ECOHYDROLOGY OF GROUNDWATER-SURFACE WATER INTERACTIONS IN A COASTAL WETLAND WITH AN EXTREMELY LOW HYDRAULIC GRADIENT

Coastal wetlands tend to have extremely low hydraulic gradients, making the identification of groundwater-surface water interactions challenging. However the exchange of water, ions, and nutrients between the two water bodies is expected to influence ecosystem function. In the coastal Everglades, hydraulic gradients are typically less than 0.0001, and elevated concentrations of phosphorus have been observed in the underlying groundwater in response to water-rock interactions within the seawater mixing zone. The objective of this research was to determine the timing and amount of brackish groundwater discharge to the coastal wetlands of the southern Everglades and to evaluate the effects of the groundwater discharge on the overlying surface water chemistry and ecosystem metabolism. The timing of groundwater discharge was determined by four techniques including a water balance, hydraulic gradient, temperature, and geochemical tracers. Groundwater discharge rates were quantified from well data using Darcy’s Law. Ecosystem metabolism was estimated as daily rates of gross primary production (GPP), ecosystem respiration (R) and net ecosystem production (NEP) from free-water, diel changes in dissolved oxygen. Over 2 years, all four techniques converged as to the timing of groundwater discharge which was greatest between May and July. Surface water chemistry was fresh from September through February, but became brackish to hypersaline between March and July, concurrent with the times of highest brackish groundwater discharge. Phosphorus concentrations as well as GPP and R were observed to spike in the surface water during the times of greatest groundwater discharge. The results of this research support the conclusions that brackish groundwater discharge effects surface water chemistry and ecosystem function in the coastal Everglades.