Fracture‐Dominated Subsurface Flow and Transport in a Sloping Reclamation Cover

Abstract

The successful performance of reclamation soil covers over saline‐sodic overburden associated with oil sands mining in northern Alberta, Canada, depends on the dynamics of water and salt migration within these covers. Subsurface flow exerts a significant control on the distribution of soil moisture and salts within reclaimed landscapes. A conceptual model for fracture‐dominated lateral subsurface flow and transport in a sloping clay‐rich reclamation soil cover over saline‐sodic shale overburden was developed based on an interpretation of field observations. This model was then verified through the use of numerical simulations. The conceptual model assumes that lateral subsurface flow is dominated by a preferential flow system and that chemical equilibration between fresh snowmelt water stored in the macropores and higher concentration pore water stored in the soil matrix is nearly instantaneous. Numerical modeling of subsurface flow indicated that the discharge rate and cumulative volume are controlled by the bulk saturated hydraulic conductivity and drainable fracture porosity, respectively. A drainable fracture porosity ranging from 3 to 4% yielded a simulated cumulative discharge similar to measured values. A pseudo‐equivalent porous medium transport model was used to simulate the Na+ concentration of collected subsurface flow with time. General agreement between measured and simulated values demonstrates that discharge concentrations increase as the depth of perched water diminishes with time and water drains through macropores associated with a matrix of higher solute concentrations lower in the cover profile.

Perched-Water Evaluation for the Deep Vadose Zone Beneath the B, BX, and BY Tank Farms Area of the Hanford Site

“…A perched water zone in the subsurface is defined as a saturated zone that is above or not directly connected to the regional water table (Freeze and Cherry 1979). Perched water zones, containing accumulated water from natural or anthropogenic sources, are separated from the water table by unsaturated materials with depths varying from just a few (Kellin et al 2009) to hundreds of meters (Wu et al 2004). In these zones, the water phase pressure must be equal to or greater than static atmospheric gas pressure at the same elevation.…”

“…The work by Flint et al (2012) supports the importance of collecting pertinent site hydrological data to constrain the numerical predictions. Nearsurface perched-water zones have been observed on hillslopes with thin soil mantles overlaying bedrock (Uchida et al 2004) and on reclaimed landscapes in oil sands regions in Canada where reclamation soil layers, with a thickness of approximately 1 m, were deposited on saline-sodic shale overburdens (Kellin et al 2009). For these cases, infiltration water moves vertically toward the soil-rock interface, followed by perching and lateral flow above the impeding bedrock or overburden material.…”

Salinization of soil over saline-sodic overburden from the oil sands in Alberta

“…2, where the profiles are roughly symmetrical about the interface and have a sigmoidal shape with background concentrations (C/C 0 00) reached at 20 to 40 cm above the overburden. Increased downslope water movement in the form of surface runoff and subsequent infiltration as well as interflow along the cover/overburden interface (Kelln 2009) may lead to salt redistribution along the slope.…”

An evaluation of air permeability measurements to characterize the saturated hydraulic conductivity of soil reclamation covers

“…The presence of secondary structures such as cracks can significantly increase the K s value. Kelln et al (2009) found the macropores and secondary structure within the cover soils at the South Bison Hills site occupied 3Á4% of the total soil volume with an average pore size from 1)10 (5 to 1)10 (3 m.…”

“…According to capillary theory, pores with diameters greater than 60 to 30 mm will be fully drained at these potentials and consequently are open to air flow. Numerous studies have shown that macroporosity can account for more than 80% of the water flow at or near saturation while it comprises B5% of the total porosity of most soils (Watson and Luxmoore 1986;Waduwawatte and Si 2004;Kelln et al 2009). Therefore, the pores contributing to air flow at field capacity are also the dominant pathways for water migration under field saturated conditions.…”