Assessment and modeling of pollution in the Nyabugogo Catchment, Rwanda
B. K. Uwonkunda a,*, I. Nhapi, U.G. Wali a, and H. Nsengimana b
a
a
Faculty of Applied Sciences, National University of Rwanda, P.O. Box 117, Butare, Rwanda
Faculty of Sciences, National University of Rwanda, P.O. Box 117, Butare, Rwanda
Corresponding author email:
kanbruce@yahoo.fr
ABSTRACT
The traditional approach to water quality management in developing countries is to develop a
monitoring network from which samples are collected periodically. Related to this is the modern
practice of environmental impact assessment, which occasionally includes water quality assessment.
What is missing from most of these practices is the development of a comprehensive system of
pollution monitoring and control, something like a water pollution control master plan that could be
used to control development by locating and highlight the pollution and its sources. This idea was
tested in the Nyabugogo catchment in Rwanda, which also includes the capital city, Kigali. The study
mapped the area in terms of pollution hotspots, covering areas of both anthropogenic and natural
pollution. The natural part is important since the hilly nature of the catchment in conjunction with high
rainfall of around 2,000 mm/year means that a lot of erosion takes place even from virgin lands.
This study will focus on the water quality assessment especially on the identification of the critical
points of pollution (hotspots), by measuring the physical parameters, chemical parameters and the use
of a water quality indices, the Prati index in order to have an idea of the level of water pollution in the
Nyabugogo River system, The water quality monitoring was conducted from October 2008 to May
2009 and covered nutrients, organic and heavy metal pollution. The sampling was done monthly and
covered NH4+-N, NO2--N, NO3--N, SO4, PO4-P, COD, BOD5, Dissolved Oxygen, TDS, Chromium,
Iron, Lead, Manganese, Zinc, Conductivity, pH, and Turbidity. The samples were collected, preserved
and analyzed using standard methods whilst TDS, conductivity, turbidity and pH were measured in the
field using Hach field kits. The land use and land cover characteristics were studied using remote
sensing images and ground truthing. A digital elevation model of the catchment was developed using
Arc-GIS software
The water quality study revealed that both urbanized and rural sub-catchments have serious but
different types of pollution. For example, the water from rural areas is heavily contaminated with
nutrients, sediments due to a lot of erosion and organic materials whilst from urban areas the
predominant pollutants are heavy metals and some nutrients. The wetlands in the urban areas seemed
to be playing a buffering role for pollutants but this will soon be exhausted, leading to the pollution of
downstream water bodies.
Keywords: Land use activities, Nyabugogo Catchment, Pollution hotspots, Water quality assessment.
1. Introduction
1.1 Background
Nowadays catchments become polluted by many different human impacts, this includes littering,
pouring chemicals down drains and water pollution from industry, all of which are washed into creeks
and storm water drains. Wikipedia defines catchment as an extent of land where water from rain or
snow melt drains downhill into a body of water, such as a river, lake, reservoir, estuary, wetland, sea
or ocean. The drainage basin includes both the streams and rivers that convey the water as well as the
land surfaces from which water drains into those channels, and is separated from adjacent basins by a
drainage divide. As part of the Nile Basin, Lake Victoria is highly polluted these days and that
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pollution is rising (Campbell et al., 2004), and pollution resulting from increased human activities is
threatening Lake Victoria, its effects being characterized by eutrophication and the occurrence of
dramatically low dissolved oxygen levels (Scheren et al. 2002), that pollution is coming from its
tributaries that pick up pollution coming from land use activities on all their paths. The city of Kigali,
capital of Rwanda has grown very fast since the 1994 genocide in Rwanda, now the city of Kigali has
a population of 1.287.020 (Kigali Conceptual master plan) and is included in the Nyabugogo
Catchment, due to this growth in population, sanitation issues are becoming complex due to poor
planning in the past, consequence to that water resources are affected because of the unsafe disposal of
untreated wastewater and a lack of sustainable sanitation infrastructure, a study conducted by
Brenniman et al. (1997) on pollution of the Nyabarongo River, showed that in and around the capital
city of Kigali there is a concentration of people farms and industries which were discharging
wastewater into the Nyabarongo River, all the waste water from city of Kigali are discharged into the
Nyabugogo swamp and from that, they went into the Nyabarongo River, small commercials activities
and agriculture are the main activities in the Nyabugogo Catchment, but there are some others
activities like Farming and industry which is only present in the city of Kigali. Most of these activities
are generating pollution that goes directly into the Nyabugogo River which is the main River to cross
the Catchment and up into the Nyabarongo, which will join the Akanyaru to form the Akagera, which
leave the country to end up in the Lake Victoria, and with the increasing of the population that
pollution is going to increase. Usanzineza (2007) made a study on the lake Muhazi and relate the
pollution of the lake coming from land use activities in the catchment, on his study on the Nyabugogo
natural wetland Nkuranga (2007) described the Nyabugogo wetland as receiving all kind of untreated
wastewaters of Kigali city including wastewaters from industries area which are mainly discharged
through two small rivers Rwanzekuma and Ruganwa, his result showed that waste water from
industrial area into the Nyabugogo wetlands are polluted by heavy metal at a level which is above the
environmental standards. Muhirwa (2008) made a study on the characterisation of wastewater from
Nyabugogo abattoir which discharge its waste waters into the Mpazi River, which is discharged into
the Nyabugogo River, his shows that the Nyabugogo abattoir is highly loaded with degradable
organics and other pollutants that pose an environmental risk to the receiving Mpazi River, where
significant pollution of the Mpazi River was observed for COD, BOD5, nutrients, Chloride, Calcium,
total coliforms and TSS. On his study on a comparative assessment of different intake systems, he
described the Yanze River with high level of flooding, erosion, sedimentation and turbidity.
