Quantifying Spatiotemporal Sediment-P Dynamics in Brush Creek
by Jacob Major
Under the guidance of my mentor, Dr. Shannon Speir from the Department of Crop, Soil, and Environmental Sciences, I delved into the complex world of sediment-phosphorus sorption dynamics. Please join me as I share the insights gained and challenges faced throughout the course of my project.
Legacy phosphorus (P), or the P stored by stream sediments in agricultural regions, can have lasting impacts by maintaining elevated soluble reactive P (SRP) concentrations, diminishing the effects of conservation practices, and degrading water quality long after terrestrial P inputs decline. My project assessed sediment-P sorption dynamics in Brush Creek by quantifying key indicators of the potential for stream sediments to serve as sinks or sources of P, including the equilibrium P concentration (EPC), P saturation ratio (PSR), and particle size distribution of stream sediments. By assessing the capacity of stream sediments to serve as sinks or sources of phosphorus (P) and relating my findings to land use and water quality data, I hope to improve understanding of the sediment-P sorption dynamics that control cycling of legacy P in Brush Creek, a subcatchment of the Beaver Lake Watershed in Northwest Arkansas (NWA).
I conducted bimonthly synoptic sampling of Brush Creek at four sites distributed longitudinally (lengthwise) throughout the watershed over the course of one year. Synoptic sampling describes the collection of samples from multiple locations over a short period of time. Like taking a “snapshot” of the watershed, synoptic sampling captured spatial variations in data collected between sites within Brush Creek. By sampling bimonthly, my project captured temporal and seasonal variations as well. On each sample date, I measured indicators of surface water quality, took filtered grab samples for SRP analysis, and collected unfiltered site water and sediment cores for determination of sediment EPC, PSR, and particle size distribution. I determined sediment EPC, a key indicator of the potential for sediments to either bind or release P, through a series of in-lab EPC assays which provided me with some interesting challenges.
EPC assays are used to determine the EPC of sediments by allowing sediments to equilibrate in a series of aqueous solutions with known initial SRP concentrations, then measuring post-equilibration SRP, and finally, regressing the change in SRP (∆SRP) of the solutions against the initial SRP of the solutions to find the x-intercept, where ∆SRP is equal zero. If the EPC of the sediments is less than the ambient SRP concentration of the solution, then sorption will occur, while if it is greater, P will be released. The initial concentrations I used in my EPC assays (0, 0.5, 1.0, and 2.0 mg/L–P) were a challenge to attain and standardize across all assays. My mentor, Dr. Speir, guided me toward using precisely calibrated volumetric flasks and electronic pipettes. By using Dr. Speir’s recommended approach, I was able to overcome this obstacle in my research, achieving precise and consistent concentrations across all EPC assays.
Although my research is not yet finished, I feel as though I have already learned so much, and I’m not referring only to my project’s area of interest. The opportunities and challenges this work has presented with me so far have taught me a lot about project management, collaboration within a team, and scientific communication that I think will serve me very well in my future endeavors, whatever those may be. I can’t thank my faculty mentor, Dr. Speir, and the rest of the Speir Lab enough for their substantial contributions to this project, as well as the Bumpers College Honors Program for the funding they provided for this research.
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