Groundwater - Current Groundwater Use
water aquifer edwards pumping
The nation's use of groundwater grew dramatically in the last several decades of the twentieth century. According to a report from the USGS (Estimated Use of Water in the United States in 2000), the United States withdrew about 34 billion gallons per day (gpd) in 1950, reached 84 billion gpd in 1980, and dropped off a bit over the next several years before reaching a new high of 84.5 billion gpd in 2000 (the latest data available). Figure 4.6 shows the percentage of groundwater allocated to various uses in the United States.
About half of the U.S. population depends on groundwater as its primary source of drinking water. The percentage is much higher in rural areas, where about 95% of the population depends on groundwater.
Plentiful groundwater is critical to the economy of the United States. According to an October 1999 USGS report (Safe Drinking Water Act, Section 1429 Ground Water Report to Congress), nationally, more than 40% of all water used for crop irrigation and livestock watering and more than 20% of water used by industries comes from a groundwater source. Seventeen states obtain more than 25% of their overall water supply from groundwater. Another seven states depend on groundwater for more than 50% of their total water supply. The thirteen states with the greatest groundwater withdrawals account for 69% of all groundwater used nationally. Of these states, California, Texas, and Nebraska are the three largest users. Groundwater withdrawal is generally the highest in the western United States, where the water is used to sustain important agricultural activities and to support a growing population.
Historically, groundwater and surface water have been managed as separate resources. Since the 1970s there has been a growing awareness that these two sources are inseparably linked. Groundwater seeps into rivers, streams, lakes, and other water bodies, and breaks the surface as springs. In some parts of the United States, especially arid regions, aquifers contribute a large portion of the water found in rivers and streams. Figure 4.7 shows groundwater contributions to surface water.
This source of surface water recharge is particularly important during dry periods. Reductions in surface water can have adverse effects on the ecology of a watershed, stressing fish populations and their food supply, wetlands, and the plants and animals living along the banks of rivers and streams. Groundwater depletion in some areas has obliterated the aquatic and semiaquatic life that depended on groundwater flow in surface water streams.
Pumping groundwater from a well always causes a decline in groundwater levels at and near the well, and it always causes a diversion to the pumping well of groundwater that was moving slowly to its natural, possibly distant, area of discharge. Pumping a single well typically has a local effect on the groundwater flow system. Pumping many wells (sometimes hundreds or thousands of wells) in large areas can have regionally significant effects on groundwater systems.
If a groundwater system is not overused, the rate of groundwater recharge and discharge balances out; however, in the United States the rate has become uneven. According to a 2000 USGS report (the latest data available), every day in 2000, 84.5 billion gallons of groundwater were withdrawn.
Groundwater is pumped for a multitude of uses, including public (swimming pools, fire fighting, and street washing), domestic (drinking water and other household uses), agricultural (irrigation and livestock), industrial, mining, and other uses. When these withdrawals exceed the rate at which the groundwater source is recharged, they result in the lowering of groundwater to levels that may impair the resource.
Overpumping groundwater can have many different effects, including:
- Neighboring wells can dry up, requiring construction of new, deeper wells or significant changes to existing wells.
- Aquifer materials can compact, causing the land above the aquifer to sink and leaving gaping holes in the land that cause damage to buildings, roads, canals, pipelines, and other infrastructure.
- Permanent loss of aquifer capacity because of compaction of aquifer materials, resulting in higher pumping costs and a decrease in well yields.
- Changes in the volume and direction of groundwater flow can induce flow of salty water and water of lower quality into a well.
- Lower stream baseflows can dry up wetlands and cause adverse effects on ecological systems that are dependent on groundwater discharge.
When Use Exceeds Replenishment
In 1940 C. V. Theis warned in "The Source of Water from Wells" (Civil Engineering, vol. 10, no. 5, 1940) that pumping from wells profoundly affects aquifers. According to the USGS, large withdrawals of groundwater have altered the flow systems and geological and chemical conditions of some of the major aquifers in the United States. Declining groundwater levels can change the location and size of recharge areas and reduce discharge rates. Some aquifers in the West have suffered major losses in aquifer storage because of overpumping.
For some Americans, droughts in the 1990s were a "wake-up call." In Texas, the 1996 drought resulted in federal disaster relief for farmers in 95% of the state's 254 counties and cost the Texas economy an estimated $5 billion. The disaster revived controversy surrounding water rights, including rule of capture, which permits a property owner to pump any amount of water from an aquifer beneath his or her land. Dispute over the control of groundwater prompted innumerable lawsuits, especially regarding the gradually depleted Edwards Aquifer (located in the San Antonio area).
