Seawater intrusion and mud volcanism impact the groundwater geochemistry of small Bornean islands

Mud-volcanic environments represent a striking departure from typical shallow aquifer settings, exhibiting geochemical processes and elemental mobilities rarely encountered in non-volcanic coastal systems. In this study, we compare the chemical composition of groundwater and solid-phase samples from mud-volcanic islands and a nearby non-volcanic control island in Northern Borneo to characterize how mud-volcanism alters the ionic composition, trace-element fluxes, and seawater intrusion dynamics through the fossil reef terrains. Although expelled muds introduce elevated concentrations of trace elements (e.g., Al., Mn, Sr) and gases into the shallow subsurface, posing potential contamination risks, the abundant siliciclastic clays derived from mud-volcanism also infill pore spaces and reduce aquifer permeability. As a result, mud-volcanic islands exhibit less seawater intrusion than a lithologically similar non-volcanic island, despite active anthropogenic pumping and tidal forcing. Geochemical mass-balance and PHREEQC-based mixing models reveal that, in mud-volcanic settings, cation-exchange reactions and localized carbonate dissolution/precipitation dominate over conservative seawater mixing, whereas on the non-volcanic island, seawater intrusion and carbonate dissolution are the primary controls on groundwater chemistry. These findings demonstrate that mud-volcanism not only mobilize pollutants but also reshapes local aquifer hydrodynamics, uncovering a dual role as both a source of trace-element contamination and a natural barrier against seawater intrusion.