Monday, August 22, 2011

Depleting our Groundwater Supplies

Whenever you pump water from a well it has to be balanced by a loss of water from storage in the groundwater aquifer. Groundwater is recharged from rain and sources of surface infiltration. The response of a groundwater aquifer to pumping depends on whether the aquifer is confined or unconfined, how much water is pumped and the geology of the area. If too much water is pumped, water tables can drop in unconfined aquifers, water pressure fall in confined aquifers, surface water and ecology could be impacted and in some locations with fine grained soils compaction and subsidence can take place. The U.S. Geological Survey’s (USGS) Groundwater Resources Program is conducting large-scale multiyear regional studies of groundwater availability in the United States. The USGS has found that the volume of groundwater stored in the earth is decreasing in many regions of the United States. The water level is falling in many areas and if this continues we could deplete our groundwater. The extent of groundwater level declines across the United States has not been monitored before now. The most recent effort to summarize the declines in artesian water levels or water tables was in 1983. Since that time our demands on the groundwater have increased and our understanding of groundwater has improved. It is now very clear we are running a groundwater deficit.

Although humans have been digging wells and tunnels for water supply for thousands of years, extensive use of ground water is relatively recent, and coincides with more effective drilling and pumping technologies during the past 75 years. The USGS is trying to determine how much ground water we have, how fast groundwater supplies are running out and where climate and human development might combine to create critical problem areas. Large-scale development and exploitation of ground-water resources and the accompanying declines in ground-water levels and other effects of pumping has led to concerns about the future availability of ground water to meet domestic, agricultural, industrial, and environmental needs. Water availability and how we manage water resources will determine the future of our nation.

During the past century, several ground-water assessments have been completed by the USGS. These national and regional evaluations have increased our knowledge about roundwater resources and groundwater in general. Our understanding of groundwater and its how it is connected to surface-water systems has expanded and new methods and technologies for resource assessment have been developed and with it the issues of concern have changed. Environmental decision making has grown more complex with increased knowledge about groundwater and its role in estuaries. Water is necessary for human use and environmental protection and preservation.

The USGS has been using long-term groundwater monitoring data, combined with groundwater models, to improve our understanding of the storage and flow of groundwater. This task is quite difficult, groundwater is not easily observed and not all the water pumped is consumed. When water is pumped from the ground and used, the water molecules are not destroyed; the water is simply moved to different places. Consumed water is assumed to be evaporated, transpired, incorporated into products or crops, consumed by humans or livestock, or otherwise removed from the immediate water environment. The rest of the water goes back into the environment, such as sewage disposal into streams, septic leaching fields back into the ground and additional recharge from excess irrigation. Even the water consumed, however, is not really lost; it goes into the atmosphere or into products or living tissue. When analyzing the amount of ground-water available, it is important to consider consumptive use and return flow as well as withdrawals.

The growing population and the effects of recent droughts have made the need for an updated status on the availability of the groundwater necessary. For over 60 years the USGS has worked with state and local agencies to compile estimates of groundwater and surface-water withdrawals for the Nation at 5-year intervals. Some water-use data, such as public supply for household uses and withdrawals by some industrial users, are obtained by direct measurement. Other permitted uses are estimated as the amount reported or allowed by permit. Many uses, such as private drinking water wells, irrigation, and some industries, are estimated. This data has been used to see how groundwater demand has changed over time. This information has been combined with water level measurement and monitoring to develop computer models and tools to forecast groundwater aquifer response to human and environmental stressors like groundwater pumping, diversion of surface water, irrigation and droughts.

The USGS has compiled all this data to try and get an idea of what our water resources are and what our demands for groundwater are. Groundwater provides half our drinking water and is essential to the vitality of agriculture and industry, as well as to the health of rivers, wetlands, and estuaries throughout the country. We need to have a sustainable water budget for the nation’s groundwater aquifer systems; however, sustainable budgets do not appear to be our nation’s strong point. This is compounded by the fact that ground-water management decisions in the United States are made at a local level. Many aquifer systems cross these political boundaries, making appropriate management extremely difficult even within our own nation.

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