Submarine groundwater discharge (SGD) delivers terrestrial solutes to coastal waters, but carbon and nitrogen are not conserved during transit through intertidal aquifers. Sediment type, by governing permeability, exchange, and redox conditions, controls dissolved carbon sources, transformation pathways, and export forms. This study compared DIC and DOC behavior in sandy and muddy intertidal groundwater in Liaodong Bay using major ion/nutrient chemistry, carbon isotopes, conservative mixing deviations, DIC-TA stoichiometry, and PHREEQC inverse modeling.

The sandy aquifer, with high permeability and strong tidal exchange (mean DO 4.78 mg/L), was shaped by freshwater input, seawater mixing, and interface reactions. Carbon transformation was driven by DOC oxidation to DIC, carbonate buffering, and nitrogen cycling; SGD exported predominantly DIC and NO₃⁻. The muddy aquifer had low permeability and slow porewater renewal (mean DO 1.11 mg/L, strongly reducing). Both DIC and DOC showed non-conservative enrichment, sourced from sedimentary organic matter mineralization, DOC release, and reductive reactions.

PHREEQC inverse modeling revealed contrasting reaction networks. In the sandy aquifer, O₂ consumption, NO₃⁻ transformation, and DNRA occurred in 94.1%, 100%, and 76.5% of selected models, coupling carbon transformation to oxygen and nitrogen cycling. In the muddy aquifer, organic carbon mineralization and Fe(III)/Mn(IV) reduction appeared in all models; SO₄²⁻ reduction with FeS precipitation occurred in 37.5% of oxic models and 100% under the no-O₂ scenario, converging toward a reduced configuration when oxygen pathways are suppressed.

The two intertidal zones thus represent functionally distinct reaction units: the sandy aquifer is an open, tidally flushed, DOC-consuming system; the muddy aquifer is a retention-dominated reducing system fueled by solid-phase organic matter mineralization. The two embody a shift in the electron acceptor cascade from aerobic oxidation to sulfate reduction, producing divergent carbon and nitrogen SGD exports. Sediment type, more than discharge magnitude, determines whether SGD delivers oxidized or reduced carbon and nitrogen to coastal waters — a dimension absent from current flux assessments.