Comprehensive Assessment Of Sediment Toxicity And Sediment Quality Of Dongjiang River Basin

As the pool and source of pollutants in the water environment, sediment is of great environmental significant. At present, polluted sediments generally exist in aquatic ecosystems all over the world. The monitoring, evaluation, remediation and subsequent management of contaminated sediments are very inconvenient and expensive. As a result, the research on biotoxicity of sediment is limited, and there is a lack of standard methods and application examples of determining sediment toxicity. Evaluations on sediment quality mainly base on chemical methods which are poor ecological correlation, and seldom conduct a comprehensive evaluation of the sediment quality.Dongjiang River is an important drinking water source in the Pearl River Delta. However the pollution of Dongjiang basin is more and more serious and the pollutants are various and complex. Although there are a great deal of chemical monitoring data of sediments in Dongjiang basin, researches on ecotoxicological effects and comprehensive evaluations on sediments are very limited, so it is difficult to evaluate the true effects of pollutants on the ecological environment.This research selected three kinds of representative organisms of different trophic level, namely luminescent bacteria, algae and zebrafish embryo to meansure the biotoxicity of sediment and pore water of 20 sampling sites in the Dongjiang basin. Two different tests, development and alteration of genes expression, were performed with zebrafish embryo to study the development changes as well as preliminary analysis of alteration of zebrafish m RNA of metallothionein protein(MT), cytochrome P450 enzyme 1A(CYP1A), vitellogenin(VTG), androgen receptor(AR) and thyroid hormone receptor beta(THRB).A comprehensive evaluation method of sediment quality was established with grey Technique for Order Preference by Similarity to Ideal Solution(TOPSIS) method, which integrates chemical analyses, laboratory toxicity tests and benthic community structure analyses and compares the relative magnitude of risks among sites. The setting of two comparison sites mekes it possible to rate each site with high, moderate, or low ecological risk. And an Excel Visual Basic for Application(VBA) program was developed which makes the comprehensive evaluation becomes simpler and more feasible. The data from chemical analyses, laboratory toxicity tests and benthic community structure analyses of Dongjiang basin sediment were then combined and the sediment qualities were comprehensively evaluated with the grey TOPSIS tool. The main results of this study are as follows:1) The pore water samples showed low biological toxicity on luminescent bacteria with the luminescent inhibition rates from–34% to 29%. The pore water samples caused algae growth inhibition rates between–114% and 79%; while most whole sediment samples caused algae growth inhibition rates between 38% and 76%. It is thought that whole sediment samples can reveal sediment toxicity better than pore water for they can reduce the disturbance of nutrients. Fish Teratogenicity Indice(FTI) of pore water and whole sediment zebrafish embryo tests were between 0-3, and many samples(especially those in the Shima River) showed very high development toxicity on zebrafish embryo.2) The m RNA expression of MT, THRB and AR decreased in most 25% whole sediment samples; while the m RNA expression of VTG, CYP1 A and HSP70 increased in most 25% whole sediment samples. The fold changes of the various genes in most samples were between 1-3, which were not significant. While the fold changes of all the genes in S12 and S13 were significantly different. Gene expressions were associated with a variety of environmental factors.3) The relative degree of biological toxicity showed by different experiments were: whole sediment zebrafish embryo development test > whole sediment algae test > pore water zebrafish embryo development test > pore water algae test > pore water luminescent bacterium test >25% whole sediment zebrafish embryo gene expression test. Comparing to pore water test, whole sediment experiments showed higher biological toxicity and reflected the actual situation of sediment better.4) Those sampling sites with lowest biological toxicity locate in reservoirs or the upstream of Xizhijiang river, those with lower toxicity locate in the downstream of Xizhijiang river or Dongjiang river, those with highest toxicity locate in the Danshui river and the Shima River.5) Results of different LOE(Line of Evidence) assessments show that the chemical pollutions of the sediments in research area are low to moderate, biological toxicities of the sediments are moderate to high, benthic community structures of the sediments are highly disrupted. Results of comprehensive evaluation show the overall risks of sediments in the study region are moderate to high, among which, the sediment quality of Shima River is the worst a, following by the Danshui river, and sediment qualities of Dongjiang River and Xizhijiang river are better.6) The comprehensive evaluation method can integrate different sediment quality evidences and can compare the relative magnitude of risks among sites and rate each site with high, moderate, or low ecological risk. The evaluation process is simple and the evaluation results are accurate.