MANAGEMENT OF RIVER BASINS UNDER WATER STRESS - EXCHANGE OF
EXPERIENCES FROM BRAZIL AND GERMANY.
Günter Gunkel1 & Maria Carmo do Sobral2
1
Technical University of Berlin, Dept. Water Quality Control, Sekr. KF 4, Strasse des 17. Juni
135, 10623 Berlin, Germany. e-mail: guenter.gunkel@tu-berlin.de
2
Universidade Federal de Pernambuco (UFPE), Departamento de Engenharia Civil, Av. Acadêmico
Hélio Ramos, s/n Cidade Universitária, 50.740-530, Recife/PE, Brazil. e-mail: msobral@ufpe.br
Abstract
MANAGEMENT OF RIVER BASINS UNDER WATER STRESS - EXCHANGE OF
EXPERIENCES FROM BRAZIL AND GERMANY.
As a consequence of global warming a reduction of available water will occur in many
watersheds and conflicts concerning water use as well as impact on ecosystems will take place,
this situation is already typical in semiarid areas such as in Northeast Brazil, where a large
number of reservoirs are used for water storage. Focus of investigations is the development and
optimization of reservoir and river basin management in the Northeast of Brazil, because the
increased energy demands, the development of the economic basis of human society and
climate change are leading to severe and increasing pressure on the aquatic systems. Today, the
environmental policy of many countries such as Brazil gives priority to the construction of new
reservoirs for hydropower use, and an adopted reservoir and river basin management has to
minimize environmental effects. Moreover, the production of energy plants for ethanol and biodiesel will promote the development of new dam projects.
The Itaparica Reservoir is a nearly 20 year old, large reservoir (828 km2) in the São Francisco
River, Northeast of Brazil, and was built in 1988 for hydroelectric power generation, but
nowadays also serves to develop large areas of irrigation agriculture, abstraction of drinking
water, fishery and aquaculture. Significant environmental impacts are recognizable such as
increased sedimentation in the inflow area, damage of the littoral zone by operational water
level variation, water losses by evaporation and infiltration, missing degradation of inundated
vegetation, a trophic upsurge with severe eutrophication related processes (occurrence of
cyanobacteria, deficit of oxygen in the hypolimnion, development of organic rich sediments,
mass development of macrophytes (Egeria densa)). The sensibility of the reservoir to
eutrophication and contamination processes is determined by longitudinal and lateral mixing
processes as well as by thermal stratification and seasonal variation of phytoplankton or
macrophyte dominance.
1
Resumen
MANAGEMENT OF RIVER BASINS UNDER WATER STRESS - EXCHANGE OF
EXPERIENCES FROM BRAZIL AND GERMANY.
Como consequência do aquecimento global, em muitas bacias hidrográficas ocorrerá uma
redução da água disponível e um aumento dos conflitos pelo uso da água e dos impactos nos
ecossistemas aquáticos, esta situação já é típica em zonas semi-áridas como no Nordeste do
Brasil, onde o armazenamento de água é feito por um grande número de reservatórios. O
enfoque das investigações é o desenvolvimento e otimização da gestão de reservatórios e bacias
hidrofráficas no Nordeste brasileiro, tendo em vista que o aumento da demanda por energia, o
desenvolvimento de atividades econômicas pela sociedade e a mudança do clima, conduzem a
um aumento das pressões nos ecossitemas aquáticos. Atualmente, as políticas ambientais em
muitos países, como o Brasil, dão prioridade para construção de novos reservatórios para
produção de energia e uma gestão adaptada dos reservatórios e bacias hidrográficas deve
reduzir os impactos ambientais. Além disso, a producão de plantas energéticas para fabricacão
de etanol ou biodiesel promoverá a construção de novos reservatórios.
O reservatório de Itaparica é um grande reservatório (828 km2)de aproximadamente 20 anos
localizado no rio São Francisco, Nordeste do Brasil, e foi construído em 1988 para produção de
energia elétrica, mas hoje em dia também é utilizado para o desenvolvimento de grandes áreas
de agricultura irrigada, captação de água para abastecimento público, pesca e aquicultura.
Significativos impactos ambientais são reconhecidos como o aumento da sedimentação na área
de entrada de água, danos na zona litoral pela operação de variacão do nível da água, perdas de
água por evaporação e infiltração, falta de degradação da vegetação inundada, aumento
repentino do estado trófico com processos intensivos de eutrofização (ocorrência de
cianobactérias, déficit de oxigênio no hipolímnio, desenvolvimento de sedimentos ricos em
material orgânico, grande desenvolvimento de macrófitas (Egeria densa)). A sensibilidade do
reservatório para processos de eutroficación e contaminação é determinada pelos processos de
mistura longitudinal e lateral, bem como, pela estratrificação térmica e variação sazonal do
fitoplâncton ou dominância de macrófitas.
