NUTRIENT CRITERIA DEVELOPMENT WITH A LINKED MODELING SYSTEM: WATERSHED AND ECOLOGICAL MODEL APPLICATION AND LINKAGE

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. The criteria were based on assessing “natural” background and “cultural” eutrophication in 14 “ecoregions” in the country. Within ecoregions EPA developed criteria to reflect average reference conditions (i.e., conditions that describe the characteristics of waterbody segments least impacted by human activities) for each of three waterbody types (rivers and streams, lakes and reservoirs, wetlands).Agency guidance documents encourage states and tribes to develop nutrient criteria that recognize the ecoregion-based criteria as a starting point, and subsequently refine the values by using additional scientifically defensible methods and data. This paper describes an approach that incorporates three of EPA's recommended elements for refining nutrient criteria (i.e., historical data, use of predictive models, expert judgment). The demonstration methodology uses readily available types of data (e.g., meteorological, land use, topography, streamflow) in conjunction with a linked mechanistic modeling system that includes a watershed model and an ecological effects model.The demonstration was performed using the watershed model HSPF and the aquatic ecosystem model AQUATOX, which are both components of EPA's BASINS watershed and waterbody assessment package. To provide the foundation of the demonstration study, the linked watershed/ecological effects modeling system was applied to two large Minnesota watersheds (Blue Earth River, Crow Wing River), each of which is representative of a different nutrient ecoregion. HSPF was used to estimate sediment, nutrient, dissolved oxygen and BOD loadings based on watershed characteristics and land use practices. AQUATOX was used to estimate instream aquatic nutrient concentrations and to relate these concentrations to the concentrations of “response variables” (e.g. chlorophyll-a).A second objective of the demonstration study was to use the linked system to assess the attainability of standards derived from 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 might be used to define standards.This paper discusses the watershed model development and application efforts for both the Blue Earth and Crow Wing watersheds, along with the model results and linkage procedures to AQUATOX. Separate companion papers present the overall project methodology and demonstration study results (Carleton et al., 2005) and the calibration and validation efforts for AQUATOX under alternative nutrient conditions (Park et al., 2005).