WTP20 WORLD BANK TECHNICAL PAPER NUMBER 20 April 1984
Water Quality in Hydroelectric Projects Considerations for Planning in Tropical Forest Regions Public Disclosure Authorized
Camilo E. Garzon Public Disclosure Authorized Public Disclosure Authorized
*a =,q,e ,. .,,; 3', 7 .'py ...j. -;
TD 326.5 *G37 1984 Public Disclosure Authorized c2 WORLD BANK TECHNICAL PAPERS
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( ) Indicates numbers assigned after publication WORLDBANK TECHNICALPAPER NUMBER 20
Water Quality in Hydroelectric Projects Considerations for Planning in Tropical Forest Regions
Camilo E. Garzon
The World Bank Washington, D.C., U.S.A. Copyright Oc 1984 The International Bank for Reconstruction and Development / THE WORLDBANK 1818 H Street, N.W. Washington, D.C. 20433, U.S.A.
First printing April 1984 All rights reserved Manufactured in the United States of America
This is a document published informally by the World Bank. In order that the information contained in it can be presented with the least possible delay, the typescript has not been prepared in accordance with the procedures appropriate to formal printed texts, and the World Bank accepts no responsibility for errors. The publication is supplied at a token charge to defray part of the cost of manufacture and distribution.
The views and interpretations in this document are those of the author(s) and should not be attributed to the World Bank, to its affiliated organizations, or to any individual acting on their behalf. Any maps used have been prepared solely for the convenience of the readers; the denominations used and the boundaries shown do not imply, on the part of the World Bank and its affiliates, any judgment on the legal status of any territory or any endorsement or acceptance of such boundaries.
The full range of World Bank publications, both free and for sale, is described in the Catalog of Publications; the continuing research program is outlined in Abstracts of Current Studies. Both booklets are updated annually; the most recent edition of each is available without charge from the Publications Sales Unit, Department T, The World Bank, 1818 H Street, N.W., Washington, D.C. 20433, U.S.A., or from the European Office of the Bank, 66, avenue d'Iena, 75116 Paris, France.
Camilo E. Garzon is a Doctor of Engineering candidate at the Resource Policy Center of the Thayer School of Engineering, Dartmouth College, and an environmental engineering consultant to the World Bank.
Library of Congress Cataloging in Publication Data Garzon, Camilo E., 1948- Water quality in hydroelectric projects. '7 (World Bank technical paper ; no. 20) Includes bibliographical references. 1. Water quality management--Tropics. 2. Hydroelectric power plants--Environmental aspects--Tropics. 3. Reser- voir ecology--Tropics. 4. Forests and forestry--Environ- mental aspects--Tropics. I. Title. II. Series. TD326.5.G37 1984 333.91'4 84-7312 ISBN 0-8213-0363-5 - iii -
ABSTRACT
This paper identifies and describes the studies necessary to predict water quality changes, at an early state of planning, in large tropical reservoirs with long retention times. Emphasis is placed on both the reservoir area and the region downstream. The need for defining the
"baseline" environment is presented as a requirement for conducting studies associated with the flooding and operating stages. These studies are classified according to the stage of project development.
In the reservoir area, aspects such as biomass quantification, reservoir thermal stratification, water circulation, dissolved oxygen consumption and reservoir recovery are of major importance. Downstream from the project, the stress is placed on river recovery capacity, water uses and conflicts, and flow requirements. The results obtained from the studies serve as the basis for deciding the extent of forest clearing and other mitigatory measures.
The paper illustrates that biological degradation in tropical reservoirs follows a significantly different path from that in reservoirs in temperate zones, thus, conventional approaches to reservoir clearing and filling may not be adequate for projects in forested tropical regions. Two approaches - for project feasibility and project design - are suggested in order to meet the need for successive refinement in the results, and to take advantage of the increasing availability of project and environmental information. - iv - ABSTRAIT
Ce document identifie et decrit les etudes A effectuer pour predire,
A un stade peu avance de la planification, les variations de la qualite de l'eau dans les grands reservoirs tropicaux A longue dur6e de retention. I1 met l'accent A la fois sur la superficie du reservoir et sur la region en aval. La necessite de definir les elements de base de 1'environnement est pr6sentee comme une condition necessaire A l'ex6cution des 6tudes li6es aux stades de la submersion et de l'utilisation. Ces etudes sont class6es selon le stade d'avancement du projet.
Vis-a-vis de l'6tendue couverte par le r6servoir, certains aspects tels que l'6valuation quantitative de la biomasse, la stratification thermique du reservoir, la circulation de l'eau, la consommation d'oxygene dissous et la remontee du niveau de l'eau sont particulierement importants. En aval du pro- jet, l'accent est mis sur la capacit6 de retablissement du debit du cours d'eau, sur les utilisations de l'eau et les conflits A ce sujet, ainsi que sur les besoins en matiere de debit. Les resultats de ces etudes servent de base aux decisions concernant l'etendue de forat a deboiser et autres mesures des- tin6es a am6liorer la qualit6 de l'eau.
