A Modeling System of Nutrient Loads to Long Island Sound from Connecticut Watersheds
Over the past 15 years, state and federal efforts have focused on improving the water quality and natural resources of Long Island Sound (LIS), with a specific emphasis on addressing the issue of hypoxia. Hypoxia, usually defined as levels of DO less than 3.0 mg/l, has been the focus of the development of a comprehensive multi-phase management plan under the LIS Study (LISS) to reduce nitrogen loads entering the Sound. It has been estimated that the load of nitrogen delivered to LIS has more than doubled since pre-colonial times, and that discharges from sewage treatment plants, atmospheric deposition, and nonpoint runoff from the LIS watershed are the primary sources of nitrogen enrichment to LIS.
In this study, funded by EPA and the Connecticut Department of Environmental Protection (CT DEP), a statewide watershed model was developed as a tool to help quantify all sources of key nutrients to LIS, using the U.S. EPA Hydrological Simulation Program - FORTRAN (HSPF) and the U.S. Geological Survey's graphical user interface GenScn. The Connecticut Watershed Model (CTWM) was developed in this effort to evaluate nutrient sources and loadings, including Total Nitrogen, Total Phosphorus, and Total Organic Carbon, within each of six nutrient management zones that lie primarily within the state of Connecticut, and assess their delivery efficiency to LIS. The CTWM evolved by first performing calibration and validation on three small test watersheds across the state (Norwalk, Quinnipiac, and Salmon) representing a range of land uses, including urban, forest, and agricultural. The model was then extended to three major river calibration basins (Farmington, Housatonic, and Quinebaug) and subsequently expanded to a statewide model by using the most spatially applicable set of calibrated watershed parameters in non-calibrated areas.
Based on urban growth rates predicted to occur by the year 2020, buildout scenarios and resulting loads to LIS were developed for the six management zones. To evaluate potential impacts of alternative best management practices (BMPs), pollutant removal efficiencies were extracted from the literature for BMP categories determined to be technically feasible and effective at removing the pollutants of concern. Composite pollutant removal efficiencies were estimated for BMPs relevant to both urban and agricultural lands. Alternative levels of BMP implementation were represented by defining increasing fractions of the urban and agricultural areas served by BMPs. Ultimately, three nonpoint source BMP implementation scenarios were developed and simulated, in addition to the buildout scenario with and without BMPs; and the associated nutrient reductions were evaluated. The user-friendly interface and framework of the CTWM was specifically designed to promote continuing use by CT DEP staff to assess additional BMPs, implementation levels, and relative impacts of point source controls for nutrient reductions to LIS.
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