Evaluation of source water contribution to tidal marshland using stable isotopes (²H, ¹⁸O, ¹⁷O) of water
Date
2025
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Publisher
University of Delaware
Abstract
Sea-level rise is increasing saltwater intrusion into coastal marshes, altering porewater chemistry and threatening ecosystem functions such as nutrient cycling and carbon storage. Bulk salinity tracers (e.g., EC, Cl⁻) capture tidal mixing but struggle to distinguish precipitation inputs and evaporative enrichment. This study investigates whether stable water isotopes (δ²H, δ¹⁸O, δ¹⁷O) paired with end-member mixing analysis (EMMA) provide finer resolution of water sources and their biogeochemical impact than salinity alone. Porewater was sampled along a forest-to-channel transect across depths, seasons, and spring/neap tides. This work analyzed isotopes, EC, redox (Eh), and major ions, and compared simple two-endmember models with end-member mixing analysis (EMMA) using (a) isotopes only and (b) isotopes+EC, and the calculation of an Evaporative Enrichment Index (EEI). Direct comparison of isotope- and EC-based models revealed strong agreement at intermediate seawater fractions but divergence in interior zones. The isotope-only EMMA retained clear seasonal and tidal variability tied to recharge and evaporation, while the isotope + EC EMMA collapsed to a conservative, salinity-dominated axis. Incorporating an Evaporative Enrichment Index (EEI) corrected EC-derived estimates by isolating isotopic enrichment from evaporation and transpiration, producing a process-aware mixing framework. Spatial and chemical patterns aligned with marsh zonation: near-channel sites showed rapid flushing and redox oscillation; the transition zone exhibited prolonged residence, evaporative enrichment, and mobilization of Fe, Mn, and P. Isotope and salinity-informed models capture complementary process signals and not interchangeable estimates. Integrating isotopic corrections such as EEI enhances salinity-based models, providing a mechanistic framework for predicting how hydrologic and redox gradients reorganize as marsh zones compress under rising sea levels.
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Keywords
Isotope tracers, Marsh migration, Saltwater intrusion, Evaporative Enrichment Index