Cette etude montre que la degradation biologique qui se produit dans les reservoirs tropicaux suit une vole sensiblement differente de celle que l'on observe dans les r6servoirs des zones temperees, de sorte que les m6thodes classiques de deboisement et de remplissage pourraient ne pas conve- nir aux projets d'amenagement de r6servoir dans les regions tropicales. Deux formules sont preconis6es - pour les etudes de faisabilit6 et la conception des projets - afin de repondre A la necessite d'apporter plusieurs am6liora- tions successives aux resultats et de tirer profit de la disponibilite crois- sante d'informations sur les projets et sur l'environnement. - v -
EXTRACTO
En este trabajo se identifican y describen los estudios necesarios para predecir, en una etapa inicial de la planificacion, los cambios de la calidad del agua que ocurren en los embalses grandes con periodos de retencion prolonga- dos que se construyen en zonas tropicales, dandose especial importancia a los estudios tanto de la zona del embalse como de la situada aguas abajo del pro- yecto. Ademas, se subraya la necesidad de definir el medio ambiente "basico" para la realizacion de los estudios relacionados con las etapas de inundaci6n y funcionamiento. Estos estudios se clasifican de acuerdo con la respectiva etapa de ejecuci6n del proyecto.
En lo que se refiere a la zona del embalse, aspectos tales como la cuanti- ficaci6n de la biomasa, la estratificaci6n termnica, la circulaci6n del agua, el consumo de oxigeno disuelto y la recuperaci6n revisten primordial importancia.
En cuanto a la zona situada aguas abajo, se hace especial hincapie en la capaci- dad de recuperaci6n del rio, en los usos del agua y los posibles conflictos al respecto y en los requisitos en materia de caudal. Los resultados que se obtie- nen con los estudios sirven de base para las decisiones relati;vas al alcance del desbroce de la zona forestal y otros paliativos.
Se muestra graficamente en el trabajo que la degradaci6n biol6gica en los embalses de las zonas tropicales sigue una trayectoria notablemente diferente de la que sigue en los de las zonas templadas; por lo tanto, es posible que los metodos usuales de desbroce y llenado no sean adecuados para los proyectos que se llevan a cabo en regiones forestales tropicales. Se sugieren dos enfoques
--para la evaluaci6n de la viabilidad y el disefiode los proyectos-- a fin de satisfacer la necesidad de refinamiento sucesivo de los resultados y de aprove- char la disponibilidad cada vez mayor de informacion sobre los proyectos y los aspectos ambientales.
- vii -
TABLE OF CONTENTS
ABSTRACT
PREFACE
INTRODUCTION ...... 1
1. PROBLEM OVERVIEW ...... 3
1.1 Upstream Area ...... 3 1.2 Reservoir Area ...... 3 1.3 Downstream Area ...... 6
2. RESERVOIR WATER QUALITY PREDICTIONS ...... 8
2.1 Biomass Decomposition ...... 8 2.2 Hydrothermal Behavior and Circulation Patterns ...... 13 2.3 Oxygen Balance ...... 20 2.4 Reservoir Recovery ...... 24
3. RIVER WATER QUALITY PREDICTIONS ...... 26
4. SUMMARY OF PROPOSED APPROACHES ...... 30
REFERENCES ...... 32 - viii -
TABLES
Table 1 Aspects Related to Water Quality Management ...... 4
Table 2 General Composition of Plant Tissues ...... 9
Table 3 Dissolved Oxygen Balance during the Filling Process .. 21
FIGURES
Figure 1 Water Quality as a Function of Reservoir Retention Time and Area. 2
Figure 2 Simplified Organic Carbon Cycle of a Typical Freshwater Lake ...... 12
Figure 3 Schematic Arrangement of Thermal Lake Types ...... 15
Figure 4 Oxygen Isopleths in a Length Profile of the Reservoir along the Former Suriname River (dry period) .16
Figure 5 Oxygen Isopleths in a Length Profile of the Reservoir along the Former Suriname River (rainy period) .17
Figure 6 Temperature and Oxygen Content at Different Depths at Kabelstation, Suriname ...... 18
Figure 7 Water Densities for Various Temperatures ...... 19
Figure 8 Reservoir Filling Process ...... 22
Figure 9 Oxygen Consumption ...... 23
Figure 10 Reservoir Recovery Process ...... 25
Figure 11 Dissolved Oxygen Profile of the Lower Rio Sinu (flow 400 m3 /sec) .27
Figure 12 Dissolved Oxygen Profile of the Lower Rio Sinu (flow 50 m3 /sec) ...... 28
Figure 13 Dissolved Oxygen Profile of a Hypothetical River ..... 29
Figure 14 Proposed Water Quality Considerations ...... 31 - ix - PREFACE
This paper is a summary of material presented at a World Bank
Seminar on March 1, 1983, sponsored by the Energy and Industry Staff, and by the Office of Environmental Affairs. The intent is to outline tropical reservoir water quality management as related to the various aspects of the planning process.
The paper presents an analytical appraisal and predictions stemming from poor water quality conditions as a result of the decomposition of great amounts of biomass flooded by their associated reservoirs. This issue is important because of the increasing number of hydroprojects being planned in regions with tropical wet forests. The predictions presented serve as a basis for the decision and extent of forest clearing and the need for structural and managerial remedial measures.
The World Bank Seminar and paper resulted from the World Bank's requirement for environmental consideration of the Urra Hydroproject, located in northeastern Colombia, which is expected to begin operating in
1988. The environmental studies conducted by the author clearly show both the technical aspects of the project layout and the elements of the surrounding environment. The need for further quantitative analysis and the complexity of the phenomena involved suggest the need for an innovative and technologically advanced approach. The efforts made at the
Urra Project represent a step in this direction.
I wish to thank Mr. R. Goodland and Mr. J.J. Fish for their kind invitation to present the seminar, to write this paper and their most helpful and detailed improvements. Several people involved with the Urra
Project have contributed to the ideas introduced, and their contribution is gratefully acknowledged. The views presented here are personal and should not be attributed to the World Bank.
