A list of surface water bodies, including lakes, reservoirs and streams was obtained from the California Department of Health Services; this list denotes points for water distribution. This data was investigated further to determine the ultimate water source(s) for the State's surface drinking water supply. We have identified 232 individual sources and have data on nearly 50% of these waterbodies. Where available, data includes, an estimate of the population served from each source, volume of the waterbody, depth of water intake, recreational use, values fro MTBE measurements, etc. Each waterbody does not necessary have exclusive use by a single water supplier. In general the surface water supplies could be placed into the following categories: (1) water taken directly from lake or reservoir - location of intake found at single or multiple depths; (2) water taken from a stream or river after discharge from an upstream reservoir/lake - at times the river water was a blend of discharge from multiple lakes/reservoirs; and (3) water was taken from a downstream reservoir which received flow from an upstream reservoir.

Approximately 80% of the waterbodies for which we have currently surveyed have some amount of MTBE concentration data from water quality sampling. Nearly one-half of these are below the analytical limit of detection; however, the detection limit can be as high as 5 parts per billion. Additionally, the location of the water quality sample was not always located adjacent to the water intake pipe (e.g. if sampling were done in a marina). Further analysis of this data will help determine the overall distribution of MTBE in the surface drinking water bodies in California, and will help shed light on the appropriateness of the current sampling protocol.

Fate and Transport in Surface Waters

Despite recent environmental policy decisions regarding the continued use of MTBE, few published studies exist on sources, fate and transport of MTBE in surface waters. Complete vertical profiles of MTBE in Donner Lake, California were taken for nearly a year to determine the (1) relative contribution of motorized watercraft as a source of MTBE, (2) its seasonal distribution, (3) loss from the water column, (4) extent of transport from surface waters into deeper portions of the lake, and (5) persistence between years. Donner Lake is a subalpine, multiple-use lake in the Sierra Nevada Mountains; however, on the basis of our analysis to date of the state-wide MTBE data base for surface waters and discussions with other investigators conducting the few comprehensive studies which are being done on this topic, it appears that a number of the processes we have observed at Donner Lake may likely apply to other lakes/reservoirs. Current efforts are being directed towards looking at these processes further in other lakes where data is available to quantify their magnitude of importance.

A summary of the Donner Lake study showed that recreational boating was clearly the most important source of MTBE in this lake, and it appeared that engine exhaust and not spills was the major factor. Statistically, 86% of the variation in the seasonal trend of total lake MTBE was explained by variation in the use of motorized watercraft. Neither urban runoff nor precipitation contributed significantly. Analysis of fresh snow fall indicated that in all samples MTBE was less than 0.1 parts per billion (ppb). MTBE concentration ranged from < 0.1 ppb to a high of 12 parts ppb measured in the surface waters just after the July 4th holiday. Between July 1-7, 1997, total lake MTBE rose dramatically from 115 kg to 365 kg. By the following January, levels declined to a minimum of 15 kg suggesting little, if any, persistence between years. The major loss of MTBE from Donner Lake appeared to be volatilization at the air-water interface. Loss was calculated during two distinct periods. During the boating season, and following the seasonal maximum, total lake MTBE declined at a rate of 1.2 kg.d-1 (193 day half-life). However, once boating drastically declined at the end of the summer recreation season, MTBE loss increased to 8.1 kg.d-1 (14 day half-life). These loss values will vary as a result of magnitude and seasonality of boat use and other in-lake conditions. As part of the UC Davis SB 521 effort we will analyze this further using the existing state-wide data base and newly created mathematical models (see abstract in this series by G. Schladow). The influence of mean daily wind speed on the seasonal distribution of MTBE was minimal. The density gradient in the lake produced by thermal stratification was an effective barrier against MTBE transport to deeper depths.

Lake Tahoe Study

A focused assessment of the impact(s) of MTBE on the Tahoe Basin environment was specifically required in SB 521. This is being done within the context of a much larger investigation at Lake Tahoe by a multi-agency team investigating the effect of motorized watercraft on water quality and air quality. The design and the pre-project coordination has been completed, with majority of the data collection scheduled for the period June-October. Members of the UC Davis MTBE team have been involved with the coordination of each of the five major focus areas: (1) MTBE monitoring in the surface waters of seven nearshore and two offshore locations - first survey in July [U.S. Geological Survey; Tahoe Regional Planning Agency]; (2) MTBE monitoring in deep waters - water samples from thirteen depths from surface (0 m) to near bottom (450 m) in Lake Tahoe have been collected on a monthly basis since January 1998 [UC Davis, Tahoe Research Group]. These data provide information on deep-water concentrations, year-to-year persistence of MTBE, and establish a comprehensive baseline prior to the boating season. Results to date show MTBE concentrations at all depths to be less than 0.1 parts per billion. Efforts to reduce the analytical limit of detection to extremely low levels revealed actual concentrations to be extremely low and near true background at 0.030-0.050 parts per billion; (3) In-lake testing of MTBE emission during boat use - this work looks at MTBE loading from motorized watercraft under actual lake conditions and first survey is scheduled for June 1998 [Lahontan Regional Water Quality Control Board and University of Nevada-Reno]; (4) Tank testing of MTBE emission from spark ignition marine engines - the goal of this project is to test outboard, jet boat and personal watercraft engines for air and water emissions - MTBE will be one of many exhaust products evaluated under controlled experimental conditions [California Air Resources Board]; (5) Lake Tahoe boat use survey - data from this survey will allow for the estimation of projected whole-lake MTBE loading from this source [Nevada Department of Waterways, California Department of Boating and Waterways, Tahoe Regional Planning Agency]. Members of the UC Davis MTBE project team will coordinate data analysis and reporting phases of this study.