Quantifying groundwater discharge through fringing wetlands to estuaries: Seasonal variability, methods comparison, and implications for wetland‐estuary exchange

Abstract

Because groundwater discharge along coastal shorelines is often concentrated in zones inhabited by fringing wetlands, accurately estimating discharge is essential for understanding its effect on the function and maintenance of these ecosystems. Most previous estimates of groundwater discharge to coastal wetlands have been temporally limited and have used only a single approach to estimate discharge. Furthermore, groundwater input has not been considered as a major mechanism controlling pore-water flushing. We estimated seasonally varying groundwater discharge into a fringing estuarine wetland using three independent methods (Darcy's Law, salt balance, and Br Ϫ tracer). Seasonal patterns of discharge predicted by both Darcy's Law and the salt balance yielded similar seasonal patterns with discharge maxima and minima in spring and early fall, respectively. They differed, however, in the estimated magnitude of discharge by two-to fourfold in spring and by 10-fold in fall. Darcy estimates of mean discharge ranged between Ϫ8.0 and 80 L m Ϫ2 d Ϫ1 , whereas the salt balance predicted groundwater discharge of 0.6 to 22 L m Ϫ2 d Ϫ1 . Results from the Br Ϫ tracer experiment estimated discharge at 16 L m Ϫ2 d Ϫ1 , or nearly equal to the salt balance estimate at that time. Based upon the tracer test, pore-water conductivity profiles, and error estimates for the Darcy and salt balance approaches, we concluded that the salt balance provided a more certain estimate of groundwater discharge at high flow (spring). In contrast, the Darcy method provided a more reliable estimate during low flow (fall). Groundwater flushing of pore water in the spring exported solutes to the estuary at rates similar to tidally driven surface exchange seen in previous studies. Based on pore-water turnover times, the groundwaterdriven flux of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and NH to the estuary was ϩ 4 11.9, 1.6, and 1.3 g C or g N m Ϫ2 wetland for the 90 d encompassing peak spring discharge. Groundwater-induced flushing of the wetland subsurface therefore represents an important mechanism by which narrow fringing marshes may seasonally relieve salt stress and export material to adjacent water masses.Identifying the hydrological factors affecting chemical fluxes within and through wetlands is critical to understanding wetland function and maintenance within the landscape. Intertidal wetlands have been suggested as transformers and potential regulators of nutrient fluxes with nearby coastal

Mediation of benthic–pelagic coupling by microphytobenthos: an energy- and material-based model for initiation of blooms of Aureococcus anophagefferens

“…The importance of groundwater as a source of nutrients in shallow estuaries, and the Long Island bays in particular, has been noted before (La Roche et al, 1997;Tobias et al, 2001;Gobler et al, 2002) and Aureococcus has been shown to utilize the high-molecular-weight nitrogenous compounds in the pore-water (Berg et al, 2003). However, exchange of both dissolved and particulate material between the sediment and water column is modulated by the benthic flora.…”

Salt marsh ecohydrological zonation due to heterogeneous vegetation–groundwater–surface water interactions

“…Few models or experiments have examined lateral (2-D-XY) or three-dimensional (3-D) salt marsh groundwater hydrology. Notable exceptions are the groundwater tracer experiments by Tobias et al [2001] and Jordan and Correll [1985] in 2-D-XY and 3-D, respectively, and the 3-D archetypal marsh-and-channel model by Xin et al [2011].…”

Hydrologic Controls of Coastal Groundwater Discharge in Southern Taylor Slough, Everglades National Park, Florida

“…Some other studies of GW discharge in other areas have been performed in saltmarshes with minimal relief and more homogeneous surface and landcovers (e.g. Michael et al, 2005, Tobias et al 2001. The homogeneity of GW/SW interactions in regions like saltmarshes therefore makes estimates of regional phenomena more powerful because more homogeneous flow can be assumed.…”

“…Numbers of observations correspond with Table 3 Table 7: Correlation matrix comparing deep and shallow density corrected groundwater discharge potential (PGD EFW), evapotranspiration (ET), and rainfall at TS6 over time. ..... 50 Table 8 (Tobias et al 2001). Discharge of GW can contribute a significant volume of water and entrained constituents to the regions in which the discharge occurs (Burnett et al 2003;Moore 2006;Tobias et al 2001;Winter et al 1998).…”

“…Since GW discharge has also been shown to be an important contributor of not only brackish water but also nutrients to the receiving SW body (Burnett et al 2003;Moore 2006;Tobias et al 2001;Winter et al 1998), the influence of GW discharge on the SW of the southern Everglades was also investigated. A better understanding of the interactions between GW and SW can help support research on nutrient cycling (Sutula et al 2001) because GW and SW interactions are an important component of the hydrologic cycle (Winter et al 1998).…”