Keywords: eutrophication, Itaparica, reservoirs, river basin management, São Francisco River
1. Introduction
About 7 years ago, a cooperation between the Technical University of Berlin, Germany,
Department Water Quality Control, and the Universidade Federal of Pernambuco, Recife,
Brazil, Departamento de Engenharia Civil, was established with its focus on developing
environmental sciences, especially aquatic ecosystem protection and river basin management.
In Germany, anthropogenic impact on aquatic systems has been a focus of investigations for
several decades, and environmental sciences as well as aquatic system management plays a key
role in education and research, while in Brazil, these disciplines are also becoming increasingly
significant, due to severe water quality problems in many rivers and reservoirs (Bouvy et al.,
2000; Andeoli & Carneiro, 2005; Gunkel & Sobral, 2007).
In Brazil, the construction of reservoirs for energy production and flood control is of high
importance and necessitates water management to guarantee sustainable water use. But the
experiences of the last decades point to increasing problems due to sewage input as well as to
impact by climate chance. River basins under water stress due to climate change is a new
2
experience for many European countries, and the development of a network of countries with
areas under water stress can provide a basis for the evaluation and management of future
climate change related problems.
Many Brazilian reservoirs already constructed or still in the planning process, are mainly for
hydroelectric power and flood regulation, both of high significance to guarantee a better quality
of life. But nowadays, due to increasing importance of the environmental policy, a more
differentiated use of water must also be implemented, as a consequence of socio-economical
development and the protection of drinking water and of the natural aquatic ecosystems all
having a high priority and not just energy production.
The operation of the reservoirs has already been provoking conflict in relation to the multiple
uses of water. These conflicts will increase during extreme climate periods such as extended
droughts or intensive floods. The multiple use of reservoirs, that means generation of
hydroelectric energy, flood regulation, urban and industrial water supply, irrigation agriculture,
abstraction of water for aquaculture, net cage culture in the reservoir, fishery, navigation,
recreation and tourism implies an integrated river basin management, a regionally adopted
water management system, which includes land use and social-economic development, water
use and terrestrial as well as aquatic ecosystems dynamics (Tundisi & Matsumura-Tundisi,
2004; Andreoli & Carneiro, 2005; Gunkel & Sobral, 2007).
Goals for integrated river basin management are the reduction of water quality impact to
guarantee long term water use (Straškraba & Tundisi, 1999; Tundisi & Matsumura-Tundisi,
2004), the reduction of the impact on nature , mainly of biodiversity as well as of contamination
level, and on a global point of view, the emission of greenhouse gases (Rosa et al., 2002;
Fearnside, 2005).
Water storage in reservoirs is of main importance in the Northeast of Brazil, because ground
water resources are small and over exploited, and alternatives such as water harvesting and
subsurface dams are not sufficient for water supply to larger urban areas (Gunkel & Sobral,
2007); but the use of reservoirs in the tropical and semiarid zone implies many problems
concerning water quality and environmental impact (Gunkel et al., 2003a; 2003b).
The evaluation of the environmental impacts of the 20 years old Reservoir Itaparica in the River
São Francisco is target of a German-Brazilian research group (Gunkel & Sobral, 2007); the
River São Francisco has an length of 3,160 km and is the 25th largest river worldwide,
stretching from Minas Gerais, the rainy southwest of Brazil, to the dry zone of Northeastern
Brazil. There are 8 reservoirs in the middle and lower-middle course of the São Francisco
River. Apart from the generation of electrical energy, the reservoirs are used for flood control
and water abstraction; according to the Brazilian National Water Agency (ANA, 2003), the
main use of water in the lower-middle course of the São Francisco River is irrigated agriculture,
which represents 50.5 % of the usage.
After 20 years of operation, it is possible to make a comparative evaluation between the
environmental impacts foreseen in the environmental study and the environmental situation
nowadays (Sobral, 1991; 2007; Gunkel, 2007). Up to now, economic activities such as
irrigation agriculture and aquaculture have been developed for the 40,000 local population close
to the reservoirs banks, which intensify the risk of degradation of aquatic ecosystems and of the
water quality due to uncontrolled use of agro-chemicals and absence of sewage treatment.
