Without water there can be no life. Water is our most valuable resource and how we manage its use or allow its abuse may determine the fate of mankind. The earth's total water supply is vast, estimated to be about 333 million cubic miles of water, over 96 % of which is saltwater. Fresh water represents only 4% of the total water of the earth. Over two thirds of the freshwater on earth (68%), is locked up in ice and glaciers, about 30% of freshwater is in the ground as groundwater, and thus, surface-water sources (such as rivers) only represent about 2% of the fresh water and 1/10,000th of 1% of total water, yet rivers are the source of most of the water people use. The interaction between surface water and groundwater is complex, site specific and not fully understood.
According to the US Geological Survey about 26 % of the freshwater used in the United States in 2000 came from ground-water sources; the other 74 % came from surface water. Groundwater is an important natural resource, especially in those parts of the country that don't have ample surface-water sources, such as the arid West. Groundwater is a renewable resource, but not in the way that sun light is. Groundwater recharges at various rates from precipitation and other sources of infiltration. The US Geological Survey estimated that the nation receives about a trillion gallons of recharge to the groundwater aquifers each day. (USGS circular 415). The recharge is not spread evenly across the nation or even where the water is needed.
There are costs and limits to the amount of groundwater available for extraction from the aquifer. Wells need to be drilled, pumps installed and operated and water moved through a delivery system. These represent the direct expenses of groundwater pumping. There are indirect costs. The amount of groundwater removed from an aquifer needs to be sustainable and should ideally match the recharge rate. Water captured by pumping a well will result in changes in the local or regional hydraulic balance- a reduction in discharge to surface water at some other location, an increase in recharge from surface water, or a loss of storage in the aquifer by falling water table or some combination of these effects. Changing the recharge rate by diverting water from the system can change the entire water balance and ecology of a region. Pulling large quantities of groundwater from one well rather than a series of smaller geographically spaced wells will have a much larger impact on the groundwater basin.
Groundwater availability and recharge rates vary locally and regionally and can be impacted by man. Over pumping of groundwater that results in compaction of the soils and subsidence which is permanent loss of water storage capacity in the region. Over pumping of groundwater in costal regions can lower groundwater tables or in a confined artesian system result in salt water intrusion. Development often is characterized by pavement and building that prevents the infiltration of precipitation that occurred before development. In some areas of the country (and world), groundwater currently being pumped entered the aquifer a millennia ago when the climate in that area was wetter. That water is not being replaced under these climate conditions and may ultimately be used up. Centralized wastewater systems further compound the problem by collecting the used groundwater, treating it and releasing the water into a stream or to the ocean in costal areas. Decentralized, managed and density controlled alternative onsite sewage systems may be a better solution for maintaining the groundwater resource, or as is done in areas of Florida and Long Island land application of the treated water from waste water systems.
Our freshwater resources need to be managed as a whole. The utilization of groundwater resources in an unsustainable manner can result in impacts to the entire region, including the decrease in water level and aquifer storage, reductions in stream flow and lake levels, loss of wetland and riparian ecosystems, land subsidence, saltwater intrusion and changes in groundwater quality. Each groundwater system or basin is unique and must be managed individually, and the data necessary to understand and manage water resources must be gathered locally over time to track and respond to changes in groundwater quantity and quality as well as stream flow. All groundwater is not equal and there a consequences of withdrawing water from an aquifer beyond its recharge rate. Water is critical to life and we need to manage this most valuable resource before our cities and western states run out.