Final Report: Effects of Air Pollution on Water Quality: Emission of MTBE and Other Pollutants From Motorized Watercraft

EPA Grant Number: R825433C024
Subproject: this is subproject number 024 , established and managed by the Center Director under grant R825433
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: EERC - Center for Ecological Health Research (Cal Davis)
Center Director: Rolston, Dennis E.
Title: Effects of Air Pollution on Water Quality: Emission of MTBE and Other Pollutants From Motorized Watercraft
Investigators: Reuter, John E. , Allen, Brant C. , Jassby, Alan D. , Goldman, Charles R.
Institution: University of California - Davis
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 1996 through September 30, 2000
RFA: Exploratory Environmental Research Centers (1992) RFA Text |  Recipients Lists
Research Category: Center for Ecological Health Research , Targeted Research


The objectives of this research project were to: (1) estimate atmospheric nutrient deposition to Lake Tahoe for inclusion in the lake's nitrogen and phosphorous input budget; and (2) estimate wet and dry atmospheric deposition and compare it to the other primary sources. These objectives are important for helping to guide restoration activities in the Tahoe Basin because increased nutrient loading increases algal growth rates which, in turn, contribute to the ongoing decrease in water clarity. We also investigated the sources, fate, and transport of the gasoline additive methyl tertiary-butyl ether (MTBE) in lakes and reservoirs as another example of the direct interaction between air quality and water quality.

Summary/Accomplishments (Outputs/Outcomes):

As we increased our understanding of Lake Tahoe's algal growth patterns, it became clear that for successful pollutant management, we needed a nutrient budget that quantified the critical sources and sinks of nutrients. In developing a preliminary nutrient input budget for Lake Tahoe, five major sources were identified: (1) direct wet and dry atmospheric deposition; (2) stream discharge; (3) overland runoff directly to the lake; (4) groundwater; and (5) shoreline erosion. Our results show that atmospheric deposition provides a significant portion of the dissolved inorganic nitrogen and total nitrogen to the annual nutrient load of Lake Tahoe. We further concluded that atmospheric deposition also contributes significant amounts of soluble reactive phosphorous and total phosphorous to the lake, but to a lesser extent.

The second portion of the research investigated sources, fate, and transport of the gasoline additive MTBE in lakes and reservoirs. This study is highly relevant to the Center for Ecological Health Research (CEHR) mission because it represents a new stressor to lakes and groundwater (before 1995, MTBE contamination in bodies of water was not known), and because its purpose was to improve air quality. This environmental "dilemma" provides an excellent example of how air quality and water quality are inexorably linked, and how policy must consider all environmental aspects when solving specific problems. Our study considered the role of motorized watercraft in MTBE-contaminated waters. These watercraft have not always been recognized as an important potential source of MTBE in lake and reservoir surface water because of, in part, the focus given to groundwater contamination. Concurrent studies, however, by the Metropolitan Water District of Southern California and the University of California at Davis (this study), clearly identified motorized recreational watercraft as the predominant source of MTBE in these lakes/reservoirs. At the same time, monitoring by the East Bay Municipal Utilities District in the San Francisco Bay Area linked MTBE levels in drinking water reservoirs to motorized boating activity.

Our study shows that different watercraft engines (two stroke vs. four stroke) release substantially different amounts of MTBE into the water. Based on the volumes of fuel used by each class of engine, and the data about MTBE emissions from various engine types, we calculated that carbureted two-stroke engines contribute a disproportionate load of MTBE, benzene, and toluene to Lake Tahoe, as compared to four-stroke engines. Two-stroke engines cumulatively contributed more than 90 percent of the MTBE to Lake Tahoe, using only about 11 to 12 percent of the total fuel used by boats in 1998. Moreover, these engines were responsible for more than 70 percent of the benzene and 80 percent of the toluene deposited during the boating season. In contrast to this, the four-stroke inboard and inboard/outboard class consumed 87 percent of the fuel used by boating on Tahoe, but was responsible for 7 percent of the estimated MTBE, 28 percent of the estimated benzene, and 17 percent of the estimated toluene loading to the lake. We were able to confirm this hypothesis directly when measured levels of MTBE in Lake Tahoe declined by 90 percent following a ban on most two-stroke marine engines.

The following activities were accomplished:

• Our study represents the first published research to conclude that atmospheric deposition provides most of the inorganic nitrogen and total nitrogen, as well as significant amounts of soluble reactive phosphorous and total phosphorous, to the annual nutrient load of Lake Tahoe. Furthermore, this study adds to the growing body of literature that has begun to document the potential importance of direct atmospheric deposition in the nutrient budgets of select bodies of water. This finding is important because it allows for a better itemized nutrient budget and a more complete understanding of the atmospheric factors contributing to decreased water clarity and increased algal growth rates in bodies of water. These conclusions have led to a more comprehensive exploration of atmospheric nutrient sources, including a study by the California Air Resources Board.

• MTBE is an oxygenate added to gasoline (10-15 % by volume) to reduce air pollutant emissions; however, the potential for drinking water resources is widespread. The U.S. Environmental Protection Agency (EPA) classifies MTBE as a possible human carcinogen. The discovery of MTBE in surface water and groundwater used for drinking water has raised scientific, public, and political concerns throughout the United States. On the basis of scientific data, both California and the U.S. EPA have called for a phase out of MTBE from fuels.

