Effects of Erratic Rainfall Patterns on Water Quality of Yanze River in Kigali, Rwanda

Abstract

Effects of erratic rainfall patterns on water quality of Yanze River in Kigali were investigated. This study was divided into survey, fieldwork and laboratory analysis where statistical analyses were used to establish the relationship between water quality and rainfall patterns. Anthropogenic activities were assessed by taking transect walks and conducting key informant interviews. Eleven parameters were considered which include, turbidity, total suspended solids, conductivity, total dissolved solids, pH and temperature were measured insitu using meters, while nitrate, phosphate, E-coli and total coliforms were analysed in the laboratory according to standard methods and procedures of water quality analysis. Trends of rainfall, water quantity and quality patterns of Yanze River were analysed by a non-parametric Mann–Kendall test and their relationship established by Pearson Product Moment correlation coefficient. The results obtained from the field work and laboratory analysis of water quality parameters for the month of December 2018 for TDS, TSS and Turbidity were 38.86 mg/l, 85.16mg/l and 190.5 NTU while those for January 2019 were 28.08mg/l, 36.08 mg/l and 57.74 NTU respectively. The concentration levels for the month of December 2018 were found to be high by 30% compared to those of the month of January 2019.The biological parameters investigated, E-coli and total coliforms were found to be in the range of 897-1710 and 1986.3- 2419.6 MPN respectively in the two months. The Mann-Kendall statistical test showed that the characteristics of water quality, rainfall and water levels of Yanze River varied across the years from 2012 to 2018, the MK test indicated that the rainfall stations in Yanze region showed a negative trend ranging between -0.137 to -0.233, (P<0.05) implying a decrease in rainfall over the period of study. The trend analysis of Yanze water level showed a positive trend of 0.187. The results also showed an increased water levels ranging between 0.55m and 0.9m in all seasons. The trend analysis of water quality of Yanze, all the eight parameters showed a positive trend except nitrate. The results indicate that E-coli, total coliforms, turbidity and total suspended solids show the highest positive trend of 0.520, 0.50, 0.47 and 0.40 respectively (P<0.05). The results briefly detail how erratic rainfall was for the past 7 years. That change was characterized by increase of rainfall in the years 2012 to 2013, 2015 to 2016 and 2017 to 2018 and a decrease of rainfall from the year 2014 to 2015 and 2017 and the characteristics of that change were affecting the water quality of Yanze River as well in the corresponding years. Correlation analysis by Pearson Product Moment between water quality and rainfall were found to be positive, ranging from -0.10 to 0.47 while their coefficient of determination r2 varied between 0.04 % and 22.39%. Thus, the precipitation has an influence on increase in concentration of water quality parameters especially turbidity, total suspended solids, total dissolved solids and electric conductivity. The r squared values in this study are low which indicate there is the attribution of other factors which contribute to the degradation of Yanze water quality such as land use and land cover, topography of the area as well as high population density coupled with poor human settlement