A two-stage passive treatment approach was assessed at a bench-scale level using two Colorado Mining Influenced Waters (MIWs). The first-stage was a limestone drain with the purpose of removing iron and aluminum and mitigating the potential effects of mineral acidity. The second stage was a sulfate reducing bioreactor composed solely of 50% corn stover and 50% walnut shells by volume. The primary difference in the two MIWs was the concentration of zinc 5-7 mg/L for the National Tunnel Adit drainage (NTA) vs. 65-75 mg/L for the Silver Cycle Adit drainage (SCA). The limestone pretreatment columns reduced the zinc in the NTA MIW to 1-2 mg/L and the SCA MIW to 38 - 56 mg/L. The two SCA biocolumns had similar zinc removal but different sulfate removal with time. The sulfate reduction rate (SRR) for the SCA columns peaked at day 50 but at 0.5 mol S/m 3/d for column 1 and 0.3 mol S/m 3/d for column 2. Average SRR after day 50 was 0.24 and 0.13 mol S/m 3/d for columns 1 and 2, respectively. The NTA columns (3 and 4) sustained an averaged SRR of 0.3 mol S/m 3/d for days 30-130. The effluent zinc after startup from the two systems were < 0.1 mg/L and <2 mg/L for the NTA and SCA treatment systems, respectively. Other significant results included startup of sulfate reduction in both sets of bioreactors without the typical "manure" inoculum. The time to start up was not negatively affected by the lack of a designated inoculum. Another important result was the longer start up time required and the overall lower sulfate reduction observed for the higher zinc MIW.