Particle dynamics of ⁷Be, ²¹⁰Pb and the implications of sedimentation of heavy metals in the Wenjiao/Wenchang and Wanquan River estuaries, Hainan, China

Radionuclides (i.e., ⁷Be and ²¹⁰Pb) can be used to trace particle and sediment dynamics and to quantify coastal oceanic processes with time scales ranging from a few days to a hundred years. Here, we study the settling dynamics of suspended particles and the implication by sedimentary heavy metals in the Wenjiao/Wenchang River and Wanquan River estuaries through the measurement of the particulate ⁷Be and ²¹⁰Pb nuclides. Activity in the particulate phase had a range of 2.1-54.5 and 4.6-67.9 Bq kg^-1 for ⁷Be and excess ²¹⁰Pb (²¹⁰Pb_xs), respectively, in the Wenjiao/Wenchang River estuary. In the Wanquan River estuary, activity is in the range of 1.2-43.5 Bq kg^-1 for ⁷Be and 6.2-194.5 Bq kg^-1 for ²¹⁰Pb_xs. At the same time, activity in the dissolved phase had a range of 0.46-1.26 and 0.30-1.17 Bq m^-3 for ⁷Be and ²¹⁰Pb, respectively, in the Wenjiao/Wenchang River estuary; ranges of 0.10-2.31 and 0.09-1.87 Bq m^-3 for ⁷Be and ²¹⁰Pb, respectively, were observed in the Wanquan River estuary. The distribution coefficients (K_d) for the two nuclides decreased within increased in suspended particle matters (SPM) concentration and/or salinity in Wanquan River estuary. The residence times of particulate ⁷Be and ²¹⁰Pb_xs had ranges of 0.4-1.6 and 1.65-5.15 days, respectively, in the Wenjiao/Wenchang River estuary; and ranges of 0.02-3.2 and 0.61-4.44 days, respectively in the Wanquan River estuary. All residence times for the two nuclides increased in the seaward direction. In the Wenjiao/Wenchang River estuary, we found that 11.8-21.0% of Cu, 3.0-9.0% of Zn and 43.2-69.9% for Cd is removed from the water column and deposited into the estuary, and 24.2-34.8% for Cu, 7.2-23.8% for Zn, and 70.0-82.5% for Cd in the Wanquan River estuary, respectively.