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Modeled summer background concentration nutrients and suspended sediment in the mid-continent (USA) great rivers
Citation:
Angradi, T., Dave Bolgrien, M. Starry, AND B. Hill. Modeled summer background concentration nutrients and suspended sediment in the mid-continent (USA) great rivers. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION. American Water Resources Association, Middleburg, VA, 48(5):1054-1070, (2012).
Impact/Purpose:
Our model-predicted concentrations for the mid-continent great rivers are a first attempt to estimate background concentrations for water quality indicators independent from thresholds based on percentiles or derived from stressor-response relationships. The estimated background concentrations can be used by tribes, states, and state consortia as lines of evidence in developing numeric water quality criteria for approved designated uses.
Description:
We used regression models to predict background concentration of four water quality indictors: total nitrogen (N), total phosphorus (P), chloride, and total suspended solids (TSS), in the mid-continent (USA) great rivers, the Upper Mississippi, the Lower Missouri, and the Ohio. From best-model linear regressions of water quality indicators with land use and other stressor variables, we determined the concentration of the indicators when the land use and stressor variables were all set to zero the y-intercept. Except for total P on the Upper Mississippi River and chloride on the Ohio River, we were able to predict background concentration from significant regression models. In every model with more than one predictor variable, the model included at least one variable representing agricultural land use and one variable representing development. Predicted background concentration of total N was the same on the Upper Mississippi and Lower Missouri rivers (350 ug l-1), which was much lower than a published eutrophication threshold and percentile-based thresholds (25th percentile of concentration at all sites in the population) but was similar to a threshold derived from the response of sestonic chlorophyll a to great river total N concentration. Background concentration of total P on the Lower Missouri (53 ug l-1) was also lower than published and percentile-based thresholds. Background TSS concentration was higher on the Lower Missouri (30 mg l-1) than the other rivers. Background TSS concentration on the Upper Mississippi River (13 mg l-1) was well below a threshold (30 mg l-1) designed to protect submerged aquatic vegetation in that river. Of the water quality indicators examined, the background concentrations of chloride (1119 mg l-1) were the most similar to current ambient concentration suggesting relatively minor increases from anthropogenic sources. Relevance of predicted background concentrations for great-river water quality criteria is discussed.
