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

Purpose

The main purpose of this study was to demonstrate the utility of the sediment fingerprinting approach to apportion surface-derived sediment, and then age date that portion using short-lived fallout radionuclides. In systems where a large mass of mobile sediment is in channel storage, age dating provides an understanding of the transfer of sediment through the watershed and the time scales over which management actions to reduce sediment loadings may be effective.

Materials and methods

In the agricultural Walnut Creek watershed, Iowa, the sediment-fingerprinting approach with elemental analysis was used to apportion the sources of fine-grained sediment (croplands, prairie, unpaved roads, and channel banks). Fallout radionuclides (⁷Be, ²¹⁰Pb_ex) were used to age the portion of suspended sediment that was derived from agricultural topsoil. Age dating was performed at two different scales: ²¹⁰Pb_ex which can date sediment to ~ 85 years and ⁷Be to ~ 1 year.

Results and discussion

Sediment fingerprinting results indicated 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 topsoil–derived portion of sediment (primarily agriculture) dated using ²¹⁰Pb_ex has ages ranging from 1 to 58 years, and using ⁷Be, a component of much younger sediment that yields ages ranging from 44 to 205 days. The occurrence of ⁷Be indicates that some portion of the sediment is young, on the order of months, whereas the dating based on ²¹⁰Pb_ex indicates that some of the surface-derived sediment has been in channel storage for decades. Published studies in Walnut Creek indicate that a large component of sediment is stored in the channel bed.

Conclusions

We conclude that the ²¹⁰Pb_ex-based ages are a reasonable estimate for the mean age of the surface-derived fraction and that ⁷Be activities are evidence that there is a smaller fraction of very young sediment in the stream. We propose a geomorphic model where agricultural soil is delivered to the channel and conveyed to the watershed outlet at three time scales: a geologic-millennial time scale, decades, and a young time scale (< 1 year).