Modeling nutrients and organic pollution in the Akagera Transboundary River
A. Ngombwa a,*, I. Nhapi a, U.G. Wali a, and H. Nsengimana b
a
b
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: napi20002000@yahoo.fr
ABSTRACT
The management of pollution in international river basins could be improved through the use of remote
sensing and river water quality modeling techniques. Often, in African countries there is no enough data
to characterize the origin and transportation of pollution. This study deals with the modeling of nutrients
and organic pollution in the Lake Victoria Basin, specifically focusing on the Akagera River. The
Akagera River drains Burundi, Rwanda, Tanzania and Uganda. This study is part of a VicRes-sponsored
research project aimed at developing a decision support system (DSS) that would be used by the basin
authorities in understanding pollution dynamics and in assessing the impact of remedial measures on the
Lake and its major tributaries. This paper focuses mainly on NH4-N, NO3-N, PO4-P, TDS, conductivity,
pH, temperature, and turbidity. Sampling was conducted monthly from March, 2008 to February, 2009
based on eleven sampling points, four of which are on the tributary rivers, Nyabarongo, Akanyaru (origin
Burundi) and Muvumba. The first parameters were analysed in the laboratory using standard methods,
whilst TDS, conductivity, temperature, turbidity and pH were measured in the field using HACH field
kits. The landuse and land form characteristics were studied using satellite imagery and ground truthing.
From this pollution reduction factors in terms of kg/ha.yr of pollution load were developed for each
landuse type and used for developing the model. Rainfall and river flow data were obtained from gauging
stations in and around the river.
The results from the water quality study show elevated levels of NH4-N which is ranged from 0 to 2.36
mg/l, NO3-N from 1.8 mg/l to 314.4 mg/l and PO4-P ranging from 0.02 mg/l to 19.3 mg/l and this agrees
with the observed activities in the catchment where proper landuse practices to control erosion and
nutrient export from cultivated areas are not in place. Because of the floodplain around the river, the river
absorbs, assimilates and releases nutrients into the papyrus wetland around it. More studies are required
upstream focusing on smaller catchments and more characteristics of the land such as slope, soil type,
rainfall patterns, etc. This would result in the development of more export coefficients, making the model
useable deeper.
Keywords: Akagera River, Export coefficients, Nutrients, Organic pollution, Transboundary river, Water
quality modelling
1. Introduction
The multiplicity of transboundary water basins in Africa has led to international cooperation and action
plans, such as the establishment of the Africa Ministerial Council on Water (AMCOW) and the Africa
Water Task Force to steer the processes. Through the New Partnership for Africa’s Development
(NEPAD), a short-term action plan (STAP) was prepared, with the aim of strengthening the enabling
environment for effective cooperative management and development of transboundary water resources,
and of initiating the implementation of prioritized programs. There are many areas that the cooperation at
the transboundary scales such as water quantity and quality. One of the problems is transboundary
pollution. This is the pollution which is emitted in one country and deposited or causing harm in another
country. Due to the absence of a supranational institution that could enforce international legislation,
transboundary externalities cannot be approached in the same way as domestic pollution problems. In the
Nile Basin region of Africa, serious water quality problems have been reported in Lake Victoria, which
receives water from Burundi, Kenya, Rwanda, Tanzania and Uganda. The rapid deterioration of water
quality in the Lake are attributed to uncontrolled human activities such as poor catchment management,
agricultural operations, transport operations, point and non-point source pollution discharges of
sewage/industrial effluents. As a result, the lake now faces numerous challenges including eutrophication,
reduced biodiversity, destruction of wetland resources, and invasion by non-native species (Kayombo and
Jorgensen, 2000). LVEMP-I was developed as a five-year program to research issues affecting the lake,
design potential solutions, and implement pilot programs to address them. Lake Victoria Environmental
Management programme -I (LVEMP –I) produced several notable outcomes, including the establishment
of a common database for fisheries research and the first comprehensive water quality monitoring
program ever undertaken on Lake Victoria. In 2003, program development began on LVEMP-II to build
on the tasks initiated under LVEMP-I and further address regional threats (Kelly, 2009).