Additionally, the Federal Ministry of National Integration has started a transposing water
project, whose objective is to carry about 30 – 60 m3 sec-1 water from the Itaparica Reservoir to
other Northeast States, to assure the socio-economic development in this semi-arid area (ANA,
2003). This project will increase conflicts of the uses of water and requiresgreat deal of
development of irrigation agriculture techniques in the semiarid and arid zone.
3
2. Material and methods
2.1. Area studied
The São Francisco River Basin is one of the main river basins of Brazil, its water basin is
640,000 km3, the average outflow 1,849 m3/s. Up to the present, 8 reservoirs have been
constructed, Sobredinho, Itaparica, Moxotó, Apolio Sales and Paulo Afonso I – IV, of which
Sobredinho and Itaparica are the largest, situated in the middle course of the São Francisco
River, all being managed by CHESF (Hydro Electric Company of São Francisco). The
Itaparica Reservoir is located between the states Bahia and Pernambuco, 290 km from the
Atlantic Ocean. The Itaparica dam was finish in 1988, constructed for hydroelectric power
generation (1,479 MW); for nearly 40,000 translocated people, a new economic base with
irrigation agriculture had to be developed.
2.2. Climate and geological settings
According to SUDENE (1990), the area is located in the ‘Depression of São Francisco’, which
means a hot and dry semi-arid climate with summer-autumn rains (BShw of Köppen climatic
classification) whose annual precipitation decreases from southeast to northeast, varying
between 448 mm on the area of the old Petrolândia city, nowadays submerged, located about 10
km south of Apolônio Sales, and 403 mm on the town of Icó, near to Icó-Mandantes. The rainy
period extends from May to July, the driest period from September to November. Annual
temperature averages around 26 °C and there is a very high annual evaporation rate of 2,386
mm (CODEVASF, 1998).
The geology of the area is mainly made up of sandy formations of the sedimentary basin of
Jatobá, forming the plain and soft wavy relief of the study area. Its altitude varies from 300 m to
slightly over 500 m. It is mostly of quartz latosoil sands (EMBRAPA, 2001). Sands in the area
from Petrolândia to Ibimirim/Pernambuco State are very deep, excessively drained, strong to
extremely acidic, and have low to very low natural fertility (Cavedon, 1986). They are
characteristic soils of the irrigation area of the village of Ico-Mandantes and permit only limited
agricultural usage.
The biom of the area influenced by the Itaparica Reservoir is ‘caatinga hiperxerófila’, with the
presence of Umbuzeiro (Tuberosa spondias), Juazeiro (Juazeiro zizyohus), Mandacaru (Cereus
famacuruos) and Acacia (Acaria spp.) as typical salt tolerant species.
2.3. Itaparica reservoir
The Itaparica Reservoir (Fig. 1) was constructed below the Sobredinho Reservoir with a
regulated outflow of 2,060 m3/sec. The Itaparica Reservoir has a length of 149 km, 828 km2
surface and with a maximum depth of 101 m (mean depth = 18 m, Tab. I). The reservoir has a
capacity of 10.7 x 109 m3, the dam wall length is 4,700 m.
The planning process for the dam, which was built in 1986 with the major goal of energy
production, took place in the 1970s, when an environmental audit was not part of the
permission requirements. Therefore, environmental protection programs were quite limited.
Later, in 1986, the Resolution CONAMA Nr. 01/1986 established the obligation of
environmental impact studies for specific projects, and the 1987 realized environmental study
of Itaparica Reservoir had a pioneer role in Northeast Brazil (Sobral, 1991).
4
Tab. I. Morphometric data of the Itaparica Reservoir.
Reservoir length
Reservoir surface
São Francisco water basin
Itaparica Reservoir sub water basin
Regulated outflow
Dam length
Installed capacity
Max. depth
Mean depth
Operation level
Max. volume
Mean water exchange time
148 km
828 km2
640,000 km2
93,040 km2
2,060 m3/sec
4,700 m
1,479 MW
101 m
18 m
299 – 304 m
10.78 km3
63 d
3. Results
3.1. Environmental impacts of reservoirs
The building of reservoirs can lead to some impacts on nature and humans which have been
discussed for many decades. The possible environmental impacts can be grouped into longrange effects, reservoir upstream effects, reservoir effects and reservoir downstream effects
(Tab. II), and should be the basis for any environmental assessment study. Good planning and
construction of a reservoir can minimize or even completely exclude the potential risk factors,
nevertheless the building of a reservoir is a political decision – and regional socio-economic
development, environmental impact and water availability has to be evaluated.
Tab. II: Possible environmental impacts of reservoirs
Long-range
effects
Change of microclimate with increase of air moisture.
Hydrology effects with change of the drainage area, water balance and river
bed erosion.
