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

Reclaiming Wetlands after Oil Sands Mining in Alberta, Canada: The Changing Vegetation Regime at an Experimental Wetland

Surface mining for oil sand results in the formation of large pits that must be reclaimed. Some of these pits are backfilled with a myriad of substrates, including tailings rich in cations and anions, to form a solid surface. Experimental reclamation of the East in-pit located on the Syncrude Canada Ltd. mine lease was initiated in 2011 with Sandhill Wetland. Here, we report on monitoring (between 2015 and 2021) of Sandhill Wetland plant communities and significant environmental features, including base cations and water tables. Multivariate analyses demonstrated that the three dominant plant communities established in 2013 have continued to be dominated by the same species nine years after reclamation was initiated, but with reduced species richness. Plant communities have shifted across the wetland in response to water table changes and increases in sodium concentrations. The stoichiometry of base cations is unlike the natural wetlands of the region, and the surficial water chemistry of the wetland is unique. In response to variability in precipitation events coupled with wetland design, water tables have been highly variable, creating shifting water regimes across the wetland. Plant community responses to these shifting conditions, along with increases in base cation concentrations, especially sodium, provide background data for future in-pit reclamation designs. The plant responses underscore the need to develop reclamation designs for landscapes disturbed by mining that alleviate extreme water table fluctuation events and decrease cation concentrations to levels that approach natural wetlands.

Optical properties of dissolved organic matter highlight peatland-like properties in a constructed wetland

Constructing novel peatland ecosystems can help to restore the long-term carbon accumulating properties of northern soil systems that have been lost through resource extraction. Although mining companies are legally required to restore landscapes following extraction, there are limited tools to evaluate the effectiveness of restoring peat accumulating landscapes. This study analyzed the spatial patterns of the first seven years (n = 575) of dissolved organic matter (DOM) optical characteristics from a pilot watershed built to restore boreal plains peatlands on a former open pit oil sands mine. A principal component analysis (PCA) indicated a very high degree of redundancy in absorption-florescence DOM properties (PARAFAC, HIX, FI, freshness index, SUVA, and peak A, B, C, T, wavelength, and intensity ratios) at this site. The leading principal component indicated a gradient of fresh protein rich inputs, which are highest near the upland region, to older highly degraded DOM, which is highest in the lowland closest to the outlet. Two functionally different reference peatlands, a poor-fen and bog system and a moderate-rich fen, had relatively similar optical DOM characteristics indicating a high level of decomposition at these sites. Over the first seven years, in some regions of the reconstructed lowland the DOM characteristics are becoming increasingly similar to the highly decomposed DOM observed at the reference sites. When combined with carbon flux measurements these findings indicate the potential for long term organic matter accumulation at this reconstructed site.

Geochemical Stability of Oil Sands Tailings in Mine Closure Landforms

Oil sands surface mining in Alberta has generated over a billion cubic metres of waste, known as tailings, consisting of sands, silts, clays, and process-affected water that contains toxic organic compounds and chemical constituents. All of these tailings will eventually be reclaimed and integrated into one of two types of mine closure landforms: end pit lakes (EPLs) or terrestrial landforms with a wetland feature. In EPLs, tailings deposits are capped with several metres of water while in terrestrial landforms, tailings are capped with solid materials, such as sand or overburden. Because tailings landforms are relatively new, past research has heavily focused on the geotechnical and biogeochemical characteristics of tailings in temporary storage ponds, referred to as tailings ponds. As such, the geochemical stability of tailings landforms remains largely unknown. This review discusses five mechanisms of geochemical change expected in tailings landforms: consolidation, chemical mass loading via pore water fluxes, biogeochemical cycling, polymer degradation, and surface water and groundwater interactions. Key considerations and knowledge gaps with regard to the long-term geochemical stability of tailings landforms are identified, including salt fluxes and subsequent water quality, bioremediation and biogenic greenhouse gas emissions, and the biogeochemical implications of various tailings treatment methods meant to improve geotechnical properties of tailings, such as flocculant (polyacrylamide) and coagulant (gypsum) addition.

