FIELD AND NUMERICAL STUDIES OF TRACER GAS TRANSPORT AND SURFACE GAS TRANSFER IN LATERALLY UNIFORM, PARTIALLY STRATIFIED ESTUARIES

Techniques for determination of reaeration rates in natural waterbodies are reviewed. The tracer gas technique for reaeration rate determination offers many advantages over other existing methods and is widely used in rivers and streams. The tracer gas method seems to be the most promising of available techniques for estuarine reaeration rate determination. The two-dimensional late-rally averaged equations describing flow and transport in estuaries are derived and discussed.;Field studies conducted as a part of the study are described. Two dye releases were made in the upper Houston Ship Channel; two dye and tracer gas releases were later made in the same region. The data from these studies are presented and analyzed. Mechanical mixing by shipping traffic proved to be the predominant mixing mechanism and a hindrance to further studies at that site. An intensive field study was conducted in the Colorado River estuary. Field data included velocities, salinity profiles, water surface elevations, and dye concentration data from three dye releases. The data from this study are used to calibrate and test the numerical model of estuarine tracer gas transport. An instantaneous and a continuous release of tracer gas are simulated, and the surface transfer rate coefficient is calculated from simulated data. Procedures for implementing the technique in estuaries are discussed relative to data analysis procedures.;A laterally averaged numerical model of estuarine hydrodynamics and transport is modified so that tracer gas releases may be simulated. Modifications include application of the fractional step technique and a sophisticated interpolation scheme to reduce numerical dispersion, implementation of a variable vertical grid, and inclusion of a surface exchange algorithm. Procedures are outlined for two-step application of the model to long reaches so that longitudinal discretization of the reach may be coarse except near the tracer gas cloud. The model is applied to a hypothetical estuary (and to an actual estuary as summarized below) to test tracer gas field and data analysis procedures. Three methods of analyzing the surface transfer rate are evaluated--a method based on using the numerical model and field data to fit a transfer coefficient to the data, a method based on the temporal rate of mass change, and an analytical solution.