INTRODUCTION
The water quality problems addressed by this paper refer mainly to the situation encountered when large amounts of tropical vegetation
(i.e., tropical rain forest) are flooded by new reservoirs. The original river water quality deteriorates so drastically as to impair human consumption and most economic uses. In hydroelectric projects which require relatively large river flows, this situation becomes exacerbated when the reservoir's retention time (mean volume/mean flow) is also large. Figure 1 shows the region of main concern. The boundary line shown between regions is arbitrary. In addition to retention time and area, other variables such as mean depth, climatic conditions and reservoir morphometry, can increase or decrease water quality problems.
This particular reservoir category has not been sufficiently studied mainly because very few hydroprojects have presented those characteristics. However, the few projects built under these conditions have developed various kinds of environmental problems--water quality being one of the most serious. A good example of this is the Brokopondo
Lake (Afobaka Dam) in Suriname, built in 1964, which is illustrated in
Section 2.2 (Heide, 1976; Panday 1977). Several other important hydroelectric projects are in the planning process in tropical developing countries, for example in the Amazon basin (Goodland, 1978) and in
Colombia's Pacific Region (DNP, 1979). They will require detailed analyses if water quality and other environmental disturbances are to be prevented. The potential for similar water quality problems also is significant in Equatorial Africa and Southern Asia. FIGURE 1
Water Quality as a Function of Reservoir Retention Time and Area
Area Covered 1,500 by Forest (kin2 )
Regiono ntes WaterQualt 1,000 ~~~~~~~~Problems
500
II I X I ,1 1 1 1 6 12 18 24 30 36 42 48
RetentionTime (Months)
WorldBank-2521 - 3 -
This paper points out the need for two successive approaches in predicting water quality changes, and answering the question of how much forest clearing should be carried out in order to ensure the required quality of water. Extensive clearing is such a costly remedial measure, that it could compromise the feasibility of the project itself.
1. PROBLEM OVERVIEW
Aspects directly or indirectly related to the water quality issue are summarized in Table 1. They are organized both by geographical location and by stage of project development. The column entitled
'Baseline Environment" comprises the studies which will become the bases for the predictions listed on the two following columns.
1.1 Upstream Area
Inflowing tributaries acquire their physical, chemical and biological characteristics in the watershed, upstream from the reservoir.
Parameters such as water temperature, nutrient concentration, pH and organic content define significant properties of the incoming rivers that will partially determine the nature of the water quality in future reservoirs. However, this influence is more noticeable in short retention-time reservoirs than in stagnant reservoirs. In the latter, the effect will be manifested over a longer time span, i.e., during the reservoir "recovery" period. (See Section 2.4) For the same reason, prediction of future changes in land use and in the ensuing water quality characteristics will become necessary.
1.2 Reservoir Area
The reservoir area, which is to be inundated, requires several descriptive studies. First, it needs a quantification of the amounts and -4-
TABLE 1
Aspects Related To Water Quality Management - Descriptive and Predictive Studies-
STAGES BASELINE ENVIRONMENT RESERVOIR FORMATION PROJECT OPERATION LOCATION
UPSTREAM WATERSHED WATERSHEI) Land Use Land Use Changes
RIVER CHARACTERISTICS RIVER CHARACTERISTICS Quantity Quality Changes Quality
RESERVOIR VEGETATION FLOODING PROCESS RESERVOIR USE AREA Amounts Areas Covered Fishing Types Duration Recreation Elemental Composition Others HYDROTHERMAL BEHAVIOR Development of Strati- HYDROTHERMAL BEHAVIOR SOIL CHARACTERISTICS fication Stratification/ Stability CLIMATIC ASPECTS DISSOLVED OXYGEN BUDGET Ambient Temperature Aerobic/Anaerobic CIRCULATION PATTERNS Solar Radiation Conditions Morphometry Relative lHumidity Cloud Cover CIRCULATION PATTERNS WATER QUALITY Wind Direction/Speed Changing Morphometry Fertilization Recovery LOW-LEVEL DISCHARGES INTAKE CONFIGURATION
DOWNSTREAM RIVER CHARACTERISTICS MINIMUM FLOWS REQUIRED HYDROLOGIC ASPECTS Biochemical Seasonal Requirements "Dry" Reaches Hydraulic, Hydrologic Water Uses Flow Fluctuations
QUANTITY-QUAI.TY RELA- WATER QUALITY WATER QUALITY TIONSHIPS Parameter Profiles Parameter Profiles Tributaries Effect of Tributaries Delta Assimilation Capacity Other Features
WATER USES RECOVERY Human Consumption Fishing Irrigation Waste Water Disposal Others -5 - types of existing vegetation. This task is better conducted in two stages, as outlined below. At the outset, an estimate of the readily degradable fraction of the vegetation will suffice. 1/ Later, for more detailed analyses, the elemental composition of the biodegradable fraction will be required in order to estimate the input of nutrients into the water. The input of the superficial soil also has to be taken into account. Although tropical forest soils are usually nutrient-poor, this characteristic varies from one region to another. Soil organic content, on the other hand, tends to be relatively high when compared to temperate forests. Estimates based on data from forest ecosystem studies conducted in similar regions have proven useful, even though forest studies are not usually conducted with the same purpose in mind.
Second, the climatic characteristics of the region should be evaluated. This is usually an easier task, since most needed data are collected routinely by weather stations in the area and by engineering studies. For instance, ambient temperature, relative humidity and wind direction and speed data are gathered this way. Solar radiation and cloud
cover may require additional measurements on the part of the water quality
analysts. Climatological data serve as the bases for predicting the hydrothermal and mixing behavior of the future reservoir.