Change of groundwater hydraulics with influence on direction of groundwater
flow and groundwater table level.
Flooding of agricultural, cultural and archaeological sites.
Seismological effects with risk of man-made earthquakes.
Reservoir
upstream
effects
Sedimentation in the inflow area and build up of a delta with increased
flooding risk.
Water level is determined by the reservoir management, mainly energy
production.
Reservoir
effects
Periodical water level fluctuations with littoral erosion.
Risk of landslides triggered by littoral erosion and earthquakes.
Sedimentation of river bed load and suspended material lead to a loss of
reservoir storage capacity.
Change of water balance with increased water losses by transpiration, evapotranspiration and infiltration as well as abstraction of water.
Remaining vegetation leads to eutrophication and an impact on boating and
fishery (no net fishery).
5
The reservoir must be regarded as a new ecosystem, and plankton, fishes,
benthic and littoral fauna are changed from those of the river; and it will take 1
– 2 decades to reach a high ecological buffer capacity.
The damming effect with trophic upsurge leads to significantly more eutrophic
conditions than in the previous stretch of river.
Eutrophication by nutrient input from the watershed occurs (= cultural
eutrophication).
Barrier to the stream flow and inhibition of fish migration.
Creation and spreading of water borne diseases.
Reservoir
downstream
effects
Regulated flow and flooding regime is determined by reservoir’s management.
Loss of nutrients by the damming effect, no fertilization during flooding and
decline in soil fertility.
Reduced water flow due to water losses and water abstraction in the reservoir.
Missing suspended sediment load leads to river bed or bank erosion.
Erosion of the river’s delta due to missing bed load.
Concerning Itaparica, in 1987, only one year before the Itaparica Reservoir fulfilment, an
environmental impact study was presented for the purpose of receiving the operational
environmental licence for this reservoir. The construction of the reservoir was already in its
final phase and therefore this study did not influence the planning and construction process; and
the environmental impacts of urban and rural resettlement were analyzed in a very superficial
way.
Before the fulfilment of the reservoir, a rescue operation was carried out to save part of the
animal population living in the area to be flooded.
Monitoring of water quality and fishing activities was carried out before and after the reservoir
fulfilment and during the first year of operation. Another, significantly more complex
monitoring program was realized during 2004 – 2005 by CHESF.
3.2. Water chemistry
The water temperature in the Itaparica Reservoir is very high and reaches 32 °C. Water
chemistry is determined by a poor buffering capacity (mean alkalinity = 31 mg L-1 CaCO3), and
low ionic content (mean conductivity = 65 - 132 µS cm-1). In the rainy period (normally June to
August) high concentrations of suspended minerals (clay, silt) are observed (up to 425 mg/l in
2005). Occasional precipitations lead to a significant change in the water chemistry, and high
values of conductivity, turbidity, P and N are representative for the rainy periods – also a
consequence of the large sub-water basin of the Itaparica Reservoir with 93,040 km2 and its
intensive erosion processes (Tab. III).
The phosphorous concentration is moderate during dry period (13 µg L-1 P-total), P maximum
occurred during rainy period with 69 µg L-1. Oxygen concentrations vary between 5,3 mg L-1
(68 % saturation) to 12,0 mg L-1 (150 % saturation). Primary production is limited by
phosphorus (N/P factor = 16 – 29 on weight basis).
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Tab. III: Physical and chemical parameters of Itaprica Reservoir water (mean of epilimnic
water, 2004, monitoring program of CHESF, (intensive precipitations occurred in April).
