Hydrogeological controls on dissolved organic matter fluxes: Source-inventory-export decoupling in intertidal groundwater.

Dissolved organic matter (DOM) is central to coastal carbon cycling, yet how sediment-controlled hydrogeology regulates DOM concentration, composition, apparent aqueous inventory, gross exchange, and residual net DOC flux in intertidal aquifers remains poorly understood. We compared sandy and muddy intertidal aquifers on the Shandong Peninsula, China, using dissolved organic carbon (DOC) measurements, optical spectroscopy, excitation-emission matrix fluorescence with PARAFAC, and groundwater-seawater exchange flux estimates. Sandy groundwater had lower DOC concentrations than muddy groundwater (6.99 ± 2.50 vs. 15.19 ± 4.20 mg C L-1) but a larger protein-like DOM contribution, whereas muddy groundwater contained more humic-like DOM and a larger apparent aqueous DOC inventory (5880 mg C m⁻²) than the sandy aquifer (1930 mg C m⁻²). Despite its lower DOC concentration and apparent aqueous inventory, the sandy aquifer exhibited substantially stronger DOC exchange, with gross DOC exchange of 2302 mg C m-2 d-1, compared with 17.80 mg C m-2 d-1 in the muddy aquifer. Under the measured operational seawater endmember, the corresponding residual net DOC fluxes were -190 and 11.0 mg C m-2 d-1, with flux-cancellation indices of approximately 0.92 and 0.38, respectively. Sensitivity analysis showed that the direction of the sandy residual DOC balance was sensitive to seawater-endmember selection, whereas the positive muddy residual was substantially less sensitive. Thus, DOC concentration, apparent aqueous inventory, gross exchange, and signed residual net flux were not aligned. Sediment-controlled hydrogeology can therefore decouple DOM supply, inventory, and export, indicating that coastal DOC output cannot be inferred from concentration or gross exchange intensity alone.