Lake Victoria has undergone substantial and very negative changes, especially over the last 30 years. One
of the driving factors is nutrient enrichment of the Lake from human activities in the catchment, which is
causing eutrophication. This has been associated with, among others, the rapid proliferation of water
hyacinth, alga blooms, and with general disruption of the lake ecosystem (Mwanuzi et al., 2002).
Nutrients input in Lake Victoria appear to originate mainly from atmospheric deposition and land runoff
through its tributary rivers (Scheren et al., 2004).
The Dissolved Oxygen (DO) level is an important parameter in defining the health of aquatic ecosystems.
The DO concentration of the lake below 4.0 mg/l these may have a potentially adverse impacts on aquatic
life (Bean et al., 2003). In the case of Lake Victoria, the DO is depleting significantly because of
decaying organic matter, especially water hyacinth (Eichhonia crassipes). DO levels of less than 2 mg/l
have been reported in Murchison Bay, which is very low than acceptable levels of manu fish species
which is about 6.2 mg/l (www.cleanlake.com).
To reverse the lake deterioration, the East African Community (EAC) Partner States agreed to implement
the Lake Victoria Environmental Management Project (LVEMP) whose major objective is to restore a
healthy, varied lake ecosystem which is inherently stable and which can support, in a sustainable way, the
many varied human activities (Kayombo and Jorgensen, 2000).
The Akagera River is the main tributary of Lake Victoria and drains some parts of Burundi, Rwanda,
Tanzania and Uganda. The River has a total length of 785 km but the Rwandan part is estimated at about
400 km (Kayombo and Jorgensen, 2000). Its average flow is estimated at 261 m3/sec at the point where it
enters Lake Victoria and it contributes 33.5% of the total inflow of Lake Victoria (Campbell et al., 2004).
In comparison, with other tributaries inflows; contribute approximately 778.3m3/sec corresponding to
66.5%. However, Kelley (2009) has shown that the major source of water in the Lake is actually the
precipitation over the Lake. Despite the importance of the Akagera River to the Lake Victoria system,
there appears to be no comprehensive studies on its water quality and on how the River is currently
handling its current loads. Knowledge of this would greatly help in understanding and projecting its
future impacts on the Lake.
Identifying and quantifying non-point source pollution will contribute to the development of sustainable
solutions for managing the Akagera Basin. In addition, the export coefficient model proposed in this
research study could give an insight into the processes related to non-point source pollution on the River
in the short and long term periods. The outputs of such a model could be used to influence policy-makers.
The general objective of this study is to assess the water quality of the Akagera Transboundary River,
with a special focus on nutrients. The focus area is the Rwandan side of the Akagera River.
2. Materials and methods
3.1 Description of the study area
The Akagera River is the largest of the 23 rivers that drain into Lake Victoria and it is the most remote
head stream of the Nile River (Bosnjakovic, 2001). The River is formed by the confluence of two rivers:
Nyabarongo and Akanyaru Rivers. The Akagera River crosses the Eastern part of Rwanda before forming
the border between Rwanda and Tanzania. Further downstream, it forms the border between Uganda and
Tanzania before flowing into Lake Victoria (Fig 1). The Akagera River Basin has an area of about 57,364
km2 and is distributed between different countries notably Burundi, Rwanda, Uganda and Tanzania as
shown in Table 1, and is estimated to have a population of 14 million people (Andjelic, 1999). The
surface area distribution within the basin is given in Table 1.
Table 1. Akagera basin distribution by country
Country
Burundi
Rwanda
Tanzania
Uganda
Total
Source: Andjelic (1999)
Area (km2)
13,854
20,977
18,643
3,891
57364
Percentage
24.1
36.6
32.5
6.8
100
Figure 1. Maps of the Akagera River basin showing (a) location in the region, and (b) landuse (source:
Holmberg et al., 2003)
The Akagera River is the largest tributary of Lake Victoria, and it contributes about 33.5% of the annual
inflows into the lake, over twice as much as the next largest river, the Nzoia in Kenya (Sutcliffe, 1999).