Parameter
Temperature (°C)
Secchi Depth (m)
Conductivity (µS/cm)
pH
Turbidity (mg/L)
Suspended material
(mg/L)
Alcalinity (mg/L CaCO3)
Depth
Epilimnion
Hypolimnion
Lake
Epilimnion
Hypolimnion
Epilimnion
Hypolimnion
Epilimnion
Hypolimnion
Epilimnion
Hypolimnion
Epilimnion
Hypolimnion
O2 (mg/L)
Epilimnion
Hypolimnion
O2 (%)
Epilimnion
Hypolimnion
NH4-N (mg/L)
Epilimnion
Hypolimnion
NO2-N (mg/L)
Epilimnion
Hypolimnion
NO3-N (mg/L)
Epilimnion
Hypolimnion
Ntotal (mg/L)
Epilimnion
Hypolimnion
Epilimnion
Hypolimnion
SRP (mg/L)
Ptotal (mg/L)
N/P (weight basis)
Chlorophyll a (mg/L)
Epilimnion
Hypolimnion
Lake
Lake
January
28,2 (s = 0,6)
27,1 (s = 0,5)
1,3 (s = 1,0)
78 (s = 32)
64 (s = 11)
7,3 - 8,0
7,2 - 7,6
52 (s = 22)
42 (s = 7)
52 (s = 22)
42 (s = 7)
29 (s = 1)
28 (s = 3,8)
7,5 - 12,0
5,6 - 9,0
95 - 150 %
71 - 113 %
0,4 (s = 0,3)
0,6 (s = 0,8)
0,4 (s = 0,6)
0,8 (s = 0,5)
22 (s = 28)
18 (s = 23)
193 (s = 63)
212 (s = 78)
1,0 (s = 1,6)
0,8 (s = 1,9)
15 (s = 6)
38 (s = 65)
20 (s = 20)
12 (s = 10)
April
28,2 (s = 0,6)
28,1 (s = 1,4)
0,3 (s = 0,1)
132 (s = 25)
132 (s = 11)
7,7 - 9,4
7,6 - 7,7
86 (s = 12)
20 (s = 7)
27 (s = 12)
37 (s = 7)
37 (s = 4)
37 (s = 2)
5,8 - 8,8
5,3 - 6,1
74 - 120 %
68 - 77 %
1,3 (s = 1,4)
1,3 (s = 0,7)
0,3 (s = 0,3)
0,3 (s = 0,4)
69 (s = 24)
80 (s = 18)
647 (s = 195)
667 (s =162)
3,3 (s = 3,2)
5,5 (s = 2,8)
51 (s = 37)
60 (s = 47)
17 (s = 9)
16 (s = 13)
July
24,4 (s = 0,4)
24,5 (s = 0,1)
1,5 (s = 0,2)
73 (s = 7)
70 (s = 1)
7,2 - 7,6
7,1 - 7,3
47 (s = 4)
45 (s = 1)
9 (s = 4)
8 (s = 2)
26 (s = 1)
25 (s = 1)
6,5 - 6,8
6,2 - 6,6
77 - 83 %
75 - 80 %
0,9 (s = 0,5)
1,3 (s = 1,2)
0,1 (s = 0,2)
0,0 (s = 0,0)
18 (s = 12)
25 (s = 7)
260 (s = 39)
263 (s = 40)
0,6 (s = 0,5)
0,3 (s = 0,2)
11 (s = 4)
10 (s = 4)
29 (s = 18)
17 (s = 21)
October
27,7 (s = 0,)
25,5 (s = 0,2)
3,6 (s = 1,0)
76 (s = 5)
74 (s = 1)
7,2 - 7,4
7,0 - 7,4
49 (s = 3)
48 (s = 1)
10 (s = 4)
8 (s = 2)
34 (s = 1)
34 (s = 1)
6,3 - 7,9
6,2 - 6,8
80 - 95 %
76 - 80 %
0,5 (s = 0,5)
0,2 (s = 0,2)
0,3 (s = 0,1)
0,3 (s = 0,1)
2,9 (s = 1,5)
2,5 (s = 0,9)
211 (s = 61)
152 (s = 25)
0,7 (s = 0,4)
0,6 (s = 0,2)
13 (s = 4)
14 (s = 5)
16 (s = 4)
nd
3.3. Environmental problems caused by the Itaparica reservoir
Nearly 20 years after damming up, some severe environmental problems and impacts can be
observed, mainly an inadequate land use with salinization problems, eutrophication of the
reservoir, increasing fish production by aquaculture in ponds and net cages, and contamination
of the reservoir with sewage.
3.3.1. Land use
The soils with higher agricultural fertility, located in the river margins, were completely
flooded by the reservoir, and higher areas were used for agriculture (dry and irrigated
agriculture, caprine farming), but the soil quality is inferior and shows a high risk of
7
salinization. 20 years after of the construction of the reservoir, there still are many problems to
be solved. The irrigation projects have expanded over the last years to about 5,000 ha. Although
they have brought benefits to the local population of the area, they have also contributed to a
health hazard and to water and soil pollution, because of intensive and indiscriminate use of
water, fertilizers and agrochemicals; the practice of inadequate irrigation has caused soil
salinization. In the semi-arid regions, intensive use of irrigation water, absence of drainage
systems and subsurface accumulation of water can cause salinization and soil degradation.
The input of drainage water as well as erosion of soils by heavy precipitation during the rainy
period become key factors for the water contamination of the reservoir, because drainage water
is contaminated by soluble fertilizers and pesticides, and the suspended material is
contaminated by its absorption of the more insoluble compounds of fertilizers and pesticides. A
primary study in the Itaparica area pointed to the use of 75 pesticides, and the predicted
environmental concentration (PEC) for some pesticides exceeds the tolerable environmental
concentration (TEC).
