Fereidoun Rezanezhad

The effects of dual porosity on transport and retardation in peat: A laboratory experiment

Rezanezhad, F., Price, J. S. and Craig, J. R. 2012. The effects of dual porosity on transport and retardation in peat: A laboratory experiment. Can. J. Soil Sci. 92: 723-732. Peatlands cover up to half of the local landscape in the Athabasca oil sands region, and peat materials are commonly used in reclamation. The toxicity of oil sands process-affected water (OSPW) derived from the Athabasca oil sands is related to the elevated concentration of naphthenic acids (NAs) and sodium (Na). However, the transport and retardation of solutes through peat is significantly delayed by sorption and by diffusion into immobile water contained in the peat matrix. Approximately 94% of the 43.5 mg L-1 of OSPW was sorbed by 1 kg of peat. For Na ~84% sorption occurred with 382 mg L-1 kg-1 of peat. The sorption and desorption of NAs and Na on peat fitted linear isotherms with distribution coefficients of 6.53 and 5.74 L kg-1, respectively. Solute breakthrough tests were performed for NaCl and the retardation due to sorption (R) of 1.73 was estimated for Na using a two-region (mobile and immobile) non-equilibrium transport model. However, the estimated mass transfer coefficient describing solute exchange between the mobile and immobile liquid regions indicated that part of retardation is attributed to solute exchange between the mobile and immobile phases. This was evident because Cl, a conservative solute (R=1, no sorption), also exhibited characteristics of dual-porosity transport. Thus, the passage of the solute front was retarded by diffusion of solute into the immobile region and by solute exchange between the mobile and immobile phases, which occurs at a rate depending on the proportion of mobile/immobile pore spaces. In this study, we showed that the complex dual-porosity structure of the peat is the important factor in attenuating solute transport where the presence of immobile phase (dead-end pores) in the system contributes to the transport and sorption mechanism of solute into this porous medium.

How Fen Vegetation Structure Affects the Transport of Oil Sands Process-affected Waters

Oil sands mining in the Athabasca oil sands region disturbs large tracts of peatlands as the vegetation-soil layer must be removed. Processing oil sands produces large volumes of wet material containing oil sand process-affected water (OSPW) that has elevated concentrations of sodium (Na) and naphthenic acids (NAs). Attempts to reclaim mined landscapes to peat-forming systems command knowledge of the transport, fate and impact of OSPW in organic soils. Four mesocosms placed in a greenhouse were randomly assigned with two treatments: 1) a moss carpet (Bryum pseudotriquetrum) and 2) graminoids (Carex aquatilis and Calamagrostis stricta). Transport of Na and NAs through peat was significantly delayed by sorption and diffusion in peat matrix. After two growing seasons of receiving OSPW, the graminoid plants continued to grow without showing stress from OSPW, while mosses showed a considerable decline in health. Microorganisms were more active under sedges than mosses and their activity varied over time either because of seasonal variation or as a consequence of variation in Na concentration. The findings of this study are limited due to the small number of replicates and the lack of a control, but represent a first step towards the creation of peatlands in the post-mined areas.

The Role of Pore Structure on Nitrate Reduction in Peat Soil: A Physical Characterization of Pore Distribution and Solute Transport

Soil type is an important factor defining terrestrial ecosystems and plays a major role for the movement of solutes and cycling of nutrients and carbon. This paper focuses on the effect of peat complex dual-porosity structure on nitrate reduction, with the main objective to show how this process is controlled by pore-scale mass transfer and exchange of nitrate between mobile and immobile pore fractions. A mesocosm experiment was conducted where input solutions of bromide (Br−) and nitrate (NO3−) were continuously supplied downward into 40xa0cm depth of peat. Br− and NO3− breakthrough curves were used to constrain transport parameters and nitrate reduction rates in the peat depth profile. The vertical distribution of potential nitrate reduction rates were compared with depth distributions of partitioning mobile-immobile pores and the exchange coefficient between the pores. The results showed that an increase of immobile pore fractions with depth increases the common interface surface area between mobile and immobile pores which constitutes to a more pronounced exchange between the two transport domains and enhances the nitrate reduction. Hence, the pore structure with mobile-immobile pore fractions and exchange rate of solutes between mobile and immobile phases play a major role in nitrate reduction in peat soils.

