Changes of water chemistry from rainfall to stream flow in Obagbile Catchment, Southwest Nigeria

Abstract

Water chemistry changes when it flows through different pathways. This study aims to characterize the differences of water chemistry of five kinds of water in Obagbile Catchment including rainwater, overland flow, soil water, groundwater, and stream water, determine the changes in water chemistry that occur as the water moves from one pathway to another, and identify the factors responsible for the water chemistry changes. To do these, we collected 50 water samples from 10 heavy storms whose rainfall are equal to or more than 10 ​mm within an hour to test the changes of water properties across various pathways in this study. The results show that the overland flow had the highest pH and electrical conductivity (EC) and the rainwater had the lowest value of the two parameters. Ca²⁺, Mg²⁺, Clˉ, and HCOŌ were found to have their highest concentrations in stream water; meanwhile, NO₃⁻, NH₄⁺, and SO₄²⁻ were found to have almost same low concentrations in all the water samples. K⁺ was only dominant in stream water; while dissolved organic carbon (DOC) was lowest in rainwater, same in overland flow, soil water, and groundwater samples, and highest in stream flow. Principal Component Analysis (PCA) showed that for all the water samples from different pathways, two factors mainly accounted for the total variances. The two factors were related to the crustal and anthropogenic sources in rainwater suggesting that the high loadings of major cations (e.g., Ca²⁺ and Mg²⁺) in rainwater samples are soil-derived. The PCA for the overland flow and soil water showed strong correlations between pH, EC, and the concentrations of Na⁺, Mg²⁺, HCOO⁻, and CH₃COO⁻, while the high loadings of all the parameters and the strong correlations among each other were evident in the stream water. In conclusion, the chemical constituents found in water are also the constituents of pathways through which the water flows. The major factors responsible for the change in the physico-chemical properties of water in Obagbile Catchment are weathering and anthropogenic activities.