Nshimiyimana and Achille (2009) described the pollution observed in the Nyabarongo as coming from
the Nyabugogo discharges.
Mukankomeje et al.,(1993) made a study on the quality of water in Lake Muhazi where the
Nyabugogo River is coming from, later on Usanzineza (2007) made also a study on the heavy metal
and nutrient level in the Lake Muhazi describing the water in the Lake as polluted with a high level of
nutrient, a report of the Nile Basin Discussion Forum (June 2007) shows that the quality of the water
in the Nyabugogo River is worsening due to water from industries, restaurants, factories releasing
untreated water directly into the River, Nkuranga (2007) made his study on the Nyabugogo swamp
describing the pollution of that swamp coming from the loading from the city of Kigali, the data on
water quality of the water bodies in the Nyabugogo Catchment and generally in Rwanda are unknown,
some data are available from studies done in some river but are not enough to determine the water
quality aspect in the Nyabugogo Catchment, it is very difficult to determine the origin of a pollutant
observed in a certain water bodies especially downstream, mostly that pollution comes from land use
and human activities, list and allocation of land use in the Nyabugogo Catchment are unknown, which
make difficulties in any planning related to water and environmental protection. A good planning
could not be made without any knowledge of the water quality and the sources of its pollution. The
main objective of this study was to assess the pollution levels and sources in the Nyabugogo
Catchment.
2. Materials and methods
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2.1 Study area description
The Nyabugogo catchment is located in the middle of Rwanda as shown in Figure 3.1 and drains a
total area of 1,647 km2. The major land use in the catchment is agriculture, which occupies about 897
km2 (about 54%) of the catchment. The Nyabugogo catchment is mostly of temperate and equatorial
type with average temperature ranging between 16°C and 23°C depending on the altitude of the area.
The annual rainfall in Rwanda in general varies between 800 and 1,600 mm,. There are normally four
seasons in Rwanda. The first is a long dry season that spans from June to September, followed by a
short rainy season spanning from October to December. This season receives 30 to 40 % of the annual
rainfall with the highest rains falling in November. The next season is a short dry season starting in
December and ending in January. The fourth season is a rainy season spanning from February to end
of May. This season receives around 60% of annual rainfall. A summary of the climatic data for the
Nyabugogo catchment is given in Table 3.1. The catchment consists of 8 districts with population
distribution as shown in Table 3.2
Figure 1. Map showing the location of theNyabugogo catcthment in Rwanda
The Nyabugogo catchment is crossed by One River, the Nyabugogo River, which receives other
inflows Rivers like the Mwange River, Rusine River and Marenge River on its upstream and later on
the Nyabugogo River is joined by others inflows that cross the Kigali city urban area, those are
Rwanzekuma River, the Ruganwa River, the Mpazi River and the Yanze River, among the activities
identified in the Nyabugogo catchment except the Nyabugogo faming flowers and the Kabuye Sugar
Works which are both located near the Nyabugogo River, the other main activity practiced in the
Nyabugogo catchment especially on its upstream are agriculture and sugar cane are the crop that is
mostly cultivated (Fig 1), but there is also other plantation like legumes plantation and rice plantation,
the Kabuye Sugar Works are not the only industry located into the Nyabugogo catchment, there are
other many industries concentrated especially on the Kigali industrial are which all discharge their
water into the Ruganwa River, there is also the UTEXRWA which discharge its wastewater into the
Rwanzekuma River, the water from the Rwanzekuma and Ruganwa before being discharged into the
Nyabugogo River, pass through a marshland which is a natural marshland composed with papyrus
vegetation, where they leave the marshland as one water body and then enter into the Nyabugogo
River, among other activities carried on the Nyabugogo catchment there is garage car, a main taxi
park, restaurant and a lot petrol gas station, the Nyabugogo River end up into the Nyabarongo River
which has its source at Mwogo in the South province, it crosses the of Nyamagabe , the Karongi area,
the Muhanga area and reach the Kigali area where it will meet the Akanyaru River from the country of
Burundi and change the name to Akagera River, the Akagera River will continue in the East and then
leave the country to the country of Tanzania which will later on end up on the Lake Victoria.
3.2 Description of sampling points
A total of 22 sampling sites (Figs 2 and 3) were selected in order to monitor the variation of pollution
in the Nyabugogo catchment. The rational for choosing these sites is that all the inflows were located
first as long as main land use activities located near the Nyabugogo River, sampling sites were put on
Inflow Rivers and after they have joined the Nyabugogo and mixed well, for land use activities nearest
the Nyabugogo sampling sites were put few kilometres after discharges from the activities have mixed
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well, a transect was created into the Nyabugogo marshland and five sampling points were selected
inside, two last point were selected on the Nyabarongo River, before and after and it has mixed with
the Nyabugogo River. The detailed description of all the sampling points is given below. A hand-held
GPS was used to mark the sampling.
Figure 2. Location of sampling points
Figure 3. Flow chart representing the Nyabugogo Catchment system
3.3 Sample collection and analysis
The water samples were collected once every month, for a period of 8 months using the grab sample
method. The water was collected and stored in a 600 ml bottle and placed in a cooler box with ice
pending analyses. The plastic bottles were rinsed before with HCl (1M) and then with distilled water.
