Candlewood Knolls Water Authority

Water

Understanding Groundwater.

As water seeps through permeable ground, it continues downward until it reaches a depth where water has filled all the porous areas in the soil or rock. This is known as the saturated zone. The top of the saturated zone is called the water table. The water table can rise or fall according to the season of the year and the amount of precipitation that occurs. The water table is typically higher in early spring and lower in late summer. The porous area between the land surface and the water table is known as the unsaturated zone.

AQUIFERS

Water-bearing rock, sand, gravel, or soil that is capable of yielding usable amounts of groundwater is called an aquifer. The water yield from an aquifer depends greatly on the materials that make it up. Mixtures of clay, sand, and fine particles yield small amounts of water because the spaces between the particles don't allow water absorption and flow. Materials sorted into distinct layers will yield high amounts of water from coarse-grained materials like large sand grains and gravel, but low amounts from fine-grained sand, silt, or clay. Bedrock aquifers will yield substantial amounts of water if there are large openings or cracks in the rock. The capacity of soil or rock to hold water is called its porosity; the capacity for water to move through the aquifer is called permeability.

There are two types of aquifers: confined, or artesian aquifers, and unconfined, or water table aquifers. Artesian aquifers contain groundwater that is trapped under impermeable soil or rock and may be under pressure. Artesian wells are wells that pierce artesian aquifers. The water in these wells usually rises toward the surface under its own pressure. If the water level in the well is higher than the land surface, it may be a flowing artesian well. A well in an unconfined aquifer has the same water level as the water table around it.

GROUNDWATER RECHARGE

Water that seeps into an aquifer is known as recharge. Recharge comes from a variety of sources, including seepage from rain and snow melt, streams, and groundwater flow from other areas. Recharge occurs where permeable soil allows water to seep into the ground. Areas in which this occurs are called recharge areas. They may be small or quite large. A small recharge area may supply all the water to a large aquifer. Streams that recharge groundwater are called losing streams because they lose water to the surrounding soil or rock.

GROUNDWATER DISCHARGE

Groundwater can leave the ground at discharge points. Discharge happens continuously as long as enough water is present above the discharge point. Discharge points include springs, stream and lake beds, wells, ocean shorelines, and wetlands. Streams that receive groundwater are called gaining streams because they gain water from the surrounding soil or rock. In times of drought, most of the surface water flow can come from groundwater. Plants can also contribute to groundwater discharge, because if the water table is close enough to the ground, groundwater can be discharged by plants through transpiration.

GROUNDWATER MOVEMENT

Groundwater usually moves slowly from recharge areas to discharge points. Flow rates within most aquifers can be measured in feet per day, though in karst bedrock the rate of flow can be measured in miles per hour. Flow rates are faster when cracks in rocks or very loose soil allow water to move freely. However, in dense soil, groundwater may move very slowly or not at all.

Groundwater typically moves in parallel paths, or layers. Since groundwater movement is slow, it doesn't create enough turbulence to cause mixing the way surface waters mix when a river or stream empties into another waterbody. That is, layers of groundwater remain relatively intact. This can be an important factor in locating and determining the movements of contaminants that might enter the groundwater supply. But eventually contaminants will disperse through part or all of an aquifer.

Wells affect groundwater flow by taking water out of an aquifer and lowering the nearby water table. Removed water is recharged from the water table, and the lowered water table caused by the well is called a cone of depression. The cone of depression from a well may extend to nearby lakes and streams, causing the stream to lose water to the aquifer. This is known as induced recharge. Streams and wetlands have been completely dried up by induced recharge from well pumping.

Fact sheet courtesy of the United States Environmental Protection Agency.