Solute pools in Nikanotee Fen watershed in the Athabasca oil sands region

Overburden and tailings materials from oil sands production were used as construction materials as part of a novel attempt to create a self-sustaining, peat accumulating fen-upland ecosystem. To evaluate the potential for elemental release from the construction materials, total elemental concentrations in the tailings sand, petroleum coke and peat used to construct a fen ecosystem were determined using microwave-assisted acid digestions and compared to a leaching experiment conducted under environmentally-relevant conditions. A comparison of solid phase to aqueous Na, Ca, S and Mg concentrations showed they were highly leachable in the materials. Given that the concentrations of these elements can affect plant community structure, it is important to understand their leachability and mobility as they migrate between materials used to construct the system. To that end, a mass balance of aqueous Na, Ca, S and Mg was conducted based on leaching experiments and materials analysis coupled with existing data from the constructed system. The data indicate that there is a large pool of leachable Na, Ca, S and Mg in the system, estimated at 27xa0t of Na, 14xa0t of Ca, 37.3xa0t of S and 8.8xa0t of Mg. Since recharge mainly drives the fen-upland system water regime, and discharge in the fen, evapo-accumulation of these solutes on the surface may occur.

Bioretention cells under cold climate conditions: Effects of freezing and thawing on water infiltration, soil structure, and nutrient removal

Bioretention cells are a popular control strategy for stormwater volume and quality, but their efficiency for water infiltration and nutrient removal under cold climate conditions has been poorly studied. In this work, soil cores were collected from an active bioretention cell containing engineered soil material amended with a phosphate sorbent medium. The cores were used in laboratory column experiments conducted to obtain a detailed characterization of the soils bioretention performance during six consecutive freeze-thaw cycles (FTCs, from -10 to +10u202f°C). At the start of each FTC, the experimental column undergoing the FTCs and a control column kept at room temperature were supplied with a solution containing 25u202fmg/L of bromide, nitrate and phosphate. Water saturated conditions were established to mimic the presence of an internal water storage zone to support anaerobic nitrate removal. At the end of each FTC, the pore solution was allowed to drain from the columns. The results indicate that the FTCs enhanced the infiltration efficiency of the soil: with each successive cycle the drainage rate increased in the experimental column. Freezing and thawing also increased the saturated hydraulic conductivity of the bioretention soil. X-ray tomography imaging identified a key role of macro-pore formation in maintaining high infiltration rates. Both aqueous nitrate and phosphate supplied to the columns were nearly completely removed from solution. Sufficiently long retention times and the presence of the internal water storage zone promoted anaerobic nitrate elimination despite the low temperatures. Dissolved phosphate was efficiently trapped at all depths in the soil columns, with ≤2% of the added stormwater phosphate recovered in the drainage effluent. These findings imply that, when designed properly, bioretention cells can support high infiltration rates and mitigate nutrient pollution in cold climates.

Benthic nitrite exchanges in the Seine River (France): An early diagenetic modeling analysis

Nitrite is a toxic intermediate compound in the nitrogen (N) cycle. Elevated concentrations of nitrite have been observed in the Seine River, raising questions about its sources and fate. Here, we assess the role of bottom sediments as potential sources or sinks of nitrite along the river continuum. Sediment cores were collected from two depocenters, one located upstream, the other downstream, from the largest wastewater treatment plant (WWTP) servicing the conurbation of Paris. Pore water profiles of oxygen, nitrate, nitrite and ammonium were measured. Ammonium, nitrate and nitrite fluxes across the sediment-water interface (SWI) were determined in separate core incubation experiments. The data were interpreted with a one-dimensional, multi-component reactive transport model, which accounts for the production and consumption of nitrite through nitrification, denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA). In all core incubation experiments, nitrate uptake by the sediments was observed, indicative of high rates of denitrification. In contrast, for both sampling locations, the sediments in cores collected in August 2012 acted as sinks for nitrite, but those collected in October 2013 released nitrite to the overlying water. The model results suggest that the first step of nitrification generated most pore water nitrite at the two locations. While nitrification was also the main pathway consuming nitrite in the sediments upstream of the WWTP, anammox dominated nitrite removal at the downstream site. Sensitivity analyses indicated that the magnitude and direction of the benthic nitrite fluxes most strongly depend on bottom water oxygenation and the deposition flux of labile organic matter.

The role of groundwater discharge fluxes on Si:P ratios in a major tributary to Lake Erie

Groundwater discharge can be a major source of nutrients to river systems. Although quantification of groundwater nitrate loading to streams is common, the dependence of surface water silicon (Si) and phosphorus (P) concentrations on groundwater sources has rarely been determined. Additionally, the ability of groundwater discharge to drive surface water Si:P ratios has not been contextualized relative to riverine inputs or in-stream transformations. In this study, we quantify the seasonal dynamics of Si and P cycles in the Grand River (GR) watershed, the largest Canadian watershed draining into Lake Erie, to test our hypothesis that regions of Si-rich groundwater discharge increase surface water Si:P ratios. Historically, both the GR and Lake Erie have been considered stoichiometrically P-limited, where the molar Si:P ratio is greater than the ~16:1 phytoplankton uptake ratio. However, recent trends suggest that eastern Lake Erie may be approaching Si-limitation. We sampled groundwater and surface water for dissolved and reactive particulate Si as well as total dissolved P for 12months within and downstream of a 50-km reach of high groundwater discharge. Our results indicate that groundwater Si:P ratios are lower than the corresponding surface water and that groundwater is a significant source of bioavailable P to surface water. Despite these observations, the watershed remains P-limited for the majority of the year, with localized periods of Si-limitation. We further find that groundwater Si:P ratios are a relatively minor driver of surface water Si:P, but that the magnitude of Si and P loads from groundwater represent a large proportion of the overall fluxes to Lake Erie.

