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Botswana International University of Science & Technology (BIUST) 2022

Groundwater depletion at the Dukwi wellfield : causes and effects

Diphofu, Abijah

Titre : Groundwater depletion at the Dukwi wellfield : causes and effects

Auteur : Diphofu, Abijah

Université de soutenance : Botswana International University of Science & Technology (BIUST)

Grade MEng of Engineering in Geological Engineering 2022

Résumé
The main issue associated with groundwater supply in Botswana are the rate of its replenishment relative to its extraction rate and quality. This study aims to identify the possible causes and effects of groundwater decline at the Dukwi Wellfield. Specifically, it aims to investigate the contributing variables for groundwater decline at the wellfield area ; the trend and significance of groundwater level decline using the linear trend model ; the influence of groundwater decline on water quality and chemistry change ; the water quality and chemistry compliance based on the Botswana Bureau of Standard natural water specifications (BOS 262:2011) and to simulate and predict land subsidence using MODFLOW SUB and ModelMuse 4. The Pearson’s correlation method was used to determine an association between various variables such as recharge and rainfall with the drop of water levels. The recharge rates were estimated using the hydrogeological model and were compared with abstraction rates. A piper plot was used for the geochemical characterization of groundwater at the wellfield area. The results from the hydrogeological model showed mean annual recharge rate of 1 149 691 m3/year which is less than the mean annual total abstraction rate of 1 253 546 m3/year. This influenced the decline of groundwater levels and a higher concentration of parameters such as Total Dissolved Solids and Electrical Conductivity. The simulated land subsidence of the study area from 2013 to 2019 ranged from 0.008 m to 0.022 m while the projected land subsidence from 2019 to 2031 will range from 0.015 m to 0.031 m. Overall, this study revealed that higher transmissivity estimates of 8.171 m2/min, hydraulic conductivities of 0.0413 m/min and storativities of 0.0003299 were experienced at the study area, which influenced more extraction of groundwater resources and mean abstraction rates as high as 355 392 m3 /year. The statistical trends showed a declining water table with the coefficient of determination of 80% and above for most of the results from the trend model. The Electrical Conductivity and Total Dissolved Solids concentrations can be measured and monitored by implementing less energy consuming methods such as the Electromagnetic method

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Page publiée le 15 décembre 2022