Long-term data reveals the importance of hydraulic load and inflow water quality for Sb removal in boreal treatment peatlands

Abstract

Antimony (Sb) is a common contaminant in natural peatlands used as treatment wetlands for water influenced by metal mining in cold-climate regions. However, while other metalloids such as arsenic have been well studied, little is known about removal and retention of Sb in northern wetlands under challenging environmental conditions. In this study we assessed short-term, long-term and seasonal variations in mobility, removal, and retention of Sb from mining-influenced water in two peat based natural wetlands with different loading and physical conditions. Analyses based on 10 years of water quality data and data on contaminant accumulation in the peat soil revealed that the wetland with significantly lower hydraulic load and Sb areal load achieved adequate Sb removal, but with a slight decline in recent years. Antimony concentrations at the wetland outlet were slightly lower in summers than in winters. Dilution due to high rainfall during summer may be the likely reason for low outlet concentrations towards the end of summer. Outlet Sb concentrations were on the rise after inlet water quality was significantly improved through enhanced pre-treatment indicating mobilization of accumulated Sb. In comparison, the smaller wetland with higher hydraulic and Sb loading had very low Sb removal and a stronger decrease in Sb concentration through dilution due to snowmelt. The results highlight the challenges in Sb retention which should get more attention when treatment wetlands are designed for Sb rich waters such as mine waters and there are changes in water treatment arrangement in specific cases.