Sandhill Fen Research Articles

The hydrochemical evolution of a constructed peatland in a post-mining landscape six years after construction

Study RegionReclamation of wetlands, including peatlands, is legally required in the Athabasca oil sands region following bitumen extraction via surface mining which leaves large open pits that are backfilled with saline tailings waste. Study FocusSix years of hydrochemical data (2013 – 2018) from the Sandhill Fen Watershed (SFW), a 52-ha upland-peatland catchment that was built upon highly saline soft tailings, were used to evaluate salinity and ion patterns and provide insight on its trajectory. In general, electrical conductivity (EC) increased throughout SFW from 1) reduced inflow and outflow, 2) changes in water table positions and 3) increased mixing of site-wide waters. New Hydrological Insights for the RegionSalinity has increased site-wide over time as EC increased by an average of 1585 and 2313 µS/cm in the wetland and margins, respectively from 2013 to 2018. The uplands were the only region where EC declined by 1747 µS/cm over the six years. There is also evidence of mixing with underlying tailings waste (Na-Cl dominant) as the chemical composition of SFW waters shifted from largely Ca-dominant in 2013 (> 90%) to Na-dominant by 2018 (> 70%). Based on its current conditions, SFW cannot support freshwater peat-forming bryophytes and is most chemically similar to naturally occurring saline fens. A shift in design strategies is recommended (replicating saline instead of freshwater peatlands) to increase the success of these systems.

Hydrological functioning of a constructed peatland watershed in the Athabasca oil sands region: Potential trajectories and lessons learned

Mine reclamation in the Athabasca oil sands region is legally required as companies must reconstruct disturbed landscapes into functioning ecosystems which previously existed in the Boreal landscape. Upland-wetland systems are relatively new in the constructed landscape and only two exist to date. The objective of this work is to understand the key hydrological changes post-management of a constructed peatland watershed and provide insight on the overall system function. Six years of hydrometric data are presented from the Sandhill Fen Watershed (SFW), a 52-ha upland-wetland catchment built on soft tailings with a pump system to provide fresh water, support drainage, and limit salinization. Wet years (seasonal precipitation > evapotranspiration) occurred in 2013, 2016 and 2018 and dry years (seasonal precipitation < evapotranspiration) occurred in 2014, 2015 and 2017 where wet years had large 5-, 10- and 100-year storms which were absent in dry years. Surface conductance and solar radiation explained most of the variation in ET fluxes. Changes in management practices drove many of the observed hydrological changes. Heavy management in 2013 muted water table (WT) responses to climate as inflow and outflow (via pumps) controlled WT response. After 2014, management efforts declined and hydrological exchanges were predominantly vertical, and saturated storage across the wetland increased. As a result, WT variability was tightly coupled to ET regardless of WT position relative to the ground surface, with greater changes related to deeper water tables, suggesting the absence of water conserving feedback mechanisms. Intra-watershed water movement was primarily towards the wetland from recharge areas in the upland swale, whereas surface runoff was rare and only occurred during extreme rain events and spring snowmelt. Peat properties were degraded compared to those observed in undisturbed peatlands, and natural stratification of the peat profile was absent. Results suggest that current conditions are not favorable for fen-peatland development as marsh-like conditions have developed, limiting water conserving functions and the ability to persist long-term in a changing climate.

The role of snow processes and hillslopes on runoff generation in present and future climates in a recently constructed watershed in the Athabasca oil sands region

AbstractMine reclamation in the Athabasca oil sands region Canada, is required by law where companies must reconstruct disturbed landscapes into functioning ecosystems such as forests, wetlands, and lakes that existed in the Boreal landscape prior to mining. Winter is a major hydrological factor in this region as snow covers the landscape for 5–6 months and is ~25% of the annual precipitation, yet few studies have explored the influence of winter processes on the hydrology of constructed watersheds. One year (2017–2018) of intensive snow hydrology measurements are supplemented with 6 years (2013–2018) of meteorological measurements from the constructed Sandhill Fen watershed to: (a) understand snow accumulation and redistribution, snowmelt timing, rate, and partitioning, (b) apply a physically based model for simulating winter processes on hillslopes, and (c) evaluate the impact of soil prescriptions and climate change projections on winter processes in reclaimed systems. The 2017–2018 snow season was between November and April and snow water equivalent (SWE) ranged between 40 and 140 mm. Snow distribution was primarily influenced by topography with little influence of snow trapping from developing vegetation. Snow accumulation was most variable on hillslopes and redistribution was driven by slope position, with SWE greatest at the base of slopes and decreased toward crests. Snowmelt on hillslopes was controlled by slope aspect, as snow declined rapidly on west and south‐facing slopes, compared with east and north‐facing slopes. Unlike results previously reported on constructed uplands, snowmelt runoff from uplands was much less (~30%), highlighting the influence of different construction materials. Model simulations indicate that antecedent soil moisture and soil temperature have a large influence on partitioning snowmelt over a range of observed conditions. Under a warmer and wetter climate, average annual peak SWE, and snow season duration could decline up to 52% and up to 61 days, respectively, while snowmelt runoff ceases completely under the warmest scenarios. Results suggest considerable future variability in snowmelt runoff from hillslopes, yet soil properties can be used to enhance vertical or lateral flows.