Sampling bottles were also rinsed twice with sample water before final collection. From October to
December 2008 the parameters analyzed were dissolved oxygen (DO), pH, Turbidity, Conductivity,
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TDS, NH4+-N, NO3-N, N02-N, PO42-P, SO42-, Fe, Mn, Zn, Pb, and Cr. From January 2009, BOD5 and
COD were added to the parameters analysed
3.5 Analysis of results
The analysis of the data involved the use of MS Excel Spreadsheets to assess the variation of
pollutants at each sampling point and the assessment of the quality of water using the Prati Index (see
Section 2.9.1) to evaluate the quality of the water as it passes through the catchment and The results
are presented as mean values ± standard deviation.
In 1970 Prati set an index as a numerical expression of the degree of pollution, and which takes into
account the various pollutants present at the same time (though measured separately). That index,
increasing with the degree of pollution, could be used for the numerical evaluation of a purely
qualitative characteristic expressed by the term "pollution".
For this study 8 parameters were considered, DO, pH, BOD5, COD, NH4+, NO3-, Mn, Fe.
3 Results and discussion
3.1 Physical water quality
3.1.1 Turbidity
The turbidity levels for all the monitored points and for the points on the Nyabugogo River stretch
only are shown in Fig 4.
Figure 4. Turbidity levels for (a) all the sampling sites and (b) sampling sites located on the
Nyabugogo River for the monitored period
The representation of the Turbidity average for all sampling sites represent a high value of Turbidity
in the upstream of the Nyabugogo catchment especially on the Rusine River 3,675 (± 6,004) and
Marenge 1,915 NTU (± 1,859), which contributes seriously to the sediment pollution of the
Nyabugogo River, Nyabugogo 13,539 NTU (± 8,398), Nyabugogo 22,700 NTU (± 6,416) this is
mainly due to the agriculture activity that is taking place in that area, the Turbidity in Kigali is very
low compared to the other site, this due to Kigali as a city, the sediment pollution is not much than in
the rural area and the Nyabugogo Marshland is contributing to reduce that pollution, Muhirwa (2008)
found a Turbidity value of 707 NTU (± 37) and linked the high value of Turbidity on the Mpazi River
to high concentration of TSS of wastewater discharged from the Nyabugogo abattoir, for this the value
of Turbidity found on the Mpazi was 647 NTU (±845), and this value is not only coming from the
Nyabugogo abattoir but from all the activities located near the Mpazi River,Nshimiyimana recorded
Turbidity of the raw of the Yanze River from from June 2005 to October 2007, the Highest value of
Turbidity recorded was in November 2006 with a value of 637 NTU, the Turbidity become very high
when approching downstream of the catchment, as most of the place the River cross some cultivated
area, the variation on the Nyabugogo River shows that Turbidity increase on Nyabugogo 1 and
Nyabugogo 2 this due to the Rusine and Marenge River, the Nyabugogo is not contributing to pollute
the Nyabarongo River in terms of Turbity as the first point on the Nyabugogo River is higher than the
second 805 NTU (±692) and 737 NTU (±534) after it has mixed with the Nyabugogo River.
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3.1.2 Conductivity
The conductivity levels for all the monitored points and for the points on the Nyabugogo River stretch
only are shown in Fig 5.
Figure 5. Conductivity levels for (a) all the sampling sites and (b) sampling sites located on the
Nyabugogo River for the monitored period
4.1.3 Total dissolved salts
The TDS levels for all the monitored points and for the points on the Nyabugogo River stretch only
are shown in Fig 6.
Figure 6. TDS levels for (a) all the sampling sites and (b) sampling sites located on the Nyabugogo
River for the monitored period
The conductivity and TDS average of all the sampled months for all he sampling months seems to
change similarly, the figure shows a high value on all the points located on the Nyabugogo River,
Rwanzekuma River, Ruganwa River and in the Nyabugogo Marshland, this due to the fact that the
Nyabugogo catchment on its upstream when it is coming from Lake Muhazi crosses area with a lot of
vegetation contributing to increase of dissolved solids which increase the conductivity, except on the
Rwanzekuma and Ruganwa River where different solids are coming from different industries located
in Kigali, Nkuranga (2007) attributd the large value of conductivity found into the Nyabugogo
marshland from the wasterwater inflow especially from the Rwanzekuma River and Ruganwa River,
Muhirwa (2008) found that the impact ot the discharge of the Nyabugogo abattoir on the Mpazi river
was the increase of conductivity from 632 µS/cm (±33) before discharge to 726 µS/cm (±77) after
discharge, his value are too high according the value of this study 261.7 µS/cm (±129.54) because of
distance dilution occuring before the water reach our sampling point, the conductivity and TDS on all
along the Nyabugogo River shows that the values are decreasing from upstream to downstrem
showing that the water leave Lake Muhazi with high amount of dissolved salts, the conductivity and
TDS of the first point on the Nyababarongo River is low than the second point, 285.9 µS/cm
(±319.66) to 376.1 µS/cm (±400.73) respectively for conductivity and 43.1 ppm (±22.6) to 55.0 ppm
(±20.0) reflecting the Nyabugogo River is slightly polluting the Nyabarongo River with Dissolved
salts.
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3.1.4 Dissolved Oxygen
The DO levels for all the monitored points and for the points on the Nyabugogo River stretch only are
shown in Fig 7.