1/ Degradable fraction of vegetation includes leaves, twigs, flowers, fruits, portions of the bark and other softer outer tissues which decompose during the first few months. Knowledge of the biodegradable fraction, expressed in tons/hectares, for example, will allow calculation of the amount of dissolved oxygen which will be extracted from the water during the decomposition process following flooding. - 6 -
During reservoir formation, increasingly larger areas of forest will be flooded. The longer the reservoir retention time, the longer this process will last. Topographic characteristics of the area (the future reservoir morphometry), together with the hydrologic behavior of the tributaries will determine the probable amounts of organic matter added to the water mass per unit time. This relatively simple calculation (see
Section 2.3) provides initial estimates of dissolved oxygen content, and consequently, of other parameters intimately related to this vital component. The quality of the waters discharged through any low-level
(i.e., deep) outlets will be impaired if anaerobic conditions developed within the water column.
After filling, the reservoir will behave in a manner resembling that of a natural lake. If large amounts of vegetation are flooded, the reservoir will undergo a slow recovery process. Depending on need, this recovery process can be somewhat accelerated by intake configuration and reservoir water level control (Garzon, 1983).
1.3 Downstream Area
Additional studies are needed in the areas located along the river, downstream from the project site as summarized in Table 1, to determine the river self-purification capacity, and the existing and potential water uses. The river self-purification capacity can be reliably estimated from its hydraulic and hydrologic characteristics.
Deoxygenation and reaeration coefficients which are functions of molecular diffusion, water velocity, depth and temperature, will serve to estimate the dissolved oxygen levels along the river course. 2/ Other features
such as incoming tributaries, interconnected seasonal lakes, estuaries and
salt water intrusion at the river delta will have to be evaluated. The
latter may play an important role if the reservoir filling process is
protracted. During this period, in the absence of major tributaries
downstream from the project site, a substantially reduced river flow can
cause a detrimental increase in salinity concentrations near the delta.
Once the predictions for both the filling and the operational
periods of the reservoir water quality have been made, estimates on the
river water quality can be derived downstream from the project.
Thereafter, possible conflicts with highly demanding uses, such as human
2/ The differential equation (simplified for our case) that describes the rate of change of oxygen concentration in the river is of the form: dO = K2 (0*-0) - K1 L dt
where 0 = concentration of oxygen (mg/l)
0* = saturation concentration of oxygen at the local temperature and pressure
K2 = reaeration coefficient
K1 = deoxygenation coefficient (temperature dependent)
L = bioquemical oxygen demand (B.O.D.)
Numerous equations have been developed to compute the reaeration coefficient. An example is the O'Conner and Dobbins approach: 0.5 K2 = (Dm v) at 200C d1.5 where Dm = molecular diffusion coefficient v = mean water velocity in the river d = mean stream depth - 8 - consumption and fisheries, should be identified. Similarly, minimum flows required during filling, and even during project operation, should be determined; and lastly, river recovery, corresponding to the slow reservoir recovery also should be established.
In developing countries, water use downstream will normally determine the water quality required from the project. River water quality studies, thus, become a critical task. Fortunately, adequate technological tools exist to make this a relatively easy and reliable operation. However, this cannot be said about reservoir water quality predictions. These predictions still pose a great challenge that will have to be at least partly circumvented both by ingenuity and by simplification of the real processes involved. The following sections deal in more detail with the major aspects to be considered.
2. RESERVOIR WATER QUALITY PREDICTIONS
There are four major topics in the reservoir water quality prediction process: a vegetation inventory and decomposition study; an analysis of the thermal stratification and wind-driven circulation patterns; an estimate of dissolved oxygen consumption within the water mass; and a projection of the recovery process.
2.1 Biomass Decomposition
Biodegradable Fraction
The great diversity of organic chemical compounds which constitute the various parts of the vegetation can hardly be overestimated. After flooding, each substance decays following unique chemical pathways, producing different intermediate compounds and interacting at various rates with other substances. Detailed predictions -9- of this process is nearly an impossible task. Great simplifications can be made however, and still reflect the general decomposition trends.
Table 2 is an example of typical ranges in the composition of plant tissue. It shows that vegetation biomass consists primarily of substances that are difficult to decompose, such as hemicelluloses, celluloses and lignin (Goldstein, 1981). Lignin is most resistant to biochemical degradation. The ability to break it down is possessed primarily by aerobic fungi. It is regarded as virtually undegradable by anaerobic processes (Hobson, 1974). In contrast, proteins, sugars and starches decompose readily and become the substances of immediate concern. The green parts of the vegetation not only have a higher proportion of biodegradable substances, but also are more vulnerable to bacterial attack due to both their large surface area/volume ratios and their softer tissues.
TABLE 2
General Composition Of Plant Tissues
Component Percentage
Carbohydrates
Sugars and Starches 1 - 05
Hemicelluloses 10 - 28
Cellulose 20 - 50
Fats, waxes, tannins 1 - 08
Lignins 10 - 30
Proteins
Simple water soluble
and crude protein 10 - 15
Brady (1974), Goldstein (1981). - 10 -
Defining the amount of vegetation (biomass) present within the future reservoir area should be the starting point in the evaluation process, since biomass density varies widely from one place to another.
Tropical wet forest biomass is generally high (between 300 and 900 ton/ha), while temperate forest biomass varies between 200 and 400 ton/ha
(Dames and Moore, 1982). Estimates could be obtained by comparing biomass in similar forest types during the feasibility stage of the project.
Field reconnaissance is needed to check the general validity of the assumptions. Later, during the design stage, nondestructive and destructive biomass sampling should be used to refine the initial estimates.
