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

Abstract

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.