3.3.2. Water balance
Using reservoirs for water storage, water losses by evaporation, transpiration and infiltration
(bank infiltration and losses to the aquifer) must be taken into account. In Itaparica Reservoir,
the water losses by evaporation amount to 66 m3 sec-1 (= 3 % of the inflow), the losses by
transpiration are of lower significance, but the losses by infiltration mainly along the sandy
bank areas of the reservoir must be assumed to be very high, but this water flux has not yet been
calculated. Even when a reservoir is built in an area with low percolation rates, local sandy
deposits can be flooded, and water losses to the aquifer can occur because the raised water level
after damming up is much higher than the ancient aquifer level; fissures and fractures can also
be responsible for water losses. The actual situation of the São Francisco River water charge is
insufficiently studied, and the extremely reduced flow rate occurring in 2008 in the lower
course of the river points out clearly the significance of a more detailed water balance.
3.3.3. Thermal stratification and reduced water mixing in shallow areas
The Itaparica Reservoir is determined by the climate variation with a rainy season (May to July)
and a dry season (September to November). During the rainy season, water temperature is
lower (26°C) and due to wind and precipitation, intensive water mixing occurs. During the dry
season, the inflow water temperature increases up to 32 °C, caused by the shallow inflow area
of about 30 km length. This shallow inflow area is filled by deposition of the bed load of the
São Francisco River, and sand banks built up leading to an increased risk of flooding during
high water charge periods.
In shallow areas of the inflow and bays of the reservoir, water heating up to 32 °C occurs
during the dry period and this promotes the development of weak thermal stratification (C ~
2-4 °C) with high phytoplankton biomasses in the epilimnion. The inflow water is also
contaminated with pesticides and sewage. Water abstraction for irrigation and drinking water
occurs near the margins in shallow water depths and with an associated high risk for
contamination (Fig. 2). During the rainy period, an inverse situation occurs, due to the high
concentration of suspended load (up to 500 mg L-1) and suspension currents are formed but its
spreading has yet not investigated.
Due to the large length of the reservoir, the water body cannot be seen as a homogenous
system, and areas with a high risk of eutrophication respectively contamination must be
differentiated from those with a lower risk. Firstly, the longitudinal differentiation of the Sao
8
Francisco inflow must be considered, which will take theoretically two months to pass through
the reservoir. Secondly, a transversal differentiation occurs, but the transversal mixing is
inhibited by shallow flooded areas with remaining trees and sometimes dense macrophytes
stands. Small inflows from the watershed (drainage water, sewage from urban areas and
contaminated water from aquaculture) lead to an increased concentration of contaminants near
the shore line due to simple dilution processes. Severe, but local, contamination problems occur
within those bays of the reservoir with high input rates from irrigated agriculture zones and
small agro villages without sewage treatment. Nowadays these processes can be observed e.g.
at Petrolandia bay and Ico-Mandantes II bay. Chlorophyll-a distribution indicates different
areas of algal development, e.g. in July in the inflow area, whereas in January and April mass
development of algae is initiated in the middle course of the Itaparica Reservoir, probably in
some isolated bays (Fig. 3).
3.3.4. Damage of the littoral zone
The regular operational level of the São Francisco Reservoir varies by about 5 m and leads to a
complete failure of the littoral zone. Periodically, large littoral areas become dry and the
sediments are mineralized, leading to high nutrient input into the reservoir (Fig. 4). The littoral
fauna and flora are impeded and the ecological value of the littoral zone is extremely reduced.
The dried littoral zone is also used for cattle farming and agriculture with a related input of
nutrients and agrochemicals following an increase in water level.
The littoral zone is characterized by these periodical water level changes and only a few pioneer
organisms develop during inundation period such as macrophytes like Egeria densa for
example, or molluscs; thus biodiversity is severely reduced.
3.3.5. Mass development of Egeria densa
The transparency of one to a few meters allows the development of submerged macrophytes,
mainly the common waterweed (Egeria densa), which can cover large areas of the reservoir.
During dry season, reservoir water level decreases to about 5 m, this leads to complete
breakdown of the submerged macrophytes, and intense oxygen consumption in the hypolimnic
water by degradation of the macrophytes is registered. Due to the reduced flow rate and the
water heating, overlying flow of the Sao Francisco waters occurs, meaning that the nutrient rich
river inflow will form the upper water body, and an intensive development of phytoplankton,
among others cyanobacteria, can be observed; the Secchi transparency decreases to < 50 cm.