Figure 7. DO levels for (a) all the sampling sites and (b) sampling sites located on the Nyabugogo
River for the monitored period
The Dissolved Oxygen variation shows a low value of DO in area with a lot plantation, the Rwesero
sampling sites 4.2 mg/l (±1.3), the Ruganwa River 2.1 mg/l (±0.8), the Rwanzekuma River 2.6 mg/l
(±0.8) The Nyabugogo marshland with value varying from 1.6 mg/l (±0.6) to 2.3 mg/l (±2.0) and the
Mbazi River 3.8 mg/l (±1.6) are the sampling sites with low DO, Nkuranga (2007) attributed the low
concentration of Oxygen to dead papyrus as organic matter present into the Nyabugogo marshland, the
variation on the Nyabugogo River shows that Dissolved oxygen is not varying much on the
Nyabugogo River, the Dissolved Oxygen variation between the two points on the Nyabugogo River is
not that big 5.0 mg/l (±1.9) for Nyabarongo 1 and 5.7 mg/l(±2.5) for Nyabugogo 2, from that we
caonclude that the Nyabugogo River is not affecting the Nyabarongo River in terms of Dissolved
Oxygen control.
3.1.5 pH
The pH levels for all the monitored points and for the points on the Nyabugogo River stretch only are
shown in Fig 8.
Figure 8. pH levels for (a) all the sampling sites and (b) sampling sites located on the Nyabugogo
River for the monitored period
The pH is not varying a lot in the Nyabugogo River system, the lowest value is 6.44 (±0.65) on the
Marenge River and the highest value is 7.43 (±0.27) on the Nyabarongo 2 River sampling site, inside
the Nyabugogo River system the highest pH value is 7.31 (±0.34) on the Ruganwa River, Unsanzineza
(2008) described the water in lake Muhazi as slightly alkaline 8.5, and find that the mean pH of the
Lake was 7.8 which very closer to the value observed at the Rwesero sampling point 7.24 (±0.18),
which is the first point after the Nyabugogo has left the Lake Muhazi, Nkuranga (2007) find a more or
less constant pH in the Nyabugogo wetland (around 7) and attribute the high value of pH in the
Rwanzekuma and Ruganwa River from alkaline reagents from UTEXRWA and other factory in Kigali
used in their process, Muhirwa (2008) concluded that the increase of pH on the Mpazi River was due
to wastewatern from the Nyabugogo abattoir which has an average pH value of 8.9 (±0.2), our pH on
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the Mpazi River is lower than the value observed by Muhirwa (2008) as it is located 3 km from his
samling point and it has already mixed with other discharged wastewater, the pH on the Nyabugogo
River is not varying a lotthe pH is not varying a lot on the Nyabarongo River 7.39 (±0.20) for
Nyabarongo 1 and 7.43 (±0.27) for Nyabarongo 2, that shows that the water on the Nyabugogo River
leaving the cacthment arre neutral in terms of alkalinity and basicity.
3.1.6 Summary on physical parameters
The Nyabugogo River system presents a high pollution as physical parameters are concerned, with a
high value of Turbidity, this is mainly due to a lot of erosion occurring in the Nyabugogo catchment,
resulting from hills with high slope relief present into the Nyabugogo catchment, agriculture activities
are contributing also to that pollution as the soil is taken into the River, conductivity is very high on
points where water has a lot of vegetation, expect for the Rwanzekuma River and the Ruganwa River
both coming from the Kigali urban area and collecting wastewater from different industries, those two
point also present a high value of conductivity this high value to heavy metal and dissolved salt
present in those wastewater discharged from different industries, this is also justified from the results
of the Total Dissolved Salts (TDS), in terms of acidity and alkalinity the pH do not vary a lot, in terms
Physical pollution, the Nyabugogo River is not polluting the Nyabarongo River, in fact except the
turbidity where the first point value is high than the second point, for the other the second point is
slightly high than the first point, but the changes is insignificant.
3.2 Chemical water quality
3.2.1 Ammonium nitrogen
The NH4-N levels for all the monitored points and for the points on the Nyabugogo River stretch only
are shown in Fig 9.
Figure 9. Turbidity levels for (a) all the sampling sites and (b) sampling sites located on the
Nyabugogo River for the monitored period
The nitrogen ammonia results show a high value when reaching Kigali city 1.45 mg/l (±0.813) for the
Rwanzekuma River, 1.88 mg/l (±0.626) and in the Nyabugogo marshland where the Nitrogen
ammonia reaches 2.36 mg/l (±1.35), for all the remaining sampling sites the nitrogen ammonia is
below 1 mg/l, this high value of Nitrogen ammonia is possibly attributed to the domestic wastewater
from Kigali urban areas, all along the Nyabugogo River, the Nitrogen is increasing on Nyabugogo 1
and Nyabugogo 2, decrease on the next point and start increasing again, the increasing is due to
discharges from the Rusine and Marenge River, the Nitrogen variation on the Nyabarongo River
shows that the first point is high than the second 0.4071 mg/l (±0.427) and 0.2563 mg/l(±0.307),
showing that there is a Nitrogen pollution on the upstream of the Nyabarongo River and that the
Nyabugogo River is not polluting the Nyabarongo River.
3.2.2 Nitrates
The NO3-N levels for all the monitored points and for the points on the Nyabugogo River stretch only
are shown in Fig 10.
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Figure 10. Nitrate levels for(a) all the sampling sites and (b) sampling sites located on the
Nyabugogo River for the monitored period
3.2.3 Sulphates
The sulphate levels for all the monitored points and for the points on the Nyabugogo River stretch
only are shown in Fig 11.