• We found that approximately 50 percent of the sampled California lakes and reservoirs used for drinking water supplies contained measurable MTBE on at least one sampling date. Of the 105 bodies of water for which data was available, 10-15 percent contained MTBE at concentrations above the public health limit of 13 µg/L on at least one date. For most bodies of water, the data were insufficient to evaluate the length of time over which MTBE exceeded this goal. Data to date suggest that although the mere presence of MTBE in California lakes and reservoirs is widespread, public health impacts to these drinking water resources is minimal overall, but may be an issue in a few select bodies of water in which violation of the stringent California secondary drinking water standard for taste and odor (5 µg/L) was more likely to occur. This finding will be useful to water resource policymakers in California, as well as to water quality regulators.

• We identified the emission of unburned fuel directly into the water from marine engines as the primary source of MTBE in lake and reservoirs. Comprehensive studies from Donner Lake, Lake Tahoe, and other bodies of water showed a close statistical relationship between MTBE concentrations and marine engine use. This conclusion was corroborated by the statewide data discussed above. Carbureted two-stroke engines at Lake Tahoe contributed a disproportionate load of MTBE relative to fuel use. This information will affect the regulation of recreational vehicles used in lakes and reservoirs.

• The major loss of MTBE from surface water bodies appeared to be volatilization at the air-water interface. In the absence of new sources, the half life of MTBE in Donner Lake was 14 days. Similarly, MTBE in Lake Tahoe dropped at the end of the boating season. Negligible levels of MTBE during the winter in both these lakes suggest little interannual persistence. This information suggests that although MTBE contamination may be a serious problem, it is relatively easily mitigated.

• Lake and reservoir managers can protect their water body from MTBE contamination by restricting the use of carbureted two-stroke engines. A ban on most two-stroke engines at Lake Tahoe resulted in a 10-fold decline in MTBE, lakewide. This level of reduction was predicted based on studies of fuel use and engine emissions.

• We found that thermal stratification retarded MTBE transport to deep depths. At turnover, MTBE was distributed with depth; however, these increases were only temporary because of volatilization. This is important because it implies that deeper waters are less likely to be contaminated and reduces the possibility that MTBE concentrations will accumulate in the deeper waters over time.

Additional Work Conducted as an Outgrowth of This CEHR Project

In a recent publication funded by the Awwa Research Foundation and the California Department of Health Services, and conducted in cooperation with the Contra Costa Water District and CH2M Hill (Oakland, CA), the University of California at Davis developed a simple spreadsheet program to estimate maximum MTBE and benzene, toluene, ethylbenzene, and xylenes (BTEX) concentrations in reservoirs. This program is based on an extensive series of runs with the dynamic lake model-MTBE model. The user inputs data for a local reservoir, chooses a compound, and the spreadsheet calculates a predicted maximum concentration for the boating season.

The estimator (MTBE_Estimator.xls) was tested on several reservoirs for which corroborating data exist. The estimator was found to predict conditions with reasonable success, with the exception of shallow reservoirs. If the reservoir is shallow enough that it is mixed to the bottom, the estimated volatile organic compound level may be low. This tool provides lake and reservoir managers with the means to determine the allowable level of watercraft use (by number and engine type) that can occur without exceeding water quality criteria for MTBE and BTEX. Further information on this model can be obtained from Geoff Schladow, Department of Civil and Environmental Engineering, University of California at Davis (

(General note: Although we believe that these conclusions have general applicability to many lakes and reservoirs, specific conditions such as accidental spills, unusual lake hydrodynamics, etc. would represent special circumstances.)

Supplemental Keywords:

ecosystem, ecosystem protection, environmental exposure and risk, geographic area, international cooperation, water, terrestrial ecosystems, aquatic ecosystem, aquatic ecosystem restoration, aquatic ecosystems and estuarine research, biochemistry, ecological effects, ecological indicators, ecological monitoring, ecology and ecosystems, environmental chemistry, restoration, state, water and watershed, watershed, watershed development, watershed land use, watershed management, watershed modeling, watershed restoration, watershed sustainability, agricultural watershed, exploratory research environmental biology, California, CA, Clear Lake, Lake Tahoe, anthropogenic effects, aquatic habitat, biogeochemical cycling, ecological assessment, ecology assessment models, ecosystem monitoring, ecosystem response, ecosystem stress, environmental stress, environmental stress indicators, fish habitat, hydrologic modeling, hydrology, integrated watershed model, lake ecosystems, lakes, land use, nutrient dynamics, nutrient flux, water management options, water quality, wetlands., RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Water, Ecosystem Protection/Environmental Exposure & Risk, Aquatic Ecosystems & Estuarine Research, Environmental Chemistry, Aquatic Ecosystem, Biochemistry, Environmental Monitoring, Ecology and Ecosystems, Drinking Water, fate and transport, human health effects, watershed management, watersheds, ecosystem monitoring, MTBE, source water, air pollution, aquatic ecosystems, environmental stress, water quality, drinking water contaminants, ecosystem stress, hydrologic modeling

Relevant Websites: Exit

Progress and Final Reports:

Original Abstract
  • 1997
  • 1998
  • 1999 Progress Report

  • Main Center Abstract and Reports:

    R825433    EERC - Center for Ecological Health Research (Cal Davis)

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
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    R825433C007 Environmental Chemistry of Bioavailability in Sediments and Water Column
    R825433C008 Reproduction of Birds and mammals in a terrestrial-aquatic interface
    R825433C009 Modeling Ecosystems Under Combined Stress
    R825433C010 Mercury Uptake by Fish
    R825433C011 Clear Lake Watershed
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    R825433C041 Inorganic Analysis
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    R825433C073 Aquatic - Rivers and Estuaries
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