Low Levels of Gasoline Additive in Urban Stormwater and Ground Water a Key
|Topic at Chemical Society Meeting in San Francisco April 13-17, 1997,|
Methyl tert-butyl ether (MTBE) and its potential effects on air and water quality were the focus of a session Wed. and Thurs, April 16-17, 1997, at the American Chemical Society annual meeting in San Francisco, California. U.S. Geological Survey scientists will report on several studies of the occurrence and distribution of this compound in surface water and ground water during the session.
MTBE, derived from natural gas, is added to gasoline in many parts of the United States to reduce carbon monoxide and ozone levels in the air (95% of total MTBE use), or to increase the octane of gasoline (5% of use).
MTBE is the most widely used oxygenate for these purposes, however, ethanol is used in many areas of the Nation. Oxygenates, such as MTBE and ethanol, reduce the need for benzene and other ozone-forming aromatic compounds in gasoline.
MTBE, and other ether oxygenates, are some of the 88 volatile organic compounds (VOCs) that are being measured by the USGS in 1997 in surface water and ground water throughout the United States as part of the USGS National Water-Quality Assessment Program.
MTBE was included in the list of compounds measured because of its:
(b) potential release, past and present, from storage tanks and other point sources,
(c) detection in shallow ground water in urban areas, and
(d) impact to some drinking-water wells.
The U.S. Environmental Protection Agency (USEPA) has classified MTBE as a possible human carcinogen and has established a draft lifetime drinking-water advisory of 20-200 ug/L (micrograms per liter).
This advisory is currently being updated. Also, MTBE imparts taste and odor to water, which can be detected by many persons beginning at levels between 40-130 ug/L. (more--ACS-97-MTBE) *** MTBE In Municipal Stormwater *** MTBE was detected in some urban stormwater samples collected in 16 cities and metropolitan areas by the USGS, but all detections of MTBE were less than the lower limit of the draft health advisory for drinking water.
At a reporting level of 1.0 ug/L for most samples, USGS scientist Greg Delzer said MTBE was detected in one or more stormwater samples in eight cities--Atlanta, Baton Rouge, Birmingham, Colorado Springs, Denver, Dallas/Fort Worth, San Antonio, and Phoenix. The detection rate in urban stormwater was> highest in samples collected during the months of October through March each year of the study (1991-95). This October-March period corresponds with the expected seasonal use of oxygenated gasoline in areas where carbon monoxide exceeds established air-quality standards.
Concentrations of 62 VOCs and other constituent groups were measured in 592 stormwater samples collected in the 16 cities in 11 states, all of which have a population greater than 100,000. MTBE was the seventh most frequently detected VOC and was detected in 41 of the 592 stormwater samples.
In decreasing order, the most frequently detected VOCs were toluene, total xylenes, chloroform, trimethylbenzene, tetrachloroethene, and naphthalene. Detections of MTBE ranged from 0.2 to 8.7 ug/L with a median of 1.5 ug/L. In each of the three cities--Phoenix, Colorado Springs, and Denver--known to use MTBE to abate air pollution, it was detected only in stormwater samples > collected during the season when oxygenated gasoline was in use.
MTBE was detected in 40 percent of the samples collected in these three cities during October through March. Detection of MTBE in cities confirmed not to use oxygenated gasoline or reformulated gasoline -- Atlanta, Baton Rouge, Birmingham, Dallas/Fort Worth, and San Antonio -- may be attributable to the use of MTBE as an octane enhancer.
This USGS work is part of an interagency assessment of the scientific basis and effectiveness of the nation's winter oxygenated fuel program, which is coordinated by the President's Office of Science and Technology Policy. It is a compilation of several USGS studies that were done to assist cities in applying > for urban stormwater permits. The data were synthesized by the USGS National Water-Quality Assessment Program, one goal of which is to provide a > comprehensive assessment of the quality of most of the Nation's water resources.
*** MTBE In Ground Water ***
The ground water component of the National Water-Quality Assessment program focuses on assessing: (a) water-quality conditions of large used aquifers, and (b) the quality of recently recharged shallow ground water associated with human land-use activities. This design is based on the need to examine ground-water quality at a range of spatial scales and to build an understanding of the potential effects of land use on ground-water quality.
At a reporting level of 0.2 ug/L, MTBE was detected in 22% of 304 shallow monitoring wells sampled in 13 urban areas. MTBE was detected in less than 3% of shallow wells in agricultural areas or deeper wells in major aquifers. MTBE was the second most frequently detected VOC (of 60 compounds measured) in urban areas. About 2% of the shallow urban wells exceeded 20 ug/L, the estimated lower limit of the USEPA draft drinking-water health advisory. However, none of these wells were used as a source of drinking water. In contrast, none of concentrations in agricultural areas or deeper wells exceeded 20 ug/L.
The following illustration may help to provide perspective on the significance of these concentrations in ground water. If all the water in an aquifer that is 30 feet thick in a 1 square-mile area were mixed with 4 gallons of reformulated gasoline, the water would have 0.2 ug/L of MTBE. It would take 400 gallons of reformulated gasoline to bring the concentrations in the water to 20 ug/L.
Paul Squillace, a USGS research hydrologist, said the detection of MTBE varied substantially among the 13 urban areas investigated. Urban areas where MTBE was most commonly detected include Denver (79% of wells), Harrisburg, PA (37%), and various cities in New England (37%). Urban areas where MTBE was not detected include Ocala and Tampa, FL, Portland, OR, and Virginia Beach, VA. Possible source of MTBE in ground water include point sources, such as leaking storage tanks, and nonpoint sources such as recharge of precipitation and stormwater runoff.
Reformulated gasoline is about 50 times more soluble in water than conventional gasoline. This higher solubility is due to the fact that reformulated gasoline consists of about 10% MTBE. Up to 5,000 mg/L (milligrams per liter) of MTBE are possible in water when contaminated by reformulated gasoline.
There are several factors that make MTBE an important ground-water contaminant. It is more mobile, persistent, and higher concentrations in water are possible, in comparison to hydrocarbons present in conventional gasoline. Also, the USEPA has classified MTBE as a possible human carcinogen.
Further information on the urban stormwater study is available in USGS Water-Resources Investigations Report 96-4145: "Occurrence of the Gasoline Oxygenate MTBE and BTEX Compounds in Urban Stormwater in the United States, 1991-95," by Gregory C. Delzer, John S. Zogorski, Thomas J. Lopes, and Robin L. Bosshart. the report is available for $1.50 for paper copy or $4.00 for microfiche, plus $3.50 for shipping and handling, from the Branch of Information Services, USGS, Box 25286, Denver, Colo., 80225.
The report is also available via the Internet on the World Wide Web at: http://wwwsd.cr.usgs.gov/nawqa/pubs/wrir/wrir96.4145/wrir.doc.html
The USGS has also published a fact sheet: "Environmental Behavior and Fate of Methyl tert-Butyl Ether (MTBE)," numbered FS 203-96, single copies of which are available free from the Denver address above. The fact sheet contains general and technical information on MTBE, its potential sources and how they can be determined, effects on surface water and ground water, and how MTBE behaves in the environment (for example, how quickly it travels in streams and rivers before it changes to a gas) and how easily MTBE can be removed from drinking-water supplies.
General information about the USGS and links to the pages listed above are available at: http:/www.usgs.gov
* * * USGS * * * (Note to Editors: USGS spokesperson on MTBE John Zogorski, can be reached at his office in Rapid City, S. Dak., at (605) 394-1780 ext 214.) ------------------------------------------------------------------------
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