Figure 11. Sulphate levels for(a) all the sampling sites and (b) sampling sites located on the
Nyabugogo River for the monitored period
The Nitrates variation show a high value on the Rusine River 34.44 mg/l (± 24.61) , Marenge River
37.33 mg/l (± 29.66), Nyacyonga 27.70 mg/l (± 18.18), Rwanzekuma River 70.55 mg/l (± 37.45),
Ruganwa River 72.55 mg/l (± 9.95), in the Nyabugogo Marshland the highest value of Nitrates
observed was on transect 4, 29.21 mg/l (± 43.41), the Nitrates variation on the Nyabugogo River show
that the Nitrates all along the Nyabugogo River is decreasing, and on the Nyabarongo River the
nitrates values are 35.54 mg/l (± 30.30) and 28.79 mg/l (±19.58) for Nyabarongo 1 and Nyabarongo 2
sampling sites respectively, that shows that there is a lot of Nitrates on those sites and the Nybugogo
River is not polluting the Nyabarongo River by nitrates, a further study is recommended to show
where that pollution is generated, Muhirwa (2008) find an increase of nitrates from 114 mg/l (± 10) to
176 mg/l (±13.3). The Sulfates results show that the average is varying between 0 and 10 mg/l except
on the Mpazi River 13.125mg/l (±17.5) and on the Rwanzekuma River, where the value is reaching
34.6 mg/l (±16.826) on average which is a lot of sulfates pollution, a further study on the UTEXRWA
factory is also recommended to see if that pollution is generated by the factory or not, the sulfates
variation shows that the sulfates are increasing on the Nyabugogo River starting to Nyabugogo 1
sampling sites, showing that the Rusine River is contributing to polluting the Nyabugogo River in
sulfates, the Nitrates and Sulfates analysis shows a high value on the first sampling point than the
second sampling point on the Nyabarongo River, 35.54 mg/l (±30.30) and 28.79 mg/l (±19.58)
respectively for nitrates, 5.23 mg/l (±3.562) and 5.375 mg/l (5.787) respectively for sulfates, showing
that the Nyabugogo River is not polluting the Nyabarongo River by Nitrates and Sulfates.
3.2.4 Nitrites
The nitrite levels for all the monitored points and for the points on the Nyabugogo River stretch only
are shown in Fig 12.
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Figure 12. Nitrite levels for (a) all the sampling sites and (b) sampling sites located on the
Nyabugogo River for the monitored period
3.2.5 Phosphates
The phosphate levels for all the monitored points and for the points on the Nyabugogo River stretch
only are shown in Fig 13.
Figure 13. Phosphate levels for (a) all the sampling sites and (b) sampling sites located on the
Nyabugogo River for the monitored period
The Nitrites variation shows that the nitrites are not varying so much in the Nyabugogo catchment,
except on the Rusine River 0.0568 mg/l (±0.072) and Ruganwa 0.0370 mg/l (±0.065) River where we
have the highest value, the nitrites all along the Nyabugogo River shows two high pics the first one
Nyabugogo 1 sampling site and the second one on the Nyabugogo 3 sampling site those high pics are
due to discharges from Rusine River for the Nyabugogo 1 sampling site and for Nyabugogo 3 this
might be due to the Nyacyonga farming flower or Rice plantation practiced in that area ,for the
Phosphates the highest value is located on the Rusine River 0.73 mg/l (±1.23), this is due to slaughter
house located near the Rusine River, which discharge their waste directly into the River, the sulfates
variation on the Nyabugogo River show an increase at the upstream and give a high pic on the the
Nyabugogo 1 sampling site due to discharge from the Rusine River, after that sampling sites the
sulfates value start decreasing again, the Nitrites results show a slight increase of Nitrites from
Nyabarongo 1 to Nyabarongo 2, 0.0203 mg/l (±0.013) to 0.0262 mg/l (±0.028), but the changes is
insignificant to conclude that the Nyabugogo River is polluting the Nyabarongo River, for the sulfates
the value on Nyabarongo 1 is high than the value on Nyabarongo 2, 5.43 mg/l (±3.562) and 5.375 mg/l
(±5.787), showing that the Nyabugogo River is not polluting the Nyabarongo River, with sulfates.
3.2.6 Biochemical Oxygen demand
The BOD5 levels for all the monitored points and for the points on the Nyabugogo River stretch only
are shown in Fig 14.
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Figure 14. BOD5 levels for (a) all the sampling sites and (b) sampling sites located on the Nyabugogo
River for the monitored period
The BOD5 result shows that the Nyabugogo River system does not have a high variation in BOD5, the
lowest value is 15.70 mg/l (±5.37) transect 4 in the Nyabugogo marshland and the highest value is also
present into the Nyabugogo marshland 35.21 mg/l (±37.60) transect 5, the remaining sampling points
present a variation between those values, the BOD5 pollution is coming from organic pollution, this
resulting from bad sanitation conditions, in the upstream of the Nyabugogo catchment people does not
use toilet and most of the time reject the feacal waste into the environment which all end up in the
Nyabugogo River, in the Kigali most of domestic wastewater are not treated and are directly into
different Rivers crossing Kigali Rwanzekuma, Ruganwa and Mpazi River, which contribute a lot to
increase the organic pollution, Muhirwa (2008) find BOD5 increasing from 161 mg/l (±24) to 629 mg/l
(±27) respectively before and after the Nyabugogo abattoir discharge their wastewater into the Mpazi
River, our result are very low comparing to Muhirwa’s result due to different location, his sampling
site are very close to the Nyabugogo abattoir, the BOD variation on the Nyabugogo shows some up
and down values of BOD, where we have some high pics on Rutunga sampling sites, Nyabugogo 2
sampling site and on the Nyabugogo sampling site, the BOD variation shows a high value on
Nyabarongo 1, 24.15 mg/l (±13.11) than Nyabarongo 2, 19.32 mg/l (±7.74), showing that there is an
organic pollution on the upstream of the Nyabarongo River.
