Storm-Driven Fresh Submarine Groundwater Discharge and Nutrient Fluxes From a Barrier Island

Abstract

Quantifying and characterizing groundwater flow and discharge from barrier islands to coastal waters is crucial for assessing freshwater resources and contaminant transport to the ocean. In this study, we examined the groundwater hydrological response, discharge, and associated nutrient fluxes in Dauphin Island, a barrier island located in the northeastern Gulf of Mexico. We employed radon (222Rn) and radium (Ra) isotopes as tracers to evaluate the temporal and spatial variability of fresh and recirculated submarine groundwater discharge (SGD) in the nearshore waters. The results from a 40-day continuous 222Rn time series conducted during a rainy season suggest that the coastal area surrounding Dauphin Island was river-dominated in the days after storm events. Groundwater response was detected about 1 week after the precipitation and peak river discharge. During the period when SGD was a factor in the nutrient budget of the coastal area, the total SGD rates were as high as 1.36 m day–1, or almost three times higher than detected fluxes during the river-dominated period. We found from a three-endmember Ra mixing model that most of the SGD from the barrier island was composed of fresh groundwater. SGD was driven by marine and terrestrial forces, and focused on the southeastern part of the island. We observed spatial variability of nutrients in the subterranean estuary across this part of the island. Reduced nitrogen (i.e., NH4+ and dissolved organic nitrogen) fluxes dominated the eastern shore with average rates of 4.88 and 5.20 mmol m–2 day–1, respectively. In contrast, NO3– was prevalent along the south-central shore, which has significant tourism developments. The contrasting nutrient dynamics resulted in N- and P-limited coastal water in the different parts of the island. This study emphasizes the importance of understanding groundwater flow and dynamics in barrier islands, particularly those urbanized, prone to storm events, or located near large estuaries.

Extreme precipitation events induce high fluxes of groundwater and associated nutrients to coastal ocean

“…Although several studies have focused on understanding the seasonal dynamics of SGD (Charette, 2007;Gwak et al, 2014;Michael et al, 2005;Rodellas et al, 2017), limited research has been conducted on SGD driven by episodic events (Adyasari et al, 2021;Wilson et al, 2011). This is mainly because of the inherent difficulties related to monitoring and sampling during and after these extreme events.…”

“…However, most of the SGD investigations are conducted under baseflow conditions, that is, in the absence of any meteorological, hydrological, or oceanographical events (e.g., storms, monsoons, sea-level anomalies) which might significantly impact the magnitude of SGD. Only a few articles have focused on the evaluation of the temporal variations in SGD induced by episodic events (Adyasari et al, 2021;Gonneea et al, 2013;Hu et al, 2006;Rodellas et al, 2020;Sugimoto et al, 2016). Extrapolating SGD estimates derived under baseflow conditions to obtain annual fluxes neglects the role of such events, which may represent an important contributor to overall SGD fluxes.…”

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

“…Ongoing climate change is contributing to an ongoing increase in the frequency of extreme precipitation events globally (Ingram, 2016) and in parts of Canada (Gizaw et al, 2021; Tan et al, 2017), and these trends are expected to continue in the future (Donat et al, 2016; Zhou et al, 2018). However, the impacts of extreme precipitation events on groundwater‐borne nitrate export to the ocean are relatively unexplored (Adyasari et al, 2021). In contrast, the impacts of heavy rainfall on nutrient export through rivers and streams is well documented.…”

Atmospheric Groundwater Forcing of a Subterranean Estuary: A Seasonal Seawater Recirculation Process

“…Su and coauthors also determined rates of SGD from radon and radium inventories, but did not distinguish between fresh and saline SGD or investigate seawater recirculation mechanisms. In nearby Mobile Bay and Saint George Sound, seawater recirculation through nutrient‐rich sediments can be a significant source of SGD nutrient loading (Adyasari et al., 2020, 2021; Beebe & Lowery, 2018; Montiel et al., 2019; Santos et al., 2009). Seawater recirculation may be a nutrient source for Little Lagoon (Su et al., 2014); however, previously described recirculation processes including tidal pumping, wave pumping, wave setup, and movement of the saline‐freshwater interface are likely dampened or absent here because of the restricted tidal range, limited fetch, and topographically limited inland head gradient.…”