A PROCESS LEVEL MODELING APPROACH TO IMPLEMENTATION OF NUMERIC NUTRIENT CRITERIA IN WATER QUALITY STANDARDS
(EPA Contract No. 68-C-01-0037, Work Assignment 3-19)
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Client:U.S. EPA Office of Water, Office of Science and Technology, Washington, DC
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.
Between 1998 and 2003 EPA developed and finalized a series of nutrient criteria documents to assist the states in adopting nutrient standards. Unlike most water quality criteria, the U.S. EPA 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. In establishing ecoregion-based criteria, EPA recommends two methods, using different percentile distributions of 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 Agency guidance documents, states and tribes have the option of developing nutrient criteria using other scientifically defensible methods and data. For example, the Minnesota Pollution Control Agency (MPCA), with funding from a U.S. EPA grant, found significant and predictable relationships among nutrients, algae, and biological oxygen demand in five medium to large rivers that had been the focus of an intensive monitoring program. These relationships will help the state establish nutrient standards and total maximum daily load (TMDL) allocations in rivers with characteristics similar to those included in the study.
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, states have a need for an alternative approach to establishing nutrient criteria.
In this study EPA, AQUA TERRA Consultants and Eco Modeling jointly investigated an approach to developing nutrient criteria that requires less aquatic and biological monitoring data than an empirical approach; instead relatively minimal data were used in conjunction with a linked mechanistic modeling system that included a watershed model and an ecological effects model. The approach was illustrated in a demonstration project that used the watershed model HSPF and the aquatic ecosystem model AQUATOX, which are both part of EPA’s BASINS package. AQUATOX was used to link aquatic nutrient concentrations with concentrations of “response variables” (chlorophyll-a, water clarity), and HSPF was 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), was the first of several geographically diverse projects developed to illustrate the utility of models for developing nutrient criteria in different parts of the country.
To provide the foundation of the demonstration study, AQUA TERRA and its subcontractor Eco Modeling applied the linked watershed/ecological effects modeling system to two large Minnesota watersheds, each of which is representative of a different ecoregion. The BlueEarthRiver watershed wasmodeled to represent Level III Ecoregion VI (Corn Belt and Northern Great Plains), and the CrowRiver watershed was modeled to represent the Level III Ecoregion VIII (NutrientPoorNorthernLakes and Forests, and the NorthCentralHardwoodsForests).
A second objective of the demonstration study was to use the linked system to assess the attainability of standards based on the model-based nutrient criteria. This was accomplished by modeling selected permit and land management scenarios with HSPF to estimate modified nutrient loadings, then using AQUATOX to estimate the effects on nutrient concentrations and other response parameters which could be used to define standards. The results of the investigation were reported as an EPA case study report.
EPA is exploring potential synergies between the methodology and results of the current demonstration study and 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.