Microtopographical and hydrophysical controls on subsurface flow and solute transport: A continuous solute release experiment in a subarctic bog

Abstract

Resource extraction and transportation activities in subarctic Canada can result in the unintentional release of contaminants into the surrounding peatlands. In the event of a release, a thorough understanding of solute transport within the saturated zone is necessary to predict plume fate and the potential impacts on peatland ecosystems. To better characterize contaminant transport in these systems, approximately 13,000 L/day of sodium chloride tracer (200 mg/L) was released into a bog in the James Bay Lowland. The tracer was pumped into a fully penetrating well (1.5 m) between July 5 and August 18, 2015. Horizontal and vertical plume development was measured via in situ specific conductance and water table depth from an adaptive monitoring network. Over the spill period, the bulk of the plume travelled a lateral distance of 100 m in the direction of the slight regional groundwater and topographical slope. The plume shape was irregular and followed the hollows, indicating preferential flow paths due to the site microtopography. Saturated transport of the tracer occurred primarily at ~25 cm below ground surface (bgs), and at a discontinuous high hydraulic conductivity layer ~125 cm bgs due to a complex and heterogeneous vertical hydraulic conductivity profile. Plume measurement was confounded by a large amount of precipitation (233 mm over the study period) that temporarily diluted the tracer in the highly conductive upper peat layer. Longitudinal solute advection can be approximated using local water table information (i.e., depth and gradient); microtopography; and meteorological conditions. Vertical distribution of solute within the peat profile is far more complex due to the heterogeneous subsurface; characterization would be aided by a detailed understanding of the site‐specific peat profile; the degree of decomposition; and the type of contaminant (e.g., reactive/nonreactive). The results of this research highlight the difficulty of tracking a contaminant spill in bogs and provide a benchmark for the characterization of the short‐term fate of a plume in these complex systems.

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

“…Precipitation totals and evaporative fluxes were determined using the methods of Balliston et al. (2018).…”

“…It is on the margin of a continental domed bog, which gently slopes towards a stream; the sitewide gradient is ∼0.004 m m −1 . The site extent is defined by the final shape of a NaCl plume generated during a 45‐d continuous solute release experiment (Balliston et al., 2018). There is a distinct hummock and hollow microtopography formed by Sphagnum mosses.…”

“…Diluted NaCl was injected into a fully penetrating well 1.5 m below ground surface (m bgs), at an average flow rate of 13,500 L d −1 , and an average NaCl mass flux of 2.7 kg d −1 . Average Cl − and Na + concentrations were 182 and 118 mg L −1 , respectively, and average specific conductance was 543 μS cm −1 (a more detailed explanation of the injection system is described by Balliston et al, 2018). Specific conductivity (SC) was used as a proxy to evaluate solute movement in the saturated and unsaturated zones; four hummocks (F1-F4) within 10 m of the injection point were instrumented with three Campbell Scientific CS 615 probes connected to a time domain reflectometer (Campbell Scientific TDR 100, Figure 1).…”

“…Contaminant releases can be unintentional (e.g., leakage from pipelines or spills via train derailment; Zoltai & Kershaw, 1995) or intentional (e.g., wastewater from mining camps; Steinback, 2012). Once released into these systems, there is potential for downgradient solute migration (Baird & Gaffney, 2000; Balliston, McCarter, & Price, 2018; Hoag & Price, 1995; McCarter & Price, 2017a), accumulation within the variably saturated upper zone (Simhayov, Weber, & Price, 2018), and in some cases vertical smearing due to water table fluctuations (Dobson, Schroth, & Zeyer, 2007; Gharedaghloo & Price, 2018). This has potential toxicity to vascular and nonvascular plants (Pouliot, Rochefort, & Graf, 2012; Rezanezhad, Price, & Craig, 2012).…”

Small-scale effects of hydrodynamics on the structure of intertidal macroalgal communities: A novel approach

“…Small-size (ca. 10 cm) pressure sensors are currently used to assess water levels and less often tidal variations (Balliston et al, 2018;Van Putte et al, 2019). In this study, small-size pressure sensors have been used as wave height recorders on intertidal rocky shores.…”