several natural and anthropogenic factors contributing to its advanced eutrophicatio n state. T he high primary production is th e source oflarge quantiti es o f dead organic matter settling to the bottom of the Lake. Algae, zooplankton and higher forms of aquati c life w hen they die settle at the bottom of the Lake, w h ere they decompose consumin g all available oxygen. This, in co mbination with the stratification of th e Lake, wh ich prevents oxygen tra nsfer to th e hypo limnion, leads to anox ic conditions below the pycnocline, w hi ch acts as a barrier to verti cal 72 71.8 716 71~ 712 71 708 oxygen transport. LOnQ•ude("W] T he en vironmental condition of the Lake has also bee n affected Figure 1. Map of Maracaibo system showing Lake by sewage and indu s tri a l Maracaibo, Maracaibo Strait, Tablazo Bay, Zapara discharges, which cause loca lly and Canonera Mouths, and the Gulf of Venezue la. u nsanitary cond i t i ons an d Also indicated are the location of the dredged co ntribute to the n utrient load of shipping lane and Maracaibo City. th e Lake. H owever, the bul k of nu t ri e nts comin g into Lake nutrient loading of the La ke, as we!J as Maracaibo is from the rivers flow ing into various pol lution discharges. the Lake. The nitrogen and phosph o rous The focus of most of the remediation in the river inflows is genera!Jy in particschemes proposed over the last forty years, ulate form. In their particulate forms, from has been the reduction of salt- water river inflows and as organic detritus fron, intrusion into the Lake. Such sc hem es primary production, n utrients enter the ranged from the constructio n of various saline hypolim ni on through sed imenengineered structures designed to block tation. Within th e hypo lim n io n bacteria sa lt-water intrusion by isolating the Lake break down the organic nitrogen and system from the Gulf of Venezuela, to the p hosphorus forming ammo nium and abandonment of th e dredged navigation dissolved reactive phosphorus resp ecti vely. chan nel and retu rn of the system to its The persistent an oxic conditions in the natural condition. Such schem es had two hypolinm ion lead to the den itrifi cation of general goa ls: a) reduce the density stratnitrates and the release of phosphate from th e sedim ents into the water co lumn. Some of the e n v ironm en tal 11 13 probl ems of the Lake have quite obvious soluti ons, like for example !:10 po int source control and watershed },. management to reduce the of nutrient '8 and BOD load coming into the La ke, and the co ntrol of toxic pollutant discharges into the Lake system. The poten ti al e n v ironme ntal restoration measures can be ca n be :[10 classified as : J,. • en gineering opt ion s in vo lvin g C) '8 physical changes in the Lake system through changes in its bathymetry and the construction of special works. Th e main obj ecti ve of the engineering options is to reduce the salin ity of the Lake, an d by redu cing its density stratFigure 2. F-Probe salinity dat a in Lake ification affect water quality. Maracaibo; a) transect from NW-SE on 24/11/98; b) Transect from NE-SW on • management options that include 24/11/98; c) transect from NW-SE on TE po int so urce control and wa tershed 25/11/98; d) Transect from NE-SW on manageme nt. The m a na ge m e nt optio ns aim primarily at red uci ng the 25/11/98 .
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WATER MARCH 2001
ification of the Lake, therefore potentially eliminating the anoxic conditions in the hypolimn io n, and b) reduce the salinity of the surface waters enough to allow their use for irrigation. Many of these salinity reductio n schemes proposed over th e years require the construction o f significan t infrastructure.
Hydrodynamic field program One of th e first tasks for CWR was to des ign a field program to provide an understandin g of the major physical processes that affect the water quality of Lake Maracaibo and to provide sufficient data for the ca libratio n and vali datio n of numerical models. Fieldwork was carried out during two separate field campaigns in 1998-99, timed to co rrespond with the wet and dry seasons of the region. PhD student Bernard Laval trave!Jed to Venezuela to deploy the CWR fine-sca le (fprobe) and mi cro-scale (Self-Contained Autonom ous Mi cro-Profiler - SCAMP) conductivity an d temperature profilers. M easurements made by other members o f the project tea m included current velocities; conductivity and temperature time-series at fixed depths; w ind speed and direction ; and water level. A water quality monitor ing campaign w as conducted at the sam e time as th e hydrodynami c ca mpai gn. Considerable historica l data ex ists on the flow an d stratification in these various regions, but th ese data m erely suggest a set of dynamics. Th e data were all co llected for a diverse set of programs over nearly a 40-year p eriod and as such form a valuable reso urce, esp eciall y for long term trends of salinity. However, these data are not co herent enou gh , nor were they co!Jcctcd with particu lar hypotheses in mind, to allow detailed dynamical interpretations in any of th e key h ydro d yna mi c pro cesses th at determine the salinity distribution in Lake M aracaibo or to validate the numerical models of these pro cesses. The fie ld program was thus designed to serve several pu rposes: (a) to provide an understanding of the maj or processes that affe ct th e salinization , stratificatio n and renewal of hypo l imnetic water o f L ake M ara caibo, (b) to provide su fficient data for the calibration and validation o f num eri cal models, and (c) to provide the modeling groups sufficient data on bou ndary and initial co ndi-