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

Estimating Pb-210 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 Pb-210 source in coastal seas from SGD is generally ignored. In the present work, we built the Pb-210 budget by analyzing the Pb-210 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 Pb-210 activity concentrations (Bq/m(3)) 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 Pb-210 flux was calculated to be (3.3 +/- 0.4) x 1010 Bq/yr (approximately 1.4 times the river input flux), which accounted for similar to 18% of the total Pb-210 sources in Qinzhou Bay. Although most of the Pb-210 (>66%) would be buried into the sediment of this coastal bay, SGD-discharged dissolved Pb-210 can account for approximately half of the ocean mixing, which is the second most important Pb-210 sink in Qinzhou Bay. Therefore, based on these estimates, we have shown that the Pb-210 input from SGD can be an important source at the coastal sea scale and needs to be accounted for in coastal sea areas.

Automated summary

Estimating Pb-210 sources/sinks is important for understanding sediment transport and chronology in coastal seas. This study investigated Pb-210 budget in a typical aquaculture bay in the Beibu Gulf and found that submarine groundwater discharge (SGD) can be a significant source of Pb-210 in coastal seas. SGD-discharged dissolved Pb-210 can not only be buried in sediment, but also contribute to ocean mixing.