What is Groundwater?
First and foremost it's water. It's water that is out of sight and below land surface. Because groundwater is unseen it is commonly misunderstood. Many people envision groundwater as flowing in underground rivers. Only in rare cases, places where caverns exist, does groundwater move in that fashion.
Normally, groundwater moves slowly through small openings in or between sand grains of subsurface materials. Materials that provide storage for groundwater are called aquifers.
Groundwater moves through fractured rock, sand or limestone aquifers much like soda does through a cup of crushed ice. The drinking straw is analogous to a well. Groundwater always moves away from high pressure toward low pressure -- downgradient. Gradients exist naturally because groundwater enters the water cycle at higher elevations and eventually flows towards the ocean. Natural gradients can be altered by withdrawing groundwater from an aquifer using wells.
We see the source of groundwater as it rains. A small portion of the rain doesn't evaporate or feed the plants and trees (transpiration), but infiltrates into the soil and deeper geologic formations (water budget). It's that water, moving at modest fractions of a foot per day, that discharges to surface water bodies and is withdrawn by wells throughout North Carolina. Usually there is plenty of groundwater to go around. Wells are widely separated and withdrawals from one well don't influence neighboring ones.
It is common in the coastal plain to withdraw groundwater from confined aquifers. Clay-rich sediments above and below the aquifer restrict vertical water flow. Water levels in wells tapping a confined aquifer rise to a level equivalent to the water pressure in the aquifer (potentiometric surface).
Sometimes this level is above land surface and the well is called a flowing well. The water is pressured by the elevation of the recharge area for that aquifer. The recharge area, or the region where water enters the aquifer, is at a higher elevation than the land surface elevation at the flowing well.
The confining layers also allow groundwater to move greater distances in response to pumping wells.
Groundwater in the coastal plain can be pumped at high rates for city and county supplies. Of course, this means that pumping wells more easily interfere with one another (creating a larger cone of depression).
In the area surrounding Kinston, North Carolina flowing wells were common in the 1920s. Between then and now, water levels in wells have fallen more than 150 feet in response to the aggregate pumping from public water systems, agriculture and industry. This situation concerns State officials who fear that aquifers could be damaged if withdrawal growth rates are left unchecked and water levels drop below the top of the aquifer. Also, water level drawdowns put pressure on communities with increasing populations to find alternate sources of water. That means use of surface water or a shallower aquifer which is more easily recharged.
Another concern for this region of the State comes from the potential for salt water intrusion either laterally from the ocean or upwards from deeper salty aquifers.
Salt water intrusion can occur if an aquifer is stressed by pumping too close to the fresh water-salt water interface.
Once salt water has invaded a portion of an aquifer more costly treatment is required to use the groundwater for potable supply.
Although the source is unseen, about 27% of North Carolina water use is from groundwater. In the coastal plain, that percentage is much higher, about 90%.
It is important for everyone to understand the source of groundwater and how it moves through the subsurface because it is a critical part of our daily lives.