SOURCE DETERMINATION AND RESIDENCE TIME INDICATIONS OF SUSPENDED SEDIMENT FOR THE MISSISSIPPI RIVER

Characterization of suspended particulates in large river systems is important and challenging. Particulates that enter the ocean from rivers are the products of integrated basin-wide processes. This study focuses on the development of a suite of proxies that give insight to the source and system-wide residence time of particulates discharged by rivers to the coastal zone. To completely characterize the suspended sediments in a major river system, regular sampling over longer periods of time is necessary. A river's suspended load is highly variable over a range of time scales; therefore high temporal resolution sampling is required. Continuous, regular sampling incorporates not only seasonal and annual change, but it also allows interpretation for differences on a sub-seasonal scale. Biweekly sampling of the Mississippi River was performed from March 2002 through December 2004.

Previous studies have used short-lived radionuclides (⁷Be, ¹³⁷Cs, and ²¹⁰Pb) to determine sediment source and residence time in small agricultural watersheds. Using the same approach in a large river system is more challenging because sources and sinks are more numerous and complex and residence times change from smaller to larger systems. As a result of these complexities, the use of short-lived radioisotopes alone was not completely diagnostic in this river system. Trends in ⁷Be and ¹³⁷Cs activities appear to be driven by mixing of old and new sediments. Therefore, both source and residence time effects control the observed radioisotope activities. Additional examinations of grain size, ²³⁴U/²³⁸U isotope activity ratios, and sediment mineralogy were done to better evaluate basin sources. Four up-basin sampling trips were also performed in Feb., Apr., July, and Nov. of 2004. The upper Mississippi, Missouri, Ohio, and Arkansas rivers were sampled each time. These end-member samples are essential to determining primary source, residence time, and are used in a semi-quantitative mass balance analysis for this large river system. When done in conjunction with basin source indicators, the mass balance approach sheds light on how the main stem of the Mississippi River influences particle retention times, long term storage, and particle alterations.