Nitrogen removal buffer capacity of the Lubigi wetland in Uganda
Introduction
Natural wetlands provide a wide variety of bio-physical and socio-economic functions such as promotion of biodiversity, preservation of water quality, storage of floods and control of erosion (Novitzki et al., 1997; Fisher and Acreman, 2004; Muraza et al., 2013). However, they are facing increasingly serious threats from watershed anthropogenic environmental degradation and human and infrastructural encroachments (Co'zar, 2007; Kayima, 2020). With increasing population, urbanization and industrialization, wetlands are increasingly becoming recipients of raw or partially treated wastewater discharges in many countries of the world (UNEP, 2013; Kayima et al., 2018a, Kayima et al., 2018b). For example, wastewater treatment plants of the urban areas of major Ugandan cities discharge their effluents into the nearby natural wetlands (Ministry of Water and Environment, Uganda, 2015), although they have capacities to remove some pollutants from water and wastewater, and thus to some extent filtering and purifying it (Kansiime, 2004; Kayima et al., 2018a, Kayima et al., 2018b; Kayima, 2020). This demand for particular urgency in their efficient management and sustainable utilisation (Ministry of Water and Environment, Uganda, 2015). However, in accordance with Gumm (2011), wetlands in Kampala are inadequately protected because of lack of accountability, conflict of interest and lack of resources, which eventually lead to misuse of the wetlands.
One of the wetlands severely strained from anthropogenic activities is the Lubigi wetland, which is one of the largest Lake Kyoga drainage basin wetlands. The anthropogenic activities affecting this wetland include illegitimate disposal of solid wastes and municipal wastewaters (African Development Fund, 2008) and construction of major projects in the wetland such as high-tension electric power line and a sewage treatment plant (Watebawa, 2012; Kayima, 2020). Lubigi wetland also receives storm water discharge from Nsooba-Lubigi drainage channel with average capacity of about 220,000 m3/day (Kayima and Mayo, 2018). All these human activities in and around the wetland has resulted into the degradation of the wetland. In accordance with Habonimana (2014), GIS mapping showed that 40% of the wetland was degraded, which is well above the national average of 30%, which may reduce its buffering capacity.
The Lubigi wetland buffers Lake Kyoga against pollution from some parts of Kampala city (Kayima, 2020). One of the important functions of the wetland is reduction of nitrogen from raw or partially treated wastewater (Metcalf and Eddy Inc.; 2003; Vymazal, 2007; Kansiime et al., 2007; Mayo et al., 2018). Excessive concentrations of nitrogen can stimulate excessive growth of aquatic flora, which can lead to blockages of receiving fresh water bodies and interfere with navigation (UNEP, 2013). Eutrophication of natural water bodies can lead to dissolved oxygen depletion and toxicity to aquatic fauna, which can cause the death of fish and render water septic and odorous and therefore unfit for human and animal use (Vymazal, 2007). High nitrate levels can lead to methamoglobinaemia in infants and increased cancer risks in human beings (Senzia, 2003).
As nitrogen exists in wastewater in various forms, transformation of nitrogen in the wetland from one form of nitrogen to another may involve volatilization of NH3, nitrification, denitrification, uptake by plants and microorganisms, sedimentation, regeneration from benthic layer, anaerobic ammonia oxidation, mineralisation, sorption and desorption (Vymazal, 2007; Mayo et al., 2018). Some of these transformation routes leads to permanent nitrogen removal. The reduction of nitrogen in wetland ecosystem is influenced by multiple factors including uptake by plant biomass and microorganisms (Lee et al., 1975; Bigambo and Mayo, 2005), sedimentation (Johnston et al., 1984; Kayima, 2020) and denitrification (Lowrance et al., 1984; Jordan et al., 1993; Mayo et al., 2018). Therefore, the performance of the wetland will depend on the wetland characteristics that enhance reduction of the pollutant (Fisher and Acreman, 2004; Vymazal, 2007). The main objective of this research study, is to investigate the wetland ecological factors, and the processes governing the fate of nitrogen in the Lubigi wetland.
Section snippets
The Lubigi wetland area
The Lubigi is one of the largest Lake Kyoga drainage basin wetlands located in the North-western part of Kampala, the capital city of Uganda (Kayima et al., 2018a). The main study area investigated in this research study is as shown in Fig. 1. The study area is a part of Upper Lubigi wetland covering an area of about 1.1 km2 at an altitude of approximately 1158 m above mean sea level, with a total drainage catchment area of about 40 km2. The wetland study area is delineated by Sentema and Hoima
Modelling process
The conceptual model (Fig. 2) was used for prediction of the variations of the state variables in time and/or space. The System Thinking Education Learning Laboratory with Animation (STELLA) II ® 9.0 software tool of 2006 (Jørgensen, 1994), was used to simulate the processes for the transformation and removal of nitrogen in the Lubigi wetland, using the law of conservation of mass. STELLA II ® 9.0 software tool was chosen because it can work well with a moderate number of parameters to be
Physical and hydraulic characteristics of the wetland
In accordance with the data collected from the field, it was observed that the main channel through the wetland has gradually curved towards the Masanafu-Namungoona edge from Ganda-Nansana edge. This is because the wetland has gently tilted at the slope of about 0.15% towards the Masanafu-Namungoona edge. The depth of water in the wetland varies from about 10 cm near the edge of the wetland to about 100 cm and 300 cm in the central channel, during the dry and wet seasons, respectively. The
Conclusions
From the Lubigi wetland final nitrogen model, the major nitrogen transformation and removal mechanisms and processes in the wetland, in descending order of importance are plants uptake (10.33 gNm−2day−1), sedimentation (2.47 gNm−2day−1) and denitrification (0.027 gNm−2day−1). The whole wetland removes approximately 987 tons of total-nitrogen (TN) per year, which is equivalent to about 3290 kg/ha/year. The analyses of the plants nitrogen contents, also indicate that significant amounts of
Declaration of competing interest
No Conflict of Interest.
Acknowledgements
All persons who have made substantial contributions to the work reported in the manuscript (e.g., technical help, writing and editing assistance, general support), but who do not meet the criteria for authorship, are named in the Acknowledgements and have given us their written permission to be named. If we have not included an Acknowledgements, then that indicates that we have not received substantial contributions from non-authors.
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