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To protect and manage rivers and streams (hereafter, collectively referred to as streams) in the United States, the U.S. Environmental Protection Agency (USEPA) is establishing regionally based nutrient criteria that reflect the natural variability in water quality. As a basic approach to establish these criteria, the USEPA has divided the country into nutrient ecoregions (delineated on the basis of natural and anthropogenic factors) to minimize variability within regions and maximize variability among regions. The USEPA has allowed states and tribes flexibility to modify or improve on this basic approach. As part of activities of a Regional Technical Assistance Group, whose role it is to examine and refine this basic approach, an alternative regionalization scheme was developed for the Upper Midwest. In this refined approach, the relative importance of various environmental characteristics affecting nutrient concentrations are determined by use of regression-tree analysis. The area is then subdivided into relatively homogeneous areas called "environmental nutrient zones" on the basis of distributions of only the most statistically significant environmental characteristics.
On the basis of data from 234 sites, the most statistically significant environmental characteristics affecting nutrient concentrations were the percentage of agriculture (or absence of forest) and factors describing the climate and geology in the watershed. Environmental nutrient zones were then delineated that incorporated distributions in land use (similar to the ecoregion approach) and also delineated with land-use information excluded so the criteria should reflect only the naturally occurring variability in water quality. With the environmental nutrient zone stratification scheme, the variability in total phosphorus concentrations among zones was reduced by approximately 50 percent compared to that among nutrient ecoregions, whereas the variability in total nitrogen concentrations was reduced only slightly. Frequency distributions of data from each zone were then used to define the potential water quality of each zone.
The environmental nutrient zone approach can be applied to specific states or nutrient ecoregions and used to develop criteria as a function of stream type. This approach can also be applied on the basis of environmental characteristics of the watershed alone rather than the general environmental characteristics from the region in which the site is located. The environmental nutrient zone approach will enable states to refine the basic nutrient criteria established by the USEPA by developing attainable criteria given the environmental characteristics where the streams are located.