Shallow floating treatment wetland capable of sulfate reduction in acid mine drainage impacted waters in a northern climate

Abstract

Floating treatment wetlands (FTW)s that can uptake nutrients and metals from water, and/or trap suspended solids in their roots, are becoming viable options to treat urban, agriculture and sewage runoffs. However, current FTW designs favor aerobic processes and short-term storage of metals, which are ineffective in acid mine drainage (AMD) environments. Many also function poorly in northern latitudes with strong seasonality and several months of sub-zero temperatures. In this study, we designed a novel FTWs with 20 cm soil profile to test its ability to sustain anaerobic microbial processes, such as iron and sulfate reduction and remain functional after freezing conditions of winter months. Three different plants, Carex lacustris, Typha latifolia, and Juncus canadensis were used to test in our FTWs, which were deployed in a mining-impacted water in Sudbury, ON, Canada. Porewater samples were acquired using built-in porewater peepers. Low to moderately reducing conditions, along with presence of ferrous iron and hydrogen sulfide in the porewater of all FTWs was prevalent, irrespective of the constituent vegetation type. Moreover, as well as a ~30% increase in sulfate-reducing bacteria (SRB) richness and ~100% increase in SRB abundance between years, was the evidence that anaerobic processes were occurring in these shallow FTWs. From this study we estimated that during its lifetime, one shallow FTW can treat ~61 m³ of sulfate-rich water, thus offering an alternative way to capture sulfate and other metals from mining-impacted waters.

Keywords: Anaerobic; Boreal landscape; Mine closure planning; Passive water treatment; Sulfate reducing bacteria.