As mentioned above, the principal reason for the biomass estimates is to determine the portion of biodegradable organic matter present in the vegetation. For this reason, the procedures and analysis will vary slightly with respect to traditional forest studies. Emphasis should be placed on the green and softer parts of the vegetation, their biodegradable substances and their elemental (i.e., nutrient) composition. The green, readily biodegradable portion of the vegetation normally constitutes 10% or less (by weight) of the total biomass density.
Elemental phosphorus, a key element in fresh water eutrophication, is often found in amounts of approximately 50 kg/ha of forest (Dames & Moore,
1982).
Analogously, the organic matter, including roots and the nutrient content of the forest superficial soils, also should be measured.
Although tropical forest soils are poor in nutrient concentrations, the amounts of humus and other decaying organic substances could become a significant variable. - 11 -
Chemical Pathways
Initially, enough dissolved oxygen will be available in the water mass for biomass decomposition to be aerobic. Facultative and aerobic bacteria will oxidize the organic matter to the stable and relatively unobjectionable products: CO29 NO3-, SO4=s P04 -.
As the available oxygen is used up, anaerobic and facultative bacteria will convert the organic matter to simpler organic and inorganic compounds. Substances like CHRV CO2 H 2S, NH3, and H2will be produced. The methane-forming bacteria are strictly anaerobic and very sensitive to acidic conditions. Cellulose, which could be decomposed to hexane, organic acids and CH4 , would remain unaltered under acidic conditions.
A simplified organic carbon cycle is presented in Figure 2. It illustrates the contrast between aerobic and anaerobic conditions. The latter normally occur in the sediments of freshwater lakes. When the water overlying the sediments becomes anaerobic, the general direction of mineral movement is reversed. Compounds, such as those containing iron, manganese, phosphorus and sulfur, redissolve in the water and can reach high concentrations. Additionally, the rates of decomposition decrease under anaerobic conditions. Similar cycles could also be presented for other important elements such as sulfur, phosphorus and nitrogen (Goldman,
1983).
The degree of anaerobiosis will also affect the relative proportion of the products. For example, some authors classify the existing reducing conditions under the following stages (Gunninson, 1981).
a. Dissolved oxygen exhaustion (Redox potential: 300 - 400 mV)
Nitrates begin to replace oxygen as inorganic electron
acceptors for microbial processes. - 12 -
Figure 2 Simpiffied Organic Carbon Cycle of a TypicalFreshwater Lake. DOC and POC =Dissolved and Particulate Organic Carbon; PS Photosynthesis;R = Respiration. (Modified from Kuznetsov,1959,1970.)
rADlochthorK)s||ouO | | DOC, POC | DC O
Dissociatio HO 3
\EternalHumic PS Substances & Littoral Flora, Phytoplonkton. Piant Residues R Autotrophic & Chemosynthetic Bacteria Groundwater
n Cellulose Hemicelluloses i > 2i) - _ U i . L ~~~~~&F'ectins {:
aan,,rganic Dissolved O | | \ 3 O o & a r H~~~~~~~~~etwrophic|A u d i f r ~Humic'_J.... I LCompounds |o
t < ~~~~~~~~~Bacteri > /
\ \\ I ~~AncierobicDecomposto L;F gctenim \ \\ ~~~~HeterotrophicBacter E/
\ Oranicomponds
World Ebank-25122 - 13 -
b. Ammonia accumulation (Redox potential: 220 - 300 mV)
Ammonia produced by nitrate reduction accumulates.
Inorganic phosphorus is released from the sediments and from
phosphorus-bearing organic matter.
c. Manganese accumulation (Redox potential: 200 - 220 mV)
Reduced manganese is released from sediments.
d. Iron accumulation (Redox potential: 120 mV)
Ferrous iron becomes soluble in water.
e. Sulfate reduction (Redox potential: - 120 - - 150 mV)
Reduction of sulfide begins.
f. Methanogenesis (Redox potential: below - 500 mV)
Methane production begins and continues until the carbon
substrate is depleted or the reservoir destratifies.
In lakes and reservoirs where the hypolimnion (the part below the thermocline) is anaerobic, the "intensity" of the reducing conditions will usually increase with depth. This is particularly important during the flooding process when minimum flows in the river downstream may require the release of poor quality, low-level discharges.
2.2 Hydrothermal Behavior and Circulation Patterns
An important aspect, too complex to be treated in detail in this paper, is the mixing regime in the reservoir. Two distinct but related phenomena determine this behavior: the thermal energy transfers and the wind driven circulation patterns.
Thermal energy is transferred primarily through the air-water interphase and, advectively, through the inflows and outflows. At the reservoir surface, evaporation and radiation are the two main exchange - 14 - mechanisms. Thus, humidity, ambient temperature, wind speed, cloud cover and optical characteristics of the water are important variables. Wind stirring and convective overturns usually mix the heat gained from the sun to depths greater than the light penetration depth.
In tropical latitudes at low elevations, deep water masses tend to stratify thermally and to remain stratified for longer periods than in temperate climates. Oligomictic behavior (i.e., water bodies with little mixing) constitutes a major difference from the better known dimictic (two annual mixing) patterns as shown in Figure 3. This fairly permanent thermal stratification becomes, in turn, an effective barrier to the transfer of mechanical (i.e., wind induced) mixing and to mass transport.
