Background and summary: a new focus on groundwater-seawater interactions

Water and chemical fluxes across the sea floor provide an important linkage between terrestrial and marine environments. Oceanographers recognize that these fluxes may act as a source of nutrients or other harmful contaminants to marine systems (e.g. Johannes, 1980; Valiela et al., 1990). These fluxes may also act as a beneficial source of freshwater for coastal marine estuaries that require relatively low salinities. Hydrologists and hydrogeologists recognize that fluxes across the sea floor comprise an important part of the water balance for coastal aquifers. Most fresh groundwater discharge to the ocean is derived from terrestrial aquifer recharge. Management of coastal aquifers requires careful estimates of recharge and other hydrological components, such as groundwater discharge. These estimates are commonly combined into a comprehensive water budget to evaluate how much groundwater might be available for municipal uses and whether saltwater intrusion may be a potential concern. Excessive groundwater withdrawals can cause saltwater intrusion by intercepting the seaward flux of freshwater that prevents saltwater from intruding a coastal aquifer. Quantitative estimates of fresh groundwater discharge toward the coast can provide a basis for determining safe withdrawal rates. Oceanographers, marine scientists, and those studying and managing saltwater intrusion in coastal aquifers, share a common goal of quantification and understanding of groundwater and seawater interactions. Submarine groundwater discharge, or SGD, has become a popular term in the literature for describing the flux of water across the sea floor. Burnett et al. (2003) specifically define SGD as the discharge of aquifer porewater across the sea floor and into the ocean. They define flow in the opposite direction as submarine groundwater recharge (SGR). SGR is the recharging flux of seawater into the aquifer. The presence of SGR does not necessarily indicate saltwater intrusion, which occurs when saline water moves into parts of the aquifer previously occupied by freshwater. A conceptual model of the types of flow patterns that are expected to exist in many coastal aquifers at the terrestrial-marine boundary is shown. Through a detailed field study, Kohout (1960) showed that as fresh groundwater flows seaward, it meets and mixes with saline groundwater before discharging into the ocean. Because of this mixing and subsequent discharge to the ocean, seawater is drawn into the aquifer to replace the saline groundwater that discharged to the ocean. Seawater drawn into the aquifer is sometimes referred to as recirculated seawater. Michael et al. (2005) suggest that the seasonality of terrestrial recharge may also act as a mechanism for recirculating seawater through a coastal aquifer. As the mixing zone moves landward and seaward in response to seasonal fluctuations in recharge, seawater may be drawn in and flushed out of the aquifer over the course of a year. SGD can also occur at the bottom of the open ocean, even in the absence of a terrestrial connection. Waves, tides, and ocean currents can create hydraulic gradients that pump seawater across the sea floor. Interest in groundwater-seawater interactions continues to receive a significant amount of attention in the literature. Saltwater intrusion, freshwater deliveries to marine estuaries, and nutrient loading are persistent problems of global importance. The problems are difficult to address, however, because of the elusive nature of SGD. Fortunately, the science is advancing. The journal Biogeochemistry, dedicated the entire November 2003 issue to the subject of SGD as did the journal Ground Water, in the December 2004 issue. Zektser & Dzhamalov (2007) released a comprehensive review on the subject of SGD and groundwater-seawater interactions in a new book: Submarine Groundwater. In their book, they suggest that SGD and related studies should be categorized into a new field called "Marine Hydrogeology". The concentrated efforts of numerous researchers from a wide range of disciplines have led to substantial advancements in characterizing SGD, but there is still more work to be done.