Low methane emissions from a boreal wetland constructed on oil sand mine tailings

Abstract. A 58 ha mixed upland and lowland boreal plains watershed called the Sandhill Fen Watershed was constructed between 2008 and 2012. In the years following wetting in 2013, methane emissions were measured using manual chambers. The presence of vegetation with aerenchymous tissues and saturated soils were important factors influencing the spatial variability of methane emissions across the constructed watershed. Nevertheless, median methane emissions were equal to or less than 0.51 mg CH4 m−2 h−1 even from the saturated organic soils in the lowlands. Although overall methane emissions remained low, observations of methane ebullition increased over the 3 study years. Ebullition events occurred in 10 % of measurements in 2013, increasing to 21 % and 27 % of measurements in 2014 and 2015, respectively, at the plots with saturated soils. Increasing metal ion availability and decreasing sulfur availability was measured using buried ion exchange resins at both seasonal and annual timescales potentially as a result of microbial reduction of these ions. Using principle component analysis, methane fluxes had a significant positive correlation to the leading principle component which was associated with increasing ammonium, iron, and manganese and decreasing sulfur availability (r=0.31, p&lt;0.001). These results suggest that an abundance of alternative inorganic electron acceptors may be limiting methanogenesis at this time.

Nutrient supply patterns across the Sandhill Fen reclamation watershed and regional reference fens in Alberta, Canada: An ion exchange membrane study

AbstractUsing Plant Root Simulator (PRS®) probes, we evaluate nutrient supply rates at the Sandhill Fen reclamation watershed across a gradient of increasing water table position and compare observations with three reference fens (a poor, moderate‐rich, and saline fen). Our objectives were threefold: (a) determine the behavior of PRS probes across multiple peatland classes, (b) determine which type of peatland Sandhill Fen is most analogous to in terms of nutrient supply patterns and site chemistry, and (c) attempt to provide a repeatable approach for assessing nutrient supply in peatlands. In terms of porewater chemistry, conditions at Sandhill Fen were quite unique, yet, most comparable to the saline fen. Consistent with porewater analysis, PRS probes had low nitrate supply at all sites, high calcium and magnesium supply at all sites (except the poor fen), and much higher sulfur supply at Sandhill Fen. Site differences in PRS probe measurements were smaller for 20‐ than 2‐day burial periods due to displacement of less strongly held elements (e.g., potassium) on ion exchange membranes. Because long‐term incubations may underrepresent supply rates for weakly held ions due to displacement by divalent ions, we recommend that reclamation assessments over a wide range of fen types should use a relatively short burial period of 1 to 3 days. With few exceptions (sulfate and iron), nutrient supply patterns at Sandhill Fen were quite comparable to the reference fens. For future wetland reclamation performance assessments in the AOSR, PRS probes could serve an important role for evaluating belowground functional components.

Evaluating How Landform Design and Soil Covers Influence Groundwater Recharge in a Reclaimed Watershed

