Investigation of submarine groundwater discharge to tidal rivers: Evidence for regional and local scale seepage

Abstract

Fluxes of submarine groundwater discharge (SGD) were investigated into two tidal rivers on the north and south shore of Long Island, NY, during July 2015. Ground‐based handheld thermal infrared (TIR) imagery, combined with direct push‐point piezometer sampling, documented spatially heterogeneous small‐scale intertidal seepage zones. Pore waters were relatively fresh and enriched in nitrogen (N) within these small‐scale seeps. Pore waters sampled just 20 cm away, outside the boundary of the ground‐based TIR‐located seepage zone, were more saline and lower in N. These ground‐based TIR‐identified seeps geochemically represented the terrestrial fresh groundwater endmember, whereas N in pore waters sampled outside of the TIR‐identified seeps was derived from the remineralization of organic matter introduced into the sediment by tidal seawater infiltration. A 222Rn (radon‐222) time‐series was used to quantify fresh SGD‐associated N fluxes using the N endmembers sampled from the ground‐based TIR pore water profiles. N fluxes were up‐scaled to groundwater seepage zones identified from high‐resolution airborne TIR imagery using the two‐dimensional size of the airborne TIR surface water anomalies, relative to the N flux from the time‐series sampling location. Results suggest that the N load from the north‐shore tidal river to Long Island Sound is underrepresented by at least 1.6–3.6%, whereas the N load from SGD to a south‐shore tidal river may be up to 9% higher than previous estimates. These results demonstrate the importance of SGD in supplying nutrients to the lower reaches of tidal rivers and suggest that N loads in other tidal river environments may be underestimated if SGD is not accounted for.

Integration of aerial infrared thermography and in situ radon‐222 to investigate submarine groundwater discharge to Pearl Harbor, Hawaii, USA

“…We used the regressions to estimate total groundwater fluxes for all plumes visible on the SST map that were not directly measured by radon time‐series deployments (Table ). For this calculation we follow Tamborski et al () and assumed that cold groundwater encompasses uniform surface layer thicknesses within each plume. The TIR regression and TIR plume sizes yielded a harbor discharge estimate of 317,810 ± 63,600 m 3 d −1 for all point‐source plumes (Table ).…”

“…We first used aerial TIR remote sensing to detect and categorize groundwater discharge to the harbor's surface waters based on plume sizes, and the point‐source or diffuse nature of SGD. Thermal infrared surveying has been used to determine sea surface temperatures (SSTs) of the top sub‐millimeter of a body of water (e.g., Emery et al ) and is applicable for SGD studies wherever temperature differences exist between discharging groundwater and receiving estuary or coastal water (e.g., Johnson et al ; Wilson and Rocha ; Kelly et al ; Tamborski et al ; Tamborski et al ). We then used surface water and mid‐ and deep‐water surveys of 222 Rn to confirm groundwater discharge locations identified in the TIR imagery and to estimate groundwater fluxes to the harbor (e.g., Tamborski et al ).…”

Intertidal spring dynamics and coastal nutrient loading revealed through geophysics, drone surveys, and in‐situ monitoring

“…Coastal springs provide a ‘short‐circuit’ pathway that rapidly conveys nutrients from terrestrial aquifers to the coastal ocean (Carvalho da Silva et al, 2023). Such springs create plumes in the surface water column that are distinct in terms of their micro‐ and macrofauna, temperature, salinity, dissolved oxygen, nutrient concentrations, dissolved organic matter, and/or other geochemical conditions (Adyasari et al, 2021; Lee et al, 2016; Tamborski et al, 2017; Waska & Kim, 2010). Intertidal springs allow for low‐tide access to sample and directly gauge focused SGD, thereby overcoming some of the challenges with diffusive SGD characterization.…”