Using the INCA-Hg model of mercury cycling to simulate total and methyl mercury concentrations in forest streams and catchments.

We present a new, catchment-scale, process-based dynamic model for simulating mercury (Hg) in soils and surface waters. The Integrated Catchments Model for Mercury (INCA-Hg) simulates transport of gaseous, dissolved and solid Hg and transformations between elemental (Hg(0)), ionic (Hg(II)) and methyl (MeHg) Hg in natural and semi-natural landscapes. The mathematical description represents the model as a series of linked, first-order differential equations describing chemical and hydrological processes in catchment soils and waters which we believe control surface water Hg dynamics. The model simulates daily time series between one and 100 years long and can be applied to catchments ranging in size from <1 to ~10,000 km(2). Here we present applications of the model to two boreal forest headwater catchments in central Canada where we were able to reproduce observed patterns of stream water total mercury (THg) and MeHg fluxes and concentrations. Model performance was assessed using Monte Carlo techniques. Simulated in-stream THg and MeHg concentrations were sensitive to hydrologic controls and terrestrial and aquatic process rates.