The transport of contaminants in groundwater system is strongly influenced by various aquifer heterogeneity factors such as spatial aquifer heterogeneity of hydraulic conductivity and reactive substances distribution. The contaminants transport can be simulated by using numerical reactive transport models, and their fate can be possibly even predicted. Furthermore, reactive transport modeling is an essential tool to get a profound understanding of hydrological-geochemical complex processes and to make plausible predictions of assessment. The goal of this work is to improve our understanding of the groundwater contaminants fate and transport processes in heterogeneous aquifer systems, with a focus on nitrate problems. A large body of knowledge of the fate and transport of nitrogen species has been achieved by previous works, however, most previous models typically neglect the interrelation of physical and chemical aquifer heterogeneities on the contaminant fate and redox transformation, which is required for predicting the movement and behavior of nitrate and quantifying the impact of uncertainty of numerical groundwater simulation, and which motivates this study. The main research questions which are answered in this work are how aquifer heterogeneity influences on the nitrate fate and transport and then, what is the most influential aquifer heterogeneity factor must be considered. Among the various type of aquifer heterogeneity, physical and chemical aquifer heterogeneities are considered. The first part of the work describes groundwater flow system and hydrochemical characteristics of the study area (Hessian Ried, Germany). Especially, data analyses are performed with the hydrochemical data to identify the major driving force for nitrate reduction in the study area. The second part of the work introduces a kinetic model describing nitrate removal by using numerical simulation. The resulting model reproduces nitrate reduction processes and captures the sequence of redox reactions. The third and fourth parts show the influence of physical and chemical aquifer heterogeneity with varying variance, correlation length scale, and anisotropy ratio. Heterogeneous aquifer systems are realized by using stochastic approach. Results, in short, show that the most influential aquifer heterogeneity factors could change over time. With abundant requisite electron donors, physical aquifer heterogeneity significantly influences the nitrate reduction while chemical aquifer heterogeneity plays a minor role. Increasing the spatial variability of the hydraulic conductivity increases the nitrate removal efficiency of the system in addition. If these conditions are reversed, nitrate removal efficiency varies by the spatial heterogeneity of the available initial electron donor. The results indicate that an appropriate characterization of the physical and chemical properties can be of significant importance to predict redox contamination transport and design long-term remediation strategies and risk assessment.

The transport of contaminants in groundwater system is strongly influenced by various aquifer heterogeneity factors such as spatial aquifer heterogeneity of hydraulic conductivity and reactive substances distribution. The contaminants transport can be simulated by using numerical reactive transport models, and their fate can be possibly even predicted. Furthermore, reactive transport modeling is an essential tool to get a profound understanding of hydrological-geochemical complex processes and to make plausible predictions of assessment. The goal of this work is to improve our understanding of the groundwater contaminants fate and transport processes in heterogeneous aquifer systems, with a focus on nitrate problems. A large body of knowledge of the fate and transport of nitrogen species has been achieved by previous works, however, most previous models typically neglect the interrelation of physical and chemical aquifer heterogeneities on the contaminant fate and redox transformation, which is required for predicting the movement and behavior of nitrate and quantifying the impact of uncertainty of numerical groundwater simulation, and which motivates this study. The main research questions which are answered in this work are how aquifer heterogeneity influences on the nitrate fate and transport and then, what is the most influential aquifer heterogeneity factor must be considered. Among the various type of aquifer heterogeneity, physical and chemical aquifer heterogeneities are considered. The first part of the work describes groundwater flow system and hydrochemical characteristics of the study area (Hessian Ried, Germany). Especially, data analyses are performed with the hydrochemical data to identify the major driving force for nitrate reduction in the study area. The second part of the work introduces a kinetic model describing nitrate removal by using numerical simulation. The resulting model reproduces nitrate reduction processes and captures the sequence of redox reactions. The third and fourth parts show the influence of physical and chemical aquifer heterogeneity with varying variance, correlation length scale, and anisotropy ratio. Heterogeneous aquifer systems are realized by using stochastic approach. Results, in short, show that the most influential aquifer heterogeneity factors could change over time. With abundant requisite electron donors, physical aquifer heterogeneity significantly influences the nitrate reduction while chemical aquifer heterogeneity plays a minor role. Increasing the spatial variability of the hydraulic conductivity increases the nitrate removal efficiency of the system in addition. If these conditions are reversed, nitrate removal efficiency varies by the spatial heterogeneity of the available initial electron donor. The results indicate that an appropriate characterization of the physical and chemical properties can be of significant importance to predict redox contamination transport and design long-term remediation strategies and risk assessment.