The River is very important in terms of hydrological regime within Rwanda, this study was conducted.
The Akagera Basin covers 67% of the total surface area of Rwanda which is equivalent to 20,977 km2, it
has a surface area of about 5 km2, with an average river depth of about 6 m and an estimated average
volume of 30 x 106 m3. The landuse of the basin is predominantly agriculture, grassland and forest as
shown in Fig 3.2. Some of the major towns of Rwanda; Kigali, Rwamagana and Kibungo are situated in
the upstream part of the basin. After the City of Kigali, the River passes through rural agricultural areas
before reaching the Akagera National Park on the Rwandan side and the forested areas on the Tanzanian
side. The Muvumba River joins the Akagera before it leaves Rwanda and Tanzania to enter Uganda at
Kagitumba.
Land use in the Akagera Basin is dominated by croplands which cover 44% of the area. Other important
land use types are deciduous forest and savanna with 18% and 14% respectively. Water bodies cover 2%
of the total area of the watershed. The remaining surface is occupied by pasture, woodland, broad leaf
forest, mixed forest, grassland, savanna and shrubland (Haguma, 2007).
The altitude in the Akagera Basin varies from 1,200 m to 1,600m above sea mean level in the east and
rises above 2,500 m in the west with peaks reaching 4,500m in the north. The area generally has four
seasons: a short dry season from January to February; a rainy season from March to May; a long dry
season from June to September; and another rainy period from October to December. Annual rainfall
varies from less than 600 mm over the eastern part of the basin up to 1,800 mm and above in the west,
where most of the runoff is generated. Steep slopes and heavy rainfall result in erosion and high river
sediment loads (Mpyisi, 2003). However, the river flows are attenuated by a number of lakes, and in
particular by floodplains and associated lakes above and below the Rusumo Falls. The peak flow occurs
in April in the upper tributaries, in May at Kigali and Rusumo Falls (Holmberg et al., 2003). The terrain is
mountainous, declining eastward toward the Tanzanian border (FAO, 2005). The central part of the basin
in Rwanda is covered by rounded hills and large valleys with an altitude between 1,500 and 2,000 m. In
the east, the hills give way to a large region with numerous lakes and marshes (Wong et al., 2005).
Rwanda is divided into two hydrographical basins with a separating line called Congo-Nile Ridge, in the
East there is the Nile basin which covers 67% of the National territory and drains 90% of Rwandan waters
by three main rivers namely Akanyaru, Nyabarongo and Akagera. The latter is the main affluent of Lake
Victoria with an average outflow of 256 m3/s at Rusumo station and thus considered as the source of the
Nile. The Akagera Basin in Rwanda comprises a lot of small lakes (Bulera, Ruhondo, Cyohoha South,
Mugesera, Muhazi, Rwampanga, Mihindi, Mirayi and many others). Those lakes are not very deep (5 to 7
m deep) except for Lake Bulera and Ruhondo which are 50 to 60 m deep, respectively. The Akagera basin
in Rwanda is composed by three main watercourses notably Akanyaru, Nyabarongo and Akagera River
itself (Haguma, 2007).
2.2 Selection of sampling sites
Eleven water quality monitoring stations were located along the Akagera River (Table 2). Along the
Akagera River, stream size and permanent landscape alteration decreases (e.g., conversion of agriculture
to forests). Sites were selected to encompass incremental additions and variation of landuses. Criteria for
station selections were largely subjective and partly based on potential influences of near-stream landuse
activities. Downstream stations were selected to encompass additional landuse features such as Akagera
National Park, grazing and other small agricultural practices. Stations 2 through 6 were characterized by
intensive agricultural activities very close to the river whereas stations 1, 7 through 10 are surrounded by
a big floodplain which may play an important role of riparian buffer and sediment stripping.
