Saltwater Intrusion and Submarine Groundwater Discharge: Acceleration of Biogeochemical Reactions in Changing Coastal Aquifers

Willard S Moore,Samantha B Joye

doi:10.3389/feart.2021.600710

Abstract

Intrusion of saltwater into freshwater coastal aquifers poisons an essential resource. Such intrusions are occurring along coastlines worldwide due largely to the over-pumping of freshwater and sea level rise. Saltwater intrusion impacts drinking water, agriculture and industry, and causes profound changes in the biogeochemistry of the affected aquifers, the dynamic systems called subterranean estuaries. Subterranean estuaries receive freshwater from land and saltwater from the ocean and expose this fluid mixture to intense biogeochemical dynamics as it interacts with the aquifer and aquiclude solids. Increased saltwater intrusion alters the ionic strength and oxidative capacity of these systems, resulting in elevated concentrations of certain chemical species in the groundwater, which flows from subterranean estuaries into the ocean as submarine groundwater discharge (SGD). These highly altered fluids are enriched in nutrients, carbon, trace gases, sulfide, metals, and radionuclides. Seawater intrusion expands the subterranean estuary. Climate change amplifies sea level variations on short and seasonal time scales. These changes may result in higher SGD fluxes, further accelerating release of nutrients and thus promoting biological productivity in nutrient-depleted waters. But this process may also adversely affect the environment and alter the local ecology. Research on saltwater intrusion and SGD has largely been undertaken by different groups. We demonstrate that these two processes are linked in ways that neither group has articulated effectively to date.

Highlights

SALTWATER INTRUSION AND SUBMARINE GROUNDWATER DISCHARGESaltwater intrusion, the landward excursion of seawater in coastal aquifers, has been recognized since the 19th century as water in impacted aquifers became non-potable (Werner and Simmons, 2009)
Monitoring wells along the creek bank were sampled for numerous constituents including dissolved nutrients, carbon, chloride, sulfate, and radium (Moore et al, 2006)
Sulfate depletion is defined as the sulfate concentration observed in the groundwater minus the sulfate concentration predicted from the Cl− concentration

Summary

Introduction

SALTWATER INTRUSION AND SUBMARINE GROUNDWATER DISCHARGESaltwater intrusion, the landward excursion of seawater in coastal aquifers, has been recognized since the 19th century as water in impacted aquifers became non-potable (Werner and Simmons, 2009). Processes that channel precipitation into surface runoff rather than groundwater recharge exacerbate saltwater intrusion These processes include canal construction to drain freshwater coastal wetlands, sewers to drain golf courses, and the replacement of permeable surfaces with impermeable surfaces linked to sewers, e.g., roads, parking lots, buildings. Pier and bridge construction, installation of pilings for buildings, and harbor dredging may breach confining layers above coastal aquifers, allowing seawater to invade the aquifer. These human impacts are significantly altering coastal aquifers across the globe, accelerating saltwater intrusion at unprecedented–and largely unrealized–rates (Werner et al, 2013)

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