The cycling of mercury in the environment and in well water was studied to determine the seriousness of the threat to human health of the elemental mercury sometimes contained in water well equipment. Modeling of mercury cycling indicated that, because of unfavorable conditions, mercury in well water is highly unlikely to be methylated to the toxic methylmercury form. More important, aquatic organisms, such as fish and shellfish, that can concentrate methylmercury and could be consumed by humans are not present in well water.