Sulfate-reducing bioreactors (SRBRs) have shown promise as a costeffective option in the passive remediation of acid mine drainage (AMD). While these systems do provide the necessary conditions for increased bacterial activity, little is known about the internal dynamics and the functional lifespan of the systems in field settings. To help address these issues, two field-scale bioreactors are being monitored using an array of sampling ports distributed at varying depths throughout the treatment cells. These internal monitoring ports are located in such a way as to observe 3-D trends in activity occurring within the system. Water samples collected from the ports, as well as samples from the AMD inflow and outflow, have been analyzed for δ 34 S of sulfate as well as standard chemical parameters. Preliminary results indicate that in both systems, bacterial sulfate reduction is occurring yet the degree of reduction is not uniform throughout the cells. Within each system, areas where only a limited amount of bacterial sulfate reduction has occurred are characterized by high concentrations of sulfate coupled with δ 34 S values only slightly different than the influent AMD. In contrast, low sulfate concentrations together with large δ 34 S fractionations are found in areas where extensive bacterial sulfate reduction has taken place. The observed range in fractionation values likely reflects the development of preferential flow paths and points of stagnation within the systems. This implies that not all of the reactive substrate put into a cell will contribute to AMD treatment. The results of this study provide information not typically attainable in smaller laboratory-scale studies and point to the need for further engineering of SRBRs to optimize fieldscale applications.