Assessing Quantity & Quality

November 15, 2012

U.S. groundwater faces increasing contamination & overuse

usgs_aquifer map
Principal U.S. aquifers

Groundwater provides a valuable source of drinking water for millions of people across the U.S. But as new contaminants are discovered and droughts continue to worsen, resulting in depleted aquifers, it is increasingly important to monitor the quantity and quality of available groundwater. The U.S. Geological Survey (USGS) recently updated Water Quality Products on the status of groundwater quality in the U.S.

What is the role of the USGS in surveying and monitoring groundwater quality and availability in the U.S.?

The USGS has a long history of understanding groundwater quantity and quality in aquifers in the U.S. and around the world. In fact, the first national assessment of groundwater was published in the early 1900s. Currently, the USGS is determining water availability in 30 to 40 major aquifers as part of a national water census. These aquifer assessments provide objective scientific information about the effects of human activities on water levels, groundwater storage and discharge to surface water bodies, and explore how climate variability could affect groundwater availability.

In parallel, the USGS is assessing groundwater quality in the 22 principal aquifers that provide 90% of groundwater pumped for public supply and one-half of the water pumped by private domestic wells in the U.S. USGS assessments of groundwater quality complement the extensive monitoring of public water systems that is routinely conducted for regulatory and compliance purposes by federal, state and local drinking water programs, and the findings assist water utility managers and regulators in making decisions about future monitoring needs and drinking water issues.

Groundwater management decisions are made at a local level, but most aquifer systems cross political boundaries. These larger-scale USGS aquifer assessments provide the consistent and integrated information decision-makers need.

What are the primary contaminants of concern in U.S. groundwater?

In a series of aquifer studies, one of every five wells sampled contained at least one contaminant at a level of potential concern for human health. Overall, arsenic and manganese, two trace elements that are in the geologic materials that make up many of the aquifers, exceeded their human health benchmarks most frequently — each in about 7% of the wells the USGS sampled. Nitrate exceeded its human health benchmark in about 4% of wells, mostly in areas with intensively farmed land, such as in parts of the Midwest Corn Belt and the Central Valley of California.

Manmade contaminants, like herbicides, insecticides, solvents, disinfection byproducts and gasoline chemicals, each were measured at a concentration above a benchmark in less than 1% of wells, and most of these higher concentrations were concentrated in a few aquifers. However, these compounds were widely detected at low concentrations.  

Are groundwater contaminants usually naturally occurring or manmade?

Contaminants — trace elements like arsenic and manganese — from natural geologic sources occurred at concentrations of concern more frequently than those from manmade sources. In many instances, irrigation and pumping have changed the groundwater hydrology and chemistry and caused these trace elements to be released from the aquifer materials into the groundwater.  

High concentrations of contaminants from manmade sources occur in places where chemicals are used more intensively, such as agricultural and urban areas. In addition to contamination from the use of fertilizers and pesticides, groundwater contamination by manmade chemicals can be caused by improper disposal of hazardous substances, chemical spills and poorly maintained septic systems.

What emerging or newly discovered contaminants have raised concern in recent years?

In 2002, the USGS discovered that pharmaceuticals and personal care products were commonly found in U.S. rivers. Since then, USGS studies have focused on characterizing sources of these contaminants and potential environmental health impacts. For example, the USGS found that manufacturing facilities can be a significant source of pharmaceuticals to the environment.  

USGS field and laboratory experiments have documented that exposure of natural subsurface bacteria to antibiotics can change their bacterial ecology and affect ecosystem functions, such as denitrification, the process that removes nitrates from water. Other USGS efforts are focusing on identifying endocrine disrupters, which mimic natural hormones and can be biologically active at extremely low levels. Additionally, waterborne viruses recently have been documented in municipal wells.

Why is water testing important — especially for those using private drinking water wells?

Public water systems are required to treat drinking water to federal quality standards; however, it is up to private well owners to make sure their water is safe. There is no way to tell just by looking at groundwater whether or not it contains chemicals at levels that might pose a risk for human health.

In some principal aquifers, the average number of contaminated samples was especially high. For example, nitrate exceeded its human health benchmark in 60% of the private drinking water wells sampled in agricultural areas of the Piedmont and Blue Ridge carbonate rock aquifers.

What is the status of groundwater availability in the U.S.?

The volume of groundwater in storage is decreasing in many areas of the U.S. in response to high-volume pumping. Declines cause increased pumping costs, deterioration of water quality, reduction of water in streams and lakes, and land subsidence. Groundwater depletion has been a concern in the Southwest and High Plains for many years, but increased demands on our groundwater resources have overstressed aquifers in many areas. In addition, groundwater depletion occurs at scales ranging from a single well to aquifer systems underlying several states.

In Nassau and Suffolk counties in Long Island, N.Y., the pumping of water for domestic supply has lowered the water table in the Atlantic Coastal Plain, which has eliminated baseflow in streams and caused saline groundwater to encroach inland.

In the Gulf Coastal Plain, groundwater pumping in Baton Rouge, La., increased more than tenfold between the 1930s and 1970, resulting in groundwater level declines of approximately 200 ft. In the Houston area, extensive groundwater pumping to support economic and population growth has caused water-level declines of about 400 ft, resulting in extensive land surface subsidence of up to 10 ft.

The High Plains aquifer, which includes the Ogallala aquifer, underlies parts of eight states and has been intensively developed for irrigation. Since predevelopment, water levels have declined more than 100 ft.

In the Chicago-Milwaukee area, where groundwater has been used since at least 1864, long-term pumping has lowered groundwater levels by as much as 900 ft.

How will groundwater availability affect the millions who use groundwater sources for drinking water?

Groundwater is one of the nation's most valuable natural resources. It supplies about half of the population with drinking water; provides about 40% of irrigation water; sustains stream flow between precipitation events and during protracted dry periods; and helps maintain a variety of aquatic ecosystems that are dependent on groundwater discharge to streams, lakes and wetlands. The USGS is striving to understand the effects of competition for groundwater from human and environmental use. This understanding is needed to respond to basic questions about current availability and long-term sustainability.

Even when water is plentiful, it is not truly available unless the quality is acceptable for the intended use. Both water quantity and quality are essential to maintaining water supply for municipal, domestic, agricultural and recreational use and for aquatic ecosystems. Where water quality is poor, there are options to reduce or remove harmful constituents, but they come with costs.

What can people do to help conserve groundwater resources?

Groundwater conservation involves both conserving water and preventing contamination. Excessive pumping can cause the water table to be lowered, and if groundwater levels decline too far, the individual well will have to be deepened. As the depth to water increases, the water must be lifted higher to reach the land surface, which requires energy and can become prohibitively expensive.

To conserve groundwater, individuals can identify the parts of their routines where the most water is used and then identify the areas where water use can be limited.

To protect water from contamination, well owners should position wellheads a safe distance from potential contamination, update septic system inspection and cleaning, conduct annual well inspections, properly dispose of hazardous material and decommission any abandoned wells using a professional.

Herb Buxton, Kevin Dennehy and Bill Wilber are scientists for the U.S. Geological Survey (USGS). For more information, contact Kara Capelli, public affairs specialist for USGS, at kcapelli@usgs.gov or 703.648.5086.

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