2253 Environmental Toxicology and Chemistry, Vol. 26, No. 11, pp. 2253–2259, 2007  2007 SETAC Printed in the USA 0730-7268/07 $12.00 + .00 Environmental Chemistry METHYL-TERT-HEXYL ETHER AND METHYL-TERT-OCTYL ETHER AS GASOLINE OXYGENATES: ANTICIPATING WIDESPREAD RISKS TO COMMUNITY WATER SUPPLY WELLS JEFF SNELLING,*† MARK O. BARNETT,† DONGYE ZHAO,† and J. SAMUEL AREY‡ †Department of Civil Engineering, Auburn University, Auburn, Alabama 36849, USA ‡Laboratory of Computational Chemistry and Biochemistry, Swiss Federal Institute of Technology, Lausanne, Switzerland ( Received 18 September 2006; Accepted 9 April 2007) Abstract—The widespread contamination of groundwater resources associated with methyl-tert-butyl ether (MtBE) use has prompted a search for replacement oxygenates in gasoline. Among the alternatives currently under development are higher methyl-tert-alkyl ethers, notably methyl-tert-hexyl ether (MtHxE) and methyl-tert-octyl ether (MtOcE). As was the case with MtBE, the introduction of these ethers into fuel supplies guarantees their migration into groundwater resources. In the present study, a screening-level risk assessment compared predicted well water concentrations of these ethers to concentrations that might cause adverse effects. A physicochemical model which has been successfully applied to the prediction of MtBE concentrations in community water supply wells (CSWs) was used to predict well water concentrations of MtHxE and MtOcE. The results indicate that these ethers are likely to contaminate water supply wells at slightly lower levels than MtBE as a result of migrating from leaking underground fuel tanks to CSWs. Because very little data is available on the physicochemical and environmental properties of MtHxE and MtOcE, estimation methods were employed in conjunction with the model to predict well water concentrations. Model calculations indicated that MtHxE and MtOcE will be present in many CSWs at concentrations approaching the concentrations that have caused widespread public health concern for MtBE. Based on these results and the possibility that MtHxE and MtOcE are potential carcinogens, testing of the toxicological properties of these ethers is recommended before they are used to replace MtBE in gasoline. Keywords—Groundwater contamination Physicochemical transport model Gasoline oxygenates Screening-level risk assessment INTRODUCTION Homologues of methyl-tert-butyl ether (MtBE) are cur- rently being developed as replacement oxygenates for MtBE in gasoline [1,2] and have also been proposed for use as ul- traclean diesel fuels [3]. Increasing the hydrocarbon content of an ether reduces the water solubility [4] and increases the organic carbon–water partition coefficient (K OC ) [5]. Therefore, we might expect these higher carbon number ethers to be less likely to migrate from the site of a leaking underground fuel tank (LUFT) and contaminate groundwater resources than MtBE. Two homologues of MtBE are of particular interest because they can be prepared from readily available olefinic feedstocks. These two homologues are methyl-tert-hexyl ether (MtHxE) and methyl-tert-octyl ether (MtOcE). Beginning in 1995, limited quantities of MtHxE were introduced into gas- oline in Finland as a supplementary oxygenate to MtBE [6]. Commercially, MtBE is prepared almost exclusively by the methanolation of the four-carbon olefin isobutylene, the iso- butylene being available in commercial quantities from petro- leum refinery cracking streams. Similarly, the methanolation of higher olefins (carbon numbers C 6 and C 8 ) is the most likely industrial route to MtHxE [7,8] and MtOcE [9]. However, the quantity of higher olefins in refinery cracking streams is lim- ited and decreases with increasing carbon number after C 5 [10]. It may be that the most feasible industrial route to these ethers is by the methanolation of the dimerization products of pro- pylene [11] and isobutylene [12], respectively. Both propylene * To whom correspondence may be addressed (jeff@watse.co.com). Published on the Web 7/6/2007. and isobutylene are readily available from cracking streams, and are easily dimerized to dipropylene (a mixture of 2,3- dimethyl-1-butene and 2,3-dimethyl-2-butene) and di-isobu- tylene (a mixture of 2,4,4-trimethyl-1-pentene and 2,4,4-tri- methyl-2-pentene), respectively. Although MtHxE has three isomers and MtOcE has 17 [13], the methanolation of dipropylene leads only to the methyl- tert-hexyl ether designated as 2-methoxy-2,3-dimethylbutane (CAS 26356-10-5), while the methanolation of di-isobutylene leads only to the methyl-tert-octyl ether designated as 2-meth- oxy-2,4,4-trimethylpentane (CAS 62108-41-2), in accord with Markovnikov’s rule. Figure 1 presents the chemical structures of these compounds. Because of the potential importance of higher ethers as high production volume fuel blend compo- nents, a number of studies of the preparation of MtHxE and MtOcE have recently appeared in the literature [3,9,11,12,14– 16]. As was the case with MtBE, the introduction of these ethers into fuel supplies guarantees their introduction into the envi- ronment as well. In an earlier paper we addressed issues of environmental concern related to the atmospheric dispersion and deposition of MtHxE and MtOcE as a result of evaporative and tailpipe emissions [17]. Although the greatest potential for human exposure to environmental MtBE is thought to be through inhalation, the threat posed by leakage of MtBE from LUFTs and its subsequent migration into groundwater re- sources has been the source of far greater public concern (http://www.arb.ca.gov/fuels/gasoline/oxy/mtbebp.pdf). Since MtHxE and MtOcE form part of a homologous chemical series with MtBE, the basic mechanisms of subterranean transport