with seeping groundwater if it intersects the water table. Pumping water from the well causes a local depression in the surface of the water table around the well called a cone of depression.
Excessive pumping from a well or series of wells can result in large, expanding cones of depression that cause the local water table to fall enough to leave some wells dry.
Laterally extensive bodies of rock or sediment that form important reservoirs for extractable groundwater are called aquifers.
Unconfined aquifers are those that extend all the way to the surface.
Confined aquifers are those that are overlain and underlain by impermeable aquiclude layers. The water in the lower part of a confined aquifer is often under pressure from the weight of the water in the upper part of the aquifer. This pressure will cause water to rise up in a well beyond the upper limit of the confined aquifer.
Confined aquifers can create artesian wells and artesian springs if the pressure in the aquifer is sufficient to raise water above the land surface.
Karst landscapes
In regions underlain by limestone, flowing groundwater causes the dissolution of large cavities in the subsurface, forming caves, systems of caves (caverns) and sinkholes. This type of landscape is called Karst topography.
Long Island’s Aquifers
Long Island’s aquifers are both confined and unconfined. The glacial sands and gravels that the island is built from form a large unconfined glacial aquifer. This aquifer is recharged by rainfall on Long Island.
Because it is in direct contact with the surface, the glacial aquifer is an unconfined aquifer. The glacial aquifer recharges very quickly from water percolating down from the surface.
These layers include the following aquifers:
Jameco aquifer: a gravelly deposit of Quaternary age, capped by the relatively impermeable Gardiners Clay.
Magothy aquifer: a sequence of sands sandwiched within layers of clay that act as localized aquicludes.
Lloyd aquifer: a deep sand and gravel aquifer separated from the overlying Magothy aquifer by an interval of clay called the Raritan clay.
Groundwater use on Long Island has progressed through several stages through time. These stages can also be observed geographically as one moves from the populated western part of the island to the rural eastern part.
Rural stage: At first, Long Islanders were few and far between. Each household pumped water from shallow wells in the upper glacial aquifer. Sewage was returned to the aquifer via individual cesspools and septic tanks. Much of rural eastern long Island is still at this stage.
At the rural stage, impact on the water table is minimal because most water removed is recharged via the septic systems, and the amount of sewage being introduced into the water table is not significant.
Suburban stage: However, with increasing population growth, the number of septic systems grows to overwhelm the capacity of the aquifer sediments to filter and purify the seeping sewage. In western Suffolk, Nassau, Queens, and Kings counties, urbanization has polluted the shallow glacial aquifer with sewage, household detergents, and industrial toxins. In these places the glacial aquifer is no longer usable.
In Nassau and western Suffolk counties, water is pumped from deep public-supply wells that tap the lower confined aquifers. These deeper, confined aquifers have not yet been invaded by the polluted groundwater in the glacial aquifer.
Urban stage: In western Nassau county, large municipal sewage systems have been built to accomadate the septic discharge of high density populations. These sewage systems are good in that the sewage is not put into the aquifer, but is instead treated and pumped into the sea. However, this creates another problem.
Sewer systems rapidly deplete aquifers because so much of the water that is discharged is not recharged, but is instead lost to the sea. This is especially the case with Long Island’s deeper confined aquifers, which recharge very slowly from above.
On Long Island excessive discharge causes infiltration of the water table by sea water. Because freshwater is lighter than saltwater, fresh groundwater forms a barrier to saltwater flow, lying between the land surface and deeper infiltrating saltwater. However, excessive removal of freshwater from the aquifer pulls the saltwater farther inland and closer to the surface, where it can find its way into wells.
Saltwater infiltration is becoming a big problem in western Nassau county where many deep wells near the shore are becoming too salty for use. New wells must be sunk farther inland.
Brooklyn and Queens no longer pump groundwater, having long ago contaminated their aquifers with sewage and saltwater. They get their water from upstate New York. Because of the current absence of exploitation, the aquifers in this area are beginning to recharge and recover.
As Long Islanders, we will have to decide how to manage our water supply in the future. We have been blessed with a large freshwater aquifer, but if we do not use it wisely it will not last.
The main problems are these:
We pump (discharge) more water from the aquifers than they can naturally recharge. If we do this for a long period of time, then the aquifer shrinks and is replaced by salt water, which is unusable.
To solve the above problem we need to return (recharge) the water we use into the aquifer. However, we tend to pollute the water we use, either with sewage or with industrial wastes. When we recharge this water we pollute the aquifers.
We already have in place and extensive system of storm drains and sumps or recharge basins throughout Long Island. These allow water that might otherwise run into the sea to find its way back to the water table. Infiltration through the loose sands and gravels of the surface of Long Island is rapid and any pollutants picked-up by the runoff is carried down toward the drinking water supply. This includes fertilizers, pesticides, road oil, and anything anyone happens to dump down these drains.
Fortunately, studies have shown that if contaminants in the runoff are not excessive, runoff is greatly filtered and purified by the sediments as the runoff infiltrates back into the water table.
Reclaim the glacial aquifer by collecting and treating all sewage and returning treated water to shallow recharge basins.
Problems: It will take time for the glacial aquifer to recover. Also, unless the water is treated to remove dissolved minerals, evaporation loss during flow into recharge basins will cause eventual concentration of dissolved minerals. This is getting very expensive...
Conserve water resources and carefully manage natural recharge
To maintain our natural supply of clean drinking water what we really need to do is carefully manage the aquifer system. Natural recharge is the only way to get clean water back into the aquifer. To allow natural recharge to work, we need to do the following:
Avoid placing industrial development and land uses with high pollution potential (i.e. landfills, gas stations) in areas of high groundwater recharge (in the middle of Long Island). Put these in areas of discharge.
Limit residential use of groundwater contaminants (i.e. pesticides, Chemlawn!)
We need to consider limiting additional development, unless we can find the water to supply the increased use and population.
We may also need to consider switching to sewer systems instead of septic tanks as communities grow.
Presently, Long Island does not have a comprehensive water managment policy. Local governments follow their own managment policy, if they have one at all. In Suffolk county there is an overall managment authority that coordinates local efforts, but in Nassau it’s every town for itself.