Oxygen, which could be gained from the atmosphere and from photosynthesis, is confined to the upper layers by the thermocline. Figures 4, 5 and 6 illustrate the situation that developed in the Brokopondo Reservoir. The confinement of oxygen to the upper layers and its close correspondence to the water temperature profile are clearly shown in these figures. The role of simple molecular oxygen diffusion across the thermocline has not been fully determined, although most authors contend that it can be neglected. An example of the small contribution attributed to this mass transfer mechanism is given by Fisher (1979) with reference to the
Wellington Reservoir in Western Australia. The importance of a five degree centigrate difference in the water density at different temperature ranges is illustrated in Figure 7. Clearly, at high temperatures related to tropical conditions, a five-degree difference imparts more stability to the stratified water column than at low temperatures. - 15 -
FIGURE 3
Schematic Arrangement of Thermal Lake Types
'6000-
5000-
4000-
E- -°° I 30-0
I-
1000 $..
90 80 70 60 50 40 30 20 10 0 DEGREES LATITUDE Basedon Hutchinsonand Loffler, 1956. - 16 -
FIGURE 4
Oxygen Isopleths in a Length Profile of the Reservoir along the Former Suriname River (dry period)
Mamadam Koenkoen Rapids Rapids Grankreek Bedoti Kabelstation Afobaka Depth Meters
*3 000i5n035445 50 A5;
28 FORMERSURINAME RIVER BED: / / / / /.
.5 10 15 20 25 30 35 40 45 50 55 60 65 70 RiverKilometers
Note: Reconstructionof oxygen isoplethsin a length profileof the lake along the FORMER SURINAMvERIVER. on 30.XI.-3.X11.1965.Figures refer to mg 02/I. Theposition of the intake gates of the hydra-electricpower station wasalmost completely in the hypolimnionzone. Thewater passingthe turbinescontained very lite or no oxygen & caused fish mortality& oxygendeficiency over a large distance downstream. World Bank-26020 - 17 -
FIGURE 5
Reconstructionof Oxygen Isoplethsin a Length Profile of the Lake along the Former Suriname River (rainy period)
Mamadam Koenkoen Ra ids Rapids Grankreek Bedoti Kabelstation Afobaka
Depth 0 7
82
txtesf / <~~~~~~......
16-
20:
24: FORMERSURINAME RIVER BED 28: - 1-5.111.1966-
32
36L --- ' :3::::.. :.-... 5 10 15 20 25 30 35 40 45 50 55 60 65 70 River Kilometers
Note: Reconstructionof oxygen Isoplethsin a length proflleof the lake along the FORMER SURINAMERIVER, on 1-5.111.1966.Flgures refer to mg 02/1. Source: Heide(1976) World Bank-26021 FIGURE 6
Temperature and Oxygen Content at Different Depths at Kabelstation, Suriname
1C temp. KABELSTATION1964-1967 I ~~~Temperature& Oxygen Content at DmferentDepths I 340 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~34
32- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~32 b 03
8.5~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~2 26 1 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~26
1964 1965 1 1966 1967 omiIiII IV V 1\AlV\ll IX XXI XII I 1 III IV VV\AV\1VIII IX XXI XIII 111III IV VVIVmlllIIX XXI Xi I I IIIIIIV V °2 rng/1
10 I I 10
I 1 SaturationPoint 8
5 0
Note:Sampling point kABELSTATION. Temperature & oxygen content at differentdepths during the whole obsevatlon perlod, Feb. 1964-May 1967. The data areplotted at intervalsof twoweeks.
Source:Fbiede (1976) WorldBank-26019 - 19 -
FIGURE 7
Water Densities for Various Temperatures
1. coooOO
0.99950-. , 02
0. 90000
0.99850 ' \
0.99800 . \
0.99750- a.997000.99808 I I I \ _ o.sssso. ~~~~~~I I , . 0.99B50 I \
0.99500 I
0.99S000J 5 10 - 5 20 25 30 35
TEMPERATURE,OC - 20 -
Hydrothermal analysis and wind driven circulation thus become crucial aspects in predicting water quality changes. The former has been studied longer and is easier to simulate (Harleman, 1982; Orlob, 1981).
Wind driven circulation, on the other hand, has not been as intensively studied, because at least two dimensions (i.e., vertical and horizontal) need to be simulated. This, presently, is a matter of active research
(Bloss, 1980; Johnson, 1980; Krenkel, 1982; Thompson, 1980). Water quality models for stratified reservoirs could have several degrees of complexity, depending on the number of parameters simulated. Therefore, only key parameters, such as oxygen and phosphorus, should be considered initially in this type of predictive model. 3/
The overall conclusion obtained from the above is that the hypolimnion behaves like a stagnant body of water with practically one source of oxygen--the advective underflows caused by the incoming rivers.
2.3 Oxygen Balance
In order to obtain order-of-magnitude estimations on the main sources and sinks of dissolved oxygen, a simplified oxygen budget can be derived. The reservoir, as it first fills, resembles a reactor with varying input streams of river water and biodegradable organic matter
(i.e., the vegetation being flooded). Thus, the outcome of this process can be easily predicted. An example of this calculation was done for the
Urra Hydroelectric Project in northwestern Colombia. From comparative studies, a 30 ton/ha of readily biodegradable biomass was assumed. The area flooded at successive 3-month intervals was obtained from the
3/ An example of this simplified approach is presented by Klomp (1980). Other examples that introduce different perspectives can be found in Ford (1980), Gunnison and Branon (1980), Shanahan and Harleman (1982), Wang and Harleman (1982) and Wu and Alhert (1980). - 21 - project area-capacity curves and the most probable (based on mean monthly flows) filling curve.
Figure 8 shows the predicted filling process. Area and volume in this case correspond to organic matter and dilution water, respectively.
Figure 9 illustrates the great oxygen deficits originating from vegetation decomposition. Table 3 shows the variation for each time period. The second and fourth columns represent the oxygen inputs and demands for each time period. For example, between the 3rd and the 6th month, approxi- mately 240,000 tons of oxygen are required. This high value is due to the large area flooded during this time span.
