Environmental Engineering Reference
In-Depth Information
faster than do other constituents of gasoline. Public and private wells have
been contaminated in this manner. Non-point sources (such as recreational
watercraft) are most likely to be the cause of small amounts of contamina-
tion in a large number of shallow aquifers and surface waters. Air deposition
through precipitation of industrial or vehicular emissions may also contrib-
ute to surface water contamination. The extent of any potential for build-up
in the environment from such deposition is uncertain.
Based on the limited sampling data currently available, most concentra-
tions at which MtBE has been found in drinking water sources are unlikely
to cause adverse health effects; however, the USEPA is continuing to evalu-
ate the available information and is doing additional research to seek more
definitive estimates of potential risks to humans from drinking water. There
are no data on the effects on humans of drinking MtBE-contaminated water.
In laboratory tests on animals, cancer and noncancer effects have occurred at
high levels of exposure. These tests are conducted by inhalation exposure or
by introducing the chemical in oil directly to the stomach. The tests support
a concern for potential human hazard; however, because the animals were
not exposed through drinking water, significant uncertainties exist concern-
ing the degree of risk associated with human exposure to low concentrations
typical found in drinking water.
The very unpleasant taste and odor of MtBE make contaminated drinking
water unacceptable to the public. Studies conducted on the concentrations of
MtBE in drinking water determined the level at which individuals can detect
the odor or taste of the chemical. Humans vary widely in the concentrations
they are able to detect. Some who are sensitive can detect very low concen-
trations. Others do not taste or smell the chemical, even at much higher
concentrations. The presence or absence of other natural or water treatment
chemicals sometimes masks or reveals the taste or odor effects. Studies to
date have not been extensive enough to completely describe the extent of this
variability or to establish a population response threshold. Nevertheless, we
conclude from the available studies that keeping concentrations in the range
of 20 to 40 µg/L of water or below will likely avert unpleasant taste and odor
effects, recognizing that some people may detect the chemical below this.
The USEPA Advisory recommends control levels for taste and odor accept-
ability that will also protect against potential health effects. Concentrations
in the range of 20 to 40 µg/L are about 20,000 to 100,000 (or more) times lower
than the range of exposure levels in which cancer or noncancer effects were
observed in rodent tests. This margin of exposure lies within the range of
margins of exposure typically provided to protect against cancer effects by
the National Primary Drinking Water Regulations under the federal Safe
Drinking Water Act—a margin greater than such standards typically pro-
vided to protect against noncancer effects. Protection of the water source
from unpleasant taste and odor as recommended also protects consum-
ers from potential health effects. The USEPA observed that occurrences of
groundwater contamination at or above this 20- to 40-µg/L taste and odor
