Water and Hydrogeology of Watersheds

The following blog post is a summary of material found in Chapter 3, Water and Hydrogeology of Watersheds from the text by Kaufman, M., Rogers, D., and Murry, Kent. 2011. Urban Watersheds; Geology, Contamination, and Sustainable Development. Taylor and Francis Group, LLC. Boca Raton, FL.

Clean water is essential to maintain health of all species on earth. Water and geology help to shape our natural environment. We are learning more and more about how human impacts in the natural and built environmental are impacting our ground and surface waters.

Groundwater in Watersheds

Groundwater is defined as “any water beneath the surface of the ground”. Apart from the bound water in icecaps and glaciers, over 95% of all freshwater sources on Earth come from groundwater. Imagine poring all the earth’s groundwater out onto the US land surfaces, it would spread to a depth of a half-mile. On a global scale, if water covered all land surfaces, the water would be 150’ deep. The total amount of groundwater on Earth is 100 times more than all the visible surface water in lakes, streams, rivers, and swamps.

Much of the groundwater of the US is found in aquifers. An aquifer is considered a “mappable geologic unit” that is created by water-saturated porous media, usually sands and gravels, which have the capacity to store and move large amounts water. Finer grained materials like clay and silt may not transmit water quickly enough to be considered as an aquifer media. Up to 50% of the US population obtains its drinking water from groundwater sources. In addition, agriculture uses groundwater at about 40% for irrigation of crops.

Groundwater is in constant motion under the force of gravity and moves from higher to lower areas of pressure. Movement occurs through a system of passageways of unsaturated pore spaces in soils and sediment. This zone of aeration is known as the vadose zone.

Sourced: http://water.usgs.gov/edu/graphics/wcinfiltrationsoilzone.gif

Pressure is naturally higher under mountains and hills due to mass and valleys are under lower pressure; resulting in a pressure gradient where groundwater flows to the surface and can interact with surface water.

Sourced: http://water.usgs.gov/edu/graphics/wcgwdischarge.jpg

Groundwater and Surface Waters

Sourced: http://water.usgs.gov/edu/watercyclesummary.html

Groundwater and surface waters are interconnected in watersheds. Climate, vegetation, topography, rainfall, and geology all influence patterns of surface water flow and drainage density. 

Ground water provides an important influx of water to surface waters that serves to protect aquatic life through periods of low precipitation and drought. Streams, lakes and wetlands can gain groundwater; conversely surface waters can flow into groundwater. Soils with low conductivity in the vadose zone can impair ground and surface water communication. Additionally, a lowering of the water table due to drought, local geology, or excessive groundwater use can impair connectivity.

Important research must be done on ground and surface water connectivity within urban areas. Toxins and pollutants, like those found in the first flush of stormwater runoff can contaminate groundwater that is close to the surface. Groundwater contamination also has the ability to reach surface waters. Additionally, removal of large groundwater quantities near a surface water body can cause some loss of the surface waters; impairing aquatic functions.

Urban area watershed influences are numerous and can include: runoff from impervious surfaces creates increased erosion and sedimentation, petroleum-based contaminants from vehicles, groundwater contamination from industry, wetland destruction, wastewater discharge, combined sewer overflows (CSOs), aging infrastructure, stream bed alterations, other point and non-point pollution sources, dam construction altering groundwater levels, and natural drainage system alterations.