AbstractLandscape‐scale reconstruction and prescription of soil cover systems following oil sands mining is challenging due to the quality of available reclamation materials and the subhumid climate of the Boreal Plains of Canada. In an experimental reclaimed watershed (Sandhill Fen Watershed), basin‐scale upland landforms (i.e., hummocks) were designed to provide groundwater to adjacent lowlands, necessitating adequate recharge following establishment of forest vegetation. Volumetric water contents, soil water pressure heads, and groundwater levels were monitored for four years throughout the watershed and used to calibrate and verify numerical models in HYDRUS. Using a variably saturated two‐dimensional domain, we identified a threshold‐like relationship between recharge (or upflux) and upland hummock height, where upland hummocks not tall enough to limit root water uptake from the saturated zone decreased recharge or resulted in net upflux. Recharge varied with soil cover texture (higher in coarser‐textured) and associated soil hydraulic parameters. Furthermore, scenario tests indicated the importance and relative influence that maximum rooting depths, forest floor placement thicknesses, and leaf area indices (all associated with forest development) had on recharge. Simulations utilizing a historical climate record indicated that interannual climatic variability was as influential as variation in soil cover texture in determining recharge. Reclamation practitioners should recognize that the water balances of reconstructed landscapes are largely influenced by the trade‐off between optimizing forest productivity and sourcing water to downgradient landscape positions.

The initial three years of carbon dioxide exchange between the atmosphere and a reclaimed oil sand wetland

Northern peatlands contain up to 20% of the ∼3000 Pg of global soil organic carbon. Carbon-rich peatlands cover upwards of 65% of the landscape in northern Canada where resource extraction activities disturb both the carbon pools and the future carbon sequestration capacity of the landscape. Previous estimates of the carbon losses from this disturbance predict a complete loss of the region’s peatland carbon pool. Mining industries operating in these sensitive environments have recently begun constructing closure landscapes which are intended to develop carbon cycle processes similar to undisturbed northern peatlands. This study investigates eddy covariance fluxes of carbon dioxide (CO2) at one of Canada’s first fully constructed boreal plains watersheds, the Sandhill Fen Watershed. During the first three years since inception, only the lowland region had an annual net ecosystem exchange of CO2 (NEE) indicative of increasing CO2 sink potential. The lowland region was characterized by saturated salvaged peat soils, standing water, thriving communities of Typha and Carex spp. and was a net CO2 sink of 77 g C m−2 in the third year. At the same time the upland and the midland regions characterized by moist salvaged peat soils and a mix of herbaceous, shrub and planted Picea glauca and Pinus banksiana remained net sources of CO2. Despite similar rates of gross primary production, ecosystem and plot-level respiration rates in the lowland were significantly lower than in the midland region, likely due to very low reduction potentials within the lowland’s saturated soils. With no other significant outflows of carbon, the lowland of the Sandhill Fen Watershed may be in the early stages of organic matter accumulation. Due to limited oxidation of the salvaged peat substrate in the lowland region, wetland reclamation employing these techniques may reduce the disturbance loss of the carbon pool in the boreal plains.

Water table dynamics in a constructed wetland, Fort McMurray, Alberta

AbstractSurface mining in northern Alberta transforms wetlands and forests into open pits, tailings ponds, and overburden. As part of their license to operate, mine operators are required to reclaim this altered landscape to a predisturbance capacity. In 2012, Syncrude Canada Limited constructed one of the first of two reclaimed wetlands, the Sandhill Fen Watershed (SFW), to evaluate wetland reclamation strategies. SFW is a 52‐ha system atop soft‐tailings that includes an inflow/outflow pump system, underdrains, upland hummocks, and a fen lowland. In this study, water table dynamics of the fen lowland were evaluated in the 2 years following commissioning (2014–2015) to assess whether this newly constructed watershed has hydrological conditions that facilitate hydric soils with water table regimes similar to reference systems. Results indicate that the location and hydrophysical properties of placed materials control water table responses to both water management and precipitation. This differential water table response in the SFW lowland drove lateral fluxes between adjacent landforms, suggesting periods of intermittent water supply from uplands to wetlands along hummock margins. As in natural systems, the lowland fen exhibited several lateral flow reversals over the 2 years depending upon water level. Water tables on‐average were greater than those observed in natural analogues. Comparison during these first 2 years following commissioning contribute to the increasing insight as to how construction and management practices support reclamation postmining.

