Comput Geosci DOI 10.1007/s10596-014-9445-8 ORIGINAL PAPER A reactive transport benchmark on heavy metal cycling in lake sediments Bhavna Arora · S. Sevinc ¸S ¸eng¨ or · Nicolas F. Spycher · Carl I. Steefel Received: 9 April 2014 / Accepted: 2 September 2014 © Springer International Publishing Switzerland 2014 Abstract Sediments are active recipients of anthropogenic inputs, including heavy metals, but may be difficult to interpret without the use of numerical models that cap- ture sediment-metal interactions and provide an accurate representation of the intricately coupled sedimentological, geochemical, and biological processes. The focus of this study is to present a benchmark problem on heavy metal cycling in lake sediments and to compare reactive transport models (RTMs) in their treatment of the local-scale phys- ical and biogeochemical processes. This benchmark prob- lem has been developed based on a previously published reactive-diffusive model of metal transport in the sediments of Lake Coeur d’Alene, Idaho. Key processes included in this model are microbial reductive dissolution of iron hydroxides (i.e., ferrihydrite), the release of sorbed metals into pore water, reaction of these metals with biogenic sul- fide to form sulfide minerals, and sedimentation driving the burial of ferrihydrite and other minerals. This benchmark thus considers a multicomponent biotic reaction network with multiple terminal electron acceptors (TEAs), Fickian diffusive transport, kinetic and equilibrium mineral precip- itation and dissolution, aqueous and surface complexation, as well as (optionally) sedimentation. To test the accuracy of the reactive transport problem solution, four RTMs— TOUGHREACT (TR), CrunchFlow (CF), PHREEQC, and PHT3D—have been used. Without sedimentation, all four B. Arora () · N. F. Spycher · C. I. Steefel Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA e-mail: barora@lbl.gov S. Sevinc ¸S ¸eng¨ or Civil and Environmental Engineering Department, Southern Methodist University, Dallas, TX 75275, USA models are able to predict similar trends of TEAs and dis- solved metal concentrations, as well as mineral abundances. TR and CF are further used to compare sedimentation and compaction test cases. Results with different sedimentation rates are captured by both models, but since the codes do not use the same formulation for compaction, the results differ for this test case. Although, both TR and CF ade- quately capture the trends of aqueous concentrations and mineral abundances, the difference in results highlights the need to consider further the conceptual and numerical models that link transport, biogeochemical reactions, and sedimentation. Keywords Reactive transport benchmark · Benthic sediments · Ferrihydrite dissolution · Sedimentation · Compaction 1 Introduction Lake sediments act as repositories for heavy metals, poly- cyclic aromatic hydrocarbons, polychlorinated biphenyls, and other contaminants [1–4]. Sediments are heterogeneous mixtures that include mineral phases, organic matter, and biota, which can therefore accumulate a variety of con- taminants through varied biogeochemical and sedimentation processes [5, 6]. Quantification of metal cycling in sedi- ments requires a thorough understanding of the interactions between microbially mediated reactions and inorganic geo- chemical processes. Hart et al. [7] reviewed some of the mechanisms affecting metal uptake by sediments, such as physico-chemical adsorption, bacterial decomposition, sed- imentation, and physical entrapment of enriched particulate matter. There are several factors that influence sediment- metal concentrations, such as sediment characteristics (e.g.,