Nutrient and mercury transport in a sub-arctic ladder fen peatland subjected to simulated wastewater discharges.

Safely treating wastewater in remote communities and mining operations in sub-arctic Canada is critical to protecting the surrounding aquatic ecosystems. Undisturbed fen peatlands have been used to minimize the release of contaminants to the aquatic ecosystems; however, there is a limited understanding of wastewater transport or polishing in undisturbed fen peatlands. To elucidate these processes, a small (9800m2 , ~250m long) ladder fen was continuously injected with a wastewater surrogate derived from a custom fertilizer blend and 38m3 day-1 of water for 51days. The simulated wastewater included sulphate (27.2mgL-1 ), nitrate (7.6mgL-1 ), ammonium (9.1mgL-1 ), phosphate (7.4mgL-1 ), and chloride (47.2mgL-1 ). Major ion, total mercury (THg) and methylmercury (MeHg) pore water concentrations were measured throughout the study period. No wastewater contaminants were detected in the site outlet (~250m down-gradient) and most wastewater contaminants, except for SO4 2- and Cl- , remained relatively immobile. Within the SO4 2- plume, MeHg and THg concentrations became highly elevated relative to background (up to 10ngL-1 , ~ three to five-fold increase) and MeHg comprised 60-100% of dissolved THg in the pore water. No MeHg or THg was exported at the outflow. The large increase in THg cannot be solely accounted for by the increase in MeHg and was likely due to enhanced decomposition of the peat substrate by increased microbial activity due to electron acceptor loading. Since the added nutrients were effectively transformed, sequestered or otherwise removed from pore waters in this experimental system, it appears that fen peatlands have a large capacity to safely treat residential wastewater nutrients; however, the inadvertent increases in THg and MeHg require further investigation and potential management.