Estimating the relative contribution of heavy metals and nutrients loads from diffuse and point sources and various hydrological pathways is a major research challenge in catchment hydrology. Understanding of the transfer, loads and concentration of these loads in basins is useful in designing and implementing policies for the managements of surface waters. In sub- Saharan countries, few studies have been done to estimate heavy metals and nutrients transfers in catchments. It is usually difficult to obtain hydrological and hydro-chemical data even for smaller catchments. This Thesis presents the estimates of loads of heavy metals and nutrients from industry and land use into two rivers that flow through an industrializing catchment, additionally presents the selection and application of a model to estimate TN and TP loads in the Kombolcha catchments. The study of the transfer contaminants from diffuse and point sources illustrates management, capacity and policy needs for the monitoring of rivers in Ethiopia, and with relevance for other sub-Saharan countries.
The study was done in the semi-arid catchments of Kombolcha city, which sits within an urban and peri-urban setting in north-central Ethiopia. The Leyole and Worka rivers drain the catchments, and receive industrial effluents from several factories in the surrounding area and wash-off from the surrounding catchment. The rivers flow into the larger Borkena River. The goal of this research was to monitor and quantify sources and transfer of heavy metals (Cr, Cu, Zn and Pb) and nutrients ((NH4 +NH3)–N, NO3–N, TN, PO4–P, TP) into the Leyole and Worka rivers, and evaluate their management/control in a data-poor catchment. The apportionments of the total nitrogen and phosphorus loads from diffuse and point sources were investigated. The work is placed in a policy context through a review of relevant policy within Ethiopia and at the wider perspective of sub-Saharan Africa.
The first set of measurements was on industrial effluent samples collected from discharge from five factories. In total, 40 effluent samples were taken in both 2013 and 2014. The second set of measurements were on waters and sediments. In total, 120 water samples were collected from the rivers in the wet season of two monitoring years of 2013 and 2014. River bed sediment samples, in total 18 samples, were taken at six stations on three occasions in the wet seasons the two monitoring years. In order to estimate the dilution capacities of the Leyole and Worka rivers, daily flow depths of the river water were recorded twice a day during the sampling campaigns of 2013 and 2014. Stage‐discharge rating curves were used to estimate the flows of both the Leyole and Worka Rivers. The heavy metals concentrations were measured using Inductively Coupled Plasma Mass Spectrometry.
The median concentrations of Cr from tannery effluents and Zn from steel processing effluents were 26,600 and 155,750 μg/L, respectively, much exceeding emission guidelines. Concentrations of Cr were high in the Leyole river water (median: 2660 μg/L) and sediments (maximum: 740 mg/kg), Cu in the river water was highest at the midstream part of the Leyole river (median: 63μg/L), but a maximum content of 417 mg/kg was found in upstream sediments. Concentrations of Zn were highest in the upstream part of the Leyole river water (median 521μg/L) and sediments (maximum: 36,600 mg/kg). Pb concentration was low in both rivers, but, relatively higher content (maximum: 3,640 mg/kg) found in the sediments in the upstream of the Leyole river. Chromium showed similar patterns of enhanced concentrations in the downstream part of the Leyole River. Except for Pb, the concentrations of all heavy metals surpassed the guidelines for aquatic life, human water supply, and irrigation and livestock water supply. All of the heavy metals exceeded guidelines for sediment quality for aquatic organisms.
Regarding nutrients, emissions from a brewery and a meat processing unit were rich in nutrients, with median concentrations of TN of 21,00–44,000 μg/L and TP of 20,000 – 58,000 μg/L. These had an average apportionment of 10% and 13%, respectively, of the total nutrient loads. In the waters, higher TN concentrations were found from sub‐catchments with largest agricultural land use, whereas highest TP was associated with sub‐catchments with hilly landscapes and forest lands. Both the TN and TP concentrations exceeded standards for aquatic life protection, irrigation, and livestock water supply. Using specific criteria to assess model suitability resulted in the use of PLOAD. The model relies on estimates of nutrient loads from point sources such as industries and export coefficients of land use, calibrated using measured TN and TP loads from the catchments. The model was calibrated and its performance was increased, reducing the sum of errors by 89 % and 5 % for the TN and TP loads, respectively. The results were validated using independent field data.
The findings of the research shows high loads of heavy metals and nutrients in rivers of the industrializing regions of Kombolcha, identified gaps in estimating heavy metals and nutrient pollution and in policy implementation. Recommended future research and policy development to address a number of key gaps in water quality protection measures include control of point and diffuse loads of heavy metals and nutrients from sources, and improvement in land managements and monitoring and regulation.