3.2.7 Chemical Oxygen Demand
The COD levels for all the monitored points and for the points on the Nyabugogo River stretch only
are shown in Fig 15.
Figure 15. COD levels for (a) all the sampling sites and (b) sampling sites located on the Nyabugogo
River for the monitored period
The Chemical Oxygen Demand analysis shows a high value of COD on the Rusine River 293.98 mg/l
(±537.28), Marenge River 131.16 mg/l (±120.40) and on the Mpazi River 127.23 mg/l (±164.52), the
Nyabugogo marshland also present a high value of COD at one point 158.64 mg/l (±255.66), the COD
pollution is coming from different chemical like Iron, Manganese, Sulfates, Phosphates and Nitrogen
which use Oxygen to be oxydized, Muhirwa (2008) find an increasing value of 213 mg/l to 852 mg/l
before and after discharging respectively, he conclude that the main contributor of that organic
material is the waste water from slaughtering area especially blood, as it is represented by 23.942
11
mg/l, the variation of an increasing value on Nyabugogo 1 sampling site where we have a high pic,
and then decrease after that sampling site, the COD variation shows also that the Nyabugogo River
increased the value of COD on the Nyabarongo River from 65.12 mg/l (±36.91) on Nyabarongo 1 to
99.20 mg/l (±85.45), this is mainly due to some heavy metal present into the Nyabugogo River (see
section 4.2.8, 4.2.9, 4.2.12), who use the oxygen for their oxydation.
3.2.8 Iron
The iron levels for all the monitored points and for the points on the Nyabugogo River stretch only are
shown in Fig 15.
Figure 16. Iron levels for(a) all the sampling sites and (b) sampling sites located on the Nyabugogo
River for the monitored period
The Iron analysis show a high value of Iron on the Rusine River 4.94 mg/l (±6.7) and on the Marenge
River 3.83 mg/l (±4.75) and that pollution is a natural pollution from the soil composition,
Unsanzineza (2007) find a mean value of Fe 0.756 mg/l (±0.734), the value found on the Rwesero
sampling points 0.848 mg/l (±1.27) is very close to Unsanzineza’s Result, the Iron variation on the
Nyabugogo River shows that the Rusine River and Marenge River are increasing the Iron value on the
Nyabugogo River, also the water from the Kigali city are contributing to increasing again the
Nyabugogo River, the Iron variation on the Nyabarongo River 2.22 mg/l (±2.359) on Nyabarongo 1
and 3.665 mg/l (±5.199), shows that the Nyabugogo River is polluting the Nyabarongo River, this can
be probably attributed to Iron from the Rusine River and the Marenge River, which are the big sources
of Iron in the Nyabugogo River system.
3.2.9 Manganese
The manganese levels for all the monitored points and for the points on the Nyabugogo River stretch
only are shown in Fig 17.
Figure 17. Manganese levels for(a) all the sampling sites and (b) sampling sites located on the
Nyabugogo River for the monitored period
12
The Manganese analysis showed a high value on the Rwanzekuma River 9.63 mg/l (±8.21) and on the
Ruganwa River 8.43 mg/l (±8.8) those high value are coming from domestic and industrial wastewater
from the Kigali city this are also contributing to increase the Manganese value on the Nyabugogo
River as we see a high pic on Nyabugogo 4 where all the water from the city of Kigali have mixed
with the water coming from upstream, another high value is also the Nyabarongo River 17.7 mg/l
(±18.1) on the Nyabarongo 1 and 18.46 mg/l (±20.9) on Nyabarongo 2, this shows that the Nyabugogo
River is slighlty polluting the Nyabarongo River, yet the big amount of Manganese is coming the
upstream of the Nyabarongo River, the main sources of that pollution can be attributed to the soil
composition, Unsanzineza (2007) found an average of 0.34 mg/l (±0.336) of Manganese into the lake
Muhazi, our value on the Rwesero sampling sites is very higher as the River have crossed a long
distance from the Lake Muhazi and picked sediments all along.
3.2.10 Lead
The lead levels for all the monitored points and for the points on the Nyabugogo River stretch only are
shown in Fig 18.
Figure 18. Lead levels for(a) all the sampling sites and (b) sampling sites located on the Nyabugogo
River for the monitored period
The data shows a variation of lead from 0.02 mg/l to 0.10 mg/l, the Mpazi River is the point that has
the highest value; this highest value is coming from different activities carried along the River, this
River like the Nyabugogo tannery which use a lot of chemical, also cross the Kigali car main park.
The lead variation on the Nyabugogo River seems to be high on the upstream and is reduced by the
plant in the Nyabugogo marshland, Unsanzineza (2008) found an average value of Pb equivalent to
0.292 mg/l (±0.045), our value on Rwesero sampling point was 0.04mg/l (±0.02), the variation on the
Nyabarongo River shows that the first point Nyabarongo 1 has a high value 0.06 mg/l (±0.03) than the
second point Nyabarongo 2 0.04 mg/l (±0.04), showing that the Nyabugogo River is not polluting the
Nyabarongo River in terms of Lead pollution.
3.2.11 Chromium
The chromium levels for all the monitored points and for the points on the Nyabugogo River stretch
only are shown in Fig 19.
