The Atmosphere - A Potential Source of MTBE to Ground Water
USGS scientists have discovered that the atmosphere is a potential source
of the low concentrations of methyl
tert-butyl ether (MTBE) detected in shallow ground water in southern
New Jersey. The scientists measured atmospheric concentrations of MTBE
from November 1996 to February 1998
"Even when concentrations of VOCs
in groundwater are low compared to regulatory concentration limits,
it is critical to know the source. If the VOCs originate from a point
source(s), concentrations in groundwater could potentially increase
over time to levels of concern as groundwater plumes evolve, whereas
if the atmosphere is the source, then groundwater concentrations would
be expected to remain at low-level concentrations not exceeding those
in equilibrium with atmospheric concentrations."
Baehr and others,
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and determined through the use of computer simulation studies that the
concentrations were high enough to cause detections in ground water, lakes,
and streams. In subsequent work the scientists studied the presence of
MTBE in the unsaturated zone
and discovered that biodegradation of MTBE in the unsaturated zone may
reduce MTBE concentrations before they can enter ground water. Variations
in the properties of the unsaturated zone and the recharge
of ground water need to be more fully understood, however, to determine
the potential of the atmosphere as a nonpoint source of MTBE to ground
water. Resource managers when conducting surveys of ambient
ground-water quality can use the information from these studies to put
the presence of MTBE in ground water at low concentrations into context
with other contamination sources.
Nonpoint-source pollution is one of the biggest threats to the quality
of the Nation's ground waters. Volatile
organic compounds (VOCs), such as MTBE, benzene, chloroform, and trichloroethylene
(TCE) are frequently detected in ground water. Land-use planners are faced
with difficult decisions concerning development of land and the protection
of ground water from these contaminants. Fundamental to developing sound
land-use policies is understanding what role the unsaturated zone plays
in protecting shallow ground water from contamination due to land-use
activities (residential, urban, agricultural), and whether or not the
frequent occurrence of VOCs is due to the coalescing of unidentified point-source
plumes, the improper disposal of
used motor oil, the flushing of contaminants from the atmosphere during
rain storms, or some other mechanism. The above studies of the fate of
MTBE have increased the knowledge of potential sources of MTBE in ground
water and the role of the unsaturated zone in protecting ground water.
This understanding can be used by land-use managers to develop sound policies.
These studies were a joint project between the Toxic
Substances Hydrology and the National
Water-Quality Assessment (NAWQA) Programs.
References
- Baehr, A.L., Charles, E.G., and Baker, R.J., 2001, Methyl
tert-butyl ether degradation in the unsaturated zone and the relation
between MTBE in the atmosphere and shallow groundwater: Water Resources
Research, v. 37, no. 2, p. 223-233.
- Baehr, A.L., Stackelberg, P.E., and Baker, R.J., 1999, Evaluation
of the atmosphere as a source of volatile organic compounds in shallow
groundwater: Water Resources Research, v. 35, no. 1, p. 127-136.
- Pankow, J.F., Thomson, N.R., Johnson, R.L., Baehr, A.L., and Zogorski,
J.S., 1997, The
urban atmosphere as a non-point source for the transport of MTBE and
other volatile organic compounds (VOC's) to shallow groundwater:
Environmental Science and Technology, v. 31, no. 10, p. 2821-2828.
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