Table 2. Summary of sampling points
Sampling station
Description and rationale
1. Nyabarongo
Located at the bridge along Butare – Kigali Road. This is expected to capture
Bridge
pollution from Kigali City and upstream part of Nyabarongo River. Acts as a
control site;
2. Nyabarongo
3. Akanyaru
4. Akagera
5. AkageraKanzenze
6. Akagera- Gashora
7. Akagera- Rusumo
8. AkageraMahama
9. AkageraKabanyana
10. AkageraKanyonza
11. Muvumba
This site on Nyabarongo River just before confluence with Akanyaru River.
It captures pollution between Site 1 and the confluence which is a cultivated
flooplain and is located in Bugesera region, very close to where it meets
Akanyaru River;
Located in Bugesera just before it meets Nyabarongo River;
Located just about 200 m after the confluence of Akanyaru and Nyabarongo
Rivers;
Located at some kilometers after the confluence;
Located in high intensive agricultural activities in Bugesera region
Located just before the border of Rwanda and Tanzania where the small
village around the River discharges its sewage directly in the Akagera River;
This site is located at the entrance of Akagera National Park where the full
mixing of pollutants from Rusumo village is considered complete;
This is the first site taken after the Akagera River come out the Akagera
National Park.
Located at the Rwanda-Tazanian boarder just before the river leaves Rwanda
to enter Uganda. This site gives an indication of the level of pollution of the
River when it leaves Rwanda without taking into consideration the pollution
contribution of Muvumba River.
This site is from the river which discharges before Akagera River leaves
Rwanda, but its pollution can mix with its from Akagera River outside the
Rwanda side because they meets near the border.
2.3 Sample collection and preservation
The study was conducted from March 2008 to February 2009, based on monthly sampling using grab
samples. Samples were collected in polyethylene bottles that has been rinsed first with HCl to avoid
contamination and pollutants adsorption and then with distilled water. Sampling bottles were rinsed three
times with the sample water before final collection. Samples were taken from the free flowing section of
the stream at each station approximately at the middle of a river and at depth of about 20 cm below the
surface. The collected samples were stored in 500 ml plastic bottles and were placed in ice box and
transported to the laboratory.
2.4 Sample analysis in laboratory
The parameters measured in laboratory were PO4-P (orthophosphate-phosphorus), NO3-N (nitrates) and
NH3-N (ammonia nitrogen). Before the analysis samples were kept in the freezer in laboratory at a
temperature of about 0oC. The samples were analysed in the water laboratory of the faculty of applied
sciences at the civil engineering department of the National University of Rwanda. The parameters tested
were ammonia, nitrates and phosphates were analyzed. Other parameters such as pH, conductivity, TDS,
turbidity and temperature were tested in the field.
Water samples were analyzed for a variety of physical and chemical characteristics. Temperature,
conductivity and salinity were measured on-site with portable combined conductivity meter. All other
analyses were conducted using previously established procedures (Deal et al., 1996). Turbidity was
measured with a turbidity meter. Determinations of pH were made with digital pH meter, calibrated with
pH 7 and pH 4 buffers. Concentrations of PO43--P, NO3--N, NH4+-N were calculated in laboratory after
samples preparations. Filtration methods followed standard procedures using (0.45 µm pore size)
membrane filters. For the analysis of nutrients notably phosphates, nitrates, ammonia, total nitrogen and
total phosphorus, different methods were used in order that all laboratory routines be satisfied and to get
reliable results. For phosphates, nitrates and ammonia analysis, HATCH method were used were samples
were analyzed colorimetrically using its appropriate reagents.
2.5 Landuse data
The land use data for the study was obtained from digital landuse map of African produce by FAO. The
digitize land use map was process in the ArcGIS environment to obtained the land use map of the
catchment. There were about 16 land used which were further regrouped and classify into 5 land uses
agriculture, grassland, forest, water bodies and bare soil. The land uses are distributed throughout the
basin and their size and percentage area distribution were estimated using ArcGIS.