TABLE 3
Dissolved Oxygen Balance During The Filling Process (tons of oxygen)
MONTH INPUT TOTAL DEMAND TOTAL OXYGEN ADVECTED DEMANDED BALANCE (1) (2) (3) (4) (5) (6)
3 9600 9600 90000 90000 -80400 6 30400 40000 240000 330000 -290000 9 0 40000 0 330000 -290000 12 20000 60000 120000 450000 -390000 15 32000 92000 123000 573000 -481000 18 8000 100000 19500 592500 -492500 21 12800 112800 43500 630000 -523200 24 23200 136000 51000 687000 -551000 27 25600 161600 34500 721500 -559900 30 17600 179200 22500 744000 -564800 33 27200 206400 21000 765000 -558600 36 13600 220000 21000 786000 -566000
The curve obtained in Figure 9 has only theoretical value. As
the conditions become anaerobic and the usual biochemical transformations Figure 8 RESERVOIRFILLING PROCESS
100 30
80 4, 2.5
60 2.06
c
40 5
Filling(Months Time 20 Dom ~4.et.I.1.0
0 0.5 0 6 12 18 24 30 36
FillingTime (Months)
World Bonk-25 123 - 23 -
FIGURE 9
Oxygen Consumption
° -tD Iu I I I . I I I I I I I I I I x A Y
M G .20M O E U N N -30000,
D T -4 F 0 N S 1 -
. . . I I I I la 20 30 - 24 - take place, some of the products will become gaseous and rise into the aerobic (superficial) strata. There, the gases will oxidize, or, in the worst case, some will escape into the atmosphere. This process will,
therefore, satisfy a great part of the biochemical oxygen demand. The
rest will be broken down through the anaerobic pathways previously mentioned. The transformation and product accumulation rates should be more closely scrutinized in detailed water quality analysis.
2.4 Reservoir Recovery
Having made a prediction of the balance of oxygen during the
filling period, it would be useful to forecast the reservoir recovery period. This depends on the former predictions. A few calculations provide an order-of-magnitude estimation in order to gain some insight on
the general process. As an example, the concentration of a hypothetical
conservative substance was simulated for the Urra Project. This, in fact,
could be any substance that does not decay or decompose (e.g., sulfates,
chlorides, etc.). A perfectly mixed reservoir was assumed with a
residence time of 36 months. 4/ An arbitrary initial concentration of 100 units (e.g., mg./l) was considered (Figure 10). This simple calculation
showed that several years are required for complete renewal of the water mass. Assuming "recovery" at 10% of the initial value, seven years would
4/ The mathematical formulation, in its simplest form, can be represented as follows:
C = Coe- (t/to) where CO : initial concentration (mg/l)
C : concentration at time t
to residence time (months or years)
e : base of natural logorithms - 25 -
FIGURE 10
Reservoir Recovery Process
100
90
80
70 z 0 60
o 50
40
30 I
20 -
I f I aI 1 L 8 9 10 1 2 3 4 5 6 7 YEARS Wod Bank-26023 - 26 - be required. This type of calculation is useful for elements such as phosphorus--a limiting nutrient--in order to predict recovery through successive eutrophication stages of the water mass. Additionally, concentrations in the river inflows can be easily accounted for.
On a more detailed analysis, other factors, such as reservoir morphometry, circulation patterns, withdrawal location (e.g., height of penstock intake in the dam), soil characteristics and nutrient recycle
(water-to-sediment), would have to be considered.
3. RIVER WATER QUALITY PREDICTIONS
River pollution analysis has been developing for several decades and there are many detailed publications on the subject. The main objective of this section is to promote greater use both of this knowledge and of the powerful analytical tools derived from it, in order to better understand and predict the project impacts. Models, such as the QUAL-II
(EPA, 1981), will become very useful in the analysis of the downstream effects. Figures 11 and 12 show selected results of the simulation process, as it was applied to the Sinu River downstream from the Urra
Project (Dames and Moore, 1982). Due to the absence of important tributaries along the river course, a rather smooth curve always developed. Figure 13, by way of contrast, shows a clear discontinuity in the dissolved oxygen profiles due to the confluence of a major tributary.
In all of the above examples, as would be expected, a constant improvement in the oxygen level was observed.
The development and application of remedial measures, such as biomass clearing, multi-level intakes and hypolimnetic reaeration, should - 27 -
FIGURE 11
Dissolved Oxygen Profile of the Lower Rio Sinu (flow 400 m3/ sec)
U.o
z O 4.0.COXYGEN DEFICIENrZONE 0
_~~~~Dg WR- 3.0 nV/ l /
- °- DOR - 2.0 motl
DOR - 1.0 nVtl/1
SOUflRAI ° rlIERlRALTJA °20motEvrERIA '50 SABANA 3SO KILOMJETERS NUEVA
Source: Dames and Moore 1982 - 28 -
FIGURE 12
Dissolved Oxygen Profile of the Lower Rio Sinu (flow 50 m3/sec)
3.0 BOO5 - O.O"I
DD5$15 MO"
6.0/
E 2 a 4.0- >1 OXYGEN DEFICIENrZONE 0
2.0
URRA 1i 50 URRA I 100 TIERRALTA 150 20 MoNTERIA 250 SABANA 300 350 KILOMETERS NUEVA
Source: Dames and Moore, 1982 - 29 -
FIGURE 13
Dissolved Oxygen Profile of a Hypothetical River
"NATURAL"LEVEL
130
155 z Li I
0 180 C ~~~~MINIMUMLEVEL
0~~~~~~~~~~~~~~~~~~~~~~~~~1
DAMSITE/ CONFWENCE(+ 80 m3/S)
DISTANCE(km)
Word Bank-26022 - 30 - modify the profiles described above until adequate water quality is obtained. The advantage of simulated predictions becomes evident if one considers the possible combinations of remedial measures and the varying cost-effectiveness values attained.
