Submarine Groundwater Discharge as a Driver of Biogeochemical Processes in Methane Seep Sediments

Submarine groundwater discharge (SGD) is an important pathway of dissolved matter transport to coastal ecosystems, yet its identification in methane seep environments remains challenging because chemical signals are modified by sedimentary biogeochemical processes. This study evaluated hydrochemical tracers of SGD in methane seep sites, bacterial mat areas, and background sediments along the southern coast of Crimea (Black Sea). The studied settings exhibited distinct water chemical characteristics. Chloride concentrations decreased from 10.6 to 11.2 g L−1 in background waters, to 8.7–9.3 g L−1 in bacterial mat pore waters and to 7.7 g L−1 in sediment–water interface waters, indicating the presence of a low-salinity water component. Dissolved silica increased by approximately one order of magnitude relative to background values at methane-associated sites. Methane concentrations ranged from 0.025 to 1058 μM, with the highest values occurring in bacterial mat areas. These zones were further characterized by sulfate depletion (down to 0.9 g L−1), elevated normalized alkalinity, ammonium concentrations reaching 8000 μg L−1, high sulfide contents, and low dissolved Fe concentrations consistent with iron sulfide precipitation. The results demonstrate that no single hydrochemical parameter is sufficient to identify SGD in methane-affected coastal sediments. Instead, the combined use of conservative tracers (Cl− and DSi) and reactive constituents (SO42−, alkalinity, NH4+, HS−, TDFe, and Mn2+) provides a robust hydrochemical framework for recognizing groundwater influence and evaluating associated biogeochemical transformations.