Phosphorus transport in a hotter and drier Midwest: in-channel release of legacy phosphorus during summer low flow conditions

Authors: DOLPH, CHRISTINE - University Of Minnesota; FINLAY, JACQUES - University Of Minnesota; Dalzell, Brent; Feyereisen, Gary

Submitted to: Hydrology and Earth System Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2024
Publication Date: 12/6/2024
Citation: Dolph, C., Finlay, J., Dalzell, B.J., Feyereisen, G.W. 2024. Phosphorus transport in a hotter and drier Midwest: in-channel release of legacy phosphorus during summer low flow conditions. Hydrology and Earth System Sciences. 28(23):5249-5294. https://doi.org/10.5194/hess-28-5249-2024.
DOI: https://doi.org/10.5194/hess-28-5249-2024

Interpretive Summary: Excess phosphorus can cause water quality problems which can lead to environmental degradation and public health risks. We need a better understanding of the many potential sources of phosphorus in watersheds in order to develop effective management strategies and minimize risk. This study relied on long term water quality monitoring data from farm fields and watersheds to help identify important phosphorus sources. Results show that: (1) wastewater treatment plants are important sources of phosphorus in watersheds where they are present. (2) agricultural tile drainage can be an important source of phosphorus, but tile drainage alone cannot account for elevated phosphorus concentrations observed during low flow conditions that occur in late summer. This leads us to conclude that (3) phosphorus stored in stream bed sediments can be released because of chemical conditions that exist during warm temperatures and low flow conditions. This is important because these stream bed phosphorus sources can last for decades or longer; we must account for them to develop effective management strategies and set realistic goals for water quality improvement.

Technical Abstract: The release of bioavailable phosphorus (P) during hot, dry summer periods when conditions are optimal for algal growth in lakes and rivers drives increased eutrophication risk. In addition to ongoing P inputs to the environment, water quality is impacted by the existing supply of “legacy P”, i.e., the historical accumulation of P in soils and sediments due to past land use practices. River networks represent a potential sink and/or source of legacy P, with many dynamic in-channel processes potentially governing the storage and mobilization of P over time. The objective of this study was to evaluate the potential contribution of in-channel release of legacy P to bioavailable P transport during summer low flow conditions across the state of Minnesota, USA. We hypothesized that in-stream release of legacy P contributes to elevated concentrations of bioavailable P (i.e., soluble reactive P, SRP) during summer in streams with strong agricultural and/or urban influence, in addition to concurrent contributions from tile drainage systems and point source discharges. We addressed this hypothesis through synthesis of three water quality datasets: 1) water quality and stream flow (Q) data collected for 143 gaged watersheds across the state of Minnesota between 2007-2021 (22,750 total samples); 2) water quality data from 33 additional ditch, stream and river sites in Minnesota sampled under low flow conditions in summer of 2014; and 3) water quality data collected from tile drainage outlets for 10 monitored farm fields between 2011-2021. We used geospatial data and a machine learning (random forest) approach to identify possible drivers of bioavailable P concentrations during summer low flows for gaged watersheds. Our analysis indicates that between one third to one half of the gaged watersheds we studied exhibited SRP concentrations during low flows in late summer above previously identified thresholds for eutrophication of 0.02 - 0.04 mg/L. For many of these watersheds, SRP concentrations in late summer were above those observed in tile drainage outlets. Elevated SRP concentrations during late summer low flows weakened concentration-discharge relationships that would otherwise appear to indicate more strongly mobilizing SRP-Q responses across other seasons and flow conditions. We found that while wastewater discharge likely contributed to elevated P concentrations for some sites, many sites did not have substantial wastewater impacts. The most important variables for predicting bioavailable P concentrations during late summer low flow conditions in a random forest model were: land use in riparian areas (particularly crop cover), soil characteristics including soil erodibility, soil permeability, and soil clay content, agricultural intensity (reflected via higher pesticide use, higher phosphorus uptake by crops, and higher fertilizer application rates), watershed precipitation, and stream temperature. These findings suggest that, for a substantial number of stream and river sites that have been heavily impacted by past and current P inputs associated with agriculture and urbanization, biogeochemical processes mediated by climate and geology result in the release of legacy P from in-channel stores during late summer low flow conditions. As summers become hotter and, at times, drier -- predicted changes in this region -- conditions for the release of legacy P stored in stream and river channels will likely become more prolonged and/or more acute, increasing eutrophication risk.