Volatile organic compounds (VOCs) are chemicals that both vaporize into air and dissolve in water. VOCs are pervasive in daily life, because they’re used in industry, agriculture, transportation, and day-to-day activities around the home. Once released into groundwater, many VOCs are persistent and can migrate to drinking-water supply wells.
Have you ever pumped gas, had your clothes dry cleaned, or used chlorine bleach in your laundry or for disinfection? Then you’re likely to have encountered VOCs. Thousands of VOCs have been manufactured for use—many of these chemicals are toxic and can pose human-health or ecological concerns in drinking water or in the environment.
Although VOCs tend to escape from surface water through volatilization (evaporation) into the air, once dissolved in groundwater they are more persistent. They can be transported through the unsaturated zone in recharge, in soil vapor, or as a non-aqueous-phase liquid. Once in the saturated zone, some highly soluble VOCs, such as the gasoline additive MTBE, move with the groundwater, whereas other VOCs, like carbon tetrachloride, are slowed when they adhere to organic carbon in the aquifer solids. Some VOCs are degraded by bacteria in the aquifer, but others resist degradation and can be transported very long distances, in some cases reaching drinking-water supply wells.
Examples of VOCs
VOCs have been used extensively in the United States since the 1940s. VOCs are common components or additives in many commercial and household products, including gasoline, diesel fuel, other petroleum-based products, carpets, paints, varnishes, glues, spot removers, and cleaners. Industrial uses include the manufacturing of automobiles, electronics, computers, wood products, adhesives, dyes, rubber products, and plastics, and VOCs are used in the synthesis of other organic compounds. VOCs also are used in dry cleaning, in refrigeration units, and in the degreasing of equipment and home septic systems. VOCs are present in some personal care products such as perfumes, deodorants, insect repellents, skin lotions, and pharmaceuticals. Some VOCs also have been applied as fumigants in agriculture and in households to control insects, worms, and other pests.
VOCs in Groundwater
VOCs were analyzed in about 3,500 water samples collected during 1985–2001 from various types of wells, representing almost 100 different aquifer studies across the U.S. The group of VOCs most frequently detected in groundwater was trihalomethanes (THMs)—and of the THMs, chloroform was the most commonly detected. THMs form when chlorine interacts with dissolved organic matter in water, which can happen when chlorine is added to drinking water for disinfection of bacteria. Because some of that chlorinated drinking water goes down the drain, THMs are often detected in wastewater.
Solvents—with consumer and industrial uses such as degreasers, paint removers, and cleaning agents—also are among the VOCs detected in groundwater. Some solvents, like chloromethane, are no longer used in consumer products, but continue to be detected in groundwater if that contaminated groundwater recharged the aquifer back when the contaminant was still in use (see Groundwater Age).
Gasoline compounds and additives are another class of VOCs that is sometimes detected in groundwater. Leaking underground gasoline storage tanks are a common, but unseen, source of gasoline VOCs to groundwater.
MTBE—A VOC With a Groundwater History
Methyl tert-butyl ether (MtBE) in groundwater illustrates the law of unintended consequences. When lead was removed from gasoline in 1979, MtBE was sometimes used as a replacement to boost octane. In the 1990s, when federal laws required that the oxygenate content of gasoline be increased to reduce air pollution, MtBE was the most popular oxygenate added. MtBE quickly became a national issue in the United States in the 1990s because of its frequent detection in groundwater—MtBE dissolves readily in water, sorbs only weakly to soils, and, once in groundwater, resists degradation by bacteria.
Although not classified as a human carcinogen, neurological effects related to MtBE have been reported in humans, and kidney and liver tumors associated with MtBE have been reported in laboratory animals. Many states reacted to the frequent detection of MtBE in public supply wells by placing partial or complete bans on MtBE. In 2005, Congress removed the oxygen requirement from gasoline, and MtBE use in gasoline declined to negligible levels by 2007.
By 2012, MtBE concentrations were starting to decrease in some groundwater wells, but were unchanged in others, and still increasing in a few. This apparent contradiction reflects the complex mixture of groundwater ages in wells. Wells with mostly young groundwater are the most likely to have decreasing concentrations of MtBE in response to ending use of the gasoline additive. Wells with older groundwater and a broad mix of groundwater ages are the most likely to have concentrations of MtBE that are unchanging or even still increasing, as groundwater recharge carrying MtBE continues to slowly make its way to the well.
