Influence of precipitation event magnitude on baseflow and coastal nitrate export for Prince Edward Island, Canada

Abstract

The export of anthropogenic nitrate to coastal waters, which depends on the interplay between many factors such as land use and meteorological forcing, is a rising concern in many regions of the world. The present study investigates the effect of precipitation event magnitude on baseflow and associated groundwaterdriven nitrate export in Prince Edward Island, Canada. Twenty-year time-series of precipitation, stream flow, and groundwater levels across the island were analysed to establish a three-way relationship between precipitation, groundwater level rise, and baseflow increase along a hydrological response pathway in this island setting. The analysis was performed by extracting hydrological responses for groundwater level and baseflow for a selected subset of relatively isolated precipitation events. The results reveal a non-linear relationship between precipitation event magnitude and baseflow change that is also observed for precipitation events associated with hurricanes and post tropical storms. A time-series of streamflow nitrate concentrations during Hurricane Dorian's passage (September 2019) was used to evaluate the relevance of these findings for nitrate export. The data show that baseflow increases after a heavy precipitation event have limited impact on streamflow concentrations, but result in substantial and sustained increase of nitrate export (calculated by multiplying concentration by flow). These observations are consistent with a recharge-induced water table rise leading to increased hydraulic gradients that drive discharge of shallow nitrate-containing groundwater. Due to the similarity of the processes governing groundwater flow to the sea and to streams, the results can be used to gain insights into the impacts of precipitation event magnitude on direct groundwater discharge to coastal waters. On Prince Edward Island, this applies to at least the 13% of the island's surface area that is closer to the ocean than to any stream and likely feeds submarine groundwater discharge pathways.

Impacts of Extratropical Cyclone Fiona on a sensitive coastal lagoon ecosystem

“…4d) arises from the time required for the recharged meteoric water to mobilize root‐zone nitrate into the rivers via interflow and shallow groundwater, and to push out deeper “old” nitrate‐rich water from the aquifer (Paradis et al 2018). The sustained loading, which is due to the transport through the groundwater system, indicates that extratropical cyclones can trigger elevated nutrient loading that persists much longer than the surface effects of high flows, as has been recently reported elsewhere (Diego‐Feliu et al 2022; Pavlovskii et al 2023), with the timing likely related to the shallow and deep nitrate transport pathways. Given that nitrate loading is the primary driver of eutrophication in Basin Head and in many other coastal waters worldwide (González‐De Zayas et al 2021), such sustained nutrient loading may trigger sustained coastal ecosystem eutrophication, at least when storms occur in summer or early fall when the water is warm.…”

“…Associated ecosystem changes due to these long‐ and short‐term forcings will also occur at disparate timelines. For example, from an event perspective, sustained, elevated nitrate loading due to extreme precipitation associated with tropical cyclones could become more of an issue in Prince Edward Island (Pavlovskii et al 2023) and in other nutrient‐sensitive coastal waters worldwide (Maúre et al 2021). Coastal lagoons like Basin Head will also experience multi‐decadal bathymetry, salinity, and thermal changes that will in turn influence the way the ecosystem responds to future coastal storms.…”

“…The springs and salt marsh groundwater system discharging to the Basin Head lagoon were monitored to analyze Fiona's subsurface impacts, as the subsurface response to storms and recovery is expected to be delayed (Cantelon et al 2022) but still exert influence on subsequent surface dynamics (Douglas et al 2022; Pavlovskii et al 2023). Three pressure transducers (Levelogger 5, Solinst) were installed in shallow piezometers in the salt marsh and dune system between the ocean and the lagoon's northeast arm to monitor saltmarsh groundwater level and temperature (Fig.…”

“…The impacts of cyclones on coastal lagoons are multifaceted (Greening et al 2006). High precipitation rates and associated elevated loading of terrestrial contaminants (Pavlovskii et al 2023) can change the physicochemical conditions throughout a lagoon ecosystem. For example, elevated freshwater discharge to transitional coastal waters can decrease salinity, deplete dissolved oxygen (DO), dilute or enhance nutrient levels, increase suspended sediment and organic matter, and mix formerly stratified zones (Steichen et al 2020; González‐De Zayas et al 2021; Walker et al 2021).…”

Vertical Saltwater Intrusion in Coastal Aquifers Driven by Episodic Flooding: A Review

“… An example of a three‐dimensional numerical model domain (adapted from Pavlovskii et al., 2021) of a sandy barrier island including the typical boundary conditions applied for simulating vertical saltwater intrusion (SWI). Modeling challenges specific to vertical SWI are identified in the boxes.…”