A critical review of recharge estimation methods used in Southern Africa

English: A new stable isotope-based technique, the Modified Amount Effect (MAE) Method, was developed during this investigation. This technique provides insight into episodic recharge processes by estimating the proportion of preferential pathway-to-matrix-derived flow entering an aquifer, and the amount of rainfall required to initiate recharge via the respective flow paths. Significantly, the proportion of bypass flow can be determined without undertaking expensive and time consuming unsaturated zone studies, both factors often of primary concern when undertaking recharge investigations in developing countries. Four recharge thresholds can be identified using the MAE Method; the low and high recharge thresholds that must be exceeded before recharge occurs via preferential pathways or the matrix, respectively. These represent threshold limits, the low value only of importance following successive months of wet weather, the high value representing the rainfall that must be received to restore an aquifer system to equilibrium after prolonged dry spells. Once these thresholds are known, the recharge history of a site can be modelled using available rainfall data by adapting the Cumulative Rainfall Departure (CRD) Method. An important finding of modelling undertaken during this investigation is that in those semi-arid to arid areas where most recharge water enters the aquifer via the matrix, the period of time that elapses between successive rainfall events that exceed the matrix recharge threshold often extends to scores of years. This has significant resource management implications for much of the region as it indicates that the current approach of basing allocations on average recharge estimates is only justified if sufficient groundwater is available for use over the entire period between recharge events. In terms of recharge estimation, the Stable Isotope (SI) Method was found to return comparable results to the Chloride Mass Balance (CMB) Method in both wetter and drier inland areas of South Africa. However, both the SI and MAE Methods were found to be sensitive to the recharge history of the site, the returned recharge estimate significantly higher when calculated immediately after recharge via the matrix had occurred. This is not to say that these estimates were wrong (indeed they were representative of site recharge processes at the time of sampling), but that rainfall in the months prior to sampling should be considered. In general though, sampling should be undertaken near the end of the dry season, which in the summer-dominant rainfall areas of Southern Africa is between September and November (allowing for a 30 to 60 day lag time between rainfall and subsequent recharge). While the geological and geomorphological limitations of the CMB Method must be clearly understood before applying the technique, it does have application within many fractured rock terrains. On a regional scale, fractured rock aquifers are commonly regarded as equivalent porous mediums for modelling purposes, a necessity given the significant variations in porosity, hydraulic conductivity, and storage that occur between adjacent areas. Thus, even where longterm water level data is available, the hydraulic conditions that contribute to the observed water table response at a given site following recharge represent an average for the area surrounding a given borehole. The CMB Method negates the need for measuring or estimating these hydraulic parameters, as it already represents a long-term average of recharge. This is not to say that water levels should not be taken, but rather that recharge calculated using water balance methods be checked using the CMB Method in those areas completely overlain by a porous unsaturated zone of significant thickness. Indeed, the comparison of results obtained using multiple estimation techniques is recommended during all recharge-based investigations, whether conducted in fractured rock or porous environments.