4. SUMMARY OF THE PROPOSED APPROACHES
The water quality studies presented in this paper should preferably follow a staged development approach. The stages should closely parallel traditional engineering studies conducted on a project.
This will facilitate the necessary data collection activities and allow for close interaction with the project staff. The flow chart presented in
Figure 14 summarizes the different aspects that require consideration under two successive levels of approximation in a detailed water quality evaluation. The complexity of the task and the major effort required to obtain the necessary data and develop the appropriate predictive models are especially evident for the "design level" approach. This poses a great challenge to those responsible for designing such projects in tropical regions. The fact that most of them will be located in developing nations calls for greater technology interchange and an increased effort to understand both tropical ecosystems and societal goals. - 31 -
FIGURE 14
Proposed Water Quality Considerations (two successive approaches)
A. FEASIBILITYLEVEL:
Stratification Conservative Substances SET AND REEVALUATE 1 < T ~~~~~~~~~~~~~~~~~OBJECTIVES
Biodegradable Flooding Recovery Predicted Fraction _ Process Process Downstream S * _ ._ | a . _ ~~~~~~~~~~~~~~~~Effects
River Reservoir ixning Water 1Morphology .Assumptions Quality
ANALYZEALTERNATIVE PREVENTATIVE/MITIGATIVE MEASURES
B. DESIGN LEVEL:
Amount of Hydrothermal Main Water Vegetation Soi Behavior Quality SET AND REEVALUATE .~~~~~~~~~ Constituents OBJECTIVES
_Bidgradable | rFloodirng Waera Quality | Rcovr I Pred c Fraction & Process Constituents Process Downstream Nutrient Input -Balances- Discharges
River Circulation Circulation Water Quality Patterns atten
ANALYZE ALTERNATIVEPREVENTIVE/MITIGATIVE MEASURES - 32 -
References
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Brady, N.C. 1974. The nature and properties of soils. New York, Macmillan. 639p.
Chapron, S.C. and Reckhow, K.H. 1983. Engineering approaches for lake management. Woburn, MA, Butterworth. 2 Vols.
Dames and Moore, Inc. 1982. Alto Sinu Project Environmental Study. Final Report (for) CORELCA, Colombia. Washington, D.C. 543p.
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EPA. 1981. Users manual for stream quality model (QUAL II). Athens, Georgia. Environmental Research Lab. EPA-600/9-81-015. February, 75pp.
Fisher, H.B., et al. 1979. Mixing in inland and coastal waters. New York, Academic Press. 483p.
Ford, D. E., et al. 1980. A water quality management model for reservoirs. Proceedings of the Symposium on Surface Water Impoundments. New York, American Society of Civil Engineers (ASCE). 1679 p.
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Goldman, C.R. and Horne, A.J. 1983. Limnology. New York, McGraw-Hill. 464p.
Goldstein, I.S. (ed.) 1981. Organic chemicals from biomass. Boca Raton, Florida, CRC Press. 310p.
Goodland, R. 1977. Environmental optimization in hydrodevelopment of tropical forest regions (10-20). in Panday, R.D. (ed.) Man-made lakes and human health. Suriname, Paramaribo, University of Suriname. 73p.
Goodland, R. 1978. Environmental reconnaissance of the Tucurui hydroproject Amazonia, Brazil. Brasilia, Eletronorte, S.A. 141p.
Gunnison, D., et al. 1981. Characterization of anaerobic chemical processes in reservoirs. in Dames and Moore, Inc, 1982. Vicksburg, TR. E-81-6. U.S. Army Engineers waterways experiment station. - 33 -
Gunnison, D. and Brannon, J.M. 1980. Conceptual model depicting anaerobic geomicrobial processes in reservoirs. Proceedings of the Symposium on Surface Water Impoundments. New York, American Society of Civil Engineers (ASCE). June, 1679 p.
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Thompson, R.O.R.Y. and Imberger, J. 1980. Response of a numerical model of a stratified lake to wind stress. in Carstens, T. and McClimans, T. (eds.) Second international symposium on stratified flows, Trondheim, Norway, The Norwegian Institute of Technology. Tapir. 562p.
Wang, M. and Harleman, D.R.F., 1982. Hydrothermal-biological coupling of lake eutrophication models. Dept. of Civil Engineering, MIT, Cambridge, Mass., R.M. Parsons Laboratory. Technical Report No. 270, April, 245 p.
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Woxid Bank The Johns Hopkins University Press, investment decisions, income WorldBatik 1966: 4th printing, 1974. 80 pages distribution, and distortions in the Publications (including 2 appendixes). pricing system of the economy. Of Related LC 66-28053. ISBN 0-8018-0646-1, TheJohns Hlopkins University Press, $5.00 (13.00) paperback. 1977; 2nd printing, 1981.382 pages Interest (including tables, maps, index). The Economics of Power LC 76-9031.ISBN 0-8018-1866-4, System Reliability and $30.00(S.13.50) hardcover, SystemngR helibiiy anISse 1BN 0-8018-1867-2,$12.95 (45.75) Studies pprak Mohan Munasinghe French: L economiede l electricite: essais et etudes diecas. Economica. A completely integrated treatment of 19e79 system reliability. 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*** TD 326.5 .G371984 C.2 GARZON, CAMILO E., 1948- WATER QUALITY IN HYDROELECTRIC PROJECTS
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