The Direct Effect of Submarine Groundwater Discharge on the Concentration of Dissolved Oxygen in Estuarine and Coastal Waters

Submarine groundwater discharge (SGD) is recognized to supply nutrient elements nitrogen and phosphorus to coastal waters. In some cases, these nutrients are essential for biological productivity; in other cases, the nutrients are in excess or change relative proportions such that they impact community structure and/or increase algal blooms. Often overlooked is the role of reducing substances in salty SGD such as H₂S, NH₄⁺, CH₄, and DOM, which create a direct demand for dissolved oxygen (DO) and lower its concentration in estuarine and coastal waters. We call this SGD-Oxygen Demand or SGD-OD. These reduced substances primarily result from the oxidation of carbon in aquifers and aquicludes by seawater sulfate. Thus, coastal aquifers transitioning from freshwater to seawater due to seawater intrusion are most vulnerable. Saturated seawater DO concentrations are on the order of 200 µM; reducing DO in coastal waters to <150 µM induces biological stress on many organisms; reducing DO to <60 µM (hypoxic conditions) can be deadly. Studies have directly correlated DO depletion with increased SGD off the coast of South Carolina and Mississippi, USA, and in the Yangtze delta, China. These depletions initially affect near-bottom dwelling organisms and may be recognized by sudden fish kills. In this talk we will review a data base of reducing substances in coastal groundwaters and illustrate how the discharge of this water could impact estuaries and coastal waters. We will show additional examples where we hypothesize SGD-OD is occurring in hopes others will have the resources to investigate these areas.