Pd/a Crsp Sixteenth Annual Technical Report

This report summarizes progress on a project to analyze water quality and hydromechanical data from the channel estuaries of the Gulf of Fonseca in Honduras and to develop suitable water quality models. The overall objective is to perform quantitative computation of the carrying capacity of the estuarine system for a viable shrimp farming industry. The study addresses Estero el Pedregal and Estero San Bernardo, which are typical of many of the river-channel estuaries within the larger Gulf of Fonseca along which shrimp farming is being developed. It has been established that hydrographic conditions in the regions in which shrimp farming operates, including tidal range and period, freshwater throughflow, physiography and morphology (especially the role of tidal flats), and tidal currents and related parameters—mixing and dispersion—are at least as important as chemical quality in the effects of farm effluent on estuary quality. Models are under development for tidal hydrodynamics, salinity, and dissolved oxygen. The lack of good field data has been the principal impediment to modeling these systems in the past, and the database has been considerably improved in the past several years due to the efforts of CRSP researchers, the Peace Corps, and concerned individuals and shrimp farms in the region. by the geographical complexity of the watercourse and how variable that watercourse is in time. Estuaries in general, and those debouching into the Gulf of Fonseca in particular, are geometrically complex. Most of the Fonseca shrimp farms are located on dendritic, confluent channel estuaries that drain a mangrove-fringed tidal delta (see Wolanski, 1993). Temporal complexity is induced by both the high tidal exchange of the region and the variability in inflow due to the tropical Pacific climatology. The present study concentrates on two of the more important channel estuaries, Estero Pedregal and Estero San Bernardo, both in the eastern arm of the Gulf of Fonseca. METHODS AND MATERIALS Evaluation of assimilative capacity requires a suitable mathematical model to determine the concentrations of key parameters that result from a given level of wasteloading. In any watercourse, the concentration of a constituent is governed by transport processes (including mixing) and kinetic processes, so a model of that constituent must include a determination of hydrodynamic transports as well as a mass balance of the water quality constituents. In an estuary, assimilative capacity is a strong function of position. There will be areas in almost any estuary that are well-circulated and subject to regular water-mass replacement, which will generally have a high assimilative capacity. There will also be areas that are poorly circulated with frequent stagnation (dead zones), which will have a low assimilative capacity. The location of the hypothetical wasteload relative to wellcirculated or poorly circulated zones, and relative to other existing wasteloads, is important to the ability of the estuary to assimilate that wasteload. Moreover, the distribution of critical regions of the estuary and the associated assimilative capacity will vary with hydrodynamic conditions. Therefore, a model is needed of space-time distribution of the concentration of controlling parameters in the estuary. SIXTEENTH ANNUAL TECHNICAL REPORT 116 While the magnitude and geographical distribution of the mass influxes of contaminants are clearly an important control, an equally important control is the hydrodynamic capacity of the estuary for dilution and transport. For an estuary, the complex geometry and complicated hydrodynamics make data analysis and model formulation especially difficult. This is why the hydrography of an estuary must be understood in order to evaluate its water quality. In other words, the hydrography of the estuary dictates the relation between mass loads of contaminants and the severity of the resulting water quality decline. An action that alters either the hydrography or the waste loading has the potential of altering water quality. Shrimp farming can do either. The work in the present project has proceeded along two separate but related directions. First is the acquisition, compilation, and analysis of data relating to the hydrography and water quality of the study estuaries. Second is the formulation and application of mathematical models. Data collection in the study area has been underway for several years, and most of these data pertain to the operation and metabolism of the shrimp ponds themselves. Estuary water was sampled at the intakes to the pond operations. In conjunction with the present study, data collection has been extended into the estuary channels. Methods and procedures of data collection activities for this phase of the work are described in Green et al. (1999). (In the present context, only those data sets being used in the analysis and modeling work are discussed.) Data analysis is based upon closing volume and mass balances for subsections of the study estuaries on various time and space scales (see Ward and Montague, 1996, and references therein). In most cases, this has been carried out in conjunction with model development. Two models are presently being applied to the study estuaries. The first is a section-mean tidal hydrodynamic model (Dronkers, 1964; Vreugdenhil, 1989). This model is a numerical solution to the differential equations of momentum and continuity. The value of this model is that it provides a means to compute the tidal currents in the estuary based upon the tidal stage, which is measured or predicted at the mouth of the estuary. The currents, rather than tide stage, are really the important hydrographic feature, since it is the currents that are responsible for transport and tidal dispersion. The numerical solution is implemented in a program designed for operation on a DOS-based personal computer (PC). The second model is a section-mean longitudinal mass budget model for the concentration of a substance along the axis of the estuary (Ward and Espey, 1971). The large-scale tidal-mean distribution of waterborne substances such as salinity, dissolved oxygen (DO), and nutrients varies on time scales of days to weeks, so this model is designed to depict these slower, long-term established concentration profiles. Again, the model is a numerical solution to the governing differential equations implemented in a code for operation on a PC platform. Preliminary development and application of both of these models were made for the Pedregal system, presented in Ward (1995), and summarized in the following section. While these applications were illuminating, practically no field data from the systems existed at that time, and guesses had to be made for fundamental input data such as water depths, crosssectional areas, zones of tidal inundation, salinity dispersion, and inflow. Moreover, the only water quality data extant for validating the models were those analyzed from intake samples at the shrimp farms, through a cooperative program between the shrimp farmers, Honduran agencies, and the CRSP (Teichert-Coddington, 1995). A prime objective of the present project is to incorporate field data from the study estuaries into the model development and application. PROJECT ACCOMPLISHMENTS AND RESULTS The systems given specific study in this project are the Pedregal and San Bernardo, two of several channel estuaries draining into the eastern arm of the Gulf of Fonseca. An important geometric feature of each of these systems is its declining cross-sectional area with distance upstream. These are horn-shaped estuaries, whose channels have a longitudinally diminishing capacity for flow, as well as an increasing resistance to flow. Another important geometric feature is the large tidal flats adjacent to the estuary, which communicate with the main tidal channel through small scoured tidal passes through the mangrove fringe. In such systems, tidal flats have the capacity to store a great amount of water on the rising tide, and release that water back to the tidal channel as the tide stage falls. The resulting tidal prism is much larger than would be anticipated based solely upon the cross section of the tidal channel. Shrimp farming concessions have been granted extending over 30 km up the length of both systems, though shrimp farm development is presently limited to about the first 20 km. These shrimp farms eliminate the tidal flats, hydraulically isolating these areas by enclosure within levees to create the shrimp ponds. The farms exchange water between pond and estuary on each tidal cycle (12.4-hr period). For modeling purposes, data on actual producing-pond areas as of 1994 for the larger operations were compiled.