cm-1) [SO4 2-] (mM)a [CH4]aq (μM)b [H2]aq (nM)c DIC (mM)d DOC (mg.L-1)e High sulfate (HS)

wells Chm94B 13.7 7.5 707 0.58 < 0.2 25 7.8 2.2 Chm96A 13.8 7.5 663 0.41 1 3 7.2 1.3 Frd94A 14.2 7.5 760 0.98 2 3 7.4 < 0.4 Iro95A 14.3 7.5 943 1.50 1 60 n/a 3.3 Iro96A 12.1 7.5 1254 4.23 1 n/a n/a n/a Iro98B 13.0 7.6 1277 4.68 3 10 6.6 43.0 Iro98D 13.6 7.8 759 0.72 19 180 7.9 1.9 Ver94A 14.4 7.5 check details 1279 4.57 2 n/a 6.7 1.8 Ver94B 13.7 7.3 1893 10.73 1 89 4.8 1.1 Low sulfate (LS) wells Chm94A 14.1 7.6 651 0.07 4 n/a 8.0 3.6 Chm95A 14.0 7.6 649 0.14 8 4 7.7 2.1 Chm95B 13.8 7.9 670 0.04 30 3 7.9 2.0 Chm95C 13.7 7.7 601 0.11 3 20 6.6 0.5 Frd94B 15.4 7.6 611 0.05 43 9 7.4 < 0.4 Iro98C 13.3 7.4 664 0.04 15 66 7.6 2.3 Ver94C 13.6 7.7 616 0.23 3 46 7.4 1.1 Ver94D 13.9 7.7 621 0.18 10 n/a 7.7 0.8 Negligible sulfate (NS) wells AnderN 14.8 7.6 617 0.02 91 144 6.6 n/a AnderS 15.1 7.1 860 0.02 1237 175 25.9 n/a CardiS 13.6 7.7 645 0.03 454 240 7.5 n/a Chm95D 14.0 7.8 625 < 0.01 220 12 7.6 1.6 Chm98A 13.7 7.7 714 < 0.01 676 24 7.9 4.2 PklndE 14.6 7.6 678 0.03 221 63 8.7 n/a PklndW 14.4 7.5 725 0.03 611 100 6.0 n/a RailRd 14.4 7.7 661 0.02 106 50 6.4 n/a a The detection limit for sulfate LY294002 nmr is 0.01 mM. b The detection limit for dissolved methane is 0.2 μM. c The detection limit for dissolved hydrogen

is 0.5 nM. d The detection limit for dissolved inorganic carbon is 0.5 mM. e The detection limit for dissolved organic carbon is 0.4 mg L–1. Figure 2 A comparison of the methane (CH 4 ) and sulfate (SO 4 2- ) concentrations

of individual wells in the Mahomet aquifer. The amount of sulfate in HS wells is > 0.2 mM, is between 0.03 and 0.2 mM in LS wells, and is less than 0.03 mM in NS wells. In contrast to what might be expected from previous work [43, 44], H2 concentrations did not increase as methanogenic conditions became predominant in the NS wells and therefore had little impact on the available energy ID-8 calculation.