Effect of submarine groundwater discharge on the carbonate system in Jiaozhou bay

Submarine Groundwater Discharge (SGD) is an important pathway for dissolved inorganic carbon (DIC) and total alkalinity (TA) transport from land to ocean. However, understanding of the role of SGD on carbonate systems in receiving waters, especially pH is very limited. In this study, to examine the effect of SGD on DIC budget and acidification, we investigate the distributions of carbonate system and radium in the seawater through two cruises in the Jiaozhou bay (January and April 2021), and in the groundwater, rivers and wastewater treatment plants during different seasons by seven field survey sampling. The results showed that the average values of DIC and TA in groundwater were (5696±3528) μmol/kg and (4981±3359) μmol/kg, respectively, about two-fold higher than that in seawater. The SGD flux in Jiaozhou Bay was estimated to be (0.73±0.12)×10⁷ m³/d, and SGD transported DIC and TA fluxes were (2.54±0.41)×10⁷ mol/d and (2.12±0.36)×10⁷ mol/d, respectively. SGD is one of the major DIC sources in the Jiaozhou bay, and its transport flux is one order of magnitude higher than that from wastewater discharge. SGD is characteristic with high DIC and low pH mean (7.09±0.46) and input will thereby reduce the buffering capacity and pH of the baywater. Submarine Fresh Groundwater Discharge (SFGD) input to the Jiaozhou bay will lower the pH by 0.001～0.035 and aggravate ocean acidification by using a two end-member mixing model. SGD is an important component of the carbon balance in the Jiaozhou bay and plays an important role in the regulation of coastal ocean acidification and buffering capacity.