SEASONAL VARIATION OF SURFACE WATER QUALITY IN THE NAIROBI RIVER SYSTEM

Abstract

Most people living downstream of Kenya’s capital use water from Nairobi River System. To evaluate surface water quality in Nairobi County, water samples were collected from ten different locations in Nairobi River System (NRS) during the dry and wet seasons. The total dissolved solids (TDS) and temperature were determined on site using a portable meter-EC 300A.Water turbidity was determined with the help of a portable turbidity meter (SGZ -B) while the concentration of trace metals (chromium, copper, iron, manganese, lead and zinc) was determined using an atomic absorption spectrophotometer. TDS increased from upstream to downstream. The most downstream end of  NRS  considered  was  at  the  Eastern  bypass,  where  dry  and  wet  seasons  TDS  was  3040  mg/l  and  840  mg/l, respectively. For fresh water, the US EPA limits TDS at 500 mg/L, which was exceeded during both seasons. Normal water  temperature  was  observed.  Turbidity  increased  from  upstream  to  downstream,  reaching  a  maximum  dry season value of 250 NTU at the Eastern  bypass. Most heavy metals exceed the Kenya Bureau of Standards (KEBS) limits for domestic and agricultural water. With exception of iron, the concentration of metals was highest during the  dry  season.  Ngong  River  had  the  highest  concentration  of  lead  (0.6 -0.9  mg/l)  and  chromium  (0.5-1.7  mg/l). Ngong  and  Nairobi  tributaries  had  approximately  the  same  levels  of  copper,  manganese  and  iron.  This  study concludes that surface water in Nairobi has excessive levels of organic matter and heavy metals which are likely to affect the quality of Athi River, a major source of water in Machakos and Makueni Counties. It is recommended that deliberate efforts should be made to stop discharging untreated effluents into Nairobi River System as this can
have serious effects on food safety and human health.

Key words: Nairobi River, wet and dry season, physico-chemical parameters, heavy metal