Varying redox potential affects P release from stream bank sediments

Authors: RAHUTOMO, SUROSO - Iowa State University; Kovar, John; THOMPSON, MICHAEL - Iowa State University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2018
Publication Date: 12/14/2018
Citation: Rahutomo, S., Kovar, J.L., Thompson, M.L. 2018. Varying redox potential affects P release from stream bank sediments. PLoS One. 13(12). https://doi.org/10.1371/journal.pone.0209208.
DOI: https://doi.org/10.1371/journal.pone.0209208

Interpretive Summary: Stream bank erosion is one of the main sources of suspended sediments in stream water. The sediments carry phosphorus (P) that may be released to the water. Depending on the chemical properties of the sediment, P can be released to the water as oxygen levels decrease, a common occurrence in summer months. With a controlled laboratory study, we investigated P retention and release by stream sediments in relation to oxygen levels in Walnut Creek stream water. Samples representing four major types of sediments were collected from the banks of Walnut Creek in Jasper County, Iowa: Camp Creek, Roberts Creek, and Gunder members of the Holocene-age De Forest Formation and Pre-Illinoian Till, a glacial deposit that is older than 500,000 y. We measured P release from the sediments under ambient air and low oxygen conditions. Results indicated that the potential for P release from the three Holocene-age sediments was similar and increased with decreasing oxygen level in the water. During a 24-day incubation, more P was released from the Camp Creek and Roberts Creek sediments under low oxygen conditions than from the other sediments. This suggests that these two sediments, which are younger material, are more likely to be P sources when oxygen levels in the stream water are lower. The capacity to retain P was greatest in the till, the oldest material. Where it is in contact with the stream water, the till is likely to serve as a sink for P in the water column. Erosion control efforts should target the younger sediments in this watershed. The results of this work will contribute useful information to watershed scientists, local environmental groups, and NRCS personnel interested in improving water quality in this and similar watersheds.

Technical Abstract: Sediments in streams that drain agricultural watersheds may be sinks that can adsorb P from the stream or sources that can release P to the stream. Sediment characteristics and environmental factors, including the oxidation-reduction potential of the water associated with the sediment, determine whether P will be adsorbed or released by the sediment. We investigated P adsorption and release by four sediments that occur in Walnut Creek, a second-order stream in Jasper County, Iowa [three Holocene-age sediments (Camp Creek, Roberts Creek and Gunder) as well as Pre-Illinoian-age till]. The effects of two redox potentials on phosphorus buffering capacity (PBC) and equilibrium phosphorus concentration (EPC) were evaluated in batch adsorption experiments. We also simulated oxic and anoxic conditions over a 24-day period and measured solution-phase P concentrations in stirred systems where the sediments were isolated from the water by dialysis tubing. The batch experiment indicated that the EPCs of the three Holocene-age sediments were similar to one another and increased with decreasing redox potential. In the stirred flow reactors, more dissolved P was released from the Camp Creek and Roberts Creek sediments under anoxic conditions than from the other sediments. This observation suggests that these two sediments, which are younger and higher in the stratigraphic sequence, are more likely to be P sources in anoxic settings. The P buffering capacity was greatest in the till. Where it is in contact with the stream water, the till is likely to serve as an adsorbing sink for P in the water column.