MINTRAN is a new model for simulating transport of multiple thermodynamically reacting chemical substances in groundwater systems. It consists of two main modules, a finite element transport module (PLUME2D), and an equilibrium geochemistry module (MINTEQA2). Making use of the local equilibrium assumption, the inherent chemical nonlinearity is confined to the chemical domain. This linearizes the coupling between the physical and chemical processes and leads to a simple and efficient two‐step sequential solution algorithm. The advantages of the coupled model include access to the comprehensive geochemical database of MINTEQA2 and the ability to simulate hydrogeological systems with realistic aquifer properties and boundary conditions under complex geochemical conditions. The model is primarily targeted toward groundwater contamination due to acidic mine tailings efiïuents but is potentially also applicable to the full range of geochemical scenarios covered by MINTEQA2. The model is tested with respect to ion exchange chemistry and with respect to precipitation/dissolution chemistry involving multiple sharp fronts. The companion paper presents two‐dimensional simulations of heavy metal transport in an acidic mine tailings environment, focusing on environmental implications.

The mobility of potentially toxic dissolved metals discharged from a mine tailings source into an aquifer is investigated by using the multicomponent reactive transport model MINTRAN. A generic aquifer resembling a site in northern Ontario is used as a basis, and a scenario analysis is performed to determine the effect of terminating the source input of acidity and dissolved metals after a finite time. It is found that for most metals the buffering reactions in the carbonate aquifer lower the dissolved metal concentrations substantially, often by several orders of magnitude. In the case of a 12‐year source duration, most metals are effectively immobilized by precipitation, and the concentrations of metals remaining in solution are well below drinking water limits. The risk of contamination of water resources by toxic metals can thus be controlled.