Vulnerability and Pollution of Groundwater in the Kisauni Area, Mombasa, Kenya

Abstract

The growing population along the coastal region as a result of natural population growth and in-migration is exerting increasing pressure on coastal resources. It is realised that the development of facilities and public services has failed to keep pace with the growing population and demand. This state of affairs prevails in the Mombasa District in general, and the Kisauni area in particular. The Kisauni area experiences inadequacies in the infrastructure and public services, notably water supply and waste management infrastructure. The largely unplanned high-density settlements and slums in Kisauni are among the most poorly served areas. This is manifested in the shortage of clean drinking water, leaving the inhabitants with groundwater to supplement their supply, or in most cases as the sole option. An assessment of the intrinsic aquifer vulnerability using the DRASTIC model employing GIS analytical tools (ESRI’s ArcView 3.2 and Spatial Analyst 2.0) indicated that the northern and south-eastern parts of Kisauni and the south-western part of the Mombasa Island are the most vulnerable to pollution. The groundwater stream flow in the Kisauni area, assessed using the modelling tool Modflow with Pmpath (Chiang & Kinzelbach, 1993), indicated that the dominant groundwater flow direction is towards the Mtwapa creek along the northern boundary and Tudor creek along the southern boundary of the study site and relatively less intense flow towards the Indian Ocean. Monitoring data on physico-chemical characteristics showed that water obtained from abstraction facilities located in the limestone geological zone is brackish and unsuitable for drinking. Whereas, within the sand geological zone, groundwater of acceptable potable standard is obtainable. It was, however, realised that groundwater in particularly the high population density Kisauni areas has raised concentrations of nitrates, which was attributed to contamination from on-site waste disposal systems, dominated by pit latrines and septic tank / soak pit systems as the mode of sewage disposal. Other sources of groundwater contamination in the area are uncollected municipal refuse. Generally, nitrate/nitrite concentrations were elevated with concentrations of NO3 -1/NO2 -1-N varying from 0.4 to 44.4 mg l-1. Relatively higher concentrations of nitrate/nitrite were recorded during the long rain season in June/July.The nitrate concentration levels, however, have not exceeded the 50 mg l-1 level set by WHO for potable water. The Kisauni area is indicated as experiencing a high degree of groundwater contamination by microbial contaminants, especially in the high-density housing settlements, attributed to onsite sewage disposal methods dominated by pit latrines and septic tank / soak pit systems. The contamination levels are more severe during the rain season when aquifer recharge is enhanced. Direct intervention by the Mombasa City local authority in conjunction with the Ministry of Water & Irrigation to alleviate the situation has consisted of supplying chlorinating agents free of charge which helps to control outbreaks of water borne diseases such as cholera and typhoid