Drought in Texas and other states continued to be a concern in 1998. An August 5, 1998, Federal Emergency Management Administration (FEMA) press release described the strain put on underground water supplies in Texas during the week beginning August 3, 1998. Water flows from the Edwards Aquifer typically amount to 125,000 gallons per minute, but during that week flow was reduced to 80,000 gallons.
EDWARDS AQUIFER: CITIES AND RURAL AREAS IN CONFLICT.
The 160-mile-long Edwards Aquifer, one of the nation's most productive, is located in south-central Texas. Conflict over its use is an example of the growing competition for water. This aquifer has supplied water to farmers since the mid-twentieth century. As the only source of water for San Antonio, the nation's eighth most populous city as of 2004, it has been designated a sole source aquifer by the EPA. Rain and runoff filter through the Edwards Aquifer before emerging in springs. However, increased pumping for irrigation, additional water use for the rapidly growing population of San Antonio, and periodic droughts are steadily depleting the vast underground reservoir. The amount of water taken from this vital aquifer doubled over two decades. The once plentiful and seemingly infinite supply of water that was previously taken for granted is now recognized as an endangered resource.
The Edwards Aquifer has been the subject of bitter dispute between farmers and San Antonio residents as to who has rights to the water. The agricultural population is dependent on the Edwards Aquifer for the production of crops and, therefore, the farmers' livelihood. Officials in San Antonio, however, contend that the water needs of the city must be protected for present and future citizens. The rural and urban interests represent two factions involved in a modern-day version of a classic Western battle over water rights.
In 1993 Texas established the Edwards Aquifer Authority (EAA) to limit water withdrawal from this aquifer. Prior to the establishment of the EAA, the governing water-use principle was that "if it's under your land, it's your water." Under the EAA, every Edwards Aquifer user, except for domestic and livestock owners using fewer than twenty-five gallons per day, had to have a permit with a specified annual withdrawal limit. The intent was to ensure fair distribution of water among all users.
In 1998 the Texas legislature adopted the Texas Critical Management Plan (CMP). The CMP allowed the EAA to further reduce withdrawals, despite the levels specified in permits, during low rainfall or critical periods by imposing restrictions on water-use activities and monthly limits on total water use. The purpose was to protect the aquifer's integrity and to conserve water. In addition, the CMP took water management to another level, requiring all Texas water suppliers and users to develop fifty-year water management plans that address both surface and water supplies. These plans were the first step in statewide water source management and conservation.
According to the EAA Web site (www.edwardsaquifer.net), conflict over pumping water from the Edwards Aquifer continues. According to the EAA, the 1993 law that created the Authority contains conflicting provisions. The law mandates that the Authority issue minimum annual pumping rights to those who could prove that they had been using the water during the previous twenty-one years, but it also set a pumping cap of 450,000 acre-feet per year. In February 2003 the Authority had already issued pumping permits for about 482,000 acre-feet, and was considering the granting of another 82,000 acre-feet in annual pumping rights for 184 pending permit applications. In March 2003 the Authority voted to support newly proposed legislation that would raise the pumping cap to 550,000 acre-feet of water per year.
Preservation of the integrity of the Edwards Aquifer and conservation of its water also have implications for endangered species. According to the U.S. Fish & Wild-life Service (USFWS), the Edwards Aquifer is a critical habitat for nine endangered species (three beetles, one harvestman, and five spiders). A critical habitat is defined by the Endangered Species Act as a specific area deemed essential for the conservation of a threatened or endangered species, which may require special management consideration or protection. An April 8, 2003, USFWS press release reported that these endangered species are underground karst-dwelling invertebrates that are found only in karst limestone areas of Bexar County, Texas. According to Dale Hall, director of the Southwest Region of the USFWS, "These species are particularly vulnerable to groundwater pollution and are an indicator species for the health of the Edwards Aquifer, which is the source of drinking water for more than one million people. Designating habitat that is critical to their survival is another step toward ensuring their future and the health of the aquifer itself."
In May 2005 voters in San Antonio approved the second of two tax increases intended to provide funds toward the purchase of lands that contribute to the recharge of the Edwards Aquifer. The goal of the tax propositions is to raise $135 million, at which point the taxes will expire—approximately nine years from their adoption ("Voters Reauthorize Sales Tax for Aquifer Protection Land Purchases," http://www.edwardsaquifer.net/news.html, May2005).