Water quality indices and multivariate statistical analysis of urban groundwater in semi-arid Sokoto Basin, Northwestern Nigeria by Wali, Saadu Umar, Alias, Noraliani, Harun, Sobri, Umar, Kabiru Jega, Abubakar Gada, Murtala, Dankani, Ibrahim Mustapha, Kaoje, Ismail Usman, Abubakar Usman, Abdulquadir