Internal P stores in offshore lakebed sediments play an important role in lake nutrient dynamics. While P stores in nearshore aquifer sediments may also be important for nutrient dynamics, it is unclear whether P accumulates in these sediments, and if so, what factors control P accumulation and its potential later release from the sediments to nearshore waters. This knowledge gap was addressed by conducting field investigations at seven nearshore sites located along the shores of Lake Erie, Lake Huron and Lake Ontario, Canada, with more detailed dissolved and sediment phase characterization completed for two nearshore sites. PO4 concentrations were observed to be higher (>50 μg/L) in the more reducing nearshore aquifers compared to more oxidizing nearshore aquifers (<20 μg/L), despite similar total solid phase P concentrations at the sites. PO4 mobility in the nearshore aquifers was found to be closely linked to redox-driven Fe cycling. In the more reducing aquifers, dissolved PO4 was highest near the redox boundary present in the shallow sediments where oxic infiltrating surface water mixes with reducing groundwater. In the more oxidizing aquifers, solid phase characterization indicated that PO4 is sequestered to Fe oxide mineral phases throughout the nearshore aquifer which explains the low dissolved PO4. While pH was not found to be important for PO4 mobility at the study sites, batch laboratory experiments indicate that increased infiltration of more alkaline surface water into nearshore aquifers may promote PO4 release from the sediments. The study findings demonstrate that while internal P storage mechanisms in nearshore aquifer sediments may currently be limiting P loads to lakes, it is possible that P stores that build up over time may result in increased P loads to lakes in the future.
Keywords: Great Lakes; Groundwater lake interactions; Internal storage; Laboratory batch experiment; Nutrients; Sediment water interface.
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