Sodium transport and attenuation in soil cover materials for oil sands mine reclamation

Reclamation soil covers are used in oil sands mine closure to support vegetative growth over tailings. Geochemical processes within these covers may impact solute transport during upward migration of oil sands process-affected water (OSPW) from the underlying tailings. In this study, we examined the geochemical processes controlling Na transport and attenuation within the peat and clay-till cover soils at Sandhill Fen in northern Alberta, Canada. We analyzed soil core samples collected along transects of this 54-ha pilot-scale oil sands mine reclamation wetland. The geochemical (Na, Ca, Mg, K, Cl, SO4, HCO3) and isotopic (δ2H, δ18O) compositions of extracted pore water were analyzed statistically to identify OSPW and fresh surface water within the cover. Depth-dependent trends in pore water sodium concentrations were not apparent, suggesting that the soil cover had been fully flushed by a mixture of OSPW and fresh surface water used to flood the fen. Relative concentrations of Na, Ca and Mg were used to define the extent of cation exchange within the clay cover. Complementary laboratory column experiments showed that cation exchange removed up to 50% of dissolved Na as the first pore volume of simulated OSPW passed through the peat and till. However, Na attenuation by these materials declined rapidly and was limited after 4 (peat) to 7 (till) pore volumes of OSPW flushing. Reactive transport modeling confirmed that cation exchange was the dominant control on Na attenuation and corresponding Ca and Mg release within the till and peat columns. Mineral precipitation-dissolution reactions also influenced dissolved Ca and Mg concentrations and, therefore, indirectly impacted Na attenuation. Overall, this study helps constrain the geochemical processes controlling Na transport and attenuation in oil sands reclamation soil covers exposed to OSPW, and indicates that the attenuation of Na from OSPW by these covers is short-lived.

Increases in salinity following a shift in hydrologic regime in a constructed wetland watershed in a post-mining oil sands landscape

Bitumen extraction via surface mining in the Athabasca Oil Sands Region results in permanent alteration of boreal forests and wetlands. As part of their legal requirements, oil companies must reclaim disturbed landscapes into functioning ecosystems. Despite considerable work establishing upland forests, only two pilot wetland-peatland systems integrated within a watershed have been constructed to date. Peatland reclamation is challenging as it requires complete reconstruction with few guidelines or previous work in this region. Furthermore, the variable sub-humid climate and salinity of tailings materials present additional challenges. In 2012, Syncrude Canada Ltd. constructed a 52-ha pilot upland-wetland system, the Sandhill Fen Watershed, which was designed with a pump and underdrain system to provide freshwater and enhance drainage to limit salinization from underlying soft tailings materials that have elevated electrical conductivity (EC) and Na+. The objective of this research is to evaluate the hydrochemical response of a constructed wetland to variations in hydrology and water management with respect to water sources, flow pathways and major chemical transformations in the three years following commissioning. Results suggest that active water management practices in 2013 kept EC relatively low, with most wetland sites <1000 μS/cm with Na+ concentrations <250 mg/L. With limited management in 2014 and 2015, the EC increased in the wetland to >1000 μS/cm in 2014 and >2000 μS/cm in 2015. The most notable change was the emergence of several Na+ enriched zones in the margins. Here, Na+ concentrations were two to three times higher than other sites. Stable isotopes of water support that the Na+ enriched areas arise from underlying process-affected water in the tailings, providing evidence of its upward transport and seepage under a natural hydrologic regime. In future years, salinity is expected to evolve in its flow pathways and diffusion, yet the timeline and extent of these changes are uncertain.

Net nitrogen mineralization in boreal fens: a potential performance indicator for peatland reclamation

The Sandhill Fen reclamation watershed, commissioned by Syncrude Canada Ltd., is the first attempt to reclaim a self-sustaining peat-forming wetland on a previously mined area. Here, we quantified net nitrogen mineralization rates at Sandhill Fen in the first and second years since initiation (2013–2014). Our main objective was to determine whether nitrogen production potentials at Sandhill Fen were similar to six regional fens sampled across an ombrotrophic–minerotrophic peatland gradient. In the second year, net nitrogen mineralization rates across Sandhill Fen (2014 mean = 20.2 mg N·m−2·day−1; 0.9 mg N·kg−1·day−1) were quite comparable with the benchmark fen sites (2013–2014 pooled means = 20.6 mg N·m−2·day−1; 5.9 mg N·kg−1·day−1). However, in areas exhibiting low gravimetric soil moisture content at Sandhill Fen, net nitrification contributed more than 50% to the net N mineralization total, an uncommon observation in natural fen type wetlands. These findings highlight the importance of managing soil moisture levels during the early stages of reclamation to (1) maintain relatively anaerobic soil conditions, and (2) facilitate microbial-mediated processes to fall within an acceptable range of variation comparable to undisturbed Albertan fens.