Variations of Groundwater Dynamics in Alluvial Aquifers with Reclaimed Water Restoring the Overlying River, Beijing, China

Some of the rivers in northern China are dried, and reclaimed water (RW) is used to restore these degraded river ecosystems, during which the RW could recharge the aquifer by river bank infiltration. From 2007 to 2018, 2.78 × 108 m3 of RW has been replenished to the dried Chaobai River (Shunyi reach), Beijing, China, which is located on the edge of one depression cone in groundwater caused by groundwater over-pumping. The groundwater hydrodynamic variations and the flow path of the RW were identified by eight-year hydrological, hydrochemical, and stable isotopic data, together with multivariate statistical analysis. The RW infiltration drastically impacts the groundwater dynamics with a spatiotemporal variation. The 30-m depth groundwater levels at Perennial intake reach increased quickly around 3 m after 2007, which indicated that they were dominated by RW infiltration. Other 30-m depth groundwater levels were controlled by precipitation recharge from 2007 to 2011, showing significant seasonal variations. In 2012, with more RW transferred to the river, the hydrodynamic impact of the RW on 30-m depth aquifer expanded downstream. However, the 50-m and 80-m depth groundwater levels showed decreasing trend with seasonal variations, due to groundwater pumping. The 30-m depth aquifer was mainly recharged by RW, being evidenced by the enriched δ2H and δ18O. The depleted δ2H and δ18O of the 50-m and 80-m depth groundwater indicated that they were dominated by regional groundwater with meteoric origin. The heterogenous properties of the multi-layer alluvial aquifer offer the preferential flow path for RW transport in the aquifers. The proportion of the RW in the aquifers decreases with depth that was calculated by the chloride conservative mixing model. The increased lateral hydraulic gradient (0.43%) contributes to the RW transport in the 30-m depth aquifer. RW usage changed 30-m depth groundwater type from Ca·Mg-HCO3 to Na·Ca·Mg-HCO3·Cl. RW preferentially recharged the 50-m and 80-m depth aquifers by vertical leakage.

Modelling the hydrologic effects of vegetation growth on the long-term trajectory of a reclamation watershed

Reclamation watersheds that integrate fen peatlands into the design require the inclusion of uplands that are capable of supporting forest development while concurrently supplying sufficient groundwater recharge to downgradient wetland ecosystems. This necessitates selecting materials with suitable soil hydraulic properties and identifying the appropriate thickness and layering to fulfill the dual function of uplands as water storage, and water conveyance features. Currently, these systems incorporate tailings sand - a mine waste material - overlain by a cover soil of fine forest-floor material. The developmental pathway of these uplands is currently unknown, and it is unclear whether these landforms will provide enough groundwater recharge once a climax vegetation community establishes. Therefore, this research attempts to estimate the maximum density of vegetation, and associated water balance fluxes of a constructed upland integrated into a peatland watershed. The numerical modelling software HYDRUS-1D simulated soil moisture dynamics using a 65-year meteorological record, and a plant water stress algorithm was used to estimate the maximum sustainable leaf area index that the upland could support. Based on the thickness of the cover soil, the upland could support an average leaf area index of 1.2. Under this vegetation density, average annual groundwater recharge was 83 mm, and predominantly supplied by snowmelt (64%). Given this quantity of recharge, the model indicates that the upland will continue to provide enough groundwater to offset the anticipated water deficit in the downgradient fen ecosystem. However, by altering the design of the upland, specifically the spatial arrangement and thickness of cover soil, the same recharge could be supplied while also allowing for a higher average vegetation density. Such a design could allow for the creation of watersheds with a higher proportion of peatland.

Effects of Capping Strategy and Water Balance on Salt Movement in Oil Sands Reclamation Soils

The success of oil sands reclamation can be impacted by soil salinity depending on the materials used for soil reconstruction and the capping strategies applied. Using both a greenhouse-based column experiment and numerical modeling, we examined the potential pathways of salt migration from saline groundwater into the rooting zone under different capping strategies (the type and the thickness of the barrier layer) and water balance scenarios. The experimental results showed that there would be salinity issues in the cover soil within several growing seasons if there was a shallow saline groundwater table and if the soil was not properly reconstructed. The thickness of the barrier layer was the most significant factor affecting the upward movement of saline groundwater and salt accumulation in the cover soil. The suitable thickness of the barrier layer for preventing the upward movement of saline groundwater and salt accumulation in the cover soil for each material varied. A numerical simulation for a 15-year period further indicates that, when the cover soil was 50 cm of peat-mineral soil mix and when wet, dry, or normal climatic conditions were considered, the minimum barrier thickness to restrain salt intrusion into the cover soil in the long term was about 75 or 200 cm for coarse tailings sand or overburden barrier material, respectively. In view of the above, to minimize salt migration into the rooting zone and ensure normal plant growth, oil sands reclamation should consider salt migration when designing soil capping strategies.