Impact of Skin on the Movement of Nitrates in a Fractured Porous Media: Numerical Investigations

Nitrate transport in a fracture-skin-matrix system under non-isothermal conditions

Subsurface leaching of agricultural runoff has been identified to pose a serious hazard to the soil-water ecosystem and human health, mostly due to the associated contamination with nitrate. Our understanding of the nature of contaminant spread in the vadose and aquifer zones has been improved from recent mechanistic models on the flow and transport of contaminants through fractured porous media. The present study aims to explore the impacts of skin formation in a fracture-matrix aquifer system onto the nitrogen species transport under non-isothermal settings using numerical modeling. A finite-difference scheme was employed to capture the nitrogen concentration profile and kinetics of transformation by solving the derived partial differential equations. The results show evidence of an additional mass transfer from fracture to skin so as to reduce the migration of nitrogen species (NO₃-N and N₂) at the fracture-matrix interface thereby reducing the peak concentration of N₂ by nearly 1.5 times in fracture after denitrification. Although the thermal conductivity of the rock matrix has a direct impact on the temperature distribution in fracture-skin-matrix profiles, the presence of skin has a cooling effect for a high-temperature influent (45 °C), which also deteriorates the propagation of organic N₂ and NO₃-N, within the fracture. An increase in the temperature coefficient of skin has resulted in an apparent reduction in nitrogen species migration, indicating the thermo-chemical feasibility of an intermediate skin favoring the mass transfer processes. The findings of this study can be extended toward realistic estimation of groundwater contamination risks and for the design of biological filters for in situ remediation.

Numerical modelling of nitrate transport in fractured porous media under non-isothermal conditions

Subsurface contamination is a frequent occurrence in fractured porous systems, posing a potential threat for the groundwater contamination. Tracking the movement of these contaminants is an inherent aspect of effective remediation strategy. The non-isothermal conditions prevailing in the subsurface environment further add to the complexity of the existing scenario. The current study focuses on simulating the concentration profiles of nitrogen species in a fracture-matrix system under non-isothermal conditions. The kinetics and biochemical thermodynamics of nitrogen transformation reactions were explicitly modelled in this study by adopting a finite differential numerical scheme. The numerical results clearly depicted the spatial-temporal profiles of the concentration of all the species in response to the observed peak values. Considering the sensitivity of the model parameters, an increase in flow velocity triggered the migration of all nitrogen species in the fracture, while an increase in matrix porosity reduced the concentration by enhancing the chemical reactions. An increase in fracture aperture also could trigger the denitrification process in the fracture to reduce the nitrate-nitrogen contamination in the fracture. The temperature variation between 25 °C and 45 °C in the fracture and the matrix essentially reduced the availability of nitrate-nitrogen and nitrogen gas in the fracture under non-isothermal conditions. Hence, an increase in the temperature coefficient can reduce the spike of nitrate-nitrogen and nitrogen gas in fracture by minimizing such transformation rates.

Confronting heterogeneous sorption and hydrodynamic dispersion on solute transport in a fracture-skin-matrix system using spatial moment analysis

Understanding the intricacies of inter-dependency of fluid flow and solute transfer at the scale of a single fracture is limited by various simplifying assumptions employed for computational purposes. In the present study, the fracture-rock matrix interface is assumed to be consisting of a skin layer with sufficient mass transfer properties where non-linear adsorption is considered to be the limiting reaction among the various interfaces. A numerical model has been developed using implicit finite difference method with varying grids at the fracture-skin interface to capture the mass transfer during solute transport in the presence of non-linear Sips adsorption. The model was used to identify the critically influencing parameters on the temporal profiles of fluid velocity, macro-dispersion coefficient and dispersivity using the method of spatial moments. The results indicate that the presence of the skin has enhanced the mixing phenomenon as well as the sorptive mass transfer rates. Summary of the sensitivity analysis provides the critical factors to be considered while employing such comprehensive models for elucidating details at a small scale. The presence of fracture-skin evades the sensitive role played by (a) the fracture adsorption coefficient (with a reduced rate constant for adsorption and an increased rate constant for desorption) at early times, while resulting in an enhanced mixing characteristics at later times, and (b) maximum sorption capacity of fracture as a function of solute velocity; and in turn, it provides an improved control over the transportation of solutes through the fracture.