ABSTRACT

KEY MTBE FINDINGS BASED ON NATIONAL WATER-QUALITY MONITORING

By

Rick Clawges, John Zogorski, and David Bender

U.S. Geological Survey

1608 Mt. View Road

Rapid City, SD 57702

The U.S. Geological Survey (USGS), in cooperation with several other
agencies and organizations, has determined a number of findings
concerning methyl *tert*-butyl ether (MTBE) in ambient ground water,
drinking-water sources (ground water, reservoirs, rivers, etc.), and
drinking water. These findings concern the frequency of detection and
concentrations of MTBE in water, and the correlation of MTBE detection
in water to its content in gasoline. The findings are either well
established or presently being confirmed at the national level.

# Large water-quality data sets for MTBE and other volatile organic compounds (VOCs) for three studies (figure 1) are the basis for the key findings. All three of these studies are currently in progress; therefore, some of the findings noted herein may be updated as more information becomes available. The three studies are listed and described below.

1\. The USGS National Water-Quality Assessment (NAWQA) Program is
designed as a resource assessment. Most of NAWQA's VOC sampling,
including analyses for MTBE, other ether oxygenates, and other VOCs, is
focused on ambient ground water. This monitoring uses a number of well
types and does not include known contaminated ground water. For example,
ground water at leaking underground storage tank (LUST) sites is not
sampled. Between 1993 and 1998, NAWQA staff sampled about 3,500 wells
for VOCs throughout the United States. Urban streams also are being
sampled for VOCs as a pilot effort.

2\.

The National MTBE Survey is being conducted for the American Water Works
Association Research Foundation by a team consisting of staff from the
Metropolitan Water District of Southern California, the Oregon Graduate
Institute, and the USGS. The study, in part, involves sampling 1,000
randomly selected source waters (ground water at the well head,
reservoirs at the withdrawal point, and rivers at the withdrawal point)
used as the supply for Community Water Systems (CWSs) in the United
States and its territories. Preliminary results from the National MTBE
Survey currently are available for 400 sources. Final results for all
1,000 systems will be available in December 2000, at which time
preparation of a report on the findings of the study will be initiated.

3\.

The 12-State Retrospective is a compilation of MTBE and other VOC data
for drinking water from a random sample of 20 percent of the CWSs in
Northeast and Mid-Atlantic States. This compilation is being performed
by the USGS for USEPA's Office of Ground Water and Drinking Water. The
compiled data are from compliance monitoring programs conducted within
each State. Most of the samples were collected between 1994 and 1998.
Results of analyses for MTBE are available for 10 of the 12 States. An
occurrence report resulting from this compilation should be available in
December 2000.

There are seven key findings resulting from these three studies. These
are listed and described below.

Finding #1: MTBE is one of the most frequently detected VOCs in ambient
ground water and urban streams.

Findings from the NAWQA Program for about 3,500 wells sampled between
1993 and 1998 indicate that MTBE is the second most frequently detected
(7.8 percent) VOC at a reporting level of 0.2 micrograms per liter
(μg/L) (figure 2). Chloroform had the highest frequency of detection (11
percent). The detection frequency of 7.8 percent for MTBE represents a
mixture of well types, well depths, land use, and MTBE-use areas. A 1996
USGS study showed that MTBE is detected most frequently in shallow urban
ground water. MTBE was found to be the most frequently detected VOC (23
percent) in urban streams at a reporting level of 0.2 μg/L, on the basis
of about 780 samples collected in 14 urban areas.

**Finding #2: The concentrations of MTBE detected in ambient ground
water, source water, and drinking water typically have been below
USEPA's consumer advisory (20-40 μg/L).**

Preliminary data from the National MTBE Survey of source water for CWSs
indicate a frequency of detection of 11 percent at a reporting level of
0.2 μg/L**;** however, less than 1 percent of these sources (2 of 398)
had MTBE detections greater than 5 μg/L (California's taste and odor
secondary standard), and none exceeded 20 μg/L (the lower limit of
USEPA's consumer advisory) (figure 3). The sources in this survey are
all active sources sampled prior to any treatment. The survey does not
include sources that have been removed from service because of excessive
MTBE levels. Most of the water sources sampled in this survey have not
been tested previously for MTBE.

Finding #3: Some CWSs have had excessive MTBE concentrations in drinking
water.

