Groundwater depletion at the Dukwi wellfield: causes and effects

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