Groundwater Recharge Estimation in Table Mountain Group Aquifer Systems with a Case Study of Kammanassie Area

GROUNDWATER RECHARGE ESTIMATION IN TABLE MOUNTAIN GROUP AQUIFER SYSTEMS WITH A CASE STUDY OF KAMMANASSIE AREA by Yong Wu Submitted in the fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Earth Sciences Faculty of Natural Sciences University of the Western Cape Cape Town Supervisor: Prof. Yongxin Xu Co-supervisor: Dr. Rian Titus August 2005 DECLARATION I declare that GROUNDWATER RECHARGE ESTIMATION IN TABLE MOUNTAIN GROUP AQUIFER SYSTE MS WITH A CASE STUDY OF KAMMANASSIE AREA is my own work, that it has not been submitted for any degree or examination in any other university, and that all the sources I have used or quoted have been indicated and acknowledge by complete references. Full name: Yong Wu Date: August 2005 Signed……………. Abstract Groundwater Recharge Estimation in Table Mountain Group Aquifer Systems with a case study of Kammanassie Area Y. Wu PhD Thesis Department of Earth Sciences Key words: Hydrogeology, hydrogeochemistry, topography, Table Mountain Group, Kammanassie area, groundwater recharge processes, recharge estimation, mixing model, chloride mass balance, water balance, cumulative rainfall departure The Table Mountain Group (TMG) sandstone is a regional fractured rock aquifer system with the potential to be come the major source of future bulk water supply to meet both agricultural and urban requirements in the Western and Eastern Cape Provinces, South Africa. The TMG aquifer including Peninsula and Nardouw formations comprises approximately 4000m thick sequence of quartz arenite with outcrop area of 37,000 km 2. Groundwater in the TMG aquifer is characterized by its low TDS and excellent quality. Based on the elements of the TMG hydrodynamic system including boundary conditions of groundwater flow, geology, geomorphology and hydrology, nineteen hydrogeological units were identified, covering the area of 248,000km2. Limited studies focused on recharge have been carried out in the TMG area in the past 10 years. The results were estimated as high as up to 55% of precipitation. These high estimates of TMG recharge recently postulated by some groundwater specialists raised huge media coverage on the groundwater potential of TMG. In order to estimate realistic recharge rates of the TMG aquifers and to understand the impacts of climatic changes on recharge and large scale groundwater development, six conceptual recharge models were developed; three major groups of topographic features identified, in which different recharge processes and groundwater regimes occur. The tendency, periodicity and anomalous events of climate were analysed in the Kammanassie area. i After a detailed discussion of influences of time lag, actual recharge area and storativity of the aquifer in question, the recharge rates were estimated in the Kammanassie area from 1994 to 2002 using mixing model of chloride mass balance, cumulative water level departure versus cumulative rainfall departure, regression of cumulative spring flux and water balance methods. In this study, much low er recharge estimates are obtained. The estimated recharge rates are 1.18-2.03%, 0.24-7.56%, 3.29-4.01% and 2.32-4.4 % of the precipitation, respectively. Even in the same area, the results are largely different using different methods. To fill the gaps in the different results, the estimated recharge rates from each method were integrated in order to qualify the recharge rates using the weights of the different methods. The integrated rates indicated 1.63% -4.75% of the precipitation as recharging the aquifer in the Kammanassie Mountain area from 1994 to 2003. For the Peninsula Aquifer in the Vermaaks River, the budget of 3-D numeric simulation model confirme d that the recharge rate could not exceed 1.96% of the rainfall at steady state. The results were further corroborated by the 1. 65%-3.30% of the annual precipitation for 95% confidence interval. This has significant implication on the water supply strategy for the Kammanassie area. Water balance approach to the spatial distribution of recharge in the TMG area is adopted based on evapotranspiration and runoff models. The average recharge rate is 29.74mm·a -1 in the TMG area. The highest recharge rate is 137.4mm·a-1 related to Mean Annual Precipitation (MAP) of 1842.06 mm·a-1; the lowest recharge rate is 0.72mm·a-1 related to MAP of 163.51mm·a-1. The regions of the recharge rate more than 35mm·a-1 occur in the hydrogeological unit 5, 6, 8 and part of the hydr ogeological unit 17. The other areas are smaller than 35mm·a-1. The recharge rate is a non-linear positively proportional to the MAP. Most high recharge percentages are related to rainfall of 300 to 1100mm·a-1. The recharge percentage would be less if precipitation is greater than 1100mm·a -1 or less than 300mm·a-1. Lower recharge estimates for the TMG suggest that the caution must be taken when a water supply strategy is formulated for the TMG area. ii ACKNOWLEDGEMENTS It was 1997 when Dr. Xu visited China I expressed my wish to go overseas for my PhD. Four years later he visited China again and gave me the good news that I had been offered an opportunity to make my dream come true. Early in the year 2002, I became a PhD student of the University of the Western Cape. This thesis was only made possible with the co-operation of many individuals and institutions. The author therefore wishes to record his sincere thanks to the following: § Prof. Yongxin Xu for all his assistance and useful advice in the research. § The Council for Geoscience in Pretoria, in particular Dr. Rian Titus, for his useful suggestions. § The Department of Water Affairs and Forestry in Bellville, in particular Mr. Mike Smart and Ms Rossida Peter for their kind help with collecting chemical data and sampling programs. § Overberg Water, in particular Shahied Solomon, Deon Haasbroek and Johan Uys, for all their assistance during the sampling programs, steering committee meetings and for providing data sets and information on request. § Reginald Domoney, Browen Honigwachs, secretary of Department of Earth Sciences; Dr. Shafick Adams and Braun Lawrence for their assistance during research in South Africa. § Caroline Barnard for her kind help during the recharge project. § The postgraduate students: An Eijkelenburg, Segun Adelana, Lixiang Lin, Haili Jia, Humberto Antonio Saeze (PhD students), Vuyolwethu Sigonyela, Riyaz Nakhwa Ignatius, Mokete Seboko, Joubert Bulasigobo and Fortress Netili (MSc students) in Groundwater Group at UWC are thanked for their frie ndship and support. § Finally, to my wife, Xianfeng Sun, my son, Zhenghao, my parents and family, a big and special thank you for all your immense support and patience. iii Abbreviations and Notations Notation in literatures c g average chloride concentration of the groundwater m chloride mass flux ? total water potential A recharge area or area of catchment a thermal diffusivity of the aquifer Ab total area of drainage Ab total area of the basin Af area of basin floor b rate of increase in surface temperature C pore water chloride concentration c0q0 heat capacity of the water cg concentration of chloride in the groundwater Ckp chloride concentration in precipitation Clgw chloride concentration in groundwater Clgw chloride concentration in groundwater at the water table Clib average chloride concentration in interstitial flow below the basin floor Clir average chloride concentration in regional interstitial flow between basin floors Clp precipitation weighted chloride concentration Clr chloride concentration of the runoff Clr chloride concentration of the runoff water to the basin floor Clsm chloride concentration in soil moisture Clt average chloride concentration in the ground water Cluz chloride concentration in the unsaturated zone cq heat capacity of the aquifer CR recession constant controlling the rate of decline of the groundwater level D dry chloride deposition D average depth of the aquifer D hydrodynamic dispersion coefficient D rate of addition within the aquifer iv DS current depth of soil water E evaporation Et(t) evapotranspiration ETgw evapotranspiration ETunsat evaporation from the unsaturated zone F volume of the i th flow path within the sample divided by the total volume of the sample FC field capacity Fw a vector in a plane h sampling depth of the water h total depth of the aquifer H total thickness of the aquifer K hydraulic conductivity of the soil matrix K(?) the unsaturated hydraulic conductivity LINRES Linear reservoir Module Maxil maximum intercept loss n aquifer porosity P annual precipitation P precipitation P(t) precipitation P av representing an open aquifer system P e effective precipitation P i rainfall for month i P t a threshold value representing aquifer boundary conditions q interstitial ground water recharge flux rate q volumetric water flux q CMB is the water flux calculated using CMB Q is the deposition of airborne chloride Q(t) runoff Q(x,y,t) the pumpage from the element Qa net discharge or abstraction from the groundwater system Qd surface flow Qgi-1*D a summation of the decay of previous groundwater contribution qi water flux of the i th flow path v qi actual water flux of the sample Qi-1*i rainfall induced flow increment Qib recharge through interstitial pores in the basin floor Qloss seepage Qmb recharge through macropores in or near the basin floor Qout natural outflow Qp groundwater abstraction Qs flux of spring in catchment Qtb total recharge through the basin floor r runoff r fraction of a CRD which contributes to recharge r run-on flux

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