2.6 Data analysis
The data collected were arranged in tables and graphs. Descriptive statistics in terms of mean, maximum,
minimum and standard deviation using Microsoft Excel was used to describe the data. The mean was
used to compare data with the rivers water quality guidelines as it is considered representative of the
whole sample. The maximum and minimum gives the range of the variability of the pollutants with space
and time. The pollutants concentrations was used to derive the pollutants loads which was then used to
determine the pollution coefficient.
3. Conceptual framework
Several water quality models used to estimate non-point water pollution into watersheds require the input
of either export coefficients (typically for rural areas) or event mean concentrations (typically for urban
areas). Event mean concentrations represent the concentration of a specific pollutant contained in storm
water runoff coming from a particular land use type within a watershed. Export coefficients represent the
average total amount of pollutant loaded annually into a system from a defined area, and are reported as
mass of pollutant per unit area per year (e.g. kg/ha.yr). Event mean concentrations are reported as a mass
of pollutant per unit volume of water (usually mg/L).These numbers are generally calculated from local
storm water monitoring data. Since collecting the data necessary for calculating site-specific EMCs or
export coefficients can be cost-prohibitive, researchers or regulators will often use values that are already
available in the literature. If site-specific numbers are not available, regional or national averages can be
used, although the accuracy of using these numbers is questionable (Beaulac and Reckhow, 1982). Due to
the lack of data on export coefficient of the Akagera River catchment, I will proceed by dividing
calculated load by the surface area of each land use category for determination of the specific export
coefficient.
3.1 Pollutant loading estimation
Loadings by source will be determined using the developed nutrient export coefficients. Land use
information in conjunction with monitoring data water quality and river discharge were used to estimate
nutrient export coefficients for non-point sources of PO43--P, NO3—N and NH4+-N by landuse category for
non-point sources within the watershed using the following steps:
1. Determine the dominant land uses within the sub catchments and the percent of the drainage area
within the whole catchment associated with these land uses;
2. Combine flow information with discrete measurements of nutrient concentrations taken to
provide mass loadings at the outlet of the country;
3. Apply statistical models to determine optimal estimates of the nutrient export coefficients for the
major agricultural land use categories within the watershed;
4. Develop an empirical source-contribution model using the nutrient export coefficients and landuse to estimate loadings by source;
3.1.1 Determination of land uses above the last sampling site
The determination of the land uses in the Akagera River basin has shown different land uses with
agricultural areas bigger compared to other land use categories Table 3.
Table 3. Land uses above the outlet point of the part of the Basin in Rwandan side (Akagera at Kanyonza
sampling site) and the drainage area sizes in the Akagera River watershed used in pollutants export
coefficient analyses.
Site
AkageraKanyonza (km2)
%
Agriculture
22823
Grassland
5872
Forest
6005
Waterbodies
1011
Bare soil
480
Total area
36193
63.06
16.22
16.60
2.80
1.32
100
We note here that the drainage areas taken into account in this distribution include not only the Rwandan
side but also the Burundian and Tanzanian sides whose rivers contributes to the pollution of the Akagera
River in Rwanda.
3.1.2 Calculation of nutrient cumulative mass loadings for each site
Grab samples were measured directly for PO4-P, NH4+-N and NO3—N. The discharge for most sites was
derived from the National Weather Service and at the outlet the mean flow corresponds to 166.9 m3/S.
Flow data were then combined with the nutrient concentration data to calculate cumulative nutrient
loadings at each site (Table 4). Grab samples were collected monthly during the study period and so the
flow. The mean concentration and mean flow were used in the determination of the load.