A Synthesis of Three Decades of Eco-Hydrological Research at Scotty Creek, NWT, Canada

Scotty Creek, Northwest Territories (NWT), Canada, has been the focus of hydrological research for nearly three decades. Over this period, field and modelling studies have generated new insights into the thermal and physical mechanisms governing the flux and storage of water in the wetland-dominated regions of discontinuous permafrost that characterises much of the Canadian and circumpolar subarctic. Research at Scotty Creek has coincided with a period of unprecedented climate warming, permafrost thaw, and resulting land cover transformations including the expansion of wetland areas and loss of forests. This paper (1) synthesises field and modelling studies at Scotty Creek, (2) highlights the key insights of these studies on the major water flux and storage processes operating within and between the major land cover types, and (3) provides insights into the rate and pattern of the permafrost-thaw-induced land cover change and how such changes will affect the hydrology and water resources of the study region.

Understanding the Coastal Ecocline: Assessing Sea–Land Interactions at Non-tidal, Low-Lying Coasts Through Interdisciplinary Research

Coastal zones connect terrestrial and marine ecosystems forming a unique environment that is under increasing anthropogenic pressure. Rising sea levels, sinking coasts, and changing precipitation patterns modify hydrodynamic gradients and may enhance sea-land exchange processes in both tidal and non-tidal systems. Furthermore, the removal of flood protection structures as restoration measure contributes locally to the changing coastlines. A detailed understanding of the ecosystem functioning of coastal zones and the interactions between connected terrestrial and marine ecosystems is still lacking. Here, we propose an interdisciplinary approach to the investigation of interactions between land and sea at shallow coasts, and discuss the advantages and the first results provided by this approach as applied by the research training group Baltic TRANSCOAST. A low-lying fen peat site including the offshore shallow sea area on the southern Baltic Sea coast has been chosen as a model system to quantify hydrophysical, biogeochemical, sedimentological, and biological processes across the land-sea interface. Recently introduced rewetting measures might have enhanced submarine groundwater discharge as indicated by distinct patterns of salinity gradients in the near shore sediments, making the coastal waters in front of the study site a mixing zone of fresh- and brackish water. High nutrient loadings, dissolved inorganic carbon, and dissolved organic matter originating from the degraded peat may affect micro- and macro-phytobenthos, with the impact propagating to higher trophic levels. The terrestrial part of the study site is subject to periodic brackish water intrusion caused by occasional flooding, which has altered the hydraulic and biogeochemical properties of the prevailing peat soils. The stable salinity distribution in the main part of the peatland reveals the legacy of flooding events. Generally, elevated sulfate concentrations are assumed to influence greenhouse gas emissions, mainly by inhibiting methane production, yet our investigations indicate complex interactions between the different biogeochemical element cycles (e.g. carbon and sulfur) caused by connected hydrological pathways. In conclusion, sea-land interactions are far reaching, occurring on either side of the interface, and can only be understood when both long-term and event-based patterns and different spatial scales are taken into account in interdisciplinary research that involves marine and terrestrial expertise.

Linking spectral induced polarization (SIP) and subsurface microbial processes: Results from sand column incubation experiments

Geophysical techniques, such as spectral induced polarization (SIP), offer potentially powerful approaches for in situ monitoring of subsurface biogeochemistry. The successful implementation of these techniques as monitoring tools for reactive transport phenomena, however, requires the deconvolution of multiple contributions to measured signals. Here, we present SIP spectra and complementary biogeochemical data obtained in saturated columns packed with alternating layers of ferrihydrite-coated and pure quartz sand, and inoculated with Shewanella oneidensis supplemented with lactate and nitrate. A biomass-explicit diffusion-reaction model is fitted to the experimental biogeochemical data. Overall, the results highlight that (1) the temporal response of the measured imaginary conductivity peaks parallels the microbial growth and decay dynamics in the columns, and (2) SIP is sensitive to changes in microbial abundance and cell surface charging properties, even at relatively low cell densities (<108 cells mL-1). Relaxation times (τ) derived using the Cole-Cole model vary with the dominant electron accepting process, nitrate or ferric iron reduction. The observed range of τ values, 0.012-0.107 s, yields effective polarization diameters in the range 1-3 μm, that is, 2 orders of magnitude smaller than the smallest quartz grains in the columns, suggesting that polarization of the bacterial cells controls the observed chargeability and relaxation dynamics in the experiments.