Hydrologic and Chemical Controls of Water Quality in the Lower Missouri River

Runoff from agricultural watersheds remains a critical concern of stream water quality in the Midwest. Using NASQAN data collected by USGS for the past 15 years, hydrologic versus chemical controls of stream water quality were examined for the Lower Missouri River above Hermann, MO. Mean DO values were 10.7-9.2 mg L-1 from upstream at Yankton, SD, and Omaha, IA, to downstream at Hermann, and also at Louisville of Platte River, a major tributary converged right after Omaha. Mean atrazine concentrations were 0.04 and 0.49 μg L-1 at Yankton and Omaha, respectively, and significantly increased to 1.08 μg L-1 at Hermann. Increased atrazine at Hermann was partially due to contributions from Platte River, which has mean atrazine concentration of 1.66 μg L-1 at Louisville. Mean atrazine concentration contributed by all agricultural watersheds between Omaha and Hermann was also comparable to Platte River at 1.30 μg L-1, as calculated based on the mass balance of flow and atrazine concentrations. There were no temporal trends from 1996 to 2009 for DO, nitrate and atrazine at all of the above locations. Seasonality of DO, nitrate and atrazine concentrations was very strong at all stream locations but Yankton, with higher DO concentrations in winter and higher nitrate and atrazine concentrations in late spring and early summer. Concentrations of DO, nitrate and atrazine were not highly correlated with both stream discharge and electric conductivity, but high values of nitrate and atrazine mostly occurred at low to moderate stream discharge. This result suggests that the management of agricultural watersheds in the Midwest needs to consider not only significant storms for flood control but also moderate storms for safeguarding surface water quality, particularly in late spring and early summer.