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Development of a deterministic and stochastic computer model for phosphorus retention in a surface flow wetland system
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Constructed and natural wetlands have been used to treat wastewater from both point and nonjoint sources. Phosphorus concentration in receiving waters is a concern due to the contribution of phosphorus to lake eutrophication and also to a regulatory framework being considered by the U.S Environmental Protection Agency for development of nutrient criteria. Few models have been developed to describe the function of wetland systems, and only some of these models have been applied to modeling phosphorus retention in wetlands. A computer-based model of moderate complexity describing phosphorus retention, as total phosphorus and orthophosphorus, was developed utilizing several commonly available parameters. This model was based on the Vollenweider equation, and was developed as three submodels. A submodel was developed for each of the wetland components important to phosphorus dynamics in wetlands: hydrology, soils, and macrophytes. Each submodel consisted of a mass balance for the physical process in the submodel (water, soil, and biomass) and a mass balance for phosphorus. A Monte Carlo simulation was conducted for cell 1 of the Stephenville system to quantify variability of the system. The model was calibrated and validated using two wetland systems: the Stephenville Wetlands System in Stephenville, Texas, and Boney Marsh Experimental Area in Highland County, Florida. The hydrologic submodel accurately predicted the effluent volume of cells receiving a low flow rate at the Stephenville site. For these cells, effluent phosphorus concentration, as total phosphorus and orthophosphorus, was also predicted accurately. Prediction of effluent volume for Stephenville cells receiving a high flow rate was much less accurate. Predictions of effluent phosphorus concentrations were accordingly inaccurate. The model was accurate in simulating flow conditions at BMEA, where flow volume was low relative to wetland volume. Prediction of total phosphorus effluent concentration was less accurate for BMEA than for Stephenville. The model was highly dependent on hydrology of the system. Prediction of phosphorus retention was sensitive to estimation of macrophage biomass and macrophyte phosphorus uptake rate. The Monte Carlo analysis was conducted with one simulation of 1000 iterations. The results of this analysis indicated process uncertainty of the system. Based on a summary of distribution of the complementary cumulative distribution functions, results indicate high probability of estimating effluent phosphorus concentrations comparable to those observed.
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Includes bibliographical references (leaves 80-84).
Issued also on microfiche from Lange Micrographics.
Verma, Rahul (1999). Development of a deterministic and stochastic computer model for phosphorus retention in a surface flow wetland system. Master's thesis, Texas A&M University. Available electronically from
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