Sandhill Fen, an initial trial for wetland species assembly on in-pit substrates: lessons after three years

Open-pit mining of oil sands removes wetland plant communities from the landscape. Sandhill Watershed, located on Syncrude Canada’s oil sands lease, is the first reclamation of a complex watershed that includes a 17 ha central wetland designed to develop into a rich fen. Here we sample the vegetation after three years. Of the 124 plant species recorded, 48% are peat-forming species, including 24 bryophyte species. We identified, using ordination techniques, four plant assemblages that vary in abundance of peat-forming plants. Each assemblage occurs in a spatially distinct area of Sandhill Fen, forming vegetation zones that are closely associated with height of water table. The plant assemblage distributed in the wettest areas has abundant marsh species. The assemblages in the driest areas of the fen have large numbers of upland and weed species and few species characteristic of fens. In between is a species assemblage with an abundance of species characteristic of natural peat-forming habitats. Two key findings are: water levels control spatial distributions of species assemblages, and non-peat-forming plant species are abundant and a concern for the establishment of peat-forming wetlands. Future designs should include plans for a number of interconnected site types such as marshes, fens, and riparian areas.

Multi-year water balance assessment of a newly constructed wetland, Fort McMurray, Alberta

Oil sands mining in Alberta transforms the boreal landscape of forests and wetlands into open pits, tailings ponds and overburden piles. Whereas reclamation efforts have primarily focused on upland forests, rebuilding wetland systems has recently become a motivation for research. Wetland creation and sustainability in this region is complicated by the sub-humid climate and salinity of underlying mining material. In 2012, Syncrude Canada Ltd. completed the construction of the Sandhill Fen Watershed (SFW), a 52-ha upland-wetland system to evaluate wetland reclamation strategies on soft tailings. SFW includes an active pumping system, upland hummocks, a fen wetland and underdrains. To evaluate the influence of management practices on the hydrology of the system, this study reports the water balance from January 2013 to December 2014, the first 2 years after commissioning. A semi-distributed approach was taken to examine the fluxes and stores of water in uplands and lowlands. Natural and artificial inputs and outputs were measured using a series of precipitation gauges and pumps, and evapotranspiration was quantified using three eddy covariance towers. A series of near surface wells recorded water table position. Both 2013 and 2014 were normal rainfall years, with 2013 having more and 2014 less snow than normal. In 2013, inflow/outflow from pumping was the predominant hydrological fluxes, resulting in considerable variability in water table position and storage changes throughout the summer. In 2014, the artificial addition of water was negligible, yet the water table remained near the surface in lowland locations, suggesting that wetland conditions could be maintained under current conditions. Evapotranspiration rates between uplands and lowlands were similar between years and sites, ranging from 2.2 ± 1.8 to 2.5 ± 1.2 mm/day and were largely controlled by climate. These rates were less than nearby older upland systems, suggesting that water balance partitioning will change as vegetation develops. Comparison between years and with natural systems provides insight on how management practices influence hydrologic dynamics and the overall water balance of the SFW. Copyright © 2016 John Wiley & Sons, Ltd.

Total and methyl mercury concentrations in sediment and water of a constructed wetland in the Athabasca Oil Sands Region

In the Athabasca Oil Sands Region in northeastern Alberta, Canada, oil sands operators are testing the feasibility of peatland construction on the post-mining landscape. In 2009, Syncrude Canada Ltd. began construction of the 52 ha Sandhill Fen pilot watershed, including a 15 ha, hydrologically managed fen peatland built on sand-capped soft oil sands tailings. An integral component of fen reclamation is post-construction monitoring of water quality, including salinity, fluvial carbon, and priority pollutant elements. In this study, the effects of fen reclamation and elevated sulfate levels on mercury (Hg) fate and transport in the constructed system were assessed. Total mercury (THg) and methylmercury (MeHg) concentrations in the fen sediment were lower than in two nearby natural fens, which may be due to the higher mineral content of the Sandhill Fen peat mix and/or a loss of Hg through evasion during the peat harvesting, stockpiling and placement processes. Porewater MeHg concentrations in the Sandhill Fen typically did not exceed 1.0 ng L−1. The low MeHg concentrations may be a result of elevated porewater sulfate concentrations (mean 346 mg L−1) and an increase in sulphide concentrations with depth in the peat, which are known to suppress MeHg production. Total Hg and MeHg concentrations increased during a controlled mid-summer flooding event where the water table rose above the ground surface in most of the fen. The Hg dynamics during this event showed that hydrologic fluctuations in this system exacerbate the release of THg and MeHg downstream. In addition, the elevated SO42− concentrations in the peat porewaters may become a problem with respect to downstream MeHg production once the fen is hydrologically connected to a larger wetland network that is currently being constructed.

