SUBMARINE GROUNDWATER DISCHARGE: LESSONS FROM A FLORIDA ESTUARY

Water and chemical fluxes from submarine groundwater discharge (SGD) depend on physicochemical characteristics of coastal aquifers, estuarine sediments, and geographic configuration. Indian River Lagoon (IRL) is a narrow back-barrier micro-tidal estuary with limited fetch, extending for approximately 250 km parallel to the east coast of Florida. Exchange with Atlantic Ocean water is limited to three inlets in the southern half, resulting in lagoon water residence times that range from 3 to 12 months. The watershed draining to the lagoon is restricted to a 10-kilometer wide area between the lagoon and an inland Pleistocene ridge limiting surface runoff. Consequently, SGD is a critical component of the water budget within the lagoon. Terrestrial SGD (meteoric origin) is <0.1% of the flux of surface water discharge, but marine SGD (recirculated seawater) is around 4 times the magnitude of surface discharge. The chemical composition of terrestrial SGD is modified because of slow flow, fluid-solid reactions, and microbial alteration of redox-sensitive elements along the aquifer flow path and within the fresh water-salt water interface. These modifications make terrestrial SGD an important source for elemental loading to the lagoon. Marine SGD is caused by exchange of lagoon water across the sediment-water interface to depths of 30 to 100 cm in the sediments. This exchange is mostly driven by bioirrigation through permeable bottom sediments at rates of 10 to >100 cm/day, which forces well-oxygenated water into the sediments where it is rapidly consumed by microbially mediated reactions. As IRL demonstrates, SGD is an important process where coastal aquifers crop out near shore and estuarine bottom sediments are permeable. Although much can be learned by studying SGD in a variety of settings with disparate physical and chemical characteristics, equally critical are in-depth studies of single locations where collective information about local processes can lead to insights otherwise unobtainable in short-term studies. Comparisons of a few well-studied locations could lead to even greater understanding of SGD and its importance to coastal zone water budgets and chemical compositions.