Investigating the causes of water-well failure in the Gaotlhobogwe wellfield in southeast Botswana

In the management of groundwater resources, identifying determinants of borehole failure is useful in planning borehole and wellfield remedial measures as well identifying appropriate water treatment mechanisms if deteriorating water quality prevails as a result of decline of borehole yield and water level. A continually declining well yield in the Gaotlhobogwe wellfield, in south eastern Botswana, triggered this investigation. The determinants of groundwater level decline, which reaches about 80% in the wellfield were identified and their contribution assessed through multivariate analysis. Even though there is a 50% of decline in the specific capacity of pumping wells within 8 years period from 1995- 2003, another 30% of the decline was found to be attributed to chemical determinants, namely as Mn, Mg, Fe NO₃, HCO₃, CO₃ and SO₄. The potential physical determinants are found to be pH, Ec and TDS. The geological and hydro-geological evidence of the possible causes of groundwater decline and the identified determinants is discussed in relation to borehole construction and completion guidelines for contractors and drillers.