Follow the links below to learn more about topics related to VOCs and to groundwater quality.
Groundwater Age
Groundwater Quality Research
Groundwater/Surface-Water Interaction
Public Supply Wells
Domestic (Private) Supply Wells
Water-Quality Benchmarks for Contaminants
Groundwater Quality in Principal Aquifers of the Nation, 1991–2010
Groundwater Quality—Current Conditions and Changes Through Time
Rapid Fluctuations in Groundwater Quality
Predicting Groundwater Quality in Unmonitored Areas
Factors Affecting Vulnerability of Public-Supply Wells to Contamination
- Overview
Volatile organic compounds (VOCs) are chemicals that both vaporize into air and dissolve in water. VOCs are pervasive in daily life, because they’re used in industry, agriculture, transportation, and day-to-day activities around the home. Once released into groundwater, many VOCs are persistent and can migrate to drinking-water supply wells.
Have you ever pumped gas, had your clothes dry cleaned, or used chlorine bleach in your laundry or for disinfection? Then you’re likely to have encountered VOCs. Thousands of VOCs have been manufactured for use—many of these chemicals are toxic and can pose human-health or ecological concerns in drinking water or in the environment.
Although VOCs tend to escape from surface water through volatilization (evaporation) into the air, once dissolved in groundwater they are more persistent. They can be transported through the unsaturated zone in recharge, in soil vapor, or as a non-aqueous-phase liquid. Once in the saturated zone, some highly soluble VOCs, such as the gasoline additive MTBE, move with the groundwater, whereas other VOCs, like carbon tetrachloride, are slowed when they adhere to organic carbon in the aquifer solids. Some VOCs are degraded by bacteria in the aquifer, but others resist degradation and can be transported very long distances, in some cases reaching drinking-water supply wells.
Examples of VOCs
Sources/Usage: Public Domain.USGS technicians collecting groundwater samples for analysis of water quality, including VOCs. (Credit: Alan Cressler.) VOCs have been used extensively in the United States since the 1940s. VOCs are common components or additives in many commercial and household products, including gasoline, diesel fuel, other petroleum-based products, carpets, paints, varnishes, glues, spot removers, and cleaners. Industrial uses include the manufacturing of automobiles, electronics, computers, wood products, adhesives, dyes, rubber products, and plastics, and VOCs are used in the synthesis of other organic compounds. VOCs also are used in dry cleaning, in refrigeration units, and in the degreasing of equipment and home septic systems. VOCs are present in some personal care products such as perfumes, deodorants, insect repellents, skin lotions, and pharmaceuticals. Some VOCs also have been applied as fumigants in agriculture and in households to control insects, worms, and other pests.
VOCs in Groundwater
VOCs were analyzed in about 3,500 water samples collected during 1985–2001 from various types of wells, representing almost 100 different aquifer studies across the U.S. The group of VOCs most frequently detected in groundwater was trihalomethanes (THMs)—and of the THMs, chloroform was the most commonly detected. THMs form when chlorine interacts with dissolved organic matter in water, which can happen when chlorine is added to drinking water for disinfection of bacteria. Because some of that chlorinated drinking water goes down the drain, THMs are often detected in wastewater.
Solvents—with consumer and industrial uses such as degreasers, paint removers, and cleaning agents—also are among the VOCs detected in groundwater. Some solvents, like chloromethane, are no longer used in consumer products, but continue to be detected in groundwater if that contaminated groundwater recharged the aquifer back when the contaminant was still in use (see Groundwater Age).
Gasoline compounds and additives are another class of VOCs that is sometimes detected in groundwater. Leaking underground gasoline storage tanks are a common, but unseen, source of gasoline VOCs to groundwater.
MTBE—A VOC With a Groundwater History
Methyl tert-butyl ether (MtBE) in groundwater illustrates the law of unintended consequences. When lead was removed from gasoline in 1979, MtBE was sometimes used as a replacement to boost octane. In the 1990s, when federal laws required that the oxygenate content of gasoline be increased to reduce air pollution, MtBE was the most popular oxygenate added. MtBE quickly became a national issue in the United States in the 1990s because of its frequent detection in groundwater—MtBE dissolves readily in water, sorbs only weakly to soils, and, once in groundwater, resists degradation by bacteria.
