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To What Extent Will Past MTBE Releases Contaminate the Nation's Community Water Supply Wells?
- Why is this article significant?
This article is important because it discusses the potential risk of
past MTBE-gasoline storage tank releases to community water supply (CWS)
wells. Some salient topics discussed include:
- Growth in the use of MTBE in gasoline
- Behavior and fate of MTBE in ground water
- Number of potential releases of MTBE from fuel tanks
- Information needed to perform CWS vulnerability assessments at local and national scales
- Factors that determine if a gasoline release will contaminate a CWS well
- Location analysis of leaking underground storage tanks (LUSTs) and CWS wells
- How many CWSs will be affected by past MTBE releases?
Why can't a number be determined at this time?
The national estimate cannot be determined because the data to run the
ground-water model are not available nationally. A complete analysis
will require more detailed information for both:
- the nature and locations of MTBE gasoline releases, and
- the hydrogeology and pumping practices of CWS wells.
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Didn't the histogram in the article suggest that 9,000 CWS wells
in 31 states would be affected by MTBE?
No. The article states that approximately
9,000 CWS wells in the 31 states have a
LUST within 1 km, based on preliminary analysis. The article points
out that not all LUST sites will be a significant source of MTBE to
ground water and to CWS wells, and that the 9,000 CWS figure is
"undoubtedly an overestimate of the number of wells in those states
with ... significant ... MTBE sites." The article points out, however,
that the number 9,000 is so large that the actual number of CWSs that
may ultimately be affected "may well be worrisome."
- What is the "pumping stress factor?
Why does the likelihood of MTBE impacting a CWS well increase as the factor increases?
The pumping stress factor is the measure of the well pumping rate
compared to the rate at which ground water would naturally flow
through the aquifer in the vicinity of the well (pumping rate/local
aquifer yield). When the pumping stress factor is low, a gasoline
plume will stay near the water table and will not be drawn down to the
well's opening. As the pumping stress factor increases the likelihood
of draw down is increased.
- What is the basis for the article's statement
that past MTBE gasoline releases will continue to threaten CWS wells
until at least the year 2010?
The logic behind this
observation is outlined in detail in the article. In brief, there are
three important considerations: - A large number of LUST
releases likely contained MTBE
- It may take 10 years for a MTBE
release to reach (flow to) a CWS well
- MTBE degrades slowly in
ground water and therefore LUST releases within the 10-year capture
zone of a CWS well would present a risk of contamination to that well.
- Are all CWS wells for the 31 states included in the location analysis?
No. The CWS wells included were a small subset of those in our data
base. Of the 105,000 SDWIS wells in our data retrieval, only 47,000
had interpretable latitude-longitude information. Of these about
26,000 were selected that were separated by more than 1-km distance
from each other to minimize double-counting in the analysis. The data
set is not a tally of potentially affected CWS wells, but was
developed to answer the national question: "What ranges of LUST densities are
common around CWS wells?"
- Which states are not included in the CWS/LUST location analysis?
Of the 50 states, the following states (shaded gray on the map below) are not included.
Our CWS data set does not include data from
Alaska, Hawaii, California, Colorado, Connecticut,
Iowa, Indiana, Maine, Montana, Oregon, Texas, Vermont, Washington and
Wisconsin. Maryland, Michigan, North Dakota, and Nebraska have
less than 10 CWSs in our data base.
Kentucky is not included since it has no leaking tank
locations in our StarView database retrieval.
- Are all leaking storage tanks included in the StarView database?
No. The amount of missing data and the location quality of the leaking
storage tank database varies from state to state, depending on the
quality and amount of information provided by the individual states
involved. No detailed quality assessment has been made of the StarView
database. Work by
Dooher and Happel
at Lawrence Livermore National
Laboratories indicates that the StarView data should be used very
carefully and verified for any site-specific analysis.
- Is my well at risk? Where can I get information about
environmental risks around specific wells?
- The data sets we used for our analysis are of national scale and
were collected from several sources. These include:
Both of these data sets were developed from information collected
and maintained by individual states, and ground water users and
utilities are urged to go to their state sources for more current and
complete information. Contacts for the Drinking Water programs in each
state can be found at:
http://www.epa.gov/safewater/dwinfo.htm
- EPA has a great deal of useful information on drinking water on
the home page of their Office of Drinking Water:
http://www.epa.gov/safewater/
- A resource to get a quick handle on what environmental risks
locally exist is EPA's maps on demand service:
http://www.epa.gov/enviro/emef/
- How many people in the US depend on ground water for drinking water?
- Domestic: 42,400,000 people served - 3,350 million gallons per day
- Public water supply: 91,200,000 people served -
15,100 million gallons per day
(From Solley, W.B., Pierce, R.R., and Perlman, H.A. 1998,
Estimated use of water in the United States in 1995:
U.S. Geological Survey Circular 1200.)
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