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Figure 19. Chromium levels for(a) all the sampling sites and (b) sampling sites located on the
Nyabugogo River for the monitored period
The data variation shows that Chromium is varying from 0.10 mg/l to 0.24 mg/l, the highest value is
located on the 9th site which is even highest pick on the Chromium variation all along the Nyabugogo
River, and this point is located on the Nyabugogo River after the Kabuye sugar factory had rejected
their waste water, the Chromium pollution is generated from that sugar factory, Usanzineza (2007)
found a mean value of Cr equal to 0.000 mg/l (±0.000), resulting that there is no Chromium into the
Lake Muhazi, our result shows an average value of 0.11 mg/l (±0.09) at the Rwesero sampling points,
the variation on the Nyabugogo River , the variation on the Nyabarongo River shows that Nyabarongo
1 is slightly high than Nyabarongo 2 , 0.13 mg/l (±0.06) and 0.12 mg/l (±0.07) respectively, this also
shows that the Nyabugogo River is not polluting the Nyabarongo River in terms of Chromium.
3.2.12 Zinc
The zinc levels for all the monitored points and for the points on the Nyabugogo River stretch only are
shown in Fig 20.
Figure 20. Zinc levels for(a) all the sampling sites and (b) sampling sites located on the Nyabugogo
River for the monitored period
The Zinc variation show that on the upstream of the Nyabugogo the value of Zinc is negligible, the
Zinc start to increase especially on the site located after the Kabuye Sugar factory where the highest
pick of Zinc level all along the Nyabugogo River is located, the Ruganwa River, the Rwanzekuma
River and the two points located on the Nyabarongo River, the Zinc pollution is mainly coming from
industries located on the Nyabugogo catchment, the values on the Nyabarongo Rivers 0.06 mg/l (±0.1)
for Nyabarongo 1 and 0.11 mg/l (±0.2) for Nyabarongo 2 shows that the Nyabugogo River is slightly
contributing to pollute the Nyabarongo River Unsanzineza (2007) find a mean value of Zn equal to
0.041 mg/l (±0.045) into the lake Muhazi, for our result no Zn were on the Rwesero sampling sites,
the first sampling site before the Nyabugogo River has left the Lake Muhazi. Ma and Lui 1999 cited
in Unsanzineza (2007) explained that lake sediments are a major repository of heavy metals, which
could be released to overlying water column from sediments when environmental conditions change,
Nkuranga (2007) attributed the high concentration of heavy metal in water bodies into the Nyabugogo
marshland to overflowing of wastewater from surrounding locations of the marshland
3.2.13 Summary on chemicals parameters
After analysing all those data it can be concluded that the source of the pollution for the Nyabugogo
River are coming from the Rusine River, Marenge River, Nyacyonga area, the Kabuye sugar factory,
Rwanzekuma River, Ruganwa River and the Mpazi River, as those points present the highest value on
most of the chemical parameters analyzed, further studies are recomanded in order to assess the origin
of the pollution present in those Rivers, the Nyabugogo River is not polluting the Nyabarongo River in
terms as far as chemical parameters are concerned, this is justified by the fact that Nyabarongo 1 has a
high value than Nyabarongo 2 except for COD, Fe, Mn, Zn, NO2-, but even for those parameters the
difference, the difference is not high to conclude that the Nyabugogo is significantly polluting the
Nyabarongo River.
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3.3 Identification of pollution hotspots
3.3.1 Prati Index result
One good way to represent result by the Prati index is to categorize the result into different pollution
levels and assign to each pollution level, this method was applied to our study and a map was
produced (Fig 21), representing the coloured scheme of all the sampling points for all the sampled
months, another map (Fig 22) was produced also to represent the average variation in order to have a
better understanding of it is happening in the Nyabugogo River system.
Figure 21. Prati Index colour scheme represented on each site for all the sampling period Oct 2008
to May 2009
Figure 22. Prati index coloured scheme represented on each site, according to the average index
value for all the months
15
For the first point Rwesero where the Nyabogogo River is just coming from the Lake Muhazi the Prati
index shows a very heavily polluted situation in December (>8), a heavily polluted situation (5-8)
from January to May and polluted situation in December (2-4). The pollution is mainly coming from
agriculture taking place around the river, where plants are even inside the water reducing then the
amount of Oxygen in the water.
Point 2 is located on the first river (Mwange River) to join Nyabugogo, point 3 is located on the
Nyabugogo after Mwange River have mixed with Nyabugogo River, the Prati index show a very
heavily polluted (>8) situation on October and February for point 2 idem for Point 3, except in
December on point 2 where we have a polluted situation (2-4), all the other month for the two sites
have a heavily polluted situation (4-8), the pollution is mainly coming agriculture activities where
sugar cane is main crop to be cultivated in that area.
Point 4 is the second River to join the Nyabugogo River and point 5 is located on the Nyabugogo
River, for the fourth point the Prati index give two situations very heavily polluted (>8) for October,
January, February and March, for November, December, April and May the situation is heavily
polluted (4-8), near the point 4 there are people living there and practicing small business like
restaurant, slaughter house, bar, they discharge their waste directly into the Rusine River contributing
significantly to the pollution of point 4.
Point 6 is located on the third River to join the Nyabugogo River and point 7 is located on the
Nyabogogo River after the Marenge River have joined the the Nyabugogo River, point 6 is heavily
polluted for January and April, very heavily polluted for all the remaining month.
Point 8 and point 9 are all located in the Nyabugogo River, point 8 is situated in the Nyacyonga area
near a flower farming factory and point 9 is located on a site where the Nyabugogo has not received
yet the water from the city of Kigali, the Prati index show a heavily polluted situation for the months
of November, March, April and May, very heavily polluted for October, December, January and
February the pollution is mainly generated by the waste from the flower farming which are left near
the Nyabugogo River.
