Assessment of Groundwater Quality in Nairobi County, Kenya

Abstract

Nairobi is one of the regions where limited resources and services are facing relentless pressure due to rapid population growth and water as a resource is of no exception. Although Water Resources Management Authority (WRMA) carries out regular monitoring of water quality in Nairobi, only a few boreholes are monitored and only very few parameters are analyzed. The objective of the present study was to assess the quality of groundwater in Nairobi to ascertain its quality and compare with World Health Organization (WHO) (2008) and National Environment Management Authority drinking water standards. Water samples were obtained from thirty selected boreholes distributed in various divisions of Nairobi during the wet and dry seasons. The study area was divided into six administrative zones namely; Kasarani/Roysambu (Zone A), Dagoretti/Kawangware (Zone B), Embakasi (Zone C), Karen/Lang’ata (Zone D), City Centre (Zone E) and Industrial area (Zone F). Physical parameters: temperature, pH, total dissolved solids, turbidity and conductivity are some of the parameters that were determined on site. Concentrations of nutrients: Na+, K+, Ca2+, Mg2+, NO3-, NO2+, Cl-, SO42-and heavy metals; Zn, Cd, Hg, Cu, Ni, As, Cr and Pb were all analyzed using standard methods. The data was analyzed using Excel software. Physical parameters varied; total dissolved solids (207-688 mg/L), turbidity (1.7-19.7 NTU), conductivity (0.32-0.99 µS/cm), temperature (22.8-27.3oC) and colour (<5-5 TCU). Chemical parameters also varied; free CO2 (0.0-27.2 mg/L), dissolved O2 (6.7-17.7 mg/L), pH (5.7-8.8), Na+ (23-207 ppm), K+ (1.9-5.7 ppm), Ca2+ (3.8-16.3 ppm), Mg2+ (0.0-4.8 ppm), NO3- (0.5-2.8 mg/L), NO2- (<0.01-0.09 mg/L), Cl- (7.6-120.8 mg/L), SO42- (1.1-30.4 mg/L), total hardness (9.6-44.8 mg/L), Zn (0.01-0.38 ppm), Cd (0.00-0.03 ppm), Cu (0.0-2.6 ppm), Ni (0.024-0.38 ppm), Cr (0.0-0.49 ppm), Pb (0.11-0.30 ppm), As (0.00-0.013 ppm) and Hg (0.001-0.003 ppm). The parameters total dissolved solids, NO3-, NO2- , Cl-, SO42-, Ca2+, Mg2+, total hardness, Zn and Hg were within the WHO (2008) consumers’ acceptable limits. Only one borehole exceeded the consumers’ acceptable colour limit of 15 TCU, all other boreholes were within the WHO (2008) drinking water standards. Highest pH mean value of 8.75 was recorded from zone D. For boreholes sampled, Cu was found to be below detection limit except for two boreholes in industrial area (A.C 29 and F.E.30) which were above WHO (2008) drinking water standards during the wet season. High levels of Cd and Cr were detected in boreholes in zones A, B and F during the dry season, with zone B recording the highest mean value of 0.03±0.01 ppm and 0.49±0.02 ppm of Cd and Cr respectively. Arsenic was within WHO (2008) drinking water standards for most sampled boreholes except for boreholes E.A.21 and M.W.25 in the city centre. All the sampled boreholes showed high concentrations of Ni and Pb which were above WHO (2008) drinking water standards during the dry season. The results suggest that groundwater from Nairobi boreholes need to be treated so that the water could meet WHO (2008) drinking water standards. Plain sedimentation or use of cloth/membrane filters may remove turbidity while nanofiltration could be used to remove heavy metals.