Factors controlling pH in coastal groundwater and seawater of a volcanic island

Abstract

The oceans are being acidified as a result of oceanic uptake of anthropogenic CO2, and global surface pH levels have already decreased by more than 0.1 units since preindustrial times. This ocean acidification may have critical consequences for marine ecosystems and biogeochemical cycles. In addition to ocean acidification driven by rising atmospheric CO2 levels, some coastal regions have experienced more rapid declines in their seawater pH than that in the open ocean as a result of anthropogenic nutrient inputs through river water and groundwater discharge. Although a number of studies have dealt with coastal pH changes related to river water discharge, studies on the effects of coastal groundwater discharge, which has recently been recognized as an important pathway for nutrients, organic matter, and other trace elements, are limited. Therefore, in this study, I aimed to investigate the pH changes in coastal groundwater itself and evaluate the effects of this groundwater discharge on coastal seawater pH.

In order to evaluate pH changes in coastal groundwater, pH (NBS scale), dissolved inorganic carbon (DIC), total alkalinity (TAlk), and δ13C-DIC were measured in groundwater of a subterranean estuary (STE) in Hwasun Bay, which is located in the southwestern part of Jeju Island that lies off the coast of South Korea. This site was chosen because the amount of submarine groundwater discharge (SGD), which includes a substantial amount of submarine fresh groundwater discharge (SFGD), is large in this area and thus water is exchanged actively between the land and ocean through a sandy sediment layer. In the STE of Hwasun, the pH values (7.4±0.2) of fresh groundwater increased sharply to ~10 and then behaved conservatively in the mixing zone between alkalified fresh groundwater and seawater. Similar patterns were observed for pH at Samyang on the northern coast and at Iho on the northwestern coast of the island. The distributions of DIC, TAlk, δ13C-DIC, Ca2+, and Mg2+ in coastal groundwater of Hwasun, along with the results from laboratory experiments on sediment columns, suggest that the increase of pH in this STE is associated with the adsorption of protons (protonation) onto sandy sediments, rather than other geochemical processes. The laboratory experiments that used sediment samples from five different sites (Hwasun, Samyang, Hyeobjae, Hamdeok, and Pyoseon) on Jeju Island showed that the protonation (1) is a common occurrence for various sediments, (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 of Hwasun Bay indicate that there is direct seepage of high pH water into the ocean. This increase in pH leads to the corresponding uptake of CO2 from the atmosphere. Thus, these results suggest 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 of Jeju. The effect of protonation of sediments occurs throughout the entire coast of Jeju beach sediments. However, in some STEs where the loading of organic matter is substantial and the residence time of SGD is long enough to allow organic matter to decompose, the decreases in pH down to 7.7 and the increases in DIC up to 2.5 mmol kg-1 were observed. Therefore, the pH in coastal groundwater of this volcanic island is controlled mainly by two factors, namely, protonation and organic matter oxidation.

In order to evaluate the effect of SGD on the coastal ocean, variations in pH of coastal seawaters in Hwasun Bay off the volcanic island of Jeju were measured. This bay is situated in the oligotrophic open ocean. In this region, salinities of all coastal waters depend primarily on SGD because of the lack of any contributions from the river or stream waters. A significant increase in pH along the lower-salinity plume zone was observed, and this zone was extended 0.5 km horizontally where it encompassed the waters from the bottom to the surface (<15m water depth). The observed data for the entire bay-water column showed a significant negative correlation (r2 = 0.82) between salinity and pH. A simple two-endmember (submarine groundwater and offshore seawater) mixing model showed that this pH increase was caused by enhanced biological production, which resulted from the SGD-driven nutrient inputs rather than from the groundwater dilution itself. Since a number of local and regional studies have shown that SGD-driven fluxes of nutrients are comparable to or higher than riverine fluxes, these results from an SGD-dominated environment suggest that SGD may have a significant influence on coastal biogeochemical changes such as acidification, deoxygenation, and eutrophication.

The change in pH of coastal groundwater discharge and the associated transport of substances (i.e., nutrients, organic matter, and inorganic carbon) may have a great impact on the coastal ecosystem and biogeochemistry of volcanic islands standing in oligotrophic ocean, which is very vulnerable to ongoing acidification. Therefore, the results presented here provides a good starting point for investigating the various effect of SGD on coastal ocean acidification over different temporal and spatial scales.