Benchmark problems for reactive transport modeling of the generation and attenuation of acid rock drainage

Abstract

Acid rock drainage (ARD) is a problem of international relevance with substantial environmental and economic implications. Reactive transport modeling has proven a powerful tool for the process-based assessment of metal release and attenuation at ARD sites. Although a variety of models has been used to investigate ARD, a systematic model intercomparison has not been conducted to date. This contribution presents such a model intercomparison involving three synthetic benchmark problems designed to evaluate model results for the most relevant processes at ARD sites. The first benchmark (ARD-B1) focuses on the oxidation of sulfide minerals in an unsaturated tailing impoundment, affected by the ingress of atmospheric oxygen. ARD-B2 extends the first problem to include pH buffering by primary mineral dissolution and secondary mineral precipitation. The third problem (ARD-B3) in addition considers the kinetic and pH-dependent dissolution of silicate minerals under low pH conditions. The set of benchmarks was solved by four reactive transport codes, namely CrunchFlow, Flotran, HP1, and MIN3P. The results comparison focused on spatial profiles of dissolved concentrations, pH and pE, pore gas composition, and mineral assemblages. In addition, results of transient profiles for selected elements and cumulative mass loadings were considered in the intercomparison. Despite substantial differences in model formulations, very good agreement was obtained between the various codes. Residual deviations between the results are analyzed and discussed in terms of their implications for capturing system evolution and long-term mass loading predictions.

Recent Developments and Applications of the HYDRUS Computer Software Packages

“…HP1 has recently been used also to solve a number of benchmark problems that were developed for model developers to demonstrate model conformance with norms established by the subsurface science and engineering community (Steefel et al, 2015). These benchmarks involved (i) multirate surface complexation and 1D dual‐domain multicomponent reactive transport of U(VI) (Greskowiak et al, 2015), (ii) generation of acidity as a result of sulfide oxidation and its subsequent effect on metal mobility above and below the water table (Mayer et al, 2015), and (iii) implementation and evaluation of permeability–porosity and tortuosity–porosity relationships associated with mineral precipitation and dissolution processes (Xie et al, 2015).…”

Reactive transport benchmarks for subsurface environmental simulation

“…The first (Benchmark problems for reactive transport modeling of the generation and attenuation of acid rock drainage [14]) uses reactive transport modeling to simulate the generation of acidity as a result of sulfide oxidation and its subsequent effect on metal mobility above and below the water table. The second benchmark focuses on metal accumulation and mobility in lake sediments downstream of mining operations (A reactive transport benchmark on heavy metal cycling in lake sediments [15]).…”

Understanding and Predicting Vadose Zone Processes

“…Considering these differences, benchmarking activities are critical to building confidence in models and providing measures of model performance. Several benchmarking studies have been conducted in the past that compared thermodynamic database and speciation (e.g., Nordstrom and Archer 2003), and others that compared conceptual and numerical capabilities of models (e.g., Mayer et al 2015). These are described in more detail elsewhere (Arora et al 2015 ;Steefel et al 2015;Dwivedi et al 2016).…”