Groundwater (Part 1) Groundwater as an erosional agent Groundwater as an equalizer of streamflow Distribution of groundwater

Groundwater Groundwater (freshwater beneath Earth’s surface) is the largest liquid freshwater source for humans.

Geologically, groundwater is important as an erosional agent. The dissolving action of groundwater slowly removes soluble rock, allowing depressions known as sinkholes to form as well as creating subterranean caverns.

Groundwater was responsible for creating these sinkholes Groundwater was responsible for creating these sinkholes. This is referred to as “Karst topography.”

Groundwater is also an equalizer of streamflow. Much of the water that flows in rivers is not direct runoff from rain and snowmelt. Rather, a large percentage of precipitation soaks in and then moves slowly underground to stream channels.

Groundwater Groundwater is thus a form of storage that sustains streams during periods when rain does not fall. When we observe water flowing in a river during a dry period, it is water from rain that fell at some earlier time and was stored underground.

Distribution of Groundwater When it rains, some of the water that soaks into the ground does not travel far, because it is held by molecular attraction as a surface film on soil particles. This near-surface zone is called the zone of soil moisture.

Distribution of Groundwater The upper limit of this zone is called the water table. Water that is not held by soil moisture percolates downward until it reaches a zone where all of the open spaces in sediment and rock are completely filled with water. This is called the zone of saturation, and water within this zone is called groundwater. The area above the water table where the soil, sediment, and rock are not saturated is called the unsaturated zone, or zone of aeration. Here the pore spaces in the material are filled mainly with air.

Arrows indicate groundwater movement through uniformly permeable material. The looping curves may be thought of as a compromise between the downward pull of gravity and the tendency of water to move toward areas of reduced pressure.

The hydraulic gradient is determined by measuring the difference in elevation between two points on the water table (h1 – h2) divided by the distance between them, d. Wells are used to determine the height of the water table.

Distribution of Groundwater permeable rock The quantity of groundwater that can be stored depends upon the porosity of the material, which is the percentage of the total volume of rock or sediment that consists of pore spaces (voids or openings in the material.) The permeability of a material is its ability to transmit water through connected pore spaces. impermeable rock

Distribution of Groundwater Aquitards are impermeable layers of material that hinder or prevent water movement. Aquifers are permeable rock strata or sediment that transmit groundwater freely.