Results from the 12-State Retrospective indicate that about 2 percent of
CWSs had samples of water with MTBE concentrations equal to or greater
than 5 μg/L. This detection frequency, for drinking water, is slightly
higher than the frequency of detection for source water from the
National MTBE Survey. Although frequencies of detection indicated in
both studies are small, the number of systems potentially affected is
significant. On the basis of data from the randomly selected CWSs in 10
Northeast and Mid-Atlantic States, it is projected that 180 CWSs within
those states potentially could have a source water with an MTBE
concentration at or above 5 μg/L, and that 80 CWSs could have a source
water with an MTBE concentration at or above 20 μg/L (figure 4). These
results and projections illustrate the need for routine monitoring of
CWSs to ensure that public exposure does not occur at excessive levels.

**Finding #4: The frequency of detecting MTBE is dependent on the
content (percent by volume, v/v) of MTBE in gasoline.**

In testimony to USEPA's Blue Ribbon Panel (1999), USGS scientists noted
that MTBE is more frequently detected in ambient ground water (NAWQA
Program) and drinking water (12-State Retrospective) in areas of the
United States where MTBE is present in reformulated (RFG) (10-11 percent
v/v) and oxygenated (OXY) (15 percent v/v) gasoline. For example, MTBE
was found in drinking water from 15 percent of CWSs in RFG-use areas
using MTBE and 3 percent elsewhere in the 10 Northeast and Mid-Atlantic
States that have MTBE analyses. Results from the NAWQA Program show that
the

detection frequency of MTBE in water correlates with the amount of MTBE
in gasoline. An analysis of MTBE data in ground water for 21
metropolitan areas across the United States (each with 25-30 wells
sampled) indicates an increasing frequency of detection of MTBE with
increasing MTBE content in gasoline (figure 5). A similar analysis of
surface-water samples for 9 metropolitan areas also shows an increasing
trend.

Finding #5: In drinking water, MTBE co-occurrence is with other VOCs and
not typically with gasoline compounds such as benzene, toluene,
ethylbenzene, and xylenes (BTEX).

Analysis of drinking-water data from the 12-State Retrospective
indicates that most of the co-occurring compounds are chlorinated
solvents, such as perchloroethene (PCE) and trichloroethene (TCE), and
trihalomethanes (particularly chloroform) (figure 6). Less than 1
percent of the CWSs that had MTBE levels in drinking water at or above 1
μg/L also had BTEX present in the samples.

These data indicate that the extent of gasoline contamination of
drinking water may have increased with the addition of MTBE to gasoline.

Finding #6: MTBE has been used in large amounts in gasoline in some
areas of the United States that currently are not high MTBE-use areas.

USEPA has noted that 85 percent of MTBE use during 1999 was in RFG-use
areas. This suggests that gasoline used outside of RFG-use areas would
have little or no MTBE. Data from a recent survey of MTBE (percent
volume) in gasoline by the National Institute for Petroleum and Energy
Research (NIPER) support this conclusion. Data compiled for metropolitan
areas for the Winter of 1997-98 show that a few cities use gasoline with
MTBE content at or above 10 percent (RFG-use areas), but the majority of
the other cities use gasoline with MTBE content at or below 1 percent
(figure 7). A different perspective is evident from a review of gasoline
composition data for the 1990s. Analysis of NIPER data for the period
1990-98 shows that a number of cities outside of RFG-use areas
historically have used MTBE in gasoline at substantially higher levels
than they currently do. Therefore, the detection of MTBE in ground water
in these areas should not be surprising, considering MTBE's persistence
in ground water and the large number of LUST releases across the United
States.

Finding #7: MTBE is detected in ambient ground water and in source
waters in non-RFG/OXY-use areas.

MTBE is being detected in ambient ground water (NAWQA Program) and in
source waters (National MTBE Survey) outside of former or current RFG-
and OXY-use areas, where most use of MTBE is assumed to have occurred
(figure 8). MTBE is (was) presumably being used for octane enhancement
in these areas. Results of the NAWQA Program indicate that 73 percent of
MTBE detections (140 wells) were located in former or current RFG- and
OXY-use areas**;** however, the other 27 percent of MTBE detections (53
wells) were located in non-RFG/OXY-use areas.

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Figure 1. Basis of key findings.

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Figure 2. VOC detection frequency in ambient ground water from the NAWQA
program, 1993-98.

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Figure 3. Frequency of MTBE concentrations in National MTBE Survey.

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Figure 4. Frequency of MTBE concentrations in 1,190 community water
systems in 10 states.

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Figure 5. MTBE in urban ground water from the NAWQA Program, 1993-98.

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Figure 6. VOCs co-occurring with MTBE from the 12-State Retrospective.

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Figure 7. Content of MTBE in gasoline, Winter 1997-98.

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height="3.5277777777777777in"}

Figure 8. MTBE detections in ambient ground water (NAWQA Program) and in
source waters (National MTBE Survey).