Hydrochemical characteristics of shallow groundwater and carbon sequestration in the Tianjin Plain

Atmospheric CO2 concentration plays a crucial role in controlling global climate change. Understanding the carbon cycle and CO2 balance with an accurate assessment of atmospheric CO2 is key to formulating regional CO2 reduction strategies and identifying new carbon sink pathways. Carbonate weathering-related carbon sink is an important subject of global carbon cycle research. This study analyzed the shallow groundwater hydrochemical characteristics, and estimated the total groundwater storage, DIC storage in groundwater, and carbonate weathering-related carbon sink capacity of groundwater in the Tianjin plain area. From the northern piedmont plain to southern alluvial plain and coastal plain, a N-S and NW-SW horizontal hydrochemical zoning pattern in shallow groundwater was apparent, as groundwater types varied from freshwater and brackish water with low TDS to brackish water with high TDS, and water hydrochemical types changed accordingly from HCO3-Ca•Na•Mg → Cl•SO4-Na → Cl•HCO3-Na → Cl-Na. The areas of freshwater, brackish water and saline water were 733, 3034 and 6564 km2, respectively, where Ca2+, Mg2+, and HCO3- in groundwater were mainly derived from the dissolution of carbonate. In the study area, the total shallow groundwater storage was 22416 million m3; the total DIC storage was 8.13×106 t; and the total carbon sink capacity was 4.11×106 t. In the freshwater, brackish water, and brackish water zones of the study area, the shallow groundwater storages were 1578 million, 62459 million and 14592 million m3, respectively; the DIC concentration ranged from 19 to 200 mg/L, 19 to 200 mg/L, and 19 to 342 mg/L, respectively; the DIC storages were 0.67×106 t, 1.65×106 t and 0.58×106 t, respectively; and the carbon sink capacities were 0.22×106 t, 0.90×106 t and 2.98×106 t, respectively. Groundwater storage, DIC storage, and carbon sink capacity increased along the groundwater flow direction across the study area. © 2022, Editorial Office of Earth Science Frontiers. All right reserved.