An assesment of wetland influence on water quality of river Mereronyi, Kenya.

Abstract

An investigation to assess the influence of a natural wetland in the middle reach of River Mereronyi , Kenya, on the river water quality was carried out between May and June, 2008. The aim of the study was to evaluate the effect of this particular wetland on nutrients (Nitrogen and Phosphorus) and removal and trapping of coliforms (E. coli, total coliforms) in the river water. Water samples were collected weekly using acid washed plastic bottles, while sediment samples were collected using a corer. During every sampling session; temperature, conductivity, Dissolved Oxygen (DO) and pH were measured in situ using Wissensschaftlich-Technische Werkstätten (WTW) microprocessor meters and probes. River discharge was estimated using velocity area method. In the laboratory nutrients; nitrogen and phosphorous were determined using colorimetric methods, while bacteriological analyses were done using pour plate technique and Most Probable Number method. Plant productivity was measured using harvest method, where a 0.25×0.25m quadrat was used to demarcate the plots and plants harvested just above the roots. The concentration of TN and TP in water and sediment was determined using the Kjeldahl digestion method. Data was analyzed using a Statistical Package for Social Sciences (SPSS version 12) where comparison of means of different variables was performed using Analysis of Variance (ANOVA). The relationship between the concentration of different variables in water and sediment samples was analyzed using simple linear regression analysis. Student’s t-test was used to determine the significant difference between selected variables at the wetland inlet and outlet. Water temperature differed significantly between the sites during the study period (t = 2.054, df =54, p < 0.05). Mean river discharge was 4.9 ± 2.2 m3s-1 at the wetland inlet and 6.7 ± 2 m3s-1 at the wetland outlet. The papyrus standing biomass was 21.84 tonnes/ha-1 . There was a significant negative relationship between phosphorus in sediment and that in plants with correlation coefficient of r = - 0.95. A significant positive relationship between total nitrogen in water and in plants with a correlation coefficient of r = 0.91 was also observed in this study. The results showed removal efficiencies by the wetland of 20% for ammonium, 32.7% for nitrate-nitrogen, 61.8% for Soluble Reactive Phosphorus, 57% for total phosphorus and 45.85% for total nitrogen. The highest number of E. coli and total coliforms of 516 MPN/100ml and 390 No/100ml was recorded in a disturbed site 3 (S3) within the wetland. There was reduction of SRP, NO3, TN and NH4 in water between the wetland inlet and wetland outlet. This study recommends that the wetland should be reclaimed by WRMA and NEMA together with the local community.