Examining sources and pathways of phosphorus transfer in a ditch-drained field.

Authors: MOSESSO, LAUREN - University Of Delaware; Buda, Anthony; COLLICK, AMY - University Of Maryland Eastern Shore (UMES); Kennedy, Casey; Folmar, Gordon

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2021
Publication Date: 4/11/2021
Citation: Mosesso, L., Buda, A.R., Collick, A.S., Kennedy, C.D., Folmar, G.J. 2021. Examining pathways of phosphorus transfer in a ditch-drained field with concentration-discharge relationships and isotope hydrograph separation. Journal of Environmental Quality. 50(3):680-693. https://doi.org/10.1002/jeq2.20226.
DOI: https://doi.org/10.1002/jeq2.20226

Interpretive Summary: Understanding the processes that mobilize and transport dissolved phosphorus during storms is central to controlling phosphorus losses from agricultural landscapes with open ditch drainage. In this study, we examined relationships between dissolved phosphorus concentrations and flow in a small drainage ditch. During small storms, dissolved phosphorus was diluted by groundwater inputs that had low phosphorus concentrations. In contrast, larger storms caused rapid rises in groundwater that flushed dissolved phosphorus from shallow soil waters into the ditch. Findings illustrate the importance of remediating phosphorus stratification in soils as means of reducing phosphorus losses to ditch drainage during large storms.

Technical Abstract: Understanding the processes that mobilize and transport dissolved phosphorus (P) during storms is critical to managing P in flat landscapes with open ditch drainage and legacy soil P. In this study, we employed routine baseflow monitoring and intensive storm sampling at a ditch-drained site on Maryland’s Lower Eastern Shore (July 2017 to September 2018) to assess whether concentration-discharge (C-Q) relationships and chemical and isotopic hydrograph separation could provide insight into the processes that mobilize and transport dissolved P in ditch drainage. Using a segmented regression model, we determined that long-term C-Q relationship for dissolved P differed above and below a discharge threshold of 6.4 L s-1. Intensive storm sampling revealed that small storms (n=3) occurring at or below the discharge threshold generally exhibited complex hysteresis and dissolved P dilution patterns that were presumably driven by deeper groundwater inputs with low dissolved P concentrations (0.04 mg L-1). In contrast, large storms occurring well above the discharge threshold (n=4) induced rising water tables and preferential flow pathways that most likely tapped near-surface soil waters enriched in dissolved P (0.89 mg L-1), producing consistent clockwise hysteresis and dissolved P flushing patterns. Notably, chemical and isotope hydrograph separation during two of the largest storms revealed significant event water fractions (59 to 68%) that strongly suggested a role for the rapid delivery of dissolved P via preferential flow pathways during these events. Findings highlight the need to mitigate vertical P stratification as a means for reducing dissolved P flushing from ditch-drained landscapes with legacy P.