The Sancho water reservoir has been impacted by acid mine drainage (AMD) since the Tharsis mine stopped activity in 1998. As a result, the water reservoir nowadays has a pH around 3.5, as well as high concentrations of sulfate, aluminum, iron and trace metals. Thus far, the expected removal of contaminants by sediment burial has not been as effective as expected in improving water quality within the reservoir. To better design remediation strategies, new tools capable of quantifying metal and acidity transfers across the water-sediment interface (WSI) are needed. A 1-D, non-steady-state biogeochemical reactive transport model with a comprehensive set of equilibrium and kinetic biogeochemical microbially-driven reactions was adapted to simulate the fate and transport of trace metal and acidity in sediments affected by AMD. Two submodels account for the spatial heterogeneity of the oxygen concentration of the bottom water. The “permanently oxic” submodel represents the sediments in areas shallower than the thermocline, while the “holomictic” submodel represents the deeper sediments where bottom water oxygen levels are affected by turnover events (which cause the bottom waters to oscillate between completely anoxic and oxic conditions). The model was calibrated with measured depth distributions of pore water and solid phase chemistry. Model results imply that under permanently oxic conditions the sediments act as a sink for acidity (H<sup>+</sup>), as well as aqueous Al, Zn, Cu, Co and Ni, but as a source of aqueous Mn, Fe and As. The latter are released to the overlying water as a result of Mn and Fe (hydr)oxide reduction. In the holomictic submodel, when bottom waters turn anoxic, metal sulfides precipitate in the sediment. When the bottom waters subsequently switch to oxic, the metal sulfides are oxidized (along the oxygen front) and the associated metals are released to the overlying water.

Metal fluxes across the water-sediment interface in a reservoir affected by acid mine drainage: Reactive transport modeling

253rd National Meeting (2017)

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