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Groundwater Definitions, Uses, and Issues

     Groundwater is the water below the surface of the earth that occupies all or part of the empty spaces or cracks in the soil, sand, or rocks (shown in Fgure 1) [1-3].  The zone where the empty spaces are completely saturated with water is called the saturated zone and above this zone is the vadose zone.  The underground geologic strata through which groundwater can easily move and supply a sufficient amount of water to a well or spring is called the aquifer [4].  There are two different types of aquifers based on physical characteristics: unconfined and confined. A confined aquifer is when the saturated zone is sandwiched between layers of impermeable material and the groundwater is without pressure [4]. If there is no impermeable layer immediately above the saturated zone, it is called an unconfined aquifer. In an unconfined aquifer the top of the saturated zone is the water table[4]. A saturated region that does not yield a sustainable amount of water is called an aquitard. 

Figure 1:  Schematic of Subsurface Geology taken from [3].

     Groundwater is distinguished from surface water but is related through the hydrologic cycle.  Surface water is the water found in large bodies like the oceans or lakes, or the water that flows overland in streams.  Recharge is water that seeps down from the land surface and adds to the groundwater [1].  Recharge can come from natural sources such as rain water and snowmelt or from water that leaks through the bottom of lakes and rivers. Recharge can also be human induced by water that leaks from water-supply systems (pipelines) and when crops are irrigated with more water than the plants use [3].

     Groundwater is considered a valuable resource and has several uses.  It supplies many wells and springs, which allows it to be extracted and serve several purposes.  One of its most important uses is for drinking water, where it is used both domestically by individual users and publically by local, city, and county water departments [5].  Water departments also extract groundwater for community uses, such as firefighting and recreational activities [1, 5].  Groundwater is used in industrial activities, such as mining, as well as in agriculture for irrigating and livestock purposes [1].  

     The USGS [5] reported that of all the water used in the United States in 2000 (about 408 billion gallons per day (Bgal/d) of fresh and saline water), about 21 percent (69.8 Bgal/d) came from groundwater sources (Figure 2). The other 79 percent came from surface water sources.   Almost 99% of groundwater came from freshwater aquifers. The 1% of saline groundwater was mostly used in industrial purposes for mining. For the fresh groundwater withdrawals, 68%, were for irrigation (Sumantra's Assignment), while another 19% was used for public systems, mainly to supply drinking water [5].

Figure 2.  The percentage of all the water used in the United States in 2000 taken from the USGS [5].

     Because of its numerous uses, groundwater has become a resource with several issues.  Issues with groundwater have been related to population increase and climate change [6] (Chip's-Assignment) and impact humans and as well as the environment.  Two of the main issues are over draft and pollution.  Many problems are associated with over draft.  Intense groundwater extraction has led to decreases in stream flow and lake levels, which influences surface water availability [7].  Intense groundwater extraction has also caused the subsurface geology to alter and the water table to become lowered [7].  A lowered water table can cause several problems: the reduction or elimination of vegetation, land subsidence, and seawater intrusion [7].  Pollution is another major issue for groundwater.  Polluted groundwater compromises its usage for the human activities listed above and impacts ecosystems.  It can harm animal and plant life and negatively influence biodiversity (Jeremy's Assignment).  Groundwater pollution is described in more detail in the following sections.  

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