FACTORS CONTROLLING P MOBILITY IN NEARSHORE AQUIFERS: THE ROLE OF SEDIMENT TRAP AND CLIMATE CHANGE

Despite considerable efforts devoted to decreasing point-source phosphorus (P) inputs into surface waters (e.g. tributaries), eutrophication primarily due to excessive P loading remains a major issue for Great Lakes ecosystem. The role of groundwater in delivering nutrients to large inland lakes, such as the Great Lakes, is poorly understood and thus this pathway is typically overlooked. Quantifying P inputs from groundwater to the Great Lakes is complicated due to existence of the reaction zone near the groundwater-lake interface in the nearshore aquifer, which may modify the delivery of P to nearshore waters. Previous marine studies observed the existence of sediment trap mechanism (“iron curtain”) whereby Fe-oxides may trap P (via sorption or co-precipitation) preventing P discharge to surface waters. The occurrence of a similar sediment trap mechanism and its impact on P mobility along Great Lakes shorelines is unknown. The goal of this study is to identify the factors controlling P mobility in nearshore aquifers and potential P loads to coastal waters, and how this may change over time as the climate changes.

This study presents field data collected at seven beaches located along the shores of Lake Erie, Lake Huron and Lake Ontario, Canada. Elevated dissolved P (up to 100 µg/L) were observed below the groundwater-lake interface near the shoreline. In reduced nearshore aquifers, dissolved P found to be high (> 35 µg/L) due to release of P from Fe oxides at the redox boundary where more oxic surface water mixed with reduced groundwater. In more oxic nearshore aquifers, dissolved phase P was low but solid phase data indicate Fe oxide minerals act to trap the P in the nearshore aquifer. Overall the study findings demonstrate that nearshore aquifers function as a sediment trap that currently acting to decrease P loads to lakes. These sediment traps act as legacy source of P and may result in increased P loads to lakes in the future as the climate changes (e.g. more extreme lake water level changes, increase in storm surges and high waves, and increase in organic matter concentrations). The study findings are needed to inform long term water quality predictions and management focused on addressing P loading to lakes.