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First-order reaction networks are often encountered in groundwater pollution problems. The members of the network transform/degrade from one species to another. In this paper a strong form meshfree numerical method named radial point collocation method (RPCM) is introduced to model such reactions. In the proposed method, the entire domain is decomposed into a number of overlapping local support domains surrounding each node. The concentrations of species at each node and its directional derivatives are approximated using multiquadrics radial basis functions (MQ-RBF) within the local support domain. 1D models are verified against existing analytical solutions. Further, the 2D model simulations are compared with widely used RT3D (Reactive Transport in 3 Dimensions) for a benchmark problem. Sensitivity analysis is also performed for model parameters. The proposed model solves species-wise governing equations and boundary conditions without operator splitting, unlike the popular grid/mesh-based models. The RPCM-based models are found to be very effective in modeling first-order reaction network problems.

Groundwater is the largest source for freshwater which plays an important role in the hydrological cycle. The pollution of groundwater is on the rise due to various natural and anthropogenic sources such as landfills, agricultural lands, and underground waste storage facilities etc. These pollutants can be subjected to reactions depending on the contaminant type and the subsurface environment along with advection and dispersion processes. As groundwater is used in various human activities such as drinking, agriculture and industrial activities, it is essential to track the contaminants in groundwater for assessing possible environmental impacts. The complex phenomena of flow and contaminant transport are represented by partial differential equations (PDEs) which are solved numerically throughout the problem domain. Although Finite Difference method (FDM) and Finite Element Method (FEM) based models are conventionally used for these simulations, these methods suffer from certain instabilities due to the presence of mesh/grid, for example, numerical dispersion and artificial oscillation for advection and reaction dominant problems. Moreover, these methods are not suitable for adaptive analysis which requires meshing and re-meshing in each simulation making the problem highly computationally expensive. Here, we present a strong form meshfree method named Radial Point Collocation Method (RPCM) for modelling flow and transport in groundwater. In contrast to mesh-based methods, the problem domain is discretised using only nodes in the proposed method. Moreover, unlike the mesh-based methods, it produces stable solutions for advection and reaction dominant problems without using special techniques such as up-winding, adaptive re-meshing or, operator splitting. The performance of the model is tested against analytical solutions, FDM and FEM based models for different reactive transport problems in groundwater involving adsorption, decay, multi-species decay network and biodegradation.

Groundwater flow and contaminant transport are complex phenomena which require partial differential equations to be solved numerically throughout the problem domain. Finite Difference method (FDM) and Finite Element method (FEM) based models are traditionally employed for these simulations. These methods suffer from certain instabilities due to the presence of mesh/grid. Stabilization techniques such as adaptive meshing/re-meshing, operator splitting and upwinding are used to counter these issues. Recently, meshless methods are being applied to a variety of groundwater flow and transport problems. In this study, a meshless method named Radial Point Collocation Method (RPCM) is demonstrated with two case studies. This method doesn’t use operator splitting for simulating decay reactions. Due to the use of local support domain, these methods are known to have better stability compared to FDM and FEM. The support domain size can be easily increased in order to enhance the model stability which is difficult to achieve in mesh/grid-based methods. A coupled reactive transport model involving single species first-order decay in an unconfined aquifer is developed for the first case study. For case study 2, a three-species decay chain-based reactive transport model is developed for an aquifer having irregular boundary. The model simulation results are compared with FDM- and FEMbased numerical models developed using well-established MODFLOW-MT3DMS and COMSOL models, respectively. The results indicate good agreement between the meshless and mesh/grid-based methods which shows the effectiveness of meshless methods.

Among the different hydrological models used for water resources assessment, conceptual models are gaining popularity due to their simple structure and satisfactory performance. In this study, perf...

Mesh/grid-based methods such as the Finite element method (FEM) and Finite difference method (FDM) are extensively used for simulation of contaminant species transport in groundwater. However, these methods in their standard form are susceptible to instabilities such as oscillations and numerical dispersion in the solution. Further, due to the use of mesh/grid, these methods are also computationally expensive. In this study, a meshfree method named Radial point collocation method (RPCM) is demonstrated for simulating reactive transport involving linked first-order reactions in a coupled flow environment. Multiquadrics radial basis functions (MQ-RBF) are used for approximation of the state variables. The model uses local support domains which produce sparse matrix systems that efficiently deal with the ill-conditioning problem which generally occurs while using global support domains. The model developed is verified against the semi-analytical solutions and its performance is compared with FEM for case studies. It is observed that the model provides accurate solutions for the problems considered and also handles advective flow conditions better than FEM. The proposed model successfully simulates the fate of contaminants in linked chain reactions.

Water resource assessment is important for integrated water resources management of a basin to which rainfall is a vital component. Generally, semi-distributed to distributed models are used for water resource management studies. Higher data requirement and simulation time restrict the use of these models. Lumped conceptual models are drawing attention these days owing to their simplicity and minimum data requirement. In the present study, the performance of conceptual models Genie Rural a 4 parametres Journalie (GR4J) and Australian Water Balance Model (AWBM) was evaluated in comparison with semi-distributed Soil and Water Analysis Tool (SWAT) model. The selected study area, Upper Godavari River Basin, is in the windward side of Western Ghats in India, which receives heavy rainfall during south-west monsoon season. However, this region faces water scarcity issues in non-monsoon period due to lack of proper water management scenarios. Five catchments in upper Godavari basin are used for the analysis. Spatiotemporal analysis of rainfall is done to understand its effect on streamflow at the catchment outlet. The efficacy of model predictions has been analysed statistically in terms of Nash–Sutcliffe Efficiency, coefficient of determination (R2) and percentage bias. Flow duration curves and time series diagram of streamflow predictions are also compared to better understand the results. All three models predicted streamflow with reasonable accuracy. Considering structural simplicity, less data requirement and simplicity in calibration process, this study proposes conceptual models over SWAT in the regions facing data scarcity.