S1 Supporting information for Impact of Ethanol on the Natural Attenuation of MTBE in a Normally Sulfate-Reducing Aquifer Doug Mackay*, Nick de Sieyes, Murray Einarson†, Kevin Feris, Alex Pappas, Isaac Wood, Lisa Jacobson, Larry Justice, Mark Noske, John Wilson‡, Cherri Adair‡, and Kate Scow Department of Land, Air & Water Resources, University of California, Davis, CA *dmmackay@ucdavis.edu †Geomatrix Consultants, Oakland, CA ‡U.S. Environmental Protection Agency, Ada, OK Summary of contents Six pages, with three tables and three figures. Method for estimating mass discharge of MTBE and TBA The mass discharge (total mass flux) of MTBE and TBA migrating within the S3 aquifer was estimated for the EH transect of monitoring wells for seven snapshot sampling sessions (2 before the onset of the injections and 5 after) using the methods described in the Supporting Information to (1). Table SI-1 presents the results for the seven snapshot samplings. The results are plotted in Figure 7 of this paper. Methods for estimating in situ transformation rate of MTBE Additional mass discharge estimates for MTBE and TBA are listed in Tables SI-2 and SI-3, organized for use in estimating the MTBE transformation rate in the ethanol-impacted lane. We used two approaches to estimate this transformation rate. Both methods are based on the assumption of a constant MTBE mass discharge emanating from the source zone; this assumption appeared reasonable based on data from two pre-experimental sampling snapshots (Table SI-2) which had a mean and standard deviation of 167 and 14 mg/d, respectively. We thus concluded it reasonable, for the present purposes, to assume the pre-experimental mass discharge from the source zone (cumulative from the ER to the ED transect) was a constant equal to the mean of the estimates from the individual snapshots. We also assumed that this MTBE emanation was constant throughout the experiment. Mass discharge was calculated using a wide array of wells in each lane in each transect to ensure no relevant MTBE or TBA flux was missed. The two methods for calculating transformation rates from this assumed constant source are outlined below. For both methods, we used data from the 12/04 and 1/05 snapshot samplings, which were assumed to best represent steady state, or near steady state, conditions.