Sediment Diagenesis Module for CE-QUAL-W2 Part 2: Numerical Formulation

Pit lakes are used worldwide as part of closure schemes for mining operations. Pit lakes have also been proposed in the Athabasca Oil Sands Region of Canada where bitumen is close enough to the surface to be recovered using conventional surface mining and chemical extraction procedures. Many of the pit lakes are designed to cap tailings with a substantial water layer. The pore water in tailings will contain various amounts of labile and refractory organic compounds, nutrients, trace metals, and ion concentrations that, ultimately, will be released to the water cap as the tailings consolidate. In addition to advective release of constituents from the tailings pore water, chemical reactions, more specifically, diagenetic processes, within the tailings will result in changes to the redox state, speciation, and phase of several chemicals. A module that quantifies diagenetic processes has been developed for the 2-dimensional hydrodynamic and water quality model, CE-QUAL-W2. The module includes processes such as biogenic gas production (build up and release of methane, hydrogen sulfide, and ammonia); physical release of bubbles through the water column; unconsolidated sediment re-suspension during bubble ebullition; and dynamic oxygen consumption at the sediment-water interface and in the water column from bubbles. This paper presents the numerical framework used to develop the diagenesis module and the chemical basis for the algorithms.