Impact of Pumping on Groundwater Resources-An Assessment through steady state modelling: Orapa Wellfield 7, Central Botswana

Abstract

Debswana Diamond Company, Orapa Letlhakane Damtshaa Mines (OLDM), undertook development works for a new wellfield, Wellfield 7 in 2012/2013. The primary objective was to establish the new wellfield to supply an additional 3Mm3/annum of water for the mines’ operations, for a 20 year period starting 2014. Abstraction from the wellfield has the potential to have significant impacts on the already existing groundwater resources around the area and thus it is important and necessary that the potential impacts are thoroughly evaluated and addressed. This research used a numerical groundwater model to simulate and predict the potential impact the large scale abstraction from Wellfield 7 will have on the aquifer and estimated the aquifer drawdown expected over the entire abstraction period. Primary and secondary data on geology, hydrogeology and geophysics were integrated to develop a conceptual model of the area. Visual MODFLOW Pro 4.3 was then used to model the aquifer system under steady state conditions. The parameters assigned to the model were derived from previous studies and pumping tests and they were then adjusted during model calibration. The model was calibrated under non-pumping scenario to static water levels. Overall, model results compared with the observed data, with the model being able to simulate observed field values to a 66% correlation coefficient and heads departing from the mean by 12m. The model results show that a daily abstraction of 8040m3 from the wellfield will result in a drawdown of up to 30m at the end of the 20 year pumping period. This is about 50% of the total available drawdown. Although the model is accurate and reasonable, the limitations of the model should be taken into consideration before it is applied as a management tool. This is because of, among other things, uncertainties arising from the assumptions made to simplify the complex aquifer system.