Abstract:
Three spatial structure groups of radionuclides in U and Th series, 210Pb-excess and 137Cs,
and 40K were found based on analyzing temporal and spatial datum of their content by factor
analysis with oblique rotation in Nhatrang bay.
U and Th spatial structure with their contours decreased toward the offshore, ran longshore
and divided seawater of bay into two parts with strong gradient on both sides. Inside part
located from center of Nhatrang bay toward the seashore with three main deposit centers of
their contents higher than 23 Bq/kg.dry for 238U and 40 Bq/kg.dry for 232Th, indicated
unstability of shoreline. Almost sediments coming from river extended toward the offshore,
were stopped and transported toward southeastern. The outside part was less than above
mentioned content. The boundary line between two parts superposed with the constantly
limit line of turbid plume in the rainy season. Direct influence of the continental runoff was
limited by the 9 Bq/kg.dry contour of 238U, 19 Bq/kg.dry contour of 232Th. Longshore current
was a predominant process whereas lateral transport as sifting and winnowing process of
finer grains in sediments of Nhatrang bay. Areas that had very low content of 137Cs and 210
Pb-excess adjoining shoreline showed areas being eroded. Accumulation of 137Cs and 210 Pbexcess
nearby river mouth characterized for fine compositions of sediments controlled by
seasonal plumes and sites further toward the south indicated finer materials transported
from river and accumulated in lack of hydrodynamic process. Near shore accumulation of
40K revealed the sediments there originated from bed erosion.
