1 Institute for Groundwater Studies, University of the Free State, Bloemfontein, 9300, South Africa
2 CONACyT-Tecnológico Nacional de México/CENIDET, Interior Internado Palmira S/N, Col. Palmira, C.P. 62490, Cuernavaca Morelos, México
3 Department of Chemical Engineering, University of Chemical Technology and Metallurgy, Sofia, Sofia 1756, 8 Kliment Ohridsky, blvd, Bulgaria
4 Institute for Groundwater Studies, Faculty of Natural and Agricultural Sciences University of the Free State, Bloemfontein 9300, South Africa
5 Department of Mathematical Sciences Federal University of Technology, PMB 704, Akure Ondo State, Nigeria