Groundwater- the Basics

Groundwater is the subsurface water that fills the spaces, pores or cracks in soils and rocks.   Aquifers are the name given to any body of rock or sand that contains a usable supply of water. The upper surface of this water-filled area, or "zone of saturation", is called the water table. The saturated area beneath the water table is called an aquifer, and aquifers are huge storehouses of water for mankind and the planet itself. A good aquifer must be both porous enough to hold water and permeable enough to allow the water to flow and the continuous recharge of water to a well.

Most of the void spaces in the rocks below the water table are filled with water. Depending on type, soils and rocks have different porosity and permeability characteristics, which means that water does not move around the same way in all rocks below ground. Combined together geology and rainfall determine the character of and the quantity of the groundwater.

Groundwater is replenished by the seepage of precipitation that falls on the land and infiltrates into the water table and the aquifer, and sometimes by surface water. Mankind can artificially deplete or replenished groundwater. There are many geologic, meteorologic, topographic, and human factors that determine the extent and rate to which aquifers are refilled with water or used up.

As the US Geological Survey points out: “Nearly all surface-water features (streams, lakes, reservoirs, wetlands, and estuaries) interact with ground water. These interactions take many forms. In many situations, surface-water bodies gain water and solutes from ground-water systems and in others the surface-water body is a source of ground-water recharge and causes changes in ground-water quality. As a result, withdrawal of water from streams can deplete ground water or conversely, pumpage of ground water can deplete water in streams, lakes, or wetlands.”

Though the type of rock will determine the water capacity of the aquifer. There is variability in at what depth rocks are found. A relationship does not necessarily exist between the water-bearing capacity of rocks and the depth at which they are found, it varies tremendously by region and continent. A very dense granite that will yield little or no water to a well may be exposed at the land surface. Conversely, a porous sandstone may lie hundreds or thousands of feet below the land surface and may yield hundreds of gallons per minute of water. On the average, however, the porosity and permeability of rocks decrease as their depth below land surface increases because the weight of the overlying rocks compresses pores and cracks in rocks at great depths.

Geologic conditions also control the distribution of what are called structural belts of the earth’s surface that were formed with the mountains. These belts influence groundwater flow, recharge and discharge. Both geomorphology and geology determine the volumes of surface runoff and amounts and rates of infiltration. Depending on geologic conditions, ground water can be directly connected to surface water or not connected with surface water. The connection with surface water affects the ability of an aquifer to be recharged. Groundwater that has lost it’s connection to surface water

Like water on the earth’s surface, groundwater tends to flow downhill under the influence of gravity and eventually discharges, or flows out of the ground, into streams or other surface water-dependent areas, such as wetlands in the geology of New England and the mid-Atlantic states.

Ground water flow and storage, often viewed as static reservoirs, are dynamic and continually changing in response to human and climatic stress [Alley et al., 2002; Gleeson et al., 2010]. Increase or decrease in precipitation patterns impacts available surface and groundwater. Man’s hand in changing the surface also impacts water resources. Land use changes that increase impervious cover more than 5-10% from roads, pavement and buildings does two things. It reduces the open area for rain and snow to seep into the ground and percolate into the groundwater and the impervious surfaces cause stormwater velocity to increase preventing water from having enough time to percolate into the earth, increasing storm flooding and preventing recharge of groundwater from occurring. 

Slowly, this can reduce water supply over time. Increasing population density increases water use. Significant increases in groundwater use and reduction in aquifer recharge can result in the slowly falling water levels over time showing that the water is being used up. Unless there is an earthquake or other geological event groundwater changes are not abrupt and problems with water supply tend to happen very slowly as demand increases with construction and recharge is impacted by adding paved roads, driveways, houses and other impervious surfaces. 

The changing land use impacts regional hydrology and groundwater recharge so the quantity of available groundwater and streamflow may decrease with the same amount of precipitation. Groundwater serves as a savings account for rivers and streams. Sustainability of groundwater is hyper-local. Little is known about the sustainability of our groundwater basins, but that is changing. Groundwater models and data from more monitoring wells can help develop a picture of the volume of the water within the groundwater basin and at what rate it is being used and at what rate it is being recharged. This can help manage water resources during drought years and wet years.