Unexpectedly high dissolved²¹⁰Pb in coastal groundwaters: Is submarine groundwater discharge important in coastal sea?

Estimating ²¹⁰Pb sources/sinks is significant for understanding the transport processes and chronology of sediment in coastal seas. Although submarine groundwater discharge (SGD) has been recognized as an important pathway for the transport of terrestrial chemical components, the ²¹⁰Pb source in coastal seas from SGD is generally ignored. In the present work, we built the ²¹⁰Pb budget by analyzing the ²¹⁰Pb activity concentrations in submarine groundwater (i.e., coastal well water and pore water), river water, and seawater in a typical aquaculture bay along the Chinese coast in the Beibu Gulf (Qinzhou Bay). The results showed that the ²¹⁰Pb activity concentrations (Bq/m³) in well water, porewater, seawater and river water were 6.0 ± 3.6 (n = 13), 3.8 ± 0.4 (n = 2), 1.2 ± 0.3 (n = 4), and 1.9 ± 0.6 (n = 3), respectively. The SGD-derived ²¹⁰Pb flux was calculated to be (3.3 ± 0.4) × 10¹⁰ Bq/yr (approximately 1.4 times the river input flux), which accounted for ∼18% of the total ²¹⁰Pb sources in Qinzhou Bay. Although most of the ²¹⁰Pb (>66%) would be buried into the sediment of this coastal bay, SGD-discharged dissolved ²¹⁰Pb can account for approximately half of the ocean mixing, which is the second most important ²¹⁰Pb sink in Qinzhou Bay. Therefore, based on these estimates, we have shown that the ²¹⁰Pb input from SGD can be an important source at the coastal sea scale and needs to be accounted for in coastal sea areas.