The objectives of the research are: i) to estimate the sedimentation rate in the estuarine area (19^o57N, 105^o60E) with mangrove forests in Hau Loc district, Thanh Hoa province (Central Viet Nam) using ²¹⁰Pb fall-out; ii) to evaluate the discharge of organochlorine pesticide residues (OCs) and polychlorinated biphenyls (PCBs) from inland into the area; iii) to evaluate the chronology of OCs and PCBs in the study area. The OCs constituents of particular interest are aldrin, dieldrin, endrin, HCB, a-HCH, b-HCH, g-HCH (lindane), heptachlor, the DDT group and other 10 PCBs congeners which were, possibly, used in agriculture, sanitary and industries, respectively.

It was shown that mangroves much improve the rate of sediment build-up in the area. Sedimentation rate inside the mangrove forest (706 to 341 T.ha^-1.y^-1) depending upon the location of sampling was higher than that outside the forest (143 T.ha^-1.y^-1). The variation in sedimentation rate in both mangrove forest and estuarine area was found to depend upon the age of mangrove trees and on the hydrodynamic conditions in the area.

All the mentioned above OCs constituents were found in the sediments sampled from the area. The concentration of OCs was approximately 9.0 ± 1.5 ng per gram of dry weight (DW) sediment. Among OCs, the DDT group and lindane were the major components of the pollution. Additionally, the ratio of p,p'-DDT to the sum of the DDT pollutant (Sp,p'-DDE, p,p'-DDD, p,p'-DDT, o,p-DDE, o,p'-DDD,  o,p-DDT) was found to be (0.16 ± 0.08) indicating the recent use of the chemicals inland. The use was thought mainly for sanitary purposes. The concentration of total PCBs 4.81 ± 1.02 ng g^-1 DW was much lower than that in sediments from other western Seas like the Mediterranean or Baltic Sea. For all the sampling locations, OCs and PCBs distributed almost evenly along the sediment cores from the surface to 30 cm depth. However, these pollutants were not found at lower depth. This confirms the fact that sediment in the study area was strongly disturbed by the sea products collecting of the local farmers. The impact of OCs and PCBs pollutants to the ecosystem along the marine coast of Vietnam needs to be further studied.  

Keywords: Fall-out radionuclide, ²¹⁰Pb, sedimentation, persistent organic pollutants, DDT, PCBs, mangrove forest, estuary.