Seasonal Groundwater Phosphorus Dynamics in an Alluvial Riparian Zone

Eutrophication and algal blooms caused by excessive phosphorus (P) loads is a major issue for freshwaters worldwide. Recent studies indicate riparian zones, a best management practice intended to reduce P loads to surface waters, only act as temporary P storage zones where P can be remobilized in the groundwater and delivered to streams under certain hydrological conditions (e.g. seasonal water table fluctuations). Prior studies that have observed "hot moments" of subsurface P release have focused on riparian zones that are predominately waterlogged with high organic content soil. It is unclear if this also occurs in alluvial riparian zones with comparatively deeper water tables and lower organic content soil. The objective of this study was to investigate the fate of subsurface P in an alluvial riparian zone and its potential delivery to Kintore Creek, an agricultural stream in the Lake Erie Basin, Ontario, across all four seasons. Electrical resistivity geophysical surveys provided high resolution subsurface characterization across the riparian zone. Groundwater samples have been collected since May 2019 from a network of piezometers and macro-rhizon samplers. Preliminary data from May 2019 to June 2020 show groundwater in the riparian zone generally has low soluble reactive phosphorus (SRP) concentrations (mean = 22 μg/L as P), likely due to the association of P with mineral phases (through mineralization or sorption). SRP in all piezometers consistently increased from May 2019 to October 2019, followed by a decrease in concentration through to January 2020. High SRP concentrations (SRP > 100 μg/L as P) were intermittently observed at select piezometers, but no clear spatial patterns or relationship to water table fluctuations was found. It is possible the overall low SRP observed is due to the more oxic subsurface environments in alluvial riparian zones compared to those with prolonged waterlogged conditions. Continued monitoring will focus on examining the potential mobilization of SRP in the shallow groundwater and unsaturated zone in the riparian zone with this information needed to determine the long-term effectiveness of alluvial riparian zones in limiting P loads to streams.