Determination of some chemically and Biologically Toxic substances in underground waters in areas sorrounding Nairobi

Abstract

30 samples were collected from areas around Nairobi during the wet and dry seasons of the year 1994. The samples were analyzed for heavy metals cadmium, copper, zinc, lead and manganese, nitrates/nitrites and dissolved organic carbon (DOC) by spectroscopic methods and for the presence of pathogenic bacteria using microbiological techniques.

The overall range for copper, zinc, lead, manganese, nitrates/nitrites and dissolved organic carbon (DOC) during the two seasons in the three divisions were: 0.01-1.30, 0.01-15.50, 0.01-1.00, 0.01-3.30, 0.02 - 15.00 and 2.00 - 30.80 respectively. The mean levels of copper, zinc, lead, manganese, nitrates/nitrites and DOC in Kangundo division during the two seasons were: 0.30, 1.90, 0.19, 0.57, 5.60 and 12.26 respectively. In Ngong division, the mean levels for copper, zinc, manganese lead, nitrates/nitrites and DOC during the wet and dry seasons were: 0.13, 0.91, 0.38, 0.21, 2.20 and 5.32 respectively. During the wet and dry seasons, the mean levels for copper, zinc, manganese, lead, nitrates/nitrites in Kikuyu division were: 0.22, 4.81, 0.68, 0.38, 7.93 and 2.20 respectively. All the levels are in pmm.

The highest levels of copper, nitrates/nitrites and DOC were found in Kangundo division. Kikuyu division had the highest levels of lead, Manganese and Zinc. Kangundo division had five wells with pathogenic bacteria; Ngong division had four while Kikuyu had two of all the eleven wells that contained pathogenic bacteria during the wet season. Only two wells in Kangundo contained pathogenic bacteria during the dry season.

Wells and boreholes in Kangundo contained more copper, dissolved organic carbon, nitrates/nitrites than those in the other two divisions. Zinc, lead and manganese were prevalent in water samples from Kikuyu division.

Pathogenic bacteria were more prevalent in water samples during the wet season. There were more bacteriological contaminated water sites in Kangundo - 5 of the 11 contaminated sites during the wet season and all the 2 contaminated sites during the dry season. Generally the levels of the heavy metals and nitrates/nitrites were high during the dry season while those of dissolved organic carbon were high during the wet season.

The results obtained show that the levels of the heavy metals and nitrates/nitrites are mostly below the maximum permissible levels set by the World Health Organisation (WHO). The highest level of nitrates/nitrites was 15ppm, which is below the WHO maximum acceptable limit of 45-50ppm. Highest levels of zinc, lead, manganese and copper were 15.5ppm, 1.0ppm, 3.3ppm and 1.3ppm, respectively. The levels of zinc, lead and manganese were above the WHO maximum permissible levels of 15ppm, 0.1ppm and 0.5ppm respectively. The highest copper level of 1.3ppm was below the WHO maximum permissible level of 1.5ppm.

The presence of some of these substances at toxic levels revealed that these waters are occasionally contaminated and as such are not as safe as is usually assumed.