Combining sediment fingerprinting with age-dating sediment using fallout radionuclides for an agricultural stream, Walnut Creek, Iowa, USA

Authors: GELLIS, ALLEN - Us Geological Survey (USGS); FULLER, CHRISTOPHER - Us Geological Survey (USGS); VAN METRE, PETER - Us Geological Survey (USGS); FILSTRUP, CHRISTOPHER - University Of Minnesota; Tomer, Mark; Cole, Kevin; SABITOV, TIMUR - Us Geological Survey (USGS)

Submitted to: Journal of Soils and Sediments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2018
Publication Date: 11/20/2018
Citation: Gellis, A.C., Fuller, C.C., Van Metre, P.C., Filstrup, C.T., Tomer, M.D., Cole, K.J., Sabitov, T. 2018. Combining sediment fingerprinting with age-dating sediment using fallout radionuclides for an agricultural stream, Walnut Creek, Iowa, USA. Journal of Soils and Sediments. p. 1-23. https://doi.org/10.1007/s11368-018-2168-z.
DOI: https://doi.org/10.1007/s11368-018-2168-z

Interpretive Summary: Determining the sources and ages of suspended sediment in streams can improve understanding of sediment transport from agricultural watersheds, and may allow managers to determine the time scales needed to reduce sediment loading. During the summer of 2013 in Walnut Creek, Iowa, sources of fine-grained sediment were apportioned among croplands, grasslands, unpaved roads, and channel banks. Naturally occurring radionuclides were used to determine these sources of sediment and their ages. Most (62%) of the fine-grained suspended sediment was from cropland, with streambanks contributing 36% and other sources summing to about 2%. The sediment sourced from cropland was of mixed age, ranging from <1 year to 58 years. But a large component of sediment originated along the channel, and may comprise nearly 100% of the annual sediment load in some years. Eroded topsoil and streambank sediments are conveyed to the watershed outlet at three time scales: millennial/geologic time, a decadal time scale, and recent time (<1 year). This research complements a multi-year and ongoing effort to document streambank erosion in this watershed. While conservationists and watershed managers are the major audience for this research, it is also important for the general public and policy makers to understand the long-term challenge involved in undertaking watershed improvement efforts.

Technical Abstract: Determining the sources and age of suspended sediment and sediment in channel storage is important for understanding the transfer of sediment through the watershed, and allowing managers to determine effective time scales that are needed to reduce sediment loading. In the agricultural Walnut Creek watershed, Iowa, the sediment-fingerprinting approach was used to apportion the sources of fine-grained sediment (croplands, grasslands, unpaved roads, and channel banks). Fallout radionuclides (7Be, 137Cs, 210 Pbex) were used to age the top-soil-derived portion of suspended sediment. 210Pbex can date sediment to ~85 years and 7 Be to ~ 1year, and 137 Cs is a time marker of sediment that was on the land surface between about 1954-74. Results indicate that the majority of suspended sediment is derived from cropland (62%) with streambanks contributing 36%, and prairie, pasture, and unpaved roads each contributing <=1%. The top-soil-derived portion of sediment (primarily cropland) using 210 Pbex has ages ranging from 1 to 58 years, and using 7 Be, has ages ranging from 23 to 202 days. The apparent difference in ages indicates that the sediments are of mixed age. Literature values support that a large component of sediment is stored in the channel bed, which can range to 100% of the annual suspended-sediment load (16,000 Mg/yr). We propose a geomorphic model where top soil is delivered to the channel system and subsequently conveyed to the watershed outlet along with channel bank sediment at three time scales: a geologic-millennial time scale, a time scale of decades, and young time scale (<1 year).