Angelos N. Findikakis1, Jőrg Imberger2,
Ian Sehested Hansen3,
Luis A. Delgado4, Reinaldo García-Martínez5 and Erich Gundlach6
1Bechtel Systems & Infrastructure Inc., San Francisco,
California, U.S.A.
2Centre for Water Research, The University of Western Australia
3Danish Hydraulic Institute – Water & Environment, Horsholm, Denmark
4Petroleos de Venezuela SA (PDVSA), Maracaibo, Venezuela
5Universidad Central de Venezuela,
Caracas, Venezuela
6E-Tech International Inc., Acton,
Massachusetts, U.S.A.
Correspondence: A.N. Findikakis, Bechtel Systems & Infrastructure Inc., (MS 333/12/C34), P.O. Box 193965, San Francisco, California 94119-3965, U.S.A.
Tel: 1-415-768-8550; Fax: 1-415-768-4898; E-mail: anfindik@bechtel.com
Abstract: A study was conducted to evaluate different environmental remediation options for Lake Maracaibo in Venezuela. Lake Maracaibo is the largest lake in South America, and communicates with the ocean through the Strait of Maracaibo and Tablazo Bay. Under favorable tidal, river flow and other conditions, salt water flow from the ocean intrudes into the Lake contributing to elevated salinity levels, varying between 3 and 5 psu at the surface, and up to 11 psu at the bottom. High nutrient loads from the rivers discharging in the Lake are the major source of the eutrophication of the Lake. The combination of high organic matter concentrations and density stratification lead to anoxic conditions in the hypolimnion. The objectives of the study included a compilation of the existing knowledge about the source of the major environmental problems of the Lake, collection of additional environmental data, and develop analytical modeling tools that would allow the evaluation of alternative remediation strategies.
Keywords: Lake Maracaibo, stratification, eutrophication, anoxic hypolimnion, remediation
This is the first in a series of five papers
describing different aspects of a recent study on the environmental remediation
of Lake Maracaibo. The study, supported by Petroleos de Venezuela
SA (PDVSA), had as specific objectives to synthesize the existing knowledge
about the source of the major environmental problems of the Lake, collect
additional environmental data, and develop analytical modeling tools that would
allow the evaluation of alternative remediation strategies. Because of the
direct impact of different options involving changes to the present navigation
channel on the on-land facilities serving the oil and other industries, the
study also included an analysis of
alternative pathways to maintain the transport of oil, coal and general
merchandize. On the water quality side, one of the many issues addressed
was the role of salinity and stratification
on the overall environmental quality of the Lake. A basic question posed
repeatedly over the years is whether reducing the salinity of the Lake would
help reduce nutrient levels, increase dissolved oxygen concentrations in the
Lake, and improve the overall environmental quality of the Lake.
The present paper provides general background information and an overview of the study, while subsequent papers describe the specifics of the field program and the numerical models developed in the context of the study.
Lake Maracaibo,
located in western Venezuela, is the largest lake in South America and a water
body of great economic and ecological importance. The Lake Maracaibo area is the
largest oil production center in Venezuela. The Lake is about 160 km long in the
north-south direction and 110 km wide in the east-west direction, and has an
area of about 12,000 km2, a maximum depth of 30 m, and a volume of
300 109 m3. It is connected with the Gulf of Venezuela
through the Strait of Maracaibo and Tablazo Bay. Several rivers flow into the
Lake draining a watershed of approximately 89,000 km2, the largest of
which is the Rio Catatumbo accounting for close to 60 percent of all fresh water
flow into the Lake. The mean fresh water flow from all rivers draining in the
Lake is about 52 109 m3 per year, or about one sixth of
the Lake volume. River flows vary also seasonally, being highest in May, and in
October-November, and lowest during the January-March dry season. Flows also
vary from year to year. During a 19-year period (1976-1990) for which there are
systematic estimates of the total river flow into the Lake, the monthly flows
varied between 0.3 and 2.7 times the mean flow for that period (CGR, 1993).
Fresh water coming into the Lake from the rivers mixes with its surface waters
and an equal volume of water leaves the Lake through the Strait.
Tidal action in combination
with several other factors brings saline water from the Gulf of Venezuela into
Tablazo Bay and the Strait of Maracaibo. The extent of the salt wedge intrusion
entering the system depends on the combination of several factors, which,
besides the tides, include the rate of fresh water flow into the Lake, wind
direction and speed, and atmospheric pressure distribution. Under low river flow
conditions, combined with favorable wind and barotropic conditions at the
entrance to the system, the salt wedge can advance to the south end of the
Strait of Maracaibo, and from there to the bottom of the Lake. This process
raises the salinity of the Lake, especially in the hypolimnion, and contributes
to its density stratification.
