A preliminary analysis of the groundwater recharge to the Karoo formations, mid-Zambezi basin, Zimbabwe

Abstract

A preliminary analysis of the groundwater recharge to the Karoo formations, midZambezi basin, ZimbabweLarsen, F; Owen, R; Dahlin, Torleif; Mangeya, P; Barmen, Gerhard Link to publication Citation for published version (APA): Larsen, F., Owen, R., Dahlin, T., Mangeya, P., & Barmen, G. (2002). A preliminary analysis of the groundwater recharge to the Karoo formations, mid-Zambezi basin, Zimbabwe. Physics and Chemistry of the Earth, 27(11-22), 765-772. DOI: 10.1016 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal AbstractA multi-disciplinary study is being carried out on recharge to the Karoo sandstone aquifer in the western part of Zimbabwe, where recharge is controlled by the presence of a thick, confining basalt layer. The aquifer is geographically extensive, and has been identified throughout the southern part of the mid-Zambezi basin (Fig. 1). The potential for groundwater abstraction seems to be huge.The key issues in this part of the study are the extent of the recharge area and the recharge rates. The direct recharge area has previously been considered to be the area of outcrop of Karoo Forest sandstone, before it dips below an impervious basalt cover. However, resistivity profiling shows that the basalt at the basin margin is weathered and fractured, and probably permeable, while the basalt deeper into the basin is fresh, solid and impermeable. Field and laboratory analysis of 22 groundwater samples support this extension of the recharge area to include a large area below the fractured basalt. CO 2 gas pressures, calculated with the code PHREEQC using field measurements of pH and alkalinity, show that below the fractured basalt the groundwater is an open system in contact with atmospheric CO 2 . The 14 C and nitrate concentrations in this groundwater also indicate that recent infiltration takes place.The chloride contents of the rainfall and the groundwater in the recharge area have been measured to calculate direct recharge from rainfall. These data indicate that the direct recharge is in the range of 10-130 mm/yr, with an average value of 25 mm/yr. Preliminary results of recharge estimate using 36 Cl data suggests lower direct infiltration rates, but further studies are needed.The combination of hydro-chemical, isotopic and geophysical investigations show that the recharge area extends well beyond the sandstone outcrop area, northwards beneath the basalt some 20 km beyond the basalt margin.

Rainfall–interception–evaporation–runoff relationships in a semi-arid catchment, northern Limpopo basin, Zimbabwe

“…9(b) -as reported by Butterworth et al (1999). The estimated groundwater recharge of 100 mm year -1 is within the range reported from elsewhere in Zimbabwe by Larsen et al (2002), but somewhat higher than those reported by Farquharson & Bullock (1992), Sibanda et al (2009) or Wright (1992. However, it should be remembered that the modelled season (2007/08) had higher than average rainfall (see Table 3).…”

Comparison of groundwater recharge estimation methods for the semi-arid Nyamandhlovu area, Zimbabwe

“…The total chloride deposition from the atmosphere, mainly from precipitation and measured over only two years in the study area, ranged between 0.5 and 1.0 mg/L. Larsen et al (2002) proposed for the total chloride deposition from the atmosphere an average value of 0.5 mg/L for western Zimbabwe. For the computation of the recharges a value of 0.6 mg/L has been used.…”

“…Larsen et al (2002) predicted for adjoining areas in the Mid Zambezi basin recharge rates on the order of 20-25 mm/year, Obakeng (2007) found for its similar region in northeast Botswana a recharge rate of 15 mm/year, and Beekman et al (1996) adopted recharge values for Botswana in the range of 10-25 mm/year. Comparing with previous recharge studies carried out in the Nyamandhlovu area, it can be concluded that the computed recharge rates under this study are not far off from the estimates by Martinelli and Hubert (1996) ranging from 14 to 28 mm/year.…”

“…Larsen et al (2002), Gieske (1992), Beekman and Sunguro (2002) concluded that recharge in semi-arid regions in Southern Africa is a small component of the water balance (usually <5% of the average annual rainfall). Nyagwambo (2006) demonstrated that as the potential evapotranspiration is higher than the rainfall, the recharge is dependent on rainfall intensity and the existence of fractures, fissures and cracks in the tropical crystalline basement aquifers of Zimbabwe.…”

Global Isotope Hydrogeology―Review

“…Unlike northern Africa and Europe, positive and near‐zero ∆δ 18 O late‐Pleistocene values exist away from the coasts, including the eastern portion of the Ntane Sandstone aquifer (Botswana; Osenbrück et al, ; Stadler et al, ), the Omatako Basin (northern Namibia; Külls, ) and the Stampriet Aquifer (eastern Namibia; Stute & Talma, ). Other aquifers systems have negative ∆δ 18 O late‐Pleistocene values, such as those in the Kalahari Desert of southern Botswana (Kulongoski et al, ), southwestern Zimbabwe (Larsen et al, ), and the Uitenhage Aquifer located on the southern coast of South Africa (Heaton et al, ), signifying that impacts of deglacial climate changes on the southern Africa's water cycle varied spatially.…”