A critical review of recharge estimation methods used in Southern Africa

[摘要] English: A new stable isotope-based technique, the Modified Amount Effect (MAE) Method, wasdeveloped during this investigation. This technique provides insight into episodic rechargeprocesses by estimating the proportion of preferential pathway-to-matrix-derived flow enteringan 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 expensiveand time consuming unsaturated zone studies, both factors often of primary concern whenundertaking recharge investigations in developing countries.Four recharge thresholds can be identified using the MAE Method; the low and high rechargethresholds that must be exceeded before recharge occurs via preferential pathways or the matrix,respectively. These represent threshold limits, the low value only of importance followingsuccessive months of wet weather, the high value representing the rainfall that must be receivedto restore an aquifer system to equilibrium after prolonged dry spells. Once these thresholds areknown, the recharge history of a site can be modelled using available rainfall data by adaptingthe Cumulative Rainfall Departure (CRD) Method. An important finding of modellingundertaken during this investigation is that in those semi-arid to arid areas where most rechargewater enters the aquifer via the matrix, the period of time that elapses between successive rainfallevents that exceed the matrix recharge threshold often extends to scores of years. This hassignificant resource management implications for much of the region as it indicates that thecurrent approach of basing allocations on average recharge estimates is only justified if sufficientgroundwater 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 comparableresults to the Chloride Mass Balance (CMB) Method in both wetter and drier inland areas ofSouth Africa. However, both the SI and MAE Methods were found to be sensitive to therecharge history of the site, the returned recharge estimate significantly higher when calculatedimmediately after recharge via the matrix had occurred. This is not to say that these estimateswere 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-dominantrainfall areas of Southern Africa is between September and November (allowing for a 30 to 60day lag time between rainfall and subsequent recharge).While the geological and geomorphological limitations of the CMB Method must be clearlyunderstood before applying the technique, it does have application within many fractured rockterrains. On a regional scale, fractured rock aquifers are commonly regarded as equivalentporous mediums for modelling purposes, a necessity given the significant variations in porosity,hydraulic conductivity, and storage that occur between adjacent areas. Thus, even where longtermwater level data is available, the hydraulic conditions that contribute to the observed watertable response at a given site following recharge represent an average for the area surrounding agiven borehole. The CMB Method negates the need for measuring or estimating these hydraulicparameters, as it already represents a long-term average of recharge. This is not to say that waterlevels should not be taken, but rather that recharge calculated using water balance methods bechecked using the CMB Method in those areas completely overlain by a porous unsaturated zoneof significant thickness. Indeed, the comparison of results obtained using multiple estimationtechniques is recommended during all recharge-based investigations, whether conducted infractured rock or porous environments.

[发布机构] University of the Free State