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Journal of Environmental Quality Abstract - Surface Water Quality

Total Phosphorus Concentration Trends in 40 Iowa Rivers, 1999 to 2013


This article in JEQ

  1. Vol. 45 No. 4, p. 1351-1358
    Received: July 16, 2015
    Accepted: Feb 08, 2016
    Published: July 7, 2016

    * Corresponding author(s): keith-schilling@uiowa.edu
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  1. Chao Wanga,
  2. Kung-Sik Chana and
  3. Keith E. Schilling *b
  1. a Dep. of Statistics and Actuarial Science, Univ. of Iowa, Iowa City, IA 52242
    b Iowa Geological Survey, Univ. of Iowa, Iowa City, IA 52242
Core Ideas:
  • Phosphorus concentrations were assessed at 40 Iowa rivers.
  • Phosphorus concentrations were decreasing by 4.1% at 12 sites.
  • Across Iowa, P concentrations are declining annually by 2.6%.
  • Reductions are most likely attributable to conservation.


Excessive phosphorus (P) in rivers is prompting states to develop strategies to reduce P concentrations and export. The goal of this study was to assess the current condition by analyzing trends in total P (TP) concentrations at 40 river monitoring sites in Iowa for the period 1998 to 2013. We used monthly river monitoring data collected by the State of Iowa at ambient sites located beyond the influence of point sources or cities. Study objectives were to assess the presence of any linear trends in the TP concentration data using a time-series method that accounted for temporal correlation in the data and discharge and to combine the trend information from individual sites into an assessment of the statewide rate of change in river TP concentrations. Results indicated that annual TP concentrations were significantly decreasing at 12 sites (p < 0.05), with concentrations ranging from 7.5% (Maple River) to 2.6% (Boyer River) and averaging 4.1% for the 12 sites. No statistically significant change was measured at 28 sites (p > 0.05), although all but four of these sites had decreasing concentrations. As a population of sampled rivers across Iowa, TP concentrations were declining at an annual rate of approximately 2.6%. The decreasing trends are consistent with reported reductions in sediment concentrations measured in the Raccoon River and with decreasing trends detected in other midwestern rivers. We attribute the decreasing TP trends primarily to improvements in conservation and land management. Although reducing nonpoint source TP concentrations in Iowa rivers will be a tremendous challenge for the agricultural community, progress is being made toward meeting this goal with existing programs.

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