Passage of the Clean Air Act Amendments in 1990 imposed requirements on gasoline composition in the United States. Impacts to ground water are affected by the provisions that required oxygenated additives and limited benzene concentration. Reformulated and oxygenated gasoline were required to contain an oxygenated additive at 2.0 wt % and 2.7 wt %, respectively. In most cases, the additive initially was methyl tert-butyl ether (MTBE). The amount of benzene in both reformulated and conventional gasoline was limited: reformulated gasoline contains less than 1 % benzene by volume, while benzene levels in conventional gasoline were set by producer baselines. The allowable benzene levels vary among these producers and, unlike reformulated gasoline, are not tied to use at specific locations in the U.S. In 2000, states began to pass bans on MTBE, other ethers, and/or alcohols; consequently, production and use of MTBE in reformulated gasoline declined. The Energy Policy Act of 2005 removed the oxygenate requirement from reformulated gasoline and industry responded by removing ethers from U.S. gasoline, with some limited exceptions.