Dissolved Carbon Concentrations in Bohai Sea Seawater, Adjacent River Waters, and Coastal Groundwater

Two field campaigns were conducted in April (dry season) and August 2017 (wet season). A total of 514 water samples (83 river samples, 91 groundwater samples and 340 seawater samples) were collected to analyze DIC, DOC, and TA. Our observations cover the entire Bohai Sea and its coastline to capture the full range of hydrogeological conditions. Saline groundwater samples were extracted from the intertidal zone at depths of 0.5–2 m below the ground surface using push-point sampler (MHE instruments) and a peristaltic pump, while fresh groundwater samples were collected from inland monitoring wells at ~10 m. Seawater, fresh river water and estuary water samples were collected ~ 0.5 m below the surface. DOC samples underwent filtration through 0.7 µm GF/F filters and acidification to pH < 2 with H3PO4 before storage in borosilicate vials. DIC samples were collected with no headspace and preserved with 100 µL of saturated HgCl2 solution. Salinity, oxidation-reduction potential (ORP), and pH were measured in situ for all samples using a handheld multi-parameter water quality meter (HI9829, HANNA, Italia). DOC and DIC concentrations were measured using a total organic carbon/nitrogen analyzer (Multi N/C 3100 Analyzer, Germany). Triplicate measurements were performed for all DIC and DOC samples, showing a relative standard deviation below 3%. TA was measured in situ using a digital titrator (Hach) with 0.08 N H2SO4 with analytical precision of 2 μmol L-1.