Fresh Water Pollution,risk Assessment And Phytoremediation Potential In Kenya

Kenya is a water scarce country with less than 1000 m3 water supply per capita per annum.Less than 20%of the country receives adequate rainfall that can sustain agricultural production while a significant proportion of the population is concentrated in a relatively small area leaving a vast area that is arid and semi-arid unoccupied.Water resources are from five major water towers that are served by Ewaso Nyiro,Tana River,Rift Valley,Athi River and Lake Victoria basin.Rapid population growth,unsupported by a corresponding economic expansion has led to unsustainable extraction of natural resources,especially forest reserves.This has had devastating effects in headwaters of most rivers originating from destroyed forest reserves.Reduced water levels in the headwaters coupled with over abstraction is causing drying of some rivers.Geogenic and anthropogenic pollution of meager remaining water resources compounds an already dire shortage.The objectives of current study were to investigate concentration of potentially toxic elements(heavy metals and trace elements)in Kenya fresh water,their distribution,health risk that they pose to consumers,recycling of waste water and its associated risks and potential application of macrophytes in phytoremediation.In furtherance of aforementioned,studies were conducted to determine heavy metal and nutrient concentration in surface water of Nairobi River and macrophyte bioremediation potential;suitability of recycled water for irrigation from Thika water treatment plant and sustainable use and risk assessment of Tana River water.Twenty-eight water samples were collected in the rain season of 2015 and dry season of 2016 in Nairobi River.Cd,Cu,Cr,Zn,As,Pb,Fe,Ni,Mn,NO3-,and TP were analyzed.Only Cr,Pb,Fe,and Mn had concentrations exceeding the WHO permissible limit for drinking water.Out of the 28 sites examined in the study,one site had Pb exceeding the WHO recommended level.Similarly,three sites exceeded the same level for Cr.Only three sites were within WHO permissible limits for drinking water for Mn while just four sites were within USEPA limit for Fe.Industrial effluent,domestic sewerage,agricultural activities,and solid waste were the main sources of pollution.Significant spatial variation of both heavy metals and nutrients concentration was observed and was due to point source pollution.Eleven out of 31 macrophytes species that were identified along the river and its tributaries are effective heavy metal and nutrient bioaccumulators and may be used in phytoremediation.To assess sustainable use and risk assessment of Tana River water,57 and 53 water samples were collected in wet and dry seasons of 2018,respectively.Cd,Cr,Ni,Pb,Hg,As,Mn,Zn,Cu,Al,B,Se,F-,Cl-,TP and NO3-were analyzed.Water quality index(WQI)was used to classify water quality into four categories based on pollution level while hazard quotient(HQ)and hazard index(HI)were used to assess non-carcinogenic risk posed to human health.Multivariate statistical analysis was applied to deduce associations and identify pollution sources.As,Cd,Pb,Ni,Se and Hg were not detected while Mn,Cl-and Al were the principal pollutants in the two seasons.26.3%of all studied sites recorded HQ>1 due to high Mn contamination.Tana River is at risk of eutrophication since total phosphorus concentration detected exceeded recommended threshold.Leached fertilizer from encroached riparian zone,fuel from leaking irrigation pumps and cleaning of motor bikes on water ways was the main source of anthropogenic pollution.Thika waste water treatment plant study was conducted to assess remediation efficiency of heavy metals and human health risk posed by consumption of vegetables irrigated with its reclaimed waste water compared to vegetables supplied in a local market,Makongeni.Concentration of Cu,Zn,Cr,Ni and Pb was assessed in the sludge,waste water and vegetables,spinach(Spinacia oleracea L.),kales(Brassic oleracea var.acephala)and coriander(Coriandrum sativum L.).Thika waste water treatment plant was efficient in heavy metal remediation.Effluent heavy metal concentration was within recommended concentration for irrigation.However,sludge was heavily polluted and requires regular dredging to reduce accumulated heavy metals that maybe resuspended and contaminate effluent water.Waste water should be utilized cautiously since low heavy metal concentrations in reclaimed waste water may build-up over time in soil and contaminate crops under production and jeopardize human health.Dietary intake of heavy metals and target hazard quotient were used to evaluate health risk posed to consumers and were within World Health Organization permissible limit.Phytoremediation which is a promising,low cost and environment friendly technique can be applied in restoration of Nairobi and Tana Rivers.Cyperus articulatus L.which has been noted,past studies,to be an effective hyper accumulator of major pollutants detected in Nairobi and Tana River was naturally growing on the banks of Nairobi River.Eichhornia crassicaulis Schltdl.,though an invasive species,was equally found to be effective in nitrate decontamination in Nairobi River.Restoration of infringed riparian zone can significantly mitigate adverse effects of non-point pollution in the two rivers.