The objectives of this investigation are to 1) determine the potential ecological risk to aquatic biota in California surface waters, and 2) determine the potential exposure of humans to MTBE and other fuel oxygenates by contact with surface or ground water contaminated with MTBE. This investigation will utilize information generated by the other components of the project including the concentrations of MTBE in surface and groundwater based on the fate and transport investigations, and the toxicity of MTBE to terrestrial and aquatic biota. The ecological risk assessment (ERA) will be a screening level assessment based on U.S. EPA guidelines and involves three primary steps; problem formulation, analysis, and risk characterization. Problem formulation involves three steps: establishing the water-based exposure pathways (ingestion, inhalation, dermal) for aquatic biota, developing the ecological scenarios to evaluate (e.g., lake with pelagic zone and game fish), and developing conceptual site model (i.e., appropriate food chain model). The analysis phase of the ERA includes calculating the expected exposure of each ecological receptor to MTBE, and determining the potential effects of that exposure. The analysis will be performed using Monte Carlo or modified Monte Carlo (Latin hypercube sampling) techniques. We will develop the assumptions for analysis (e.g., distributions of the size of the population exposed vs. concentration of MTBE and other oxygenates, constant vs. variable environmental concentrations of oxygenates over time), obtain distributions/constants for Monte Carlo analyses, and obtain data on concentration of MTBE (and other fuel oxygenates) in water supplies (groundwater/surface water). The risk characterization phase combines the two aspects of the analysis phase to produce an estimate of the probability that the exposure will result in adverse ecological impacts. The human health exposure requires the same basic information as the exposure analysis of the ecological risk assessment, i.e., determine who is exposed to MTBE, establish the water-based exposure pathways (ingestion, inhalation, dermal) for that population, develop the assumptions for analysis (e.g., distributions of the size of the population exposed vs. concentration of MTBE and other oxygenates, constant vs. variable environmental concentrations of oxygenates over time), obtain distributions/constants for Monte Carlo analyses, and obtain data on concentration of MTBE (and other fuel oxygenates) in water supplies (ground water/surface water). Several exposure scenarios will be developed based on the potential management options available. The exposure scenarios will be evaluated within the framework of the costs and benefits of the management options.