Research papersThe transport dynamics of chloride and sodium in a ladder fen during a continuous wastewater polishing experiment

- First controlled continuous solute transport study in fen peatlands.
- Microtopography had less of an impact on transport than hydraulic conductivity.
- Immobile water in the dual porosity peat and adsorption caused solute retardation.
- Preferential flow pathways propagated the solute plumes, as did pools to a lesser extent.
- Demonstrated the potential for ladder fens to be used for wastewater polishing.

Ladder fen peatlands have excellent potential for wastewater polishing as they naturally contain both open water (pools) and subsurface (peat) treatment landforms; however, there is a poor understanding of solute transport in ladder fens with and without the increased hydrological load imposed by wastewater discharge. To better understand solute transport in ladder fens under wastewater polishing conditions a continuous solute (NaCl) tracer experiment (38 m3 day−1 of water, chloride - 47.2 mg L−1, and sodium - 25.3 mg L−1) was conducted during the summer of 2014 (day of year 192-243) in a small ladder fen in the James Bay Lowland. The transmissivity distribution and effective porosity (average 0.5) of the peat ribs were determined through repeated bail tests and the drainable porosity of 18 peat cores at −100 mb, respectively. Water samples were taken at least every 7 days to capture the solute (sodium and chloride) plumes. Both solute plumes never reached the site outflow (∼250 m downgradient) and displayed complex plume morphology, typically following the patterns of higher hydraulic conductivity within the upper 0.1 m of the saturated peat, rather than the microtopography. Based on the 50% breakthrough isotherms, sodium and chloride were transported at an average solute velocity of 1.9 and 1.1 m day−1, respectively (average linear groundwater velocity = 2.1 m day−1); thus, the solutes were retarded by a factor of 2.1 and 1.2 for sodium and chloride, respectively. Due to the inherent retardation of solutes into inactive pores and relatively high solute residence times, this study demonstrates the potential for wastewater polishing in ladder fens.