Although not classified as a human carcinogen, neurological effects related to MtBE have been reported in humans, and kidney and liver tumors associated with MtBE have been reported in laboratory animals. Many states reacted to the frequent detection of MtBE in public supply wells by placing partial or complete bans on MtBE. In 2005, Congress removed the oxygen requirement from gasoline, and MtBE use in gasoline declined to negligible levels by 2007.
By 2012, MtBE concentrations were starting to decrease in some groundwater wells, but were unchanged in others, and still increasing in a few. This apparent contradiction reflects the complex mixture of groundwater ages in wells. Wells with mostly young groundwater are the most likely to have decreasing concentrations of MtBE in response to ending use of the gasoline additive. Wells with older groundwater and a broad mix of groundwater ages are the most likely to have concentrations of MtBE that are unchanging or even still increasing, as groundwater recharge carrying MtBE continues to slowly make its way to the well.
- Science
Follow the links below to learn more about topics related to VOCs and to groundwater quality.
Groundwater Age
The age of groundwater is key in predicting which contaminants it might contain. There are many tracers and techniques that allow us to estimate the age—or mix of ages—of the groundwater we depend on as a drinking water supply.Groundwater Quality Research
Every day, millions of gallons of groundwater are pumped to supply drinking water for about 140 million people, almost one-half of the Nation’s population. Learn about the quality and availability of groundwater for drinking, where and why groundwater quality is degraded, and where groundwater quality is changing.Groundwater/Surface-Water Interaction
Water and the chemicals it contains are constantly being exchanged between the land surface and the subsurface. Surface water seeps into the ground and recharges the underlying aquifer—groundwater discharges to the surface and supplies the stream with baseflow. USGS Integrated Watershed Studies assess these exchanges and their effect on surface-water and groundwater quality and quantity.Public Supply Wells
Are you among the more than 100 million people in the U.S. who relies on a public-supply well for your drinking water? Although the quality of finished drinking water from public water systems is regulated by the EPA, long-term protection and management of the raw groundwater tapped by public-supply wells requires an understanding of the occurrence of contaminants in this invisible, vital resource...Domestic (Private) Supply Wells
More than 43 million people—about 15 percent of the U.S. population—rely on domestic (private) wells as their source of drinking water. The quality and safety of water from domestic wells are not regulated by the Federal Safe Drinking Water Act or, in most cases, by state laws. Instead, individual homeowners are responsible for maintaining their domestic well systems and for monitoring water...Water-Quality Benchmarks for Contaminants
How does the water quality measure up? It all depends on what the water will be used for and what contaminants are of interest. Water-quality benchmarks are designed to protect drinking water, recreation, aquatic life, and wildlife. Here you’ll find links to some of the most widely used sets of water, sediment, and fish tissue benchmarks and general guidance about their interpretation.Groundwater Quality in Principal Aquifers of the Nation, 1991–2010
What’s in your groundwater? Learn about groundwater quality in the Principal Aquifers of nine regions across the United States in informative circulars filled with figures, photos, and water-quality information.Groundwater Quality—Current Conditions and Changes Through Time
Is groundwater the source of your drinking water? The USGS is assessing the quality of groundwater used for public supply using newly collected data along with existing water-quality data. Learn more about this invisible, vital resource so many of us depend on.Rapid Fluctuations in Groundwater Quality
We think of groundwater as moving slowly, and groundwater quality as changing slowly—over decades or even centuries. But in some parts of some aquifers, groundwater quality can fluctuate rapidly, sometimes over just a few hours. Are such changes part of a long-term trend, or just part of a short-term cycle? And what does that mean for suitability for drinking?Predicting Groundwater Quality in Unmonitored Areas
Groundwater provides nearly one-half of the Nation’s drinking water, and sustains the steady flow of streams and rivers and the ecological systems that depend on that flow. Unless we drill a well, how can we know the quality of the groundwater below? Learn about how the USGS is using sophisticated techniques to predict groundwater quality and view national maps of groundwater quality.Factors Affecting Vulnerability of Public-Supply Wells to Contamination
More than 100 million people in the United States—about 35 percent of the population—receive their drinking water from public-supply wells. These systems can be vulnerable to contamination from naturally occurring constituents, such as radon, uranium and arsenic, and from commonly used manmade chemicals, such as fertilizers, pesticides, solvents, and gasoline hydrocarbons. Learn about the... - Publications