Phosphorus input to Georgian Bay via a discontinued septic system plume and the surrounding aquifer

Session: Nutrient Sources, Transport, and Internal Cycling (4)

Uwe Schneidewind, Western University, Civil and Environmental Engineering, [email protected]
James Roy, Environment and Climate Change Canada, [email protected]
Clare Robinson, The University of Western Ontario, Civil & Environmental Eng., [email protected]
Jason Gerhard, Western University, [email protected]

Abstract

The role of direct groundwater discharge in delivering phosphorus (P) to the Laurentian Great Lakes is poorly understood, although this pathway may contribute to nearshore eutrophication. This study evaluates the transport of P in a sandy beach aquifer and its potential discharge to Georgian Bay, Lake Huron, considering both a plume emanating from a septic system decommissioned over 30 years ago and the surrounding aquifer. Groundwater chemistry, including soluble reactive P (SRP), was evaluated along three 60 m shore-parallel transects located 0, 20, and 40 m away from the lake shoreline in Autumn 2015 and Spring 2016.  A distinct plume of elevated SRP was identified in which the mass flux of SRP and also ammonium was found to increase steadily towards the shoreline. The data suggest additional localised SRP input to the shallow groundwater, possibly from animal excreta on the beach or mineralization of lake-derived organic matter originating from lake water recirculating through the nearshore sediments. The field data are combined with reactive transport modeling to evaluate the possible sources of P and subsequent P fluxes to the lake. The study findings provide important insights into factors affecting P delivery to lakes via groundwater discharge at beaches.