Stable Water Isotopes Reveal Non-Conservative Mixing and Redox Dynamics During Saltwater Intrusion in Tidal Marshes

Saltwater intrusion (SWI) alters water movement and redox-sensitive biogeochemical processes in tidal marshes.  It remains challenging to distinguish conservative freshwater–seawater mixing from process-driven effects such as evaporation, residence time, and redox reactions.  Electrical conductivity (EC) is commonly used to trace mixing but provides limited insight into how these processes modify porewater chemistry across space and time.  Here, we evaluate whether stable water isotopes and an Evaporative Enrichment Index (EEI) improve the interpretation of porewater mixing and redox-sensitive responses along a marsh transect experiencing SWI.  

Porewater was sampled along a forest–marsh transect at the St. Jones Reserve (Delaware, USA) across seasons, depths, and tidal settings. Mixing was quantified using stable water isotopes (δ²H, δ¹⁸O, δ¹⁷O), EC, and end-member mixing analysis (EMMA).  Results from isotope-only and isotope+EC EMMA were compared, and EEI was applied to isolate non-conservative isotopic modification associated with evaporation, transpiration, and prolonged residence time.  Mixing metrics were related to redox-sensitive variables, including redox potential, nitrate, iron, and manganese. 

Mixing fractions calculated from isotopes and EC both captured the freshwater–seawater gradient but diverged most strongly in the marsh transition zone.  Isotope-only EMMA preserved seasonal and tidal variability that was dampened when EC was included.  EEI exhibited strong seasonal structure and was negatively correlated with redox potential, indicating that isotopic enrichment coincides with more reducing conditions.  Near-channel sites showed conservative mixing and consistent nitrate decline with increasing seawater fraction, whereas the transition zone exhibited enhanced nitrate loss and

depth-dependent, nonlinear iron and manganese responses associated with extended inundation and residence time. 

These results demonstrate that isotope tracers, when combined with EEI, provide process-level insight beyond EC by resolving evaporative modification and hydrologic isolation.  EEI helps identify when and under what hydrologic conditions redox-sensitive nutrient and metal transformations occur during saltwater intrusion.