Preliminary Investigation of Tracer Gas Reaeration Method for Shallow Bays
Baker, Sarah H. and Edward R. Holley
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Accurate estimates of surface exchange rates for volatile pollutants in bays are needed to allow predictions of pollutant movement and retention time. The same types of estimates can be used to calculate reaeration rates. The tracer gas technique has been used to measure surface gas transfer rates in rivers, and to a lesser extent, in estuaries. Application of the technique to bays would be extremely useful, but it is complicated by differences in the hydrodynamics and the density stratification that can exist due to fresh river water overriding heavier saline ocean water. The objective of this research has been to investigate field procedures for application of the tracer gas technique to shallow bays. The modified tracer technique was used with propane for the tracer gas and Rhodamine-WT, a fluorescent dye, for the "conservative" tracer. The propane was injected through porous tile diffusers, and the dye was released simultaneously. The propane acts as a model for the surface exchange of other gases and volatile compounds. Three four-day field trips to Lavaca Bay on the Texas coast of the Gulf of Mexico were made during the course of the study. A variety of experimental techniques was investigated. One was to make a short-duration injection (10-30 minutes) and sample the dye cloud as it moved through the bay. Another was to use a long-duration injection (3 to 4 hours) to obtain quasi-steady conditions. The long injections were discontinued because there appeared to be no practical method of determining the travel time for the samples taken from the tracer plume. Drogues (floats) which are normally used for this purpose consistently drifted downwind from the tracer plumes. Pluses of a second fluorescent dye for determining time of travel became too diffuse to be used for this purpose. The second most promising method appears to be the short-duration injection method with a large pulse of the second dye released during the injection to mark the middle of the tracer cloud. A special injection device was designed to prevent mixing of the tracers with heavier saline water near the bed of the bay during injecting. A field fluorometer was used both in the field to track the tracer cloud and in the laboratory to measure dye concentrations in the field samples. Propane concentrations were determined with gas chromatography. The same equipment was used to perform laboratory studies to test performance of the equipment in the field and to aid in understanding field test results. These were apparently the first tests to be performed using the tracer gas technique in bays. As a result, the emphasis was on the development of techniques rather than on obtaining data. It appears that workable techniques have been developed, but they now need to be tested under a variety of conditions. Also, the preliminary results which were obtained for gas transfer coefficients indicate some anomalous results. After the completion of the project, it was learned that the addition of formalin to the field tracer gas samples may be affecting the samples adversely. Thus, some laboratory tests are now needed to investigate the behavior of tracer gases in bay water. In summary, the method appears to be very promising, but some additional developmental work is required before it can be used on a routine basis.