This alteration of phytoplankton to macrophyte dominance is promoted by the water level
change: with increasing water level after and during the rainy season, the submerged
macrophytes grow and stretch up to 4 m and build up a high biomass.
In Itaparica Reservoir, Egeria densa build up a high biomass with increasing water level,
Carillo et al. (2006) already report in the high mountain of Colombia a biomass of 1,155 g m -2
dry weight from the Nneusa Reservoir, about 10 kg wet weight m-2. With decreasing water
level, the plants experience depths < 1 m, limiting for their survival. The macrophyte stands
break down and cause severe impact due to O2 consumption and mechanical clogging of turbine
inlets.
Egeria densa is a pioneer plant with adventitious roots in the sediment, but nutrient uptake from
the sediment is without significance, thus sandy littoral zones without organic material can be
settled on. The plants occur at 1 – 7 m depth, 10 – 32 °C and are regulated by sediment removal
due to waves or water currents (Getsinger & Dillon, 1984; Bini & Thomaz, 2005).
9
3.2.6. Eutrophication
The eutrophication process is a well known phenomenon for a new dammed up reservoir called
the trophic upsurge. This trophic upsurge is caused by the change of the hydrological conditions
and by nutrient release from the inundated areas. The inundated vegetation and soils contain
high amounts of nutrients which will continuously be released by mineralisation after flooding.
It must be taken into account that the deposited soils along a river are normally very rich in
nutrients, because these areas were used for agriculture with fertilizer application.
The change of running water to a dammed up segment of the river, the reservoir, will lead to
new physical, chemical and biological conditions, more comparable to a lake. The river
morphology is changed to increased width and huge shallow littoral zones where water heating
occurs. The flow regime is changed to a lenitic (= standing water) environment, with increased
sedimentation. The oxygen budget is burdened by reduced physical water re-aeration and
increased oxygen consumption due to intensive mineralisation processes of the inundated
vegetation and autochthon production. This can cause, at least periodically, anoxic conditions in
the deep water body, and redox chemical removal of phosphorous and a emission of methane
must be expected.
The migration of people into the water basin, especially to the reservoir’s margin zone, leads to
a so called cultural eutrophication due to increased agricultural use with emission of irrigation
water after percolation, or of drainage water, by sewage input and by other activities such as
aquaculture or keeping of livestock. In semiarid areas, main nutrient input is from scarce but
intensive precipitation, which causes severe soil erosion, mainly of the silt and clay fraction rich
in nutrients; this process is called sandification.
After completion of the Itaparica Reservoir a trophic upsurge and eutrophication related
processes were registered such as the occurrence of cyanobacteria (among others
Cylindrospermopsis raciborskii, Microcystis aeruginosa), the occurrence of reduced oxygen
saturation conditions in the deep water and the mass development of submerged macrophytes,
mainly the common water weed, Egeria densa.
Eutrophication also leads to the development of the water borne disease, Schistosomiasis,
because the population growth of the intermediate host, snails of the genus Biomphalaria spp.
increase, due to the intensive development of the macrophytes.
3.3.7. Aquaculture
Aquaculture is regarded in some countries such as Brazil as an opportunity to improve the
economic situation, but the implement of aquaculture techniques such as tanks or ponds at the
shore line or net cages in the lake for tilapia culture lead to severe nutrient input into the
reservoir. For example, the net cage culture system ‘Jovens Criadores de Peixes’ of Moxotó, a
reservoir below Itaparica consists of 65 cages (Fig. 5), each one with 4 m3 and 1,000 kg fish
(Tilapia). The production cycle is only 6 months. Based on a food conversion factor of 1.3 and
a production of 130 tons of fish, 170 tons food are used yearly. This leads to an estimated
annual input of 0.9 t P, 54 t N and 27 t organic materials (food rests, faeces) as well as an
oxygen consumption (BOD5) of 7.2 t. This calculation is based on data from Lahmusluoto &
Tarcyznska (2002) with a utilization rate of phosphor about 15 – 30 % and of nitrogen about 21
– 30 %. The input of dry substance is lower and amounts about 15 % of the feed used
(Gyllenhammar & Håkanson, 2005). This contamination of the reservoir just by one single cage
culture system corresponds to 3,200 people equivalents – with benefits to only a few families.
10
Investigations about the net cage culture system ‘Péda Água’ in Mogotó Reservoir showed a
decrease of oxygen saturation values in the water, especially near the ground, an accumulation
of organic material and of phosphorous in the sediments below the cages, and a mass
development of macrophytes in the inner part of the bay.
4. Discussion
Up to now, only a few investigations are available which compare predicted environmental
impact and its environmental effects a few decades later (e. g. Curuá Una, Gunkel et al., 2003a).
