Flow Directions and Ages of Subsurface Water in a Salt Marsh System Constrained by Isotope Tracing

• Published in: Estuaries and coasts 2023-09, Vol.46 (6), p.1417-1437
• Main Authors: Grande, Emilio, Visser, Ate, Oerter, Erik, Arora, Bhavna, Seybold, Erin C., Tatariw, Corianne, Braswell, Anna, Montalvo, Maya, Zimmer, Margaret
• Format: Article
• Language: English
• Subjects: Age; Biogeochemical cycle; Biogeochemical cycles; Brackishwater environment; Chemical analysis; Coastal hydrology; Coastal Sciences; Constraints; Earth and Environmental Science; Ecology; Electrical conductivity; Electrical resistivity; Environment; Environmental Management; ENVIRONMENTAL SCIENCES; Estuaries; Evaporation; Evapotranspiration; Freshwater & Marine Ecology; Groundwater; Growing season; Hydrologic systems; Hydrology; Isotopes; Isotopic tracer; Pore water; Porewater exchange; Pressure gradients; Recharging; Salinity; Salinity effects; Salt marsh hydrology; Salt marshes; Saltmarshes; Seasons; Seawater; Stable isotopes; Subsurface water; Surface water; Surface-groundwater relations; Terrestrial environments; Tidal cycles; Tides; Tracers; Transpiration; Tritium; Tritium age; Water analysis; Water and Health; Water depth; Water salinity
• ISSN: 1559-2723; 1559-2731
• DOI: 10.1007/s12237-023-01237-3

Salt marshes are dynamic hydrologic systems where terrestrial groundwater, terrestrial surface water, and seawater mix due to bi-directional flows and pressure gradients. Due to the counteracting terrestrial and marine forcings that control these environments, we do not comprehensively understand water fluxes in these complex coastal systems. To understand the water sources, flow directions, and velocities in salt marsh porewater, we employed a combination of geochemical tracers and analytical models across a hillslope-to-salt marsh continuum in a salt marsh experiencing daily inundation of estuarine surface water (SW) from tides and mixing of fresh seasonal groundwater. We used tritium ( 3 H) as a hydrologic tracer to assess porewater ages and stable water isotope ( δ 2 H and δ 18 O) analyses to separate isotopically distinct estuarine and terrestrial groundwater across different depths and landscape positions in the study transect. We employed electrical conductivity to constrain the role of source mixing and evapotranspiration in salt marsh hydrology. Salinity and stable isotopes revealed that transpiration, rather than evaporation, increased subsurface water salinity to concentrations above estuarine SW during summer. Elevated salinity at depth indicated that salt marsh subsurface water is recharged during the dry growing season. Seasonal recharge patterns drive long-term deep subsurface water dynamics across the salt marsh, with 3 H ages of 3–7 years, and daily tidal cycles drive short-term shallow porewater dynamics with 3 H ages of 0 ± 3.6 years. Our conceptual understanding of the spatiotemporal changes in SW-subsurface water interactions at the terrestrial-marine interface quantifies the hydrological constraints we are missing to improve our understanding of biogeochemical cycles within the salt marsh.