Table 4. Nutrient loadings measured at Akagera River sampling sites during the study period
SITE
AkageraKanyonza
Flow
(m3/S)
166.9
NH4+-N
PO4-P
Concentrati
on (mg/l)
Load
(Kg/S)
2.957
0.493
Concentrati
on
(mg/l)
0.883
Load
(Kg/S)
0.147
NO3—N
Concentrati
on
(mg/l)
79.658
Load
(Kg/S)
13.295
3.1.3 Determination of pollutants export coefficients for different land areas
Most of the sampling sites represent predominately rural or agricultural land uses with urban area
comprising a small portion of the total watershed area. Site 1, located in downstream of Kigali city, was
the only sampling site representing a small portion of urban land. Typically export coefficients are
determined by monitoring land uses, such as forest, row crops or water bodies. While monitoring single
land-use watersheds may be ideal, most watersheds are comprised of a variety of land uses except sites 9
and 10 which are dominated by forest as the continuity of Akagera National park. The sampling network
in the Akagera River watershed was designed to monitor nutrients, organic pollutants and flow at stream
sites with upstream drainage areas comprised of mixtures of land uses.
To isolate the loading contribution from these mixed land-use drainage areas, multiple regression
techniques were used to develop the pollutant export coefficients for the major agricultural land uses in
the watershed. The dependent variable was the pollutants loading at the last sampling site as the
representation of the mean pollution level leaving the country (Table 3) and the independent variables
were the fraction of the drainage area above the sampling site taken into consideration represented by
each land-use category (Table 4). The coefficients from the resulting multiple regression models define
optimized export coefficients across the entire basin for each land use category for the time period
evaluated. Dividing the load by the upstream catchment area of the last sampling site (Kanyonza site), the
export coefficient of each land use category can be obtained. Using the load and drainage areas calculated
above for different land uses, the average export coefficient of each land use type can be obtained (Table
5).
Table 5. An average export coefficient for different land uses (kg/ha.year)
Land use category
Agriculture
Grassland
Forest
Water bodies
Bare soil
PO4-P
6.718
26.112
25.532
151.66
319.021
NH4-N
2.003
7.785
7.613
45.221
95.123
NO3—N
181.187
704.177
688.541
4089.912
8603.211
4. Results and discussion
4.1 Pollution levels along the Akagera River
The observed pollutant concentration was changing with time and distance downstream depending on
several factors such riparian buffers, high human pressure and others pollutant drivers.
4.1.1
Variation of Nitrates
The levels of nitrates for the monitored period varied greatly as shown in Fig 2. In general, N-NO3
concentrations were higher than 100 mg/L (well above those considered as being in the polluted water
category). However, levels exceeding 1 mg/L indicate anthropogenic influence, e.g. agricultural runoff
(European Environment Agency, 1995). The first three sampling sites are highly nitrates concentrated and
the concentration decreases with distance which may be the influence of riparian buffers. This is also
because those regions are predominately agricultural area where nitrogen is applied as ammonium-nitrate.
The others reason of high concentration at Kagitumba site is the high intensive pasture activities around
the site.
Figure 2. Nitrates variation versus distance
4.1.2
Variation of Ammonia
The levels of ammonia for the monitored period varied greatly and generally increased downstream as
shown in Fig 3. The variation of ammonia concentration is varying inversely to nitrates concentrations.
This is maybe due to reducing conditions along the Akagera River way.
Figure 3. Ammonia variation versus distance
4.1.3 Variation of Phosphates
The levels of phosphates for the monitored period varied greatly as shown in Fig 4. The variation of
phosphates is showing the higher values at Rusumo sampling station. This site is polluted by discharges
from the around river households which discharge directly in the River. Those household wastes may be
form detergent use and other sources of phosphorus. The high phosphorus level at Muvumba site may be
explained by the lack of floodplains around the River compared to Akagera River. Soil erosion of
fertilized fields and lawns can also be a reason because of high mountains in the region. The other reasons
of high phosphates concentration at Rusumo sampling site may be the phosphates concentration in the
inflow of Ruvubu River from Burundi which is not monitored in this study; it joins Akagera River at
Rusumo point. The main characteristics of the Akagera River Basin that would account for its importance
as a diffuse source of phosphorus were due not only to human intervention, but also to the runoff, which
causes significant erosion because of the topography and type of soils in this area. The soils under crop
cultivation and livestock production have steep slopes where water and wind erosion process are speeded
up by the farming practises.
