Monitoring the Impact of Surface Water Flooding on Groundwater Quality around Nyabarongo River in Rwanda
Abstract
Rwanda exhibits a climate characterized by two rainy seasons in which erosions and inundations are likely to occur and cause the flooding of rivers which threaten the quality of waters. Most of rivers are connected hydrologically with groundwater aquifers which allow the recharge. This study aimed to monitor the impact of surface water flooding on groundwater quality around Nyabarongo River in Rwanda. Two parameters namely turbidity and color which are directly influenced by flooding of water bodies were monitored between January 2017 and June 2020. Laboratory analyses for turbidity on an hourly and color on a monthly basis were conducted at Nzove Water Treatment Plant Laboratory and the computed monthly average values were used. The laboratory results for surface water and groundwater were compared through graphical presentation which indicated that there is a relationship between the change in quality of both waters due to the fact that the trends in variation of the quality of both waters correlate in the same periods or groundwater quality changes similarly just a little bit after surface water quality has changed. This observation has led to the conclusion that the changes in water quality of Nyabarongo River which is mainly exacerbated by the flooding of March to May and October to December rainy seasons affect the quality changes of the groundwater recharged by this river. This finding indicates the need and urgency of implementation of Nyabarongo catchment rehabilitation and management plans.
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Introduction
Rwanda is located within the equatorial belt with a modified humid climate including rainy forest and Savannah types. The rainfall characteristics for Rwanda are known to exhibit large temporal and spatial variation due to varied topography and existence of large water bodies throughout the country as well as the influence of climate change [10]. However, two rainy seasons are generally distinguishable, one centered on March – May and the other around October – December [17]. The existing literature indicate that temporal variability of the rainfall in terms of intensity and frequency in some occasions has resulted in extreme events such as the floods and frequent droughts that have far reaching socio-economic impacts to the country and water resources in particular used to get increasingly polluted [17] and Meteo-Rwanda official website. Water resources in Rwanda occupy a total of 135,000 ha or 8% of the country’s surface area. These include 101 lakes (1,495 km2), 861 rivers totaling 6,462 km and a network of disconnected wetlands [17] while ground Water accounts for 4.554 billion m3, Rainfall Water for 27.5 billion m3 per annum, Ground water recharge being 4.5 billion m3 per annum, Total renewable water is 6.8 billion m3 per annum while Renewable water availability per capita is 670 m3 and artificial water storage is 2.5 m3 [10], [15], [17].
Studies indicated that groundwater aquifer around Nyabarongo river is generally shallow occurring between 3-5m and 12m, with a thickness of at most 8m and is recharged by the river [1]; Nyabarongo floodplain gets completely inundated during the rainy season where the floodwater rise can completely submerge the river, the boreholes and wetland in general, the Nyabarongo River channel is not permanent; thus it significantly changes meanders across its entire flood plain. This has resulted in some boreholes being washed away and lost due to erosion as a result of river encroachment due to river erosion [1], [12] and [14]. This study has monitored the changes in turbidity and color of both surface and groundwater between 2017 and 2020 with a target to examine whether there is a relationship in the changes of both categories of water quality. It is expected that the findings will provide decision makers in water resources management and water supply a baseline information for further mitigation actions.
Conclusion
This study has used the results of laboratory analysis of turbidity, color and Total coliform parameters recorded in the period between January 2017 and June 2020 to assess the impact of the changes in Nyabarongo river quality as a result of flooding to the quality of groundwater. The findings indicated that turbidity and color highly rise most of the times in the rain seasons of March to May and October to December as a result of the sediments and other substances loaded in the river and its area during overflow and flooding [3]. The similar trend in water quality changes for both parameters on groundwater was observed which indicate that the pollution of surface water is impacting on groundwater provided the fact that the two water sources are hydrologically connected by the recharge [4]. The laboratory analyses were recorded on an hourly and daily basis which indicates the reliability of the results. However, the analysis did not cover the whole river length and the whole ground aquifer but the results of this study may represent the whole length based on the similarity in terms of physical and hydrological characteristics [1]. Another limitation is that the study did not analyze all parameters can be transferred through communication between surface and groundwater [9]. The findings of this study can serve as a reference for taking appropriate mitigation measure for protecting rivers in Rwanda from erosion and related pollution as well as for protection groundwater such as strategies for enhancing the natural treatment process of groundwater. This study can inform the institutions exploiting the river and its aquifer to incorporate technical measures to lessen the impacts on water quality such as to raise the wellhead at each borehole to a given height upward so that it is not submerged during flooding and installation of a groundwater telemetry system with for real time data capture and real time decision making.
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