The environmental condition of the Lake is a function of the combination of several natural and anthropogenic factors contributing to its advanced eutrophication state. The high primary production is the source of large quantities of dead organic matter settling to the bottom of the Lake. Algae, zooplankton and higher forms of aquatic life when they die settle at the bottom of the Lake, where they decompose consuming all available oxygen. This, in combination with the stratification of the Lake, which prevents oxygen transfer to the hypolimnion, leads to anoxic conditions below the pycnocline, which acts as a barrier to vertical oxygen transport.
The environmental condition of the Lake has also
been affected by sewage and industrial discharges, which cause locally
unsanitary conditions and contribute to the nutrient load of the Lake. However,
the bulk of nutrients coming into Lake
Maracaibo is from the rivers flowing into the Lake. The nitrogen and phosphorous
in the river inflows is generally in particulate form. In their particulate
forms, from river inflows and as organic detritus from primary production,
nutrients enter the saline hypolimnion through sedimentation. Within the
hypolimnion bacteria break down the organic nitrogen and phosphorus forming
ammonium and dissolved reactive phosphorus respectively. The persistent anoxic
conditions in the hypolimnion lead to the denitrification of nitrates and the
release of phosphate from the sediments into the water column.
Because the thermal stratification of the Lake
is relatively weak, the salinity stratification is the main contributor to the
density stratification of the Lake and is viewed by many as the primary source
of anoxic conditions in the bottom. Because
the return of the bathymetry of the system to the conditions prior to dredging
of the navigation channel has been the subject of considerable debate, it is
important to consider the relatively few early studies of the system, which
provide valuable insights into the conditions of the system prior to the
construction of the channel.
Gessner (1954) observed in 1953 that Lake Maracaibo is one of the productive systems in the world. Redfield et al. (1955) reported some of the earliest systematic water quality data in the Lake. The nitrogen and phosphorus levels reported in these studies are comparable to those observed subsequently. Also, Gessner reported the existence of anoxic conditions in the hypolimnion prior to the deepening of the navigation channel, when the salinity of the lake was lower, and at a time that the lake was isohaline. According the to Redfield and Doe (1964) the thermal stratification was enough to cause anoxic conditions in the hypolimnion. Specifically they observed that in September 1956, “when the lake was isohaline, the deep water was again anoxic. The thermal stratification which had developed during the summer apparently stabilized the water sufficiently to allow the anoxic condition to develop”.
As already
mentioned, the construction of a deeper navigation channel facilitated the flow
of denser salt water into the Lake and led to a gradual increase of the salinity
of the Lake, until the system reached a new dynamic equilibrium for the system.
Over the last forty years the increase in the salinity of the Lake has been the
subject of concern and the subject of many studies. The salinity of the Lake varies from year to year
primarily in response to hydrologic conditions, increasing in dry years and
decreasing during wet years. In general, surface salinities are vary between 3
and 5 psu and bottom salinities can be as high 11 psu.
Lake Maracaibo has
been the subject of many studies over the last forty years. Besides the already
mentioned work by Redfield, other examples of significant contributions to the
subject are the work by Battelle (1974), Parra Pardi (1979), Herman de Bautista
(1997) and Rodriguez (2000).
Some of the environmental problems of the Lake have quite obvious solutions, like for example point source control and watershed management to reduce the of nutrient and BOD load coming into the Lake, and the control of toxic pollutant discharges into the Lake system.
The potential environmental restoration measures can be can be classified as:
l Engineering options involving physical changes in the Lake system through changes in its bathymetry and the construction of special works. The main objective of various engineering options proposed over the years was to reduce the salinity of the Lake, reducing thus its density stratification and affecting water quality.
l Management options that include point source control and watershed management. The management options aim primarily at reducing the nutrient loading of the Lake, as well as various pollution discharges.
These two types of options can of course be pursued in parallel.
The focus of most of the remediation schemes proposed over the last forty years, has been the reduction of salt-water intrusion into the Lake. Such schemes ranged from the construction of various engineered structures designed to block salt-water intrusion by isolating the Lake system from the Gulf of Venezuela, to the abandonment of the dredged navigation channel and return of the system to its natural condition. Such schemes had two general goals: a) reduce the density stratification of the Lake, therefore potentially eliminating the anoxic conditions in the hypolimnion, and b) reduce the salinity of the surface waters enough to allow their use for irrigation. Many of these salinity reduction schemes proposed over the years require the construction of significant infrastructure.
The salinity reduction options are classified in three groups according to the extent to which each option restricts navigation:
(1) options with no navigation restrictions, involving for example construction of hydraulic works that would reduce the salinity of the system without reducing the navigable depth of any channels,
(2) options leading to partial navigation restrictions, for example limiting the depth of the channel or constructing hydraulic works that would require a draft restriction, and
(3) the option to close the channel and let the system return to its bathymetry prior to the dredging of the navigation channel, restricting thus completely the navigation of large vessels.
