A three-dimensional reactive transport model for sediments, incorporating microniches

Łukasz Sochaczewski; Anthony Stockdale; William Davison; Wlodek Tych; Hao Zhang

doi:10.1071/EN08006

RESEARCH ARTICLE

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A three-dimensional reactive transport model for sediments, incorporating microniches

Łukasz Sochaczewski ^A , Anthony Stockdale ^A , William Davison ^A ^B , Wlodek Tych ^A and Hao Zhang ^A

+ Author Affiliations

- Author Affiliations

^A Department of Environmental Science, Lancaster Environment Centre (LEC), Lancaster University, Lancaster, LA1 4YQ, UK.

^B Corresponding author. Email: w.davison@lancaster.ac.uk

Environmental Chemistry 5(3) 218-225 https://doi.org/10.1071/EN08006
Submitted: 17 January 2008 Accepted: 22 April 2008 Published: 19 June 2008

Environmental context. Modelling of discrete sites of diagenesis in sediments (microniches) has typically been performed in 1-D and has involved a limited set of components. Here we present a new 3-D model for microniches within a traditional vertical sequence of redox reactions, and show example modelled niches of a range of sizes, close to the sediment–water interface. Microniche processes may have implications for understanding trace metal diagenesis, via formation of sulfides. The model provides a quantitative framework for examining microniche data and concepts.

Abstract. Most reactive transport models have represented sediments as one-dimensional (1-D) systems and have solely considered the development of vertical concentration gradients. However, application of recently developed microscale and 2-D measurement techniques have demonstrated more complicated solute structures in some sediments, including discrete localised sites of depleted oxygen, and elevated trace metals and sulfide, referred to as microniches. A model of transport and reaction in sediments that can simulate the dynamic development of concentration gradients occurring in 3-D was developed. Its graphical user interface allows easy input of user-specified reactions and provides flexible schemes that prioritise their execution. The 3-D capability was demonstrated by quantitative modelling of hypothetical solute behaviour at organic matter microniches covering a range of sizes. Significant effects of microniches on the profiles of oxygen and nitrate are demonstrated. Sulfide is shown to be readily generated in microniches within 1 cm of the sediment surface, provided the diameter of the reactive organic material is greater than 1 mm. These modelling results illustrate the geochemical complexities that arise when processes occur in 3-D and demonstrate the need for such a model. Future use of high-resolution measurement techniques should include the collection of data for relevant major components, such as reactive iron and manganese oxides, to allow full, multicomponent modelling of microniche processes.

Additional keywords: diffusion, early diagenesis, high resolution, organic matter, redox reactions, sulfide, trace metals.

Acknowledgements

We thank members of the European Union (EU) TREAD Program for early input in model development and the EU for financial support. A. Stockdale was supported by the UK Natural Environment Research Council (NER/S/A/2005/13679).

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Accessory publication

Further details of model implementation and additional Tables and Figures. This material is provided as an Accessory publication on the journal’s website.