SEDIMENTARY SLEUTHING: IDENTIFYING SEDIMENT SOURCES IN THE MINNESOTA RIVER

Sediment “fingerprinting” is unlikely to be reduced to a routine boilerplate process; every situation is unique and requires a different combination of geologic methods. However, tandem use of fallout and cosmogenic isotopes shows promise in tracing sediment sources in various systems.

The Minnesota River is deeply incised, and sits in a broad river basin. Total daily sediment loads (TDSL) are above EPA standards, and deposition is 5-10x higher than pre-settlement. Gauging records indicate that the majority of sediment is from the three incised southernmost tributaries. The state government shows great interest in mitigating the high sediment load, a particular advantage in studying sediment source response to remediation efforts.

The initial goal was to distinguish between sediment sourced from topsoil erosion in the flat uplands and bluff erosion in the deeply incised rivers. The lithologic variation in source material between the topmost Des Moines till and underlying old gray tills suggest the use of major and minor element composition, magnetic characterization, and mineral point counts as tracers. Pilot project work also includes sorbed nutrients, [e.g. N and P], fallout radionuclides [210Pb and 137Cs], and cosmogenic isotope ratios [26Al:10Be]. An ideal outcome would be overcharacterization of the system, and separate evaluation of the effectiveness of each of the tracers.

Recent work has revised the goal to include characterization of upland gullying, which is accelerated through tiling systems draining directly to the tributaries. Gullying complicates the sediment sourcing goal - like the topsoil, it is also Des Moines till. A combination of 26Al:10Be ratios and concentrations should help distinguish sediment sources. High ratios indicate either a soil or gullying source, low a bluff source. High concentrations indicate soil, intermediate either gullying or a mixture of soil and bluff, and low concentrations bluff. Fallout radionuclides, which should be highest in the soils, should further distinguish between soils and gully sources. A particular challenge in combining fallout and cosmogenic isotopes is the difference in grain-sizes used in the techniques (suspended vs. sand-sized) - sediment size distributions in source material must be well-characterized. Also, acquiring equivalent river and core samples can be a challenge.