Options that require navigation restrictions would also require the relocation of some, or all port and industrial facilities presently served by the navigation channel, and the construction of new such facilities, outside the Lake.
The field work and data collection program conducted as part of the study had two general objectives: a) advance the understanding of the major processes that affect the environmental quality of the system, and b) provide the required data for the calibration of hydrodynamic and water quality numerical models.
The program included the collection of hydrodynamic, hydrographic, water quality, and selected meteorological data, the compilation and use of meteorological data collected by others, and the estimation of river flows in the Lake. A comprehensive effort was made to document different basic aspects of the hydrodynamics of the system with state-of-the-art equipment, never before used in the Lake Maracaibo system.
The hydrodynamic data collection work was
conducted in two phases, the first phase during the wet season (October-November
1998) and the second phase during the dry season (March-April 1999). The program
included measurements for both neap and spring tide conditions. It included the
collection of continuous velocity profiles at different parts of the system over
several tidal cycles using 4 bottom-placed Acoustic Doppler Current Profilers (ADCP),
velocity profiles with propeller current meters, several F-probe lines (CTD
profiles) in different parts of the system, and measurements of the
stratification microstructure in the Strait and the Lake using a Self Contained
Autonomous MicroProfiler (SCAMP). The location of some of the stations and lines
used for the collection of these data is shown in Figure 1a. A bathymetric
survey was conducted in two areas of very active morphological features, the
mouths between Zapara and San Carlos Island, and the mouth between Zapara and
Cañonera Island. In addition to the existing tidal stations, two new stations
were installed at Amuay and Punta
Perret to obtain water levels at the entrance to the Gulf of Venezuela. More
details on the field program are given in Horn, et al. (2001a).
A water quality monitoring campaign was conducted with the boat “El Bergantin” operated by the Instituto Para el Control y la Conservacion de la Cuenca del Lago de Maracaibo (ICLAM). The program was conducted under the direction of Bechtel and included two complete water quality surveys of the entire system (water quality profiles at 21 stations shown in Figure 1b). Also, a series of water quality profiles at station C-11, at the center of the lake (see Figure 1b), were collected every 2 to 4 weeks between the wet and dry season campaigns.
A meteorological station was installed on the
oil platform Planta Lama in the Lake to collect wind, rainfall, and pan
evaporation data. Also, a temporary wind station was installed in Tablazo Bay
during the wet season campaign. Meteorological data from all operating land
stations were obtained and used in the study. River flows into the Lake were
estimated with the hydrologic watershed model of the Lake Maracaibo basin
developed by C.G.R. Ingeniería.
(a)
(b)
Fig.1 Hydrodynamic and water quality stations
To evaluate the water quality impacts of the
many options proposed over the years, special computer modeling tools were used
to predict the expected changes under each option. The data from the field
investigation were used to calibrate and validate the computer models.
Because of the different time and space scales of the processes affecting the environmental quality of Lake Maracaibo, as well as the computational requirements for their proper simulation, a combination of models and methods was used to evaluate the options under consideration. Two different models were used for this purpose: a) the three-dimensional model hydrodynamic, water quality and eutrophication model MIKE 3, and b) the one-dimensional dynamic reservoir simulation and water quality model DYRESM-WQ. Two different setups of MIKE 3 were developed, the Regional and the Lake model. The Regional model included the entire Gulf of Venezuela, Tablazo Bay, the Strait and Lake Maracaibo. It was developed primarily to analyze the hydrodynamics of the entire system, and predict the mixing and flow exchange processes between its major components. Special emphasis was placed in the analysis of flows and salt-water fluxes between the Strait and the Lake. The Lake model was developed primarily to perform ten-year simulations of the most promising options, using as boundary conditions flows and salt fluxes into the Lake derived from the Regional MIKE 3 model simulations. The development, calibration and validation of MIKE 3 are described in Hansen, et al (2001a, 2001b).
DYRESM-WQ was used both as a diagnostic and a predictive tool to provide longer-term assessments and sensitivity analysis, which, because of high computational requirements, were not practically possible with the three-dimensional models. The calibration and validation of DYRESM-WQ for Lake Maracaibo is discussed in Horn, et al. (2001b).
The Lake Maracaibo system contains roughly $2 billion of port and petroleum-related transport infrastructure developed over the past 50 years. The costs of the evaluated options range from very low for the case of maintaining the existing channel to very high, of the order of $2.5 billion, for the case of allowing the navigation channel to silt in and relocating all major facilities. The costs considered were those associated with the various proposed hydraulic works as well as those associated with infrastructure relocation and development, and exclude those for domestic and industrial point source control, which should supplement any option.
