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Submarine groundwater discharge (SGD) in reefs can be controlled regionally by geologic structures. Since such structures are poorly characterized, their general importance to SGD is likely underappreciated. This study helps fill this gap by analyzing the effects and implications of faults and associated fractures on offshore hydrogeologic processes in coral reefs. The study area in Bolinao, northwestern Philippines, is an extensive reef flat complex fringing a small, limestone island. The highly linear morphology of the island's western reef front is thought to indicate a fault which may serve as a pathway for SGD. Focusing on its northwestern (NW) side, we integrated measurements of seawater Rn-222 activity, salinity, and subsurface electrical resistivity (ER) to assess the nature and magnitude of offshore fault-controlled regional SGD. In situ Rn-222 activities comparable to previously reported reef flat values with corresponding bottom water freshening were observed at several NW reef front sites. ER features consistent with freshwater-saturated sediments were also detected underlying these sites. These fresh subsurface ER features occur continuously along the NW reef front and discretely in the northern reef front, coinciding with the orientation of suspected faults in each of these reef areas. The multifaceted observations support the claim that faults across the area provide long and connected pathways for SGD in the reef front, which is in contrast with the typically lower SGD influence in offshore marine environments. The regional fault-controlled SGD analyzed here is likely to be important for other similar coastal areas.

Plain Language Summary It has been established that groundwater in sediments can seep or flow into the ocean. Such flow mostly occurs in coastal areas but has also been reported offshore through faults and fractures as pathways. We studied this phenomenon offshore at the northwestern reef front of a reef fringing a limestone island. This area was chosen because of its highly linear outline, which we believe to be a surface manifestation of a fault. In this study, we used methods that distinguish groundwater from seawater through its relatively enriched Rn-222 activity, fresher salinity, and higher electrical resistivity. We found several sites at the northwestern reef front showing spatial correlation of bottom seawater with relatively enriched Rn-222 and fresher salinities with high subsurface resistivity interpreted as sediments saturated with freshwater, suggesting the presence of groundwater flow offshore. The presence of fresh groundwater colocated with the suspected fault orientations at the northwestern and northern reef front also highlights a key role played by faults as conduits for groundwater flow.