Groundwater constitutes two thirds of the world’s freshwater resources (Chapman, 1996). According to NGWA (2014), 99% of the usable water comes from groundwater. Groundwater has been extracted for multiple uses however agricultural use accounts for 60% of the total extraction globally (ngwa, 2014). India, China, United States, Pakistan and Iran are the top five extractors of the world (ngwa, 2014).
Groundwater can be present in fractures of the hard rock or aquifers. Rock fractures may be caused by folds, faults, moves and expansion and water may run off or infiltrate into the fractures (California Department of Water Resource, n.d.). According to California Department of Water Resources (n.d.), the volume of water present in the fractures ranging from few millimetres to centimetres can be two percent of the rock volume however the depth, location, and interconnection of the fractures affect the water volume.
Aquifers can be confined and unconfined. Confined aquifer is groundwater between impermeable layers of rock with less movement of water and replenished by rain or stream water some distance away from the aquifer (Tas.gov, 2014). In contrast, unconfined aquifer has direct contact with the atmosphere through the porous soil or rock layer and replenished directly by rain or stream water from the porous layer (Tas.gov, 2014).
Gravity causes groundwater to move from the areas of recharge to areas of discharge however porosity, permeability and slope of the underground formation influence groundwater movement (Alberta Environment, 2009). Porosity has been defined as the percentage of the openings or pores in a given volume of the rock which contain water (Chapman, 1996). Highly porous rock surface contains more volume of water however only a portion of water can be extracted. The ratio of the volume of water that can be extracted to the total saturated volume has been defined as specific yield of the material (Chapman, 1996).
Alberta Environment (2009) Groundwater Basics [Online] Available at http://www.environment.gov.ab.ca [Accessed on 12 June 2014].
California Department of Water Resources (n.d.) Ground Water in Fractures Hard Rock[Online] Available at http://www.water.ca.gov [Accessed on 12 June 2014].
Chapman, D. (1996) Water Quality Assessment – A Guide to Use of Biota, Sediments and Water in Environmental Monitoring – Second Edition [Online] Available at http://www.who.int [Accessed 15 June 2014].
National Groundwater Association (NGWA)(2014)Groundwater Fact Sheets [Online] Available at http://www.ngwa.org [Accessed on 15 June 2014].
Tasmanian Government (2014) Confined and Unconfined Aquifers [Online] Available at www.tas.gov [Accessed on 5 June 2014].