Geochemical processes governing the performance of a constructed wetland treating acid mine drainage, Central Scotland

Constructed wetland systems often represent a low cost means for the remediation of dissolved metal and SO4-rich acid mine drainage. The surface water, sediment, and porewater geochemistry of one such wetland were investigated to determine the active geochemical processes that govern wetland performance. The influent drainage has a pH of 2.6, 200–250 mg/L of dissolved Fe, and Al, Mg, and Mn concentrations of 30–100 mg/L. Sulphate concentrations are also high, ranging from 500–1000 mg/L. Sediment cores show high levels of amorphous Fe oxyhydroxides near the surface and some amorphous reduced inorganic sulphur (RIS) in deeper layers. Taken together with a decline in porewater SO4 with depth, the data suggest that bacterial SO4 reduction is occurring. All sediment cores display an increase in porewater SO4 with depth in their top sections, often forming a subsurface peak, which could be attributed to oxidative dissolution of RIS at the redox boundary. However, the increase in SO4 is also accompanied by a rise in porewater pH. Desorption experiments yielded up to 8 wt% SO4 in these top layers suggesting that pH dependant sorption is an active SO4 removal mechanism along with the precipitation of Fe oxyhydroxysulphates, which also removes dissolved Fe. The maximum treatment efficiency of the wetland is 25% for the removal of dissolved Fe, and the retention of other species is equally poor. Comparison with other wetland systems suggests that poor performance is due to the low pH and high dissolved metal concentration of the drainage together with deterioration in the wetland over time, as a result of reduced active surface area and retention time.