The onset of iron biofouling in boreholes in and around Cape Town

Iron biofouling in boreholes drilled into the Table Mountain Group has been documented, with groundwater abstracted for the Klein Karoo Rural Water Supply Scheme and irrigation in the Koo Valley hampered by clogged boreholes, pumps and pipes. A similar phenomenon has been experienced at some boreholes drilled and operationalised by the Western Cape Government in response to the onset of the crippling drought in 2017. Monitoring of groundwater levels and pumping rates has yielded data showing a gradual decrease in groundwater level as the pumping rate reduced in response to the pump becoming biofouled, with possibly the same negative impact on the borehole itself. Methods are available to rehabilitate the boreholes (mechanical scrubbing, chemical treatment and jetting), but it seems difficult to destroy the bacteria and re-occurrence of biofouling appears inevitable. In the absence of better solutions, current experience suggests an annual borehole maintenance and rehabilitation budget of R 100 000 per borehole is required. This paper presents three case studies of boreholes drilled into the Malmesbury Group and Table Mountain Group and explores possible triggers of biofouling and its manifestation in the monitoring data.