As the word implies, groundwater is water held in saturated zones below the land surface. For many, this brings to mind rapidly coursing waters running through cavernous areas underground. In a relatively few cases, that image is correct. But for the most part, a sponge would make a better analogy than an underground river.

This underground water is often the forgotten element in the hydrologic system -- nature's great water recycling system. Water leaves the atmosphere as precipitation, producing stream flow in rivers and streams and also infiltrating or "recharging" through the topsoil and land surface to become groundwater. Evaporation returns water to the atmosphere to complete the never-ending cycle. 

Though our language makes a clear distinction between surface water and groundwater, the line between the two is not always a bright one.  In the words of writer Joe Gelt, 

The classifications seem clear enough, surface water occurring above ground, ground-water found below ground. A belief in surface water here and ground-water there simplifies the making of laws and policies. Groundwater and surface water can be regulated separately....

Nature, however, was not designed for easy, simple regulation. Groundwater and surface water are not isolated phenomena occurring apart and distinct
from each other. In nature, groundwater and surface water can intermix or interconnect. ¹

"Ground water, rivers, lakes, and wetlands," explains U.S. Geological Survey consultant Judy Campbell Bird, "are interconnecting parts of a single system." ² In some locations and at some times, surface water in a lake or a river slowly seeps into groundwater reserves. The water flowing through a river, for example, may flow not only through the stream bed but also into the "alluvial" sediments below the bed and along the riverbank. 

That same water may then reemerge from beneath the ground farther downstream. Water can move back and forth several times between groundwater and surface water in one short stretch of a floodplain, or groundwater can travel 10 miles or more before discharging back to surface water.

The Groundwater/Surface Water Connection

• A 1991 USGS study found the herbicide atrazine in the Cedar River in Iowa, even during periods when there was little surface runoff. The study concluded that the pesticide came from contaminated groundwater. ³ 
• In Virginia, the drilling of a gas production well had an unexpected result. Grout pumped into the well traveled through the groundwater to a spring feeding a fish hatchery. Thousands of trout were lost; thousands more had to be transported to a clean water source. ⁴ 
• Researchers studying Montana's Flathead River have discovered that groundwater discharging into the river carries nutrients important for aquatic life and moderates surface water temperatures.  The data indicates that the upwelling of groundwater may be particularly important for successful spawning and growth of young fish. ⁵ 
• "Ground-water contribution to the Chesapeake Bay," notes a former Deputy Director of EPA's Chesapeake Bay Program, "is seemingly invisible but may be the equivalent to that of a major tributary, such as the James River in Virginia,"  -- in this case carrying excessive nutrients into the Bay. ⁶