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Regional patterns of total nitrogen concentrations in the National Rivers and Streams Assessment
Citation:
Omernik, J., Steve Paulsen, M. Weber, AND G. Griffith. Regional patterns of total nitrogen concentrations in the National Rivers and Streams Assessment. JOURNAL OF SOIL AND WATER CONSERVATION. Soil and Water Conservation Society, 71(3):167-181, (2016).
Impact/Purpose:
The purpose of this manuscript is to use nutrient data from EPA's Office of Water's national surveys to explore patterns of nutrient distribution and their relationship to ecoregions. It contains no information that will impact policies of EPA.
Description:
Patterns of nitrogen concentrations in streams sampled by the National Rivers and Streams Assessment (NRSA) were examined semi-quantitatively to identify regional differences in stream nitrogen levels. The data were categorized and analyzed by watershed size classes to reveal patterns of the concentrations that are consistent with the spatial homogeneity in natural and anthropogenic characteristics associated with regional differences in nitrogen levels. Ecoregions and mapped information on human activities including agricultural practices were used to determine the resultant regions. Marked differences in nitrogen levels were found among the nine aggregations of ecoregions used to report the results of the NRSA. We identified distinct regional patterns of stream nitrogen concentrations within the reporting regions that are associated with the characteristics of specific Level III ecoregions, groups of Level III ecoregions, groups of Level IV ecoregions, certain geographic characteristics within ecoregions, and/or particular watershed size classes. We described each of these regions and illustrated their areal extent and median and range in nitrogen concentrations. Understanding the spatial variability of nutrient concentrations in flowing waters and the apparent contributions that human and non-human factors have on different sizes of streams and rivers is critical to the development of effective water quality assessment and management plans. This semi-quantitative analysis is also intended to identify areas within which more detailed quantitative work can be conducted to determine specific regional factors associated with variations in stream nitrogen concentrations.
