From Pores to Coastlines: Multiscale Perspectives on Submarine Groundwater Discharge and Coastal Ecosystems

Xiaolong Geng^1*Amir Haroon¹Xiaolang Zhang²Hong Zhang¹Hope Kanoa¹

• ¹University of Hawai'i at Manoa, Honolulu, United States
• ²Florida Atlantic University, Boca Raton, United States

Submarine groundwater discharge (SGD) is a major pathway linking terrestrial aquifers to the coastal ocean, influencing nutrient delivery, biogeochemical cycling, and ecosystem resilience. Yet a key challenge persists: connecting the fine-scale structural and reactive processes that govern flow and transport within sediments to the hydrodynamic drivers and coastal-scale discharge patterns observed in the field. This perspective highlights the need for an integrated multiscale framework that links pore-scale flow dynamics, microbial and geochemical reactions, and sediment heterogeneity to the larger-scale architecture of coastal aquifers and to hydrodynamic forcing that operates from seconds to centuries. We synthesize recent advances showing how pore geometry, facies transitions, permeability contrasts, and volcanic or sedimentary heterogeneity regulate mixing, residence times, and solute transformation. We further outline how wave swash, tidal pumping, hydroclimatic variability, and long-term coastal boundary evolution interact across temporal scales to reorganize SGD magnitude, timing, and spatial distribution. Emerging tools, including high-resolution imaging, geophysics, tracer methods, numerical modeling, and machine-learning-enabled data integration, offer new opportunities to bridge these scales. We argue that progress will require robust upscaling approaches, long-term multiscale monitoring, and coupled groundwater-nearshore models capable of representing cross-scale hydrodynamic and biogeochemical feedbacks. Such advances are essential for predicting SGD's role in nutrient fluxes, contaminant transport, carbon cycling, and coastal ecosystem response under climate and land-use change.