Surface Water Quality Assessment in Ng'ethu Water Works, Chania Catchment, Kenya

Abstract

Surface waters are prone to pollution especially from agriculture and other anthropogenic activities hence the need to establish its levels of contamination. This study focuses on Chania catchment situated on the lower side of the Aberdares water tower of Kenya that supplies 95% of water to the capital Nairobi. The aim of this study was to determine the physico-chemical and microbial quality of raw surface water in the catchment and after treatment during the wet and dry seasons. Grab water samples in triplicates were collected randomly from five strategic locations in pre-cleaned 500 ml plastic bottles. Analysis for heavy metals was done by Total-Reflection X-Ray Fluorescence (TXRF) Spectroscopy using S2 PICOFOX Spectrometer. TDS, EC, Turbidity, and pH were measured immediately after sampling using Portable TDS meter (3-Hm Digital), Portable EC meter (Lovibond SensoDirect Con200), Turbidimeter (HACH 2100N) and pH meter (HANNA HI 2211) respectively. Nitrates, Phosphates and Chloride ions were analysed by spectrophotometry method using UV-1800 UV-VIS Spectrophotometer. Analysis for aluminium was carried out by spectrophotometry method using HACH DR 3900 VIS spectrophotometer. Determination of Sodium was done by Flame Atomic Emission (FAES Model AA-6200 Shimadzu) Spectrophotometer. Free Residual Chlorine was determined by DPD Colorimetric method. Biological water quality analysis was done using multiple tube method according to the standard methods for the examination of Water and waste water (APHA, 2005). The raw water was found to contain a high number of microbial indicator counts during both dry and wet seasons with 1900±624 MPN of Coliforms recorded at Chania River sampling point during the wet season implying that the water is not suitable for drinking without treatment. Generally, most of the physico-chemical parameters were within the allowable WHO recommended Maximum Contaminant Levels (MCL) with the exception of wet season where water samples had values of turbidity higher than WHO guideline values with the highest mean turbidity of 107.89±5.85 NTU recorded at Chania River sampling point. Results of heavy metal analysis revealed that the surface water in some sampling points was polluted with Manganese, Iron, Nickel and Lead above WHO recommended levels. The highest mean concentration of Iron was 3.78±0.17 mg/L recorded at Chania River sampling point during the wet season. The mean lead concentration levels at the Thika reservoir discharge and Thika River sampling points during the wet season were found to be above 0.01 mg/L which is the WHO recommended standard with the highest Lead concentration level being recorded in Thika reservoir discharge point of 0.04±0.01 mg/L. The highest mean concentration of Manganese was 0.20±0.03 mg/L recorded at Chania River sampling point during the wet season. There was statistically significant difference in the results for most water quality parameters assessed with p = 0.05 confidence level using paired T-test; for instance the concentration of Manganese in Chania river between the dry and wet seasons, where (tcal = 7.668, ttab =2.776, d.f = 4, P=0.05). The results of this study indicated that the catchment is polluted by microbial and metal pollutants, however, after treatment the microbiological quality of water was found to be within acceptable levels. It is therefore concluded that surface water within Chania Catchment is not fit for drinking directly before treatment and that the treatment method requires improvement so as to enhance removal of iron and manganese.