Quantifying the fate of biogenic elements in mangrove aquifers: Insights from reactive transport modeling under saltwater-freshwater mixing.

Saltwater-freshwater mixing in mangrove wetlands drives complex biogeochemical processes that regulate the cycling and transformation of key elements. Yet, the detailed quantification of biogenic element cycling and transformations under saltwater-freshwater interactions remains insufficiently explored. This study developed a field-scale reactive transport model, constrained by multi-level monitoring and hydrochemical data, to investigate the migration, transformation, and fluxes of biogenic elements (C, N, S, Fe) in the Dongzhai Harbor mangrove wetland aquifer. The results reveal that freshwater-saltwater mixing and groundwater discharge enrich NH and HCO, while elevated sedimentary iron content primarily reflects Fe²⁺ accumulation in groundwater. Heterotrophic reactions, including aerobic respiration, denitrification, and nitrification, dominate in high-flow regions, while iron and sulfate reduction occur across aquifer layers, influenced by DOC availability and transport dynamics. Low molecular weight DOC entering the aquifer originates primarily from oceanic inputs and sedimentary organic matter degradation (44.8 %), with a minor contribution from terrestrial groundwater. Of this, 71.2 % undergoes microbial reactions, significantly supporting nitrate removal (1.24 × 10 mol/year) while producing HCO and NH. The aquifer is estimated to produce 2.37 × 10 mol of DOC annually. Simulations demonstrate that aquaculture wastewater, enriched in DOC, ammonium, and nitrate, enhances solute inflow and reaction activity, increasing DOC and ammonium discharge to surface waters, despite nitrate removal rates remaining high (up to 83 %). Changes in vertical permeability, related to mangrove root systems and benthic organisms, further influence nutrient cycling. Increased permeability promotes solute exchange and nitrate removal but reduces efficiency, whereas decreased permeability reduces nitrate removal but enhances its efficiency. These findings underscore the critical role of mangrove wetlands in regulating nutrient cycles and maintaining ecological stability, offering insights to support their sustainable management.