Submarine groundwater discharge supplies a continuous, often hidden pathway for dissolved substances from land to sea and can substantially alter coastal nutrient budgets. Studies using radionuclide tracers have shown that groundwater-derived nutrients may rival or exceed riverine inputs in some settings. Kenneth O. Buesseler at Woods Hole Oceanographic Institution and Peter W. Swarzenski at United States Geological Survey have used radium and radon measurements to trace these flows and demonstrate that porous sediments and seepage faces act as concentrated sources of nitrogen, phosphorus, and silica to nearshore waters. These flows are spatially patchy and temporally variable, which complicates detection and accounting in traditional budgets.
Mechanisms and controls
Nutrients reach the coastal ocean through advective flow of fresh and saline groundwater, chemical reactions within aquifers, and porewater exchange across the sediment interface. Radium tracers reveal the magnitude of exchange because radium isotopes are enriched in aquifer waters and conservative on short coastal timescales. In aquifers with long residence times, denitrification can reduce nitrate loads, while young groundwater often carries elevated nitrate from fertilizer, septic systems, or urban runoff. Karst aquifers and highly permeable volcanic or carbonate coasts concentrate discharge and therefore nutrient export, producing hotspots of enrichment along particular shorelines.
Consequences for ecosystems and societies
When groundwater delivers biologically available nitrogen and phosphorus, coastal waters can experience enhanced primary production that shifts species composition and increases the risk of eutrophication and hypoxia. Coral reefs and seagrass beds are sensitive to chronic low-level nutrient inputs common to submarine groundwater discharge, and small island states and coastal fishing communities may see declines in fisheries and tourism tied to water quality changes. Because discharge is often diffuse and beneath the sediment, management that focuses solely on river inputs can underestimate total nutrient loads and thus misdirect mitigation efforts.
Incorporating submarine groundwater discharge into coastal nutrient budgets requires combining tracer studies, hydrological mapping of aquifers, and local land use assessment. Recognizing this pathway changes the territorial scale of responsibility for coastal water quality, linking groundwater governance, agricultural practices, and wastewater management to nearshore marine health. Accounting for SGD therefore strengthens science-based policy by revealing otherwise hidden connections between terrestrial activities and coastal outcomes.