PHOSPHORUS IN DISCHARGING GROUNDWATER ALONG WASAGA BEACH: DECOMMISSIONED SEPTIC SYSTEMS AND OTHER SOURCES

Inputs of phosphorus (P) to south-eastern Georgian Bay (part of the Canadian portion of the Great Lakes) have become a concern recently due to the increasing vulnerability of some areas to algal blooms. Phosphorus contributions via groundwater are acknowledged through accounting for cottage septic system inputs, but with few direct measurements from areas with septic plumes or from background areas. In this work, we collected groundwater samples along a 6-km stretch of Wasaga Beach, where groundwater flows towards and discharges to Georgian Bay. Here, septic systems of the many hundreds of cottages have been decommissioned (i.e. switched over to sewer lines, but not dug up) between more than 30 (east end) to about 7 (west end) years ago. The aim of this on-going study is to determine if groundwater from this area has significant levels of phosphorus, and if so, to investigate i) the factors affecting the concentrations and their variability, and ii) the potential phosphorus sources. Preliminary data from shallow profiling of groundwater along the shoreline indicate significant groundwater phosphorus levels, up to 420 µg L^-1, at many locations. These were associated with reducing conditions, but with no clear trend associated with the date of decommissioning. Correlations with artificial sweeteners and chloride, tracers of wastewater, were generally poor. This may indicate the phosphorus is sourced from natural sediments, although delayed release from decommissioned septic systems cannot be ruled out.