Civil Engineering Reference
In-Depth Information
drinking water, along with their physical properties. Several of these VOCs are rec-
ognized as a threat to public health that in some instances must be dealt with by
removing the chemical from the water supply by appropriate treatment. Table 1-6 lists
the current MCL for all VOCs the EPA has determined to be of significant concern.
VOCs are seldom detected in concentrations greater than a few micrograms per
liter in surface waters, because the compounds do not occur naturally and are relatively
volatile. However, surface water subject to wastewater discharges may contain elevated
concentrations of organic solvents during periods of ice cover when volatilization of
these solvents is restricted.
The risk of groundwater contamination by VOCs is substantially more serious than
that for surface water. VOCs can enter an aquifer and be transported great distances
because they have little affinity for soils. Symons and colleagues estimated in 1975
that approximately 1 percent of the nation's groundwater was thought to be meaning-
fully contaminated by organic pollutants.
3
However, this estimate is only a rough
approximation based on incomplete surveys. Moreover, detection of groundwater con-
tamination is inherently characterized by time lags (sometimes measured in decades)
because of the distance between the site of the contamination and the wellhead.
Groundwater provides a unique environment for VOCs because:
It has limited contact with the atmosphere; hence, volatile compounds do not
evaporate quickly.
•
The surface environment below the active soil zone is relatively abiotic, allowing
little biodegradation.
•
The temperature of groundwater undergoes slow and limited fluctuations through-
out the annual climatic cycle.
•
The groundwater moves slowly, without turbulence, in a dark environment.
•
When VOCs are introduced into a groundwater system, they maintain a discrete flow
pattern because of laminar flow conditions. Dispersion and diffusion alone are respon-
sible for what limited dilution takes place. Thus, the characteristics of an aquifer tend
to preserve, rather than dissipate, VOCs.
An adequate understanding of the extent of contamination within a given aquifer
requires a thorough understanding of the aquifer geology and groundwater movement
within that aquifer, as well as analyses from nearly every groundwater discharge within
the aquifer. Contamination can be caused by a single discharge (such as a railroad
accident) and remain undetected for several years. In cases of very slow groundwater
movement, a contaminant may remain localized and impact only a small area. In any
situation, predicting the likelihood of contamination at a particular point without a
large database is extremely difficult. Often, the necessary data on aquifer contamination
can be obtained only through the development of a comprehensive groundwater mon-
itoring program. Excellent discussion of the design and installation of monitoring well
networks is available from several sources.
4
Several federal surveys, including the National Organics Reconnaissance Survey,
the National Organics Monitoring Survey, and many state surveys, have identified
VOCs in numerous groundwater-supplied potable water systems.
5
Although monitoring
has been concentrated in areas of suspected problems, particularly New England and
the mid-Atlantic states, it is now recognized that groundwater supplies in all areas of
the country are suspect. A summary of selected monitoring data from the state studies
indicating maximum contaminant levels is shown in Table 3-3.
