The twin forces of salt water intrusion (SWI) and submarine groundwater discharge (SGD) cause biogeochemical reactions in subterranean estuaries. A primary reaction is the oxidation of biogenic carbon, which releases CO2, nutrients, metals, and other byproducts. In freshwater aquifers dissolved oxygen (DO) is the primary oxidizing agent, but its low concentration (~0.28 mmol/L at saturation) limits its capacity to oxidize carbon. In freshwater aquifers oxidation largely ceases when the inherently low concentrations of oxidizing agents are exhausted. SWI supplies high concentrations of sulfate, an oxidizing agent with 200 times the oxidation capacity of dissolved oxygen in full strength seawater. The byproducts of sulfate oxidation of biogenic carbon include sulfide, ammonia, reduced metals, dissolved organic carbon and nitrogen, and radionuclide tracers, especially radium isotopes. SGD carries these reduced byproducts to coastal waters, where they may be quickly oxidized and substantially reduce the DO concentration in receiving waters. If the DO concentration falls to <60% saturation (~ 60 mmol/L), many marine organisms become stressed. Severe oxygen depletion – called hypoxia – occurs when DO concentrations fall below 30% saturation (~30 mmol/L). Such low concentrations kill or displace many organisms. Today I will focus on this often-overlooked mechanism contributing to coastal and estuarine hypoxia: the oxygen demand created by oxygen-depleting substances entering the system from coastal aquifers. I call this SGD oxygen demand (SGD-OD). This poorly recognized aspect of SWI-SGD interactions may have large implications. Today I will review documented cases at estuarine and coastal sites where SGD-OD may cause or accelerate hypoxic conditions. Without continued aeration with atmospheric oxygen, the study sites would have experienced prolonged hypoxic conditions due to SGD-OD. The presence of hydrogen sulfide supplied by SGD also impacts the ecosystem as it is poisons most organisms. Increases of SWI will exacerbate these problems.