| dc.description.abstract | For decades, Polychlorinated Biphenyls (PCBs) have been recognized as important and
potentially harmful environmental contaminants. fhe intrinsic properties of PCBs, such as
high environmental persistence, resistance to metabolism in organisms, and tendency to
accumulate in lipids have contributed to their ubiquity in the- environmental media and
have induced concern for their toxic health effects in humans and wildlife after prolonged
exposure.
This research was carried out to establish the pollution status of I gong and Mathare Rivers
with respect to the seven UNEP indicator PCBs and two additional ones (PCB 105 and
156). This was accomplished by determining their spatial and temporal distribution in
water and sediments from the two rivers and correlating the levels with physicochemical
parameters of water.
Field samples were collected monthly between vlarch and September 2009 from 7 sites
along the Ngong and Mathare rivers representing upstream, middle stream and
downstream terraines. Analysis of PCB congeners was achieved on Agilcnt Gas
Chromatograph equipped with an electron capture detector after extraction with solvent
following the USEPA methods 3510 and 3540. Mean percentage recoveries of PCB
congeners were 85.85±5.40 and 88.71±5.68 in water and sediment, respectively, while the
instrument detection limit varied between 0.000 1'2and 0.00691 ng/L for PCB 118 and 180,
respectively.
PCB congeners' concentrations 111 water ranged from BDL. to 838.23 ng/L in water
samples from Ngong River and BDL to 307.12 ng/L in sam pies from Mathare River with
the highest mean PCB levels being observed in water samples from Donholm Bridge. PCB
congeners 28 and 52 were the most prevalent in all sites an.l throughout the sampling
period with occurrence frequency of 100%.
Residue levels of PCB congeners detected in sediments ranged from BDL-34.92 ug/kg in
samples from Mbagathi Bridge, 0.32-70.30 ~g/kg in samples from Mater Bridge and 0.9188.0
I ug/kg in samples from Donholm Bridge. Fhe frequency of occurrence of the PCB
congeners in sediments was 100% for all the congeners except for PCB 138 which was not
detected in 2 sampling sites ie Mbagathi and Mater Bridges in the month of April.
There was a general increasing trend in levels of individual PCB congeners as well as the
sum of PCBs downstream of both rivers. The PCB levels were several fold higher in
sediment than in water because PCBs are hydrophobic and tend to partition to the more
organic components of the environment hence adsorb strongly to organic matter in
sediments. The concentrations were higher during the dry m nths (March, August and
September) than in the wet months (April, June. July). There \ 'as a direct relationship of
PCB levels in water with physicochemical parameters (pH, TSS, TDS) as indicated b.' the
positive Pearson's correlation coefficient values. Generally, the average PCB
concentrations were below the USEPA allowable limit of 500 ng/L. However, with
bioaccumulation and biornagnifications, the levels pose adverse potential health impacts to
human and animal health.
This study provides baseline data on the levels of PCB residues in the selected matrices
from the two rivers. This will inform policy makers on the quality of water of Nairobi
River Basin as well as supplement the country's studies as an obligation for all parties to
the Stockholm convention on POPs. | en |