Two remediation options were selected for
in-depth modeling analysis: a) to maintain the existing channel as is today, and
b) let the navigation channel to silt in to return the system to its bathymetry
prior to dredging which would require relocation of all major ports within the
Lake. Both options are accompanied by a program for the control of all domestic
and industrial point pollution sources and the development of water management
plan for the reduction of nutrients and other pollution loading coming to the
Lake. These two options bracket the range of
potential water quality changes short of sealing off the Lake from all marine
influence. The first option—maintaining the present navigation channel
configuration in combination with pollution source control measures—has a locally positive water quality
impact and serves to provide a baseline for other analyses as well as answering
policy-related questions. The option of returning to the pre-dredging
bathymetry, in combination with the implementation of a watershed
management program and pollution source control measures produces the greatest positive changes in water quality. The
assessment of the relative contribution of the closure of the navigation channel
and a watershed management program to water quality changes in the Lake is still
in progress.
At the time of the submittal of this paper,
modeling evaluation continues. Preliminary results indicate that control of
direct point source discharges by itself will not have a major effect on the
eutrophication of the Lake. Point source control is though essential for
improving water quality locally and for protecting public health. More
significant improvements can come in from the implementation of a long-term
watershed management plan, aimed at reducing the nutrient and pollutant loadings
from the rivers discharging into the Lake. Considering that the system seems to
be in a state of dynamic equilibrium, maintaining the navigation channel will
not lead to increased degradation of water quality. Closing the navigation
channel will eventually return the salinity of the Lake to its historic levels
prior to the dredging of the channel, of about 1.2 ppt., and will significantly
reduce the salinity stratification. Weaker density stratification will continue
to exist seasonally due to the variability of water temperatures. The reduction
of salinity stratification will increase dissolved oxygen levels in the Lake,
however the thermal stratification may still provide enough density gradients
during part of the year to prevent the oxygenation of the hypolimnion. The
extent of the impact of the elimination of salinity stratification on anoxic
conditions in the hypolimnion is still under
evaluation.
An essential element of the
solution to the problem of anoxic conditions in the hypolimnion is the reduction
of the dead organic matter in the Lake, which requires reduction of the primary
production (phytoplankton, i.e. algae). This in turn requires reduction of the
nutrients coming into the Lake, which can be achieved primarily through control
and management of different activities in the watershed and secondarily through
treatment of all wastewater directly discharged in the Lake.
Acknowledgements
The work reported in this paper was part of the Integral Study for the Environmental Restoration of Lake Maracaibo, conducted for PDVSA by Bechtel International. Luis Delgado and Nelson Corrie managed the study for PDVSA. The study was overseen by a Technical Committee formed by PDVSA, and which included Gilberto Rodriguez. of the Instituto Venezolano de Instituto Venezolano de Investigaciones Cientifica, Reinaldo Garcia-Martinez of the Venezuelan Central University, Marlene Arias of PDVSA, Pánfilo Masciangioli of INTEVEP, Susana Herman de Bautista, José Rincon and Sandra Viada of the Universidad de Zulia, Gonzalo Godoy of ICLAM and Roger Nava. Wayne Mikutowicz was the Bechtel project manager. Angelos Findikakis was responsible for the technical management and coordination of the field program and all modeling activities. The Bechtel team that conducted the study included several Venezuelan and international consultants. John Headland and Santiago Alfageme of Mofatt and Nichols worked on the evaluation of the port options and the first phase of the MIKE 3 simulations. Nouel Ingenieros of Caracas under the direction of Luis Nouel worked also on the port options. Sergio Canales of Soros Associates contributed to evaluation of the transportation options. Ian Sehested Hansen led a team of engineers and scientists at the Danish Hydraulic Institute (DHI) who performed three-dimensional numerical simulations with MIKE 3. Jorg Imberger of the Centre for Water Research (CWR) led the development of the field investigation design and data interpretation, as well as the DYRESM simulations. Incostas of Caracas, under the management of Juan Font, was responsible for the execution of the field program (hydrodynamic data and bathymetric survey). ICLAM was responsible for the collection and analysis of the water quality data. José Rafael Córdova of CGR Ingeniería of Caracas was responsible for the hydrologic studies. Eric Gundlach was responsible for the analysis of various environmental issues, including oil spill impacts. Alan Harding addressed navigation issues. Irina Torrey and Mazin Al Mufti of Bechtel were respectively responsible for the environmental impact analysis and the on-land infrastructure requirements. Ingeniería CAURA under the direction of Eduardo Buroz collected and analyzed most the data in support of the environmental impact assessment of new structures as part of the evaluated options. Noriko Kawamoto was responsible for the biological aspects of the environmental impact assessment.
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