NUTRIENT CRITERIA DEVELOPMENT WITH A LINKED MODELING SYSTEM: METHODOLOGY DEVELOPMENT AND DEMONSTRATION
Abstract:Nutrients (nitrogen and phosphorus) are leading causes of water quality impairment in the Nation's rivers, lakes and estuaries. To address this problem, states need the technical resources to establish nutrient criteria, adopt them into their water quality standards, and implement them in regulatory programs. In recent years EPA developed guidance documents and a series of nutrient criteria recommendation documents to assist the states in adopting nutrient standards. Unlike most water quality criteria, the nutrient criteria were not based on finding cause and effect relations between pollutant levels and adverse water conditions. Rather, the criteria were based on assessing natural background and cultural eutrophication in 14 ecoregions in the country. The criteria documents established nutrient criteria using percentiles from distributions of monitored nutrient concentrations, designed to reflect reference conditions in each waterbody type (rivers and streams, lakes and reservoirs, wetlands) in each ecoregion. However, as specified in the guidance documents, states and tribes have the option of developing nutrient criteria using other scientifically defensible methods and data. These may come into play, for example, in instances where there is neither a high volume of local data to support an empirical approach, nor an opportunity or justification for applying data and data relationships collected at another similar location.
This paper reports on an example of an approach to developing nutrient criteria that requires less aquatic and biological monitoring data than an empirical approach; instead, relatively minimal data is used in conjunction with a linked mechanistic modeling system that includes a watershed model and an ecological effects model. The approach is illustrated in a demonstration project that uses the watershed model HSPF and the aquatic ecosystem model AQUATOX, which are both part of EPA's BASINS 3.1 (U.S. EPA, 2004) package. AQUATOX is used to link aquatic nutrient concentrations with concentrations of “response variables” (chlorophyll-a, water clarity), and HSPF is used in turn to link land use practices with nutrient concentrations. The demonstration project, developed in partnership between EPA and the Minnesota Pollution Control Agency (MPCA), is the first of what may be several geographically diverse projects developed to illustrate the utility of models for developing nutrient criteria in different parts of the country. In this paper the approach is discussed, with an emphasis on overall project methodology and study results. An example use of the approach for numeric nutrient water quality criteria development is illustrated. Potential synergies with other, related efforts, such as watershed vulnerability classification, regression-based analyses of stressor-response relationships, and tie-ins with related water quality-related issues, such as Use Attainability Analyses (UAAs) and TMDLs, are discussed. Separate companion papers present the ecological model calibration and validation efforts under alternative nutrient conditions, and the watershed model development and application efforts for two Minnesota watersheds.
Document Type: Research Article
Publication date: January 1, 2005
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