Investigating groundwater abstraction impacts on surface water levels and surface water – groundwater interaction with analytical and numerical models
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Date
2023
Authors
Maluleke, Ntsako Victor
Journal Title
Journal ISSN
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Publisher
University of the Free State
Abstract
Groundwater is an important source of water for many human needs, including agriculture, water supply and industry. However, the use of groundwater may have adverse consequences. Various studies have been conducted to analyse the effect of groundwater pumping on nearby streams, and many analytical models and numerical models have been developed for various aquifer and stream conditions. Numerical models are time-consuming and require a significant amount of data input but can simulate complex systems. On the other hand, analytical models are suitable to use since they are ease to use, and have minimum data requirements and exact solutions, but they are limited to less complex problems. Streamflow impacts from groundwater pumping continue to be one of the main issues with groundwater resource development. The primary objective of this dissertation is to investigate GW-SW interaction and the impact of groundwater abstraction on surface water bodies. A secondary objective is to then develop a conceptual model that highlights the natural system behaviour of groundwater abstraction in nearby streams to evaluate alternative approaches for managing and understanding streamflow depletion.
The developed hypothetical model of an abstraction borehole near the stream is incorporated by realistic available data from the previous UFS/WRC test site along the Modder River, Free State, South Africa. Four analytical solutions, namely Jenkins (1968), Hantush (1965), Hunt (1999) and Hunt (2003) have been simulated using the developed hypothetical models (simple stream-aquifer conceptualisation to improved stream-aquifer conceptualisation) which aid in the quantification of streamflow depletion rates. To identify parameters that greatly influence streamflow depletion, parameters are varied to understand their sensitivity and effect on the rate of streamflow depletion. Furthermore, statistical analysis is used to visualise parameters that greatly influence streamflow depletion using ANOVA (Analysis of Variance). Groundwater abstraction is found to be the most sensitive parameter that greatly affects the streamflow depletion; therefore, groundwater abstraction rates should be monitored, and data collection should be accurately acquired to give out the best recommendation as this can greatly impact the surface water. Distance separation is also considered sensitive and greatly influences streamflow depletion. The period of pumping is less sensitive due to the fact that only time simulation is being extended with no variation of any parameter. The sensitivity analyses improve the understanding of how the analytical models simulate streamflow depletion rates and provide information to water managers on which parameters are most sensitive and require focused data collection.
Numerical models are applied and designed to represent the analytical solutions to assess how the solution would be implemented in a numerical model. Two approaches have been used within this dissertation namely, 1) analytical model condition which aims at creating a numerical model to simulate analytical condition as closely as possible to understand the implication of the analytical model assumptions and 2) complex numerical model conditions considering a real-world complex numerical model and assess how analytical solutions are able to represent streamflow depletion. The approximation in the numerical model, limited aquifer extent of the numerical model domain, and differences in parameter and boundary condition applied are the most significant factors that cause differences between the analytical and numerical model results. Analytical solutions overestimate and underestimate streamflow depletion when addressing a complex setting, although they fail to address complex hydrogeological settings, these analytical solutions can act as conservative tools which aid water managers in decision-making on the quantification of streamflow depletion and its mitigation.
This study's thorough investigation using different models highlights the complexities of groundwater pumping's impact on streamflow. By examining factors like borehole distance, aquifer hydraulic parameters, and model conceptualisations, it exposes limitations in simplistic analytical models while emphasising the complex nature of aquifer-stream dynamics. Understanding time delays, geological variations, and model assumptions provides crucial insights for water resource management. These findings offer valuable guidance for balancing groundwater use and preserving streamflow, urging a holistic approach integrating the strengths of numerical simulations. This research advocates sustainable strategies to address streamflow depletion, promoting responsible groundwater utilisation for the future.
Description
Dissertation (M.Sc.(Geohydrology))--University of the Free State, 2023