2006 Philadelphia Annual Meeting (22–25 October 2006)

Paper No. 4
Presentation Time: 8:55 AM


DUAH, Anthony A., XU, Yongxin and WU, Yong, Earth Sciences, University of the Western Cape, Private Bag X17, Bellville, 7535, South Africa, aduah@uwc.ac.za

The Table Mountain Group (TMG) aquifer system is exposed along the west and south coasts of South Africa. It is a regional fractured rock aquifer with the potential to be a major source of bulk water supply to meet the agricultural and urban water requirements of the Western and Eastern Cape Provinces of South Africa. The TMG aquifer system consists mainly of sandstone, shale and quartzite units and is exploited extensively for agricultural purposes. Large-scale groundwater supplies from fractured rocks in South Africa are taken from the TMG aquifer in the Kammanassie Mountain region, part of which constitutes the Vermaaks River well field. The persistent decline of water levels from the Vermaaks well field over several years even when pumping rates were reduced a number of times raised concerns about recharge estimates and the storage coefficient computed earlier.

The spatial distribution of recharge within the TMG outcrop area is calculated based on the water balance model. The recharge rates derived from the water budget in the Kammanassie area were calibrated with results from the cumulative water level departure, spring volume flow and chloride mass balance mixing models.

The average recharge rate in the TMG is about 30 mm/yr, which is generally less than values estimated earlier (Vegter, 1995). The highest derived recharge rate of 137 mm/yr is related to a mean annual precipitation (MAP) of 1842 mm/yr. The lowest rate of about 1 mm/yr is related to MAP of 164 mm/yr. The recharge percentages of MAP are from 0.28% to 12.6%.

The recharge rates based on the water balance method have a non-linear positive relationship with the MAP. The recharge rates of 7.19-57.91, 9.23-67.68, 6.25-60.07 19.83-69.25, 35.03-96.52, 19.02-115.69, and 17.45-137.42 mm/yr are associated with MAP of 400-500, 500-600, 600-700, 700-800, 800-900, 900-1,000 and 1000 mm and above respectively. Most of the high recharge percentages are related to rainfall ranges between 300 mm to 1100 mm. The low recharge percentages occur in the MAP regions of more than 1100 mm due to excess runoff and areas of less than 300 mm of MAP which could be attributed to high evapotranspiration and low effective infiltration.