Radium Isotopes and Nutrients in Razdolnaya River Estuary (Amur Bay, the Sea of Japan) in the Period of Summer Flood

The activity of dissolved radium isotopes 223 Ra, 224 Ra, 228 Ra, the concentrations of nutrients: ammonium, nitrates, nitrites, phosphates, silicates, total nitrogen, and total phosphorus are studied in the Razdolnaya River estuary at suspension concentration in river water >1000 mg/L. During flood, classical two-layer estuarine circulation was observed over the nearshore; this circulation was formed by a discharge current and compensation flow from the depth of 20 m to the depth of 8 m. The main source of 228 Ra inflow into the estuary is desorption from river suspension in the beginning of the mixing zone near the mouth bar, where its activity reached 163 ± 0.03 dpm 100 L –1 , which is 139 times that in the river water. The zone near the river mouth bar shows higher activity of 224 Ra and 223 Ra (4 and 17 times greater than that in river water) and an increase in the concentrations of total phosphorus, NH_4^ + , DSi, NO_3^ - , and total nitrogen. An extremum in the activity of 224 Ra (65.41 ± 0.68 dpm 100 L –1 ) and an increase in 223 Ra (1.97 ± 0.11 dpm 100 L –1 ) were recorded in the bottom water of the nearshore; therefore, the main source of these isotopes is bottom sediments. Direct correlation was found to exist in the pairs PO_4^3 - – 224 Ra and NH_4^ + – 224 Ra, and simultaneous extremum was recorded in 224 Ra, PO_4^3 - , and NH_4^ + in bottom nearshore waters; this extremum is attained in the area with highest density of colonies of polychaetes polychaetes―active bioirrigtors of pore waters.