An unconfined, gasoline-contaminated aquifer (2.3 to 5 m water table) in fractured sedimentary bedrock has been under remediation since 1999 in Bellingham, Washington. The central portion of the ground water plume has been difficult to remediate because of the fractured sedimentary bedrock and the presence of significant utilities that have prevented aerobic in situ cleanup methods. A 15-month field trial was conducted to determine the efficacy of enhancing the attenuation of benzene, toluene, ethylbenzene, and xylenes (BTEX) hydrocarbons at the site by continuously amending the aquifer with sulfate that was initially introduced into the aquifer using an infiltration trench in 3.8-m3 batches at a concentration of 500 mg/L. The goal was to stimulate anaerobic bacteria and enhance dissolved-phase gasoline and BTEX biodegradation rates. The subsurface microbial ecology in the aquifer was monitored during sulfate injection using Bio-Sep® biotraps (Microbial Insights Inc., Rockford, Tennessee). Bio-Sep beads have been shown to be very effective in collecting biofilms, which are more indicative of in situ microbial ecology than planktonic organisms from ground water samples. Sulfate injection was shown to increase the rates of biodegradation of BTEX components by more than twofold for toluene, for example, and about fivefold for gasoline-range hydrocarbons. The subsurface microbial community became more anaerobic in character as sulfate utilization increased as evidenced by its depletion in the aquifer. Bead biofilms were also compared to planktonic samples from ground water monitoring wells. During sulfate injection, bead biofilm biomarkers were shown to correlate with changes in ground water geochemistry, while planktonic samples from ground water remained relatively unchanged over time.