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.