A fundamental rule of ecology states that we can never simply do just one thing; that, in effect, everything is connected to everything else. Like a pebble dropped into a pond, consequences — both intended and unintended — ripple outward from all that we do.

Knowing this, most of us do our best to minimize the negative consequences of our actions and maximize the benefits — for ourselves and for others. In dealing with hazardous waste and toxic substances, we do our best to practice prevention and reduce the use of toxics in order to avoid generating waste in the first place. Because we’re not always successful, however, considerable effort is necessary to control waste and emissions, and to clean up the environment after a spill or other release. Because we can never do just one thing and because we can never know all the many consequences of what we do before we do it, sometimes our best decisions about environmental protection create new, unforeseen problems. An example worth considering involves attempts to reduce vehicle-related air emissions.

Vermont, like the rest of the United States, is highly dependent on motor vehicles for transportation. The automobile has afforded us great mobility, but at a price. Gasoline, as well as emissions resulting from the use of gasoline, are among the most prevalent hazardous substances to which we Vermonters are exposed in our everyday lives.

With the phase-out of lead from gasoline in the late 1970s (which dramatically reduced lead in the environment), compounds that contain oxygen, called oxygenates, have been added to gasoline to increase its combustibility. This use of oxygenates was expanded by the Clean Air Act Amendments of 1990, which mandated that oxygen be added to gasoline in areas where concentrations of ozone in the summer or carbon monoxide in the winter exceed established air quality standards. Vermont monitors ground-level ozone at two stations, Bennington and Underhill. Annual ground-level ozone averages consistently hover at or just below the national ambient air quality standard (Figure 1).

Ozone in the upper atmosphere helps filter cancer-causing ultraviolet radiation, but ozone near the ground damages lungs and possibly the human immune system. In the lower atmosphere, hydrocarbons and nitrogen oxides reacting in sunlight and heat form ozone. Some ground-level ozone in Vermont is formed from the nitrogen oxides blown in from midwestern, coal-burning power plants.

Methyl tertiary-butyl ether (MTBE) is a volatile organic compound derived from natural gas. It is added to gasoline either seasonally or year-round in many parts of the United States to increase the octane level and to reduce carbon monoxide and ozone levels in the air.

Nearly all gasoline sold in Vermont contains MTBE. As in other New England states, it is found at a concentration of approximately 5 to 10 percent, but may be present in higher concentrations because the same gasoline is distributed in states that require higher levels of oxygenates such as MTBE.

MTBE has entered groundwater and surface waters in Vermont through leaking underground gasoline storage tanks, spills, emissions from marine engines into lakes and reservoirs, and to some extent from air deposition. Once released into the environment, MTBE behaves differently from other gasoline compounds. MTBE does not adhere to soil particles and, because it is more soluble in water than other components of gasoline, it migrates farther and more quickly with groundwater and surface waters. In addition, MTBE resists being rendered harmless by soil and aquatic organisms (biodegradation).

The U.S. Environmental Protection Agency tentatively classifies MTBE as a possible human carcinogen. Vermont’s ground water rules recommend a limit of 40 parts per billion (ppb). This recommendation is based on preventing taste and odor complaints. MTBE has a very low odor threshold at 20 ppb, while the threshold for taste is 40 ppb. These numbers vary widely with individual sensitivity. The Vermont Agency of Natural Resources has adopted a health advisory of 40 ppb for MTBE. Water samples at or exceeding this concentration trigger the need for remediation. To date, more than 175 private and 44 public drinking water wells have been contaminated to some level by petroleum products — many of these solely by MTBE.

The number of sites contaminated by petroleum has increased steadily since the late 1980s but seems to be stabilizing due to investments in double-walled underground storage tanks (Figure 2). The Agency monitors nearly 1,200 sites contaminated by petroleum; of these, an estimated 85 percent contain MTBE.

In 1997, northeastern states and responsible parties spent about $168 million dollars to clean up gasoline spills, primarily from leaking underground storage tanks. A precise apportionment of the costs related to MTBE contamination, however, is not available because Vermont and the other states do not compile data on specific gasoline constituents.

Cleanup of waters contaminated with MTBE is particularly difficult because of its chemical properties. MTBE can spread rapidly over large areas and is very resistant to biodegradation. Consider, for example, an underground storage tank leak that occurred in Killington, a release site surrounded by vacation homes, restaurants, retail shops, and two apartment complexes. Within a 2,000-foot radius of the leaking tank are more than 80 bedrock drinking water supply wells. As the gasoline leaked from the tank, a plume of MTBE and other compounds found in gasoline spread through the bedrock aquifer and ultimately contaminated 27 water supply wells. Twenty-five of these wells were
contaminated solely by MTBE, two by other gasoline compounds. The MTBE migrated nearly 10 times the distance of the non-MTBE gasoline contaminants. Today, six years after the leak was stopped, most of the non-MTBE gasoline contaminants have been recovered, but much of the MTBE remains, beyond the reach of even the most sophisticated cleanup technologies.

Water contamination caused by MTBE has raised new questions about the desirability of this additive. The Agency, in partnership with other New England states, is working to better understand its risks and benefits.

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