Figure 4. Phosphates variation versus distance
4.1.4 Variation of Conductivity
The levels of conductivity for the monitored period varied greatly as shown in Fig 5. The values of
conductivity in the sampling sites are very high compared to normal values in rivers. The changes are
directly dependent to the rainfall patterns in the region. Those high concentrations may be related to the
geology of the area. The lower values in Bugesera region are related to low rainfall patterns of the region.
Figure 5. Conductivity variation versus distance
4.1.5 Variation of temperature
The levels of temperature for the monitored period varied greatly as shown in Fig 6. The variation of
temperature in Akagera River and its tributaries is quite low. Muvumba River is the site with low
temperature value. This is due to shadow from different classes of trees observed along the River
particularly upstream this sampling site.
Figure 6. Temperature variation versus distance
4.1.6 Variation of pH
The levels of pH for the monitored period varied greatly as shown in Fig 7. The lowest values of pH are
observed at Akanyaru and Akagera at Kanyonza level. This is the sign of high pollution of those sites.
The highest is at Nyabarongo just few meters from the City of Kigali when sewage discharge from
households and other different uses are still concentrated in effluent.
Figure 7. pH variation versus distance
4.1.7 Variation of Turbidity
The levels of turbidity for the monitored period varied greatly as shown in Fig 9. The highest turbidity
value is at Akanyaru River and the lowest at Muvumba River. The higher concentration at Gashora point
is related to mine extraction at Musha which carries a big amount of pollutants impacting on the next
sampling site and decreasing with distance due to dilution by small tributaries of the Akagera.
Figure 9. Turbidity variation versus distance
4.1.8 Variation of Total Dissolved Solids
The levels of TDS for the monitored period varied greatly as shown in Fig 10. The highest TDS
concentration is observed at Muvumba River and in the Akagera River, at Gashora point there is more
than 100 mg/L which is, may be, due to the intensive agricultural activities just on the river bank.
Activities upstream the sampling site; are carried out by the farmers in associations. The lower value is at
Akanyaru River which is generally less polluted sampling site.
Figure 10. Total dissolved solids variation versus distance
4.2 Pollutants export coefficients
4.2.1 Phosphates export coefficient
The high export coefficients for phosphorus shown in Fig 11 may be related to catchment processes such
as erosion which is intense in bare soil land cover observed in this study, and human activities around the
river. The mobilization of suspended solids (from land cultivation, forestry operations, pasture
preparations, etc) is often associated with increased phosphate exports, with phosphorus adsorption to soil
and clay particles supplying P through erosion processes.
Figure 11. Export coefficient (in kg/ha.year) of phosphates in different land uses categories
4.2.2
Nitrates export coefficient
Figure 5. 1: Export coefficient (in kg/ha.year) of Nitrates in different land uses categories
4.2.3
Ammonia export coefficient
Figure 5. 2: Export coefficient (in kg/ha.year) of Ammonia in different land uses categories
The relatively high nitrates and ammonia (NO3-N and NH4-N) catchment exports may be associated with
catchment based nitrification processes. As nitrogen enters the soils bacteria convert it to gaseous
nitrogen that rises out of the soil and into the atmosphere (i.e. reducing TN exports). Another factor that is
just as important as nutrient loads is the composition, and reactivity (digestibility), of the organic matter
source, that can contain a substantial proportion of the nitrogen and phosphorus exported to the receiving
waters. When land use is changed from native vegetation (i.e. Eucalypt forest) to grassland, the organic
matter entering the streams is very different. Grassy organic matter is much more ‘digestible’ and requires
more oxygen as part of the digestion process. This can lead to dissolved oxygen depletion in receiving
waters which can lead to major water quality problems which is the high risk threatening the Akagera
River.
It could be suggested that the influence of land use on River integrity is scale dependent. In-stream habitat
structure and organic matter inputs are determined primarily by local conditions such as vegetative cover
at a site, whereas nutrient supply and sediment delivery are influenced by regional conditions, including
landscape features and land use at some distance upstream and lateral to stream sites.
