Abstract
Water Quality Index (WQI) computation method based on quality parameters’ selection, assigning of weights, creation of sub-indices and calculation of the aggregated quality value has been used for many decades from the early 1960s to check on the pollution stati of watersheds. Today, due to rising water demand in the face of increased drought induced water shortage challenges and rampant pollution of the water sources, consumers augment their needs by using groundwater resources like in the case of Langata sub County in Nairobi city-Kenya. Little however, has been done to assess the overall potability of groundwater quality here. Accordingly, in the present study, a Water Quality Index (WQI) was developed by Weight Arithmetic Water Quality Index (WAWQI) method to fill that identified gap using a five categories’ grading scale, viz. excellent (A), very good (B), good (C), fairly good (D), suitable (E) and unsuitable (F). To realize this, chemical parameters’ concentration ranges were defined on the basis of the Kenyan Standards (KS) and International Standards of World Health Organization (WHO).Subsequently, a total of eight chemical parameters were selected based on their level of occurrence in borehole commissioning data obtained from Water Resource Management Authority’s database of the area viz; K+, Na+, Ca2+, Fe2+, F-, Cl-, SO4-2 and Ec (µS/cm). Out of a total of 137 boreholes, only 39 had complete eight chemical parameters. These 39 boreholes’ water quality assessment is taken in this study as a true representation of the entire area’s groundwater quality. The individual concentrations were spatially plotted using Surfer Software’s digital terrain model (DTM) which produced contoured maps in different chroma saturations including the overall aggregated concentration to facilitate scale ranking. The WQI for the study area is 53.18 in a scale of 1 to 100, with one being excellent. In this case, the groundwater quality was ranked as grade ‘C’. WQI calculation method is known to improve the understanding of water quality issues by integrating a suite of data into a single value which describes the status of water quality. WQI is very useful for the water management authorities because it facilitates their informing of the public on water quality in a simplified form.
Highlights
Interest to document water quality can be traced in historic writings such as the Bible where Babylon city’s wastewater discharge management is explained [1]
The results suggested that groundwater from the boreholes in Langata area need to be treated to comply with World Health Organization (WHO) guidelines on drinking water standards
Sulphate concentration in Langata groundwater ranges from 0.29-150 mg/l which is within the permissible limit of 400mg/l as per Kenyan Standards and 250mg/l as per WHO Standards
Summary
Interest to document water quality can be traced in historic writings such as the Bible where Babylon city’s wastewater discharge management is explained [1]. It is a ubiquitous fact that the literature on drinking water quality is still on a spawning spree everywhere due to the rising water demand in the face of increased drought induced water shortage challenges and pollution of the few remaining drinking water sources. The old way of knowing water quality is usually by comparing the measured physico-chemical and biological parameter values with existing standard guidelines. This methodology is only suitable in identifying the contamination sources and meeting of legal requirements on water quality. Such a procedure was able to control widespread waterborne diseases those of bacterial origin
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