Other reservoirs such as the Iraí Reservoir (Pinhais/PR) were recognized for improvement of
their severe environmental impact and were focus of intensive investigations after construction
(Andreoli & Carneiro, 2005). The World Commission of Dams (WCD, 2000) also provides
some information about the environmental impact of reservoirs.
About 20 years ago, the Itaparica Reservoir was developed as a hydroelectric power system,
without sufficient environmental impact studies and without any water resources management
plan. In the meantime, the water’s use has become more differentiated and the Itaparica
Reservoir serves for drinking water, irrigation water and in some areas for industrial water use,
aquaculture systems and recreation. Additionally, the protection of aquatic life and of the
integrity of the aquatic ecosystem has become a focus of interest. Thus an integrated river basin
management system with an adequate, local strategy has to be developed to guarantee a
sustainable use of land and water (Veltrop, 1996; Strakrabra & Tundisi, 1999; Gunkel et al.,
2003a; Andreoli & Carneiro, 2005).
Foci of interest are the eutrophication related processes such as input of nutrients (Thornton,
1987; Behrendt, 1996; IETC, 2000; Lewis, 2000), the occurrence of toxic cyanobacteria,
(Molica et al., 2005; Branco & Senna, 1994), the formation of organicrich sediments (Tipping
& Clarke, 1993), periodical anoxic conditions in the sediment and the hypolimnic water, and an
internal fertilization by redox chemical processes (Hupfer & Lewandowski, 2005; Gonsiorczyk
et al. 1997), and also the promotion of water borne diseases like Schistosomias by intensive
macrophyte growth (Gonzalez, 1989). The eutrophication process is strongly related to rivers
damming up, and a good river water quality does not give any guarantee of good reservoir
water quality. A large reservoir must be regarded as a complex system with a significant
longitudinal and traversal differentiation and a local mass development of algae or macrophytes
must be expected (Nakamoto et al., 1997). Thus new technologies like reuse of water, water
saving irrigation systems, treatment of drainage water and protection of the reservoir by a
buffer zone without any land use along the shore line (CONAMA, 2002) must be applied.
Aquaculture must also be regarded as a severe impact on a reservoir, but the strategy to develop
a capacity model can minimize eutrophication effects. Main effects of cage culture are the
increase of eutrophication processes, the accumulation of organic rich sediments below the
cages and the decrease of oxygen in the sediments, the sediment contact water zone and in the
hypolimnion during the stratification period (Tacon & Forster, 2003; Gyllenhammar &
Håkanson, 2005). But an aquaculture capacity model for reservoirs must consider the phosphate
balance, the nitrogen concentration and the N/P ratio for prediction of cyanobacteria
dominance, as well as the sedimentation processes near the fish cages to evaluate the
accumulation of organic material at the lake floor. In general, these complex interactions of
aquaculture emissions to lake biocoenosis does not allow high loading, and the development of
aquaculture systems like fish cage culture must be evaluated critically, if no future orientated
concepts like feedback systems (reuse of aquaculture waste for other applications like
irrigation) are applied.
11
Nowadays, the eutrophication level of the Itaparia Reservoir has increased, and the extraction
of drinking water, e. g. in the bay of Ico-Mandantes, has to be stopped. In future, any drinking
water supply can be ensured only with increased costs of water treatment as well as the
development of a drinking water supply in rural areas – or alternatively, the protection of the
reservoir water quality becomes intensified to avoid any further contamination of the reservoir.
Acknowledgement
This study was supported by the PROBRAL Program of Capes/Brazil (Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior) and DAAD/Germany (German Academic
Exchange Sercive); Companhia Hidro Elétrica do São Francisco (CHESF) kindly presented the
monitoring data of the Itaparica Reservoir. The authors would also like to thank Mr. R. Hatton
for the revising of this manuscript.
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Fig. 1: Itaparica Reservoir, black is the ancient river course of the Sao Francisco and grey are
less or more shallow inundated area, developed by ancient satellite images and CENA image
(Candeias 2007).
Fig. 2. Water abstraction during low water level in Ico-Mandantes II bay, Itaparica Reservoir,
water abstraction had to be replaced by a separate drinking water supply.
Fig. 3. Chlorophyll-a concentration in the Itaparica Reservoir with local increases of algal
biomass, monitoring by CHESF (2004).
Fig. 4. Littoral zone of the Itaparica Reservoir with dried Egeria densa during dry period.
Fig. 5. System of aquaculture by net cages in Moxotó Reservoir, the 65 net cages have a yearly
capacity of 130 t fish.
15