For point 10, the Prati index show very heavily polluted situation in the months of October, December
and January and a heavily polluted for the months of February, March, April and May, only in
November is the time when we have a polluted situation, this point area comes after the river has
crossed rice plantation area and a sugar cane factory which directly discharge their waste water into
the Nyabugogo River, the 11th point is located on the Rwanzekuma River which will cross the
Nyabugogo marshland and then join the Nyabugogo River, the Prati index show a heavily polluted for
only three months: November, April and May, all the remaining months are very heavily polluted, this
pollution is mainly coming from the UTEXRWA factory which is a textile factory who is dischargin
their waste water in the River without any treatment as their treatment plant is not operational, this
river is also getting waste water from Kacyiru, Nyarutarama and Gisozi and those waste water are
coming from households without any treatment, the point 11 is located on the Ruganwa River which
River cross the industrial area and get waste water from Kicukiro, Remera, Kimihurura, Kiyovu and
the center town of the city of Kigali, the Prati index shows a heavily polluted for the months of
October and May, the remaining months the water was very heavily polluted, the pollution is coming
from waste water from households and factory present in the industrial area.
For the purpose of this study, we created a transect along the Nyabugogo marshland with five points,
(point 12-16) for a good representation only one point (transect 3) were selected to show the pollution
variation in the Nyabugogo marshland, for that point the Prati index shows that the Nyabugogo
marshland is heavily polluted for October, November, February and May, very heavily polluted for
December, January and March only the month of April is showing a polluted situation, the pollution is
coming from plantation present in the marshland and waste water coming from the Kigali city.
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Point 17th is located on the Mpazi River, another River to join the Nyabugogo River, this River is
coming from different part of the city of Kigali, it get waste water from Nyamirambo, Muhima,
Nyabugogo. The Prati index show a heavily polluted situation in November and April all the
remaining months the River was very heavily polluted, this pollution is mainly coming households of
the city of Kigali which discharge their waste water without any treatment, the point 18 is located at
the Nyabugogo River after it has been joined by the Mpazi River, the Prati index show a heavily
polluted for the months of October, November, March and May, the sites was very heavily polluted in
December, January and February.
Point 19 is located on the Yanze River which is the last River to join the Nyabugogo River before it
enters into the Nyabarongo River, the Prati index shows a very heavily polluted for the months of
October, November, December and March, the water was heavily polluted for January, February,
April and March, the pollution is coming from some waste water from some households located near
the River, point 20 is the last point located on the Nyabugogo River before it enters into the
Nyabarongo River, for that point the Prati index shows a polluted situation on December, a heavily
polluted situation for November, February, April and May.
Point 21 and 22 are all located in the Nyabarongo River, point 21 is before Nyabugogo River has
joined Nyabarongo River and point 22 is after the two have mixed, the Prati show a heavily polluted
for October, January, march and April and very heavily polluted for the remaing months, for the point
22 the water was heavily polluted in November, April and May for all the remaining months the water
was very heavily polluted.
3.3.2 Summary on Prati Index result
The analysis from the Prati Index shows in general that water in the Nyabugogo River system, as most
of the sampled are in the very heavily polluted range, (>8), except some months on some sampling
points that present a heavily polluted range (4-8), some points points were also presenting polluted
category ((2-4), April for Nyabugogo 1 and Nyabugogo Transect, December for Nyabugogo 2 and
Nyabugogo 5, May Nyacyonga sampling site, the only time we observed an acceptable quality for the
water in the Nyabugogo River system was december for the sampling point Nyabugogo 1, we didn’t
record any good water possibility on any sampling points, the average representation of all the Prati
index for all the months shows a very heavily polluted situation except for three sampling points
Rwesero, Rutunga and the Nyabugogo 1, which all have heavilly polluted situation by average, , for
the full Prati analyses it is recomanded also that the missing parameters: Permanganate, Suspended
Solids, Chlorides, Alkyl Benzene Sulfonates and Carbon chloroform extract that were not analysed to
be analysed in order to have a better interpretation of the Prati index for the further studies.
5. Conclusion and recommendation
From the objectives and results of this study, the following conclusions were made:
1. The water in the Nyabugogo River system are polluted as far as physical parameters are
concerned, and the Nyabugogo River is polluting the Nyabarongo River, the two Rivers are
already polluted before they mixed.
2. The chemical parameters monitored showed consistently high levels of pollution, warranting
urgent attention to arrest further deterioration of water quality in the Nyabugogo River.
3. The Prati Index, the water in the Nyabugogo River system is highly polluted as all indices are
in the range of 4-8 and >8.
From this study, the folloing recommendations are made:
1. Erosion protection measures should be applied in the Nyabugogo catchment and these include
terraces where there is agricultural activity and tree plantations, especially around the Rusine
and Marenge Rivers, and on the upstream of the Nyabugogo River. These would reduce the
amount of sediment depositing in rivers. Industries inside the Nyabugogo catchment should
treat their wastewater before they discharge them into the environment.
17
2. Further studies should be carried out on all the tributaries of the Nyabugogo River in order to
identify potential sources of chemical pollution. All the activities carried out near the Rusine
and Marenge Rivers should be shifted and placed at a distance where they are not closer to
rivers and all wastewater (domestic + industrial) subjected to some form of treatment before
discharge into the rivers.
3. This study on the Nyabugogo catchment should continue and include the measurement also of
the hydrological data in order to be able to produce a model that could be used to predict the
situation if any changes occurred into the Nyabugogo catchment.
4. This study should be continued to include all the parameters required to have a full Prati
Index. The measurement should include adding some sampling points on each tributary after
studying the nature of the catchment and tracing suspected point sources of pollution.
Acknowledgements
The authors are very grateful to Nuffic for sponsoring this study through the WREM Project, a
collaborative capacity building project between the National University of Rwanda and the UNESCOIHE Institute for Water Education.
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