CHARACTERIZING SUBMARINE GROUNDWATER DISCHARGE AND ASSOCIATED NUTRIENT LOADING VIA GROUNDWATER SPRINGS

Coastal eutrophication has become increasingly prevalent in recent decades due to enhanced nutrient loading to coastal water bodies. Eutrophication can cause biodiversity loss, harmful algae blooms, hypoxia, and other negative effects on human health and the ecosystem. Submarine groundwater discharge (SGD) represents a significant pathway for nitrate transport to the coast; in some cases, SGD can exceed river inputs for nutrient loading to semi-enclosed coastal waters. The nature of SGD transport is diffusive and expansive, making it difficult to directly monitor. Inter-tidal springs allow for low-tide access to sample and directly gauge SGD, thereby overcoming some of the challenges with SGD characterization.

Basin Head is a coastal lagoon located in Prince Edward Island (PEI) in Atlantic Canada with a federally protected ecosystem. The watershed is dominated by intensive agricultural land use as is common throughout PEI. The main mechanism for transport of nitrate to rivers and coastal waters in PEI is thought to be groundwater given the transmissive fractured sandstone aquifers that underlie the province. At this site, various field methods have been employed to characterize nutrient loading to the lagoon via groundwater springs. Thermal imaging by an unmanned aerial vehicle (UAV) identified 31 groundwater springs discharging to the lagoon, and an analysis of the discharge-thermal plume relationship revealed similar cumulative summer flowrates between springs and freshwater tributaries. Spring discharge was also measured directly using a flume to verify these estimates. Nitrate concentrations and other water quality parameters have been monitored since 2021 at the main springs and tributaries. Nitrate data reveal that groundwater springs are characterized by higher concentrations (3.2 mg/L) than the tributaries (2.5 mg/L). UAV surveys were also conducted to map the extent of macroalgae over the warmer months. A multi-frequency broadband electromagnetic sensor was also used to map the electrical resistivity, and thus salinity distribution around the springs to better quantify freshwater input to the lagoon. These detailed multi-method investigations are critical to understanding SGD dynamics and the drivers of coastal eutrophication in these settings.