Impacts of treated wastewarer on the surface water and groundwater quality: a case study in North East Gaborone (within Notwane catchment) Botswana

Abstract

This study was conducted in the Notwane catchment where the capital city of Botswana, Gaborone is located. Botswana having unreliable rainfalls and Gaborone having shortage supply of water for drinking, this study was initiated with the aim of investigating alternative sources of water in this part of the country. Even though treated wastewater is available in the studied area, this water is not much utilized at present. The objective of the research is to investigate the impact of treated wastewater on the quality of surface water and groundwater in the north east of Gaborone, so that this water can be utilized for different purposes including as a source for groundwater recharge. The research area is located near Gaborone in south east district within Notwane catchment, which is a tributary to the Limpopo River, having an area of about 3 000 sq. kms. For this purpose forty-one surface water samples, one treated waste water sample and fifteen groundwater samples were collected from different parts of the study area. All the water samples were analysed for major cations (Ca2+, Mg2+, K+, Na+ and Li+), anions (HCO3-, Cl-, SO42-, NO3-, F-, PO43- and CO32- ) and some trace elements (As, Ba, Cd, Co, Cr, Cu, Fe, Mn, Pb, Ni and Zn). The major cations and all the trace elements were analyzed in the geochemistry laboratory of the Department of Geology, University of Botswana using Inductively Coupled Plasma Mass Spectrometry or ICP-MS. The anions were analyzed in the water quality laboratory of the Department of Water Affairs. For all the water samples alkalinity and hardness were determined using AquaChem software. Simple descriptive statistical method was utilized for the analyses of the water chemistry data. AquaChem and Suffer software’s were also used for analyses the water chemistry data. The surface water is slightly alkaline to alkaline, fresh, soft to very hard whereas the treated wastewater is slightly alkaline, fresh and hard. Groundwater is slightly acidic to alkaline, fresh to saline and soft to very hard. In all the waters, surface water, treated wastewater and groundwater based on the mean values of the chemical parameters, the cations were in the order of abundance as Na+ > Ca2+ > Mg2+ > K+ > Li+. Surface water anions were in the order of abundance as HCO3- > Cl- > SO42- > NO3- > PO43- > F- > CO32-, treated wastewater as HCO3- > Cl- > NO3- > SO42- > PO43- > F- > CO32- while in the ground water the anions reveal order of abundance as Cl- > HCO3- > SO42- > NO3- > CO32- > F- > PO43-. Based on the mean values of the chemical parameters in the surface water, the analyzed trace elements were in the order of abundance as Ni > Fe > Pb > Ba > Zn > Mn > Cd > As > Co > Cr > Cu, in the treated water Ni > Ba > Zn > Pb > Fe > Cd > Mn > As > Co > Cr and Cu having equal concentration (0 mg/l) while in the groundwater the analyzed trace element reveal order of abundance as Ni > Pb > Ba > Mn > Zn > Fe > Cd > As > Cu > Co > Cr. The results of the hydrochemical analyses of all the waters disclose that the groundwater chemistry is highly controlled by rock-water interaction and anthropogenic activities in the catchment than the chemistry of surface water and treated wastewater. The analysed cations for surface water and treated wastewater satisfy the standard of both WHO and Botswana Bureau of Standards whereas in the groundwater the cations are highly above the recommended limits of the standards set by both WHO and Botswana Bureau of Standards with the exception of lithium and potassium. The analysed anions for surface water and treated wastewater fulfil the standard of both WHO and Botswana Bureau of Standards whereas in the groundwater the concentration of Cl and NO3 were above the standards set by both WHO and Botswana Bureau of Standards. In the analysed trace elements for all the waters, almost all samples met the trace element standards set by both WHO and Botswana Bureau of Standards with the exception of nickel and lead in surface water, treated wastewater and groundwater. The overall chemical analyses of the water chemistry revealed that treated wastewater quality does not have any significant harm to both surface and groundwater quality; therefore it can be used as a source of recharge to the aquifers in the catchment.