Dependence of pH in coastal waters on the adsorption of protons onto sediment minerals

We measured pH, total alkalinity (TAlk), dissolved inorganic carbon (DIC), and C-13-DIC in groundwater of a subterranean estuary in Hwasun Bay in the volcanic island of Jeju, Korea. In the sandy aquifer, the pH values (7.4 +/- 0.2) of fresh groundwater increased sharply to approximate to 10 and then behaved conservatively in the mixing zone between alkalified fresh groundwater and seawater. The distributions of C-13-DIC, Ca2+, and Mg2+ in coastal groundwater, along with the results from lab experiments on sediment columns, suggest that the increase of pH in this subterranean estuary is associated with the adsorption of protons (protonation) onto sandy sediments, rather than other geochemical processes. Our lab experiments showed that the protonation (1) occurs commonly for various sediments of Jeju Island, (2) increases pH effectively for low-salinity (salinity<10) groundwater, and (3) depends on the relative amount of transition metal (i.e., Fe, Ti, and Mn) oxides. The chamber experiments at the interface of seawater and groundwater indicate the direct seepage of high pH water into the ocean. This increase in pH leads to corresponding uptake of CO2 from the atmosphere. Thus, our result suggests that the reaction between groundwater and coastal sediments should be considered as an important driver of pH change that regulates the magnitude of chemical species in coastal groundwater seeping into the ocean at the volcanic island, Jeju, Korea.