Heterogeneity of the peat profile and its role in unsaturated sodium chloride rise at field and laboratory scales

Abstract

Resource extraction in Canada's boreal ecozone increases the risk of contaminant release into the area's extensive bog and fen peatlands. Lateral spreading, then upwards transport of solutes into the vadose zone of these moss-dominated ecosystems, could be toxic to vegetation. To evaluate the rate and character of contaminant rise in a subarctic bog, vadose zone-specific conductance and water content were measured in four hummocks ∼5 m downslope of a 45-d 300-mg L −1 NaCl release. Four 30-cm-deep hummock peat mesocosms were extracted adjacent to the release site for an unsaturated evaporation-driven NaCl breakthrough experiment and subsequent parameterization. The field rate of solute accumulation was slower in near-surface (0-5 cm) peat, where low water contents limited pore connectivity. Solute accumulation was reduced by downward flushing by rain, though this was lesser in near surface moss where solute remained held in small disconnected pores. In the laboratory, Cl − rise reached the 15-cm depth in all mesocosms by Day 65. Sodium rise was 2.2 times slower, likely due to adsorption to the peat matrix. Rates of upwards solute movement were highly variable; the highest rates occurred in the mesocosm with small but hydrologically conductive pores near the surface, and the lowest occurred where vascular roots disrupted the physical structure of the peat. This research demonstrates that solute spilled into a bog peatland is likely to rise and be retained in the vadose zone. However, hydraulic and solute transport behaviors are sensitive to the vertical structure of peat, underscoring the need for extensive sampling and parameter characterization. Abbreviations: awt, above water table; bgs, below ground surface; PVC, polyvinyl chloride; TDR, time domain reflectometry. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Beyond fill and spill: Hydrological connectivity in a sub‐arctic bog‐fen‐tributary complex in the Hudson Bay Lowlands, Canada

“…Overwinter hydrological functioning of the bog‐fen‐tributary complex was most controlled by the presence and thickness of frost tables within the peatland units. In both the bog and fen, the frost table extended downwards from the surface through the hydrologically transmissive upper layer (Figure 4; Price & FitzGibbon, 1987), which is generally near surface (top 0–15 cm) in this area (Balliston & Price, 2020; McCarter & Price, 2017). In the absence of near surface runoff, disconnection occurred at Trib5 MOE by the end of each year despite apparently connected upgradient conditions (Figure 7).…”

Influence of vegetation‐induced water table seasonality on groundwater chloride concentration dynamics in a riparian fen peatland

“…Mixing is thus not a dichotomous process between only the two poles of rainwater and river water: this is due to the fact that there is always a depletion (or evaporation) gradient between the peatland sampling points while the same meteorological forcing constrains them. The fact that s4 always exhibits the most enriched signal can manifest as the peat soil structure being more subjected to evaporation (Balliston & Price, 2020). Also, because they follow two consecutive periods of short droughts (March/April and May 2020) and heavy rainfall (10 May 2020 and 3 June 2020 in Figure 3) with a shallow water table (above 0.5 m depth), the unique evaporated signatures observed in s2 and s3 in June 2020 suggest that water table dynamics under high atmospheric flux contrasts can lead to a greater isotopic variability.…”