Establishment of bryophytes from indigenous sources after disturbance from oil sands mining

Abstract Reclaiming Alberta's oil sands landscape has many challenges including designing and engineering totally new landscapes, as well as developing unique plant establishment regimes. Fens are a particular concern as no one has attempted to create minerogenous peat-forming wetlands. Key to the success of these peat-forming wetlands is the establishment of a ground layer of bryophytes. We surveyed Syncrude's 17 ha constructed wetland (Sandhill Fen) two years after wet-up for the occurrence of bryophytes, none of which were purposefully introduced. Twenty-one species, including 15 species of natural rich fens were found, along with five ruderal species. Species patterning was framed along a species richness gradient, suggesting that some plots were better than others for establishment. Spatial patterning of the species-rich plots was scattered within the fen with no discernible pattern. By far the most prevalent fen species was Ptychostomum pseudotriquetrum. Seasonally submerged sites had no bryophyte e...

Organic Matter Accumulation and Community Change at the Peatland–Upland Interface: Inferences from 14C and 210Pb Dated Profiles

Peatland-margin habitats with organic matter accumulation of 40–150 cm make up a significant but poorly quantified portion of Canada’s boreal forest region. Spanning the transition between non-wetland forest and fen proper, these ecosystems represent a zone of complex environmental and vegetation change, yet little is known about their ecological function or development. We here use vegetation and macrofossil analysis, traditional 14C, bomb-spike 14C, and 210Pb dating to investigate the development, organic matter accumulation, and recent vegetation history of peat margin communities at two sites in central Saskatchewan, Canada. Although similar in general shape, bomb-spike 14C and 210Pb chronologies show limited agreement in three of the four profiles examined, with 210Pb generally producing younger ages than 14C. Peat initiation and long-term organic matter accumulation at the Old Black Spruce (OBS) transect were probably driven mainly by the dynamics of Sphagnum, whereas at the Sandhill Fen (SF) transect, they were controlled by water level fluctuations in the neighboring fen. Bryophyte macrofossils suggest a recent drying of the vegetation surface at both sites, most likely triggered by regional drought in the late 1950s and 1960s. At OBS, the shift from Sphagnum- to feather moss-dominated communities continued in the 1990s, possibly reflecting effects of direct disturbance on local drainage patterns. Overall, our results suggest that community composition and C dynamics of peat-margin swamps respond dynamically to climatic and hydrologic fluctuations. However, uncertainties regarding the reliability of different chronologies limit our ability to link observed community changes to specific causal events.

00/03134 Synergistic opportunities for refining/petro-chemical plant integration

In the Athabasca Oil Sands Region in northeastern Alberta, Canada, oil sands operators are testing the feasibility of peatland construction on the post-mining landscape. In 2009, Syncrude Canada Ltd. began construction of the 52 ha Sandhill Fen pilot watershed, including a 15 ha, hydrologically managed fen peatland built on sand-capped soft oil sands tailings. An integral component of fen reclamation is post-construction monitoring of water quality, including salinity, fluvial carbon, and priority pollutant elements. In this study, the effects of fen reclamation and elevated sulfate levels on mercury (Hg) fate and transport in the constructed system were assessed. Total mercury (THg) and methylmercury (MeHg) concentrations in the fen sediment were lower than in two nearby natural fens, which may be due to the higher mineral content of the Sandhill Fen peat mix and/or a loss of Hg through evasion during the peat harvesting, stockpiling and placement processes. Porewater MeHg concentrations in the Sandhill Fen typically did not exceed 1.0 ng L−1. The low MeHg concentrations may be a result of elevated porewater sulfate concentrations (mean 346 mg L−1) and an increase in sulphide concentrations with depth in the peat, which are known to suppress MeHg production. Total Hg and MeHg concentrations increased during a controlled mid-summer flooding event where the water table rose above the ground surface in most of the fen. The Hg dynamics during this event showed that hydrologic fluctuations in this system exacerbate the release of THg and MeHg downstream. In addition, the elevated SO42− concentrations in the peat porewaters may become a problem with respect to downstream MeHg production once the fen is hydrologically connected to a larger wetland network that is currently being constructed.