4.3 Development of a pollution prevention plan
The way to tackle the high pollutant concentration is to reduce human activities close to the Akagera
River and the implementation of waste collection and treatment system. Those activities are in general
pollutant drivers towards the water bodies.
In terms of land cover, the scenario of conversion of bare soil land to forest and sometimes to agriculture
without using fertilizers, either by active afforestation or by natural succession to the forest could be
efficient because the forest land cover has been identified as low pollutants export.
4.3.1 Possible management practices
In order to reduce other pollutants loads, the following actions can be taken into account:
1.
The Akagera River Basin in Rwanda is located in the region having the two elaborated master plans;
the option to adapt to them is crucial because in their points they highlight the ways to avoid surface
water contamination and the best management of land towards rivers protection;
2.
Environmental quality will be protected by minimizing impacts from human activities and by
planning for and maintaining natural areas that contribute to the health and quality of life of
communities and people. The riparian buffers could be protected so that they can continue its
function of stripping sediments carrying a number of pollutants from hillsides.
3.
Land will be conserved that protects water quality and quantity, land providing buffers adjacent to
surface waters, such as streams, lakes and wetlands. Land will also be protected from erosion using
control measures and Best Management Practices (BMPs) when land use alterations occur.
4.3.2 Implementation strategies
To implement and monitor the elaborated management plan, the following implementation strategies can
be adopted in collaboration with local authorities:
1. The management of Akagera River particularly and its basin in general engages not only the riparian
people but also other organizations. There should be collaboration among all the stakeholders,
government departments, non-governmental organizations, private sector, community based
organizations (CBOs) and the local communities;
2. At the international level, riparian countries could establish cross-boundary water commissions or
other coordinating boards to avoid actions in one country adversely affecting water quality in other
country;
3. At national level, the Rwandan government might create inter-ministerial boards to oversee
coordination and cooperation for Akagera River water quality management. Those bodies should
include a wide representation from the authorities dealing with environment and water quality,
conservation, agriculture, forestry, fishing, mining, industry; they should also include interested
governmental and non-governmental conservation organizations.
5.
Conclusion and recommendation
6.1
Conclusion
From the results drawn above, we can give the conclusion of this study whose title is “Modelling
nutrients and organic pollution of the Akagera transboundary River” and then give the appropriate
recommendations for improving the manner the Basin is managed. The conclusions of this study are
described as follow:
1. The main sources of pollutants within Akagera River Basin are different land use activities
around the River and in the entire basin. Their change is depending on several processes and with
time where the different seasons are the main influencing factors, so temporal nutrients
concentration variability was found to be much larger than spatial variability. The level of
pollution is very high according to values observed during laboratory analysis sessions. Their
change with is dependent to such factor time and distance. Nitrates are very high concentrated in
Akanyaru River and low at Gashora station. Ammonia is high in Muvumba site and low in
Nyabarongo near Gitikinyoni point.
2. The impacts of land use activities on the Akagera River pollution is observed as one of the
influencing factor of pollution. The export coefficient values showed that some land use
categories are very influencing factor on water quality stability. Bare soil land was observed as
the most pollutants driver. The agricultural areas have been identified as the main source of
pollutants even though they are not first pollutants drivers.
3. The relationship between land use and water quality is that where the high export coefficient is
observed it implies the high concentration in receiving water bodies particularly on the nearest
sampling site. The possible way, to overcome those high concentrations of pollutants and land
uses influencing the pollutants accumulation, is to develop a pollution management plan of the
particular Akagera Basin and applying the strategies as cited in the previous chapter.
6.2 Recommendations
From this study, we propose the following recommendations in order to overcome the challenges raised in
this research work. Those are:
1. This study aimed to develop a conceptual framework of the pollution level of the Akagera River
due to the land use practices, but it should be a plan to monitor and mobilize the people living
within the basin to comply to the management practices proposed in this research project;
2. There should be a plan to carry the research with sufficient time, in order to gather required data
for model selection, calibration and validation. This may be achieved through a PhD research
project.
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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