Seasonal river channel water exchange and implications on salinity levels in sand dams: Case of semi-arid Kitui Region, Kenya

Abstract

This paper presents results of a study on the variations of salinity levels (and associated physico-chemical parameters) in sandbeds of seasonal rivers found in the semi-arid region of Kenya. The study sought to investigate the interaction between water in river sandbeds and bank storage and how this interaction influences salinity levels in sand dams and shallow wells in semi arid lands. The results of the study show that there is a significant relationship between water quality parameters in dry river sandbeds and the corresponding water quality parameters in shallow wells and sand dams. Within river sandbeds, the dry season salinity ranged from 0.9 to 12.8% while the wet season salinity ranged from 0.4 to 1.1%. The wet season total dissolved solids (TDS) concentrations ranged from 77.8 to 272 mgl^-1. The dry season TDS concentrations were high ranging from 227 to 3,320 mgl^-1. The wet season water conductivities were  however low ranging from 155.9 to 539 µS.cm^-1 as compared to the dry season conductivities that were high ranging from 455 to 6,640 µS.cm^-1. As compared to the river channel sandbeds, water in shallow wells and sand dams was characterized by relatively high levels of physico-chemical parameters with salinity ranging between 1.5% and 6.5% and TDS concentrations ranging from 396 to 1,680 mgl^-1. The level of salinity in sand dams and shallow wells tends to be much higher than those in the stream channel sandbed. Also, within the river channel sand beds, the levels of salinity were mostly within the minimum allowable limit for drinking water except in extended drought periods. The relatively high salinity, TDS concentrations and conductivity were attributed to mineralization of water due to entrapment, ground water recharge and solubisation of minerals from soils. The study shows that the flow of water within sand bed matrix is characterized by low hydraulic conductivities and water fluxes and this has important implication on the salinity levels in sand beds of seasonal rivers.

Keywords: Sandy river channel; water exchange; bank storage; salinity; Kitui, Kenya