It has recently been well documented that the volume of submarine ground water discharge (SGD) depends on the method used to evaluate the flow.  This apparent dilemma results from incomplete sampling of the various sources and drivers of SGD.  Two primary sources of SGD include meteoric water recharging and flowing through terrestrial aquifers and the circulation of marine water through coastal sediments.  Terrestrial sources are driven by gravity flow while marine sources can be driven by tidal or wave pumping or bioirrigation.  Many of these ideas are exemplified in the Indian River Lagoon on the east-central coast of Florida.  The lagoon is ~250 km long, ranges from a few hundred meters to ~5 km wide, and is separated from the Atlantic by a narrow barrier island.  Terrestrially-derived fresh-water discharge is confined to a few tens of meters from the coast on both the barrier and mainland sides of the lagoon, with upward flow rates of ~0.1 cm/day estimated from modeled depth profiles of conservative solutes in shallow (<2 m deep) pore fluids.  In contrast, seepage meter measurements and tracer studies using ²²²Rn activities in the water column reflect flow rates across the sediment-water interface of ~1 to 10 cm/day in areas of the lagoon lacking terrestrially-derived fresh water.  In these areas, chemical and thermal compositions in the upper 0.5 m of the pore water track variations in the surface water composition at time scales of hours to days, suggesting the measured flow is caused by exchange of surface and pore water.  This flow appears to be driven largely by bioirrigation considering the small tidal range (~10 cm) and wave heights and extensive number of burrowing organisms.  Mass fluxes from marine and terrestrial SGD will depend on the magnitude of water discharged as well as its original chemical composition.  Terrestrial SGD can provide new mass (e.g. pollutants and nutrients) to coastal systems but the small flow rates will limit this flux.  Mass fluxes from marine SGD will depend on local diagenetic reactions in the sediment, such as organic matter remineralization and dissolution and precipitation of metals, but could be large because of large volumes of discharge.