Assessment On And Study On Measures Against The Ecological Risk Of The River Sediment Utilization

River sediments are important grounds accumulated with nutrients and pollutants wherethere are nutritional ingredients of N、P、K and organic matter which will benefit crop growthand multiple harmful contents which will harm crop growth. The dredged river sediment willoccupy large land reserves, and the leaching loss of its harmful ingredients will produceanother contamination, resulting in the waste of resources for the inadequate use of thenutrients. Thus how to utilize the dredged river sediments scientifically has become a problemof great concern in the process of renovating lakes, rivers and land.The Tangliutang River, which lies in the lower Nandujiang River, was chosen as thetarget study area.52sampling spots were set along the Tangliutang River in order to collectdata about heavy metals, organic matter, PH value and soil texture in the sediment. Thepurpose of sampling was to assess the fertility level of organic matter and potential ecologicalrisks of heavy metals so as to evaluate land utilization of the sediments and carryout relevantexperimental researches to study ways of ensuring the safety of agricultural products and thedredged soil against contamination. This research provided scientific basis and technologicalsupport for dredging and reutilizing river sediments as an agricultural resource.First, the physical and chemical properties of52sediment samples were evaluated toobtain the fertility level of the sediments. Results showed that the average organic content inriver sediments was29.80g/kg, rich in organic matter and high in fertility level, and thehighest content of sediment organic matter appeared in middle reach. The average claycontent was15%, most of which was silty clay loam.The comparatively high fertility leveland clay content will help improve soil fertility and construct quality plough layer.Then, single factor index method, Nemerow index method and Hakanson potentialecological risk index were used to assess heavy metal contamination in the river sediments,revealing the pollution types and levels and the spatial distribution of heavy metals in thesediments of the Tangliutang River. Results showed that the contents of heavy metals in thesediments were in the range of9.79~77.07,0.89~5.04,0.13~0.56,17.30~55.11, and0.07~0.46mg/kg for Cr, As, Cd, Pb and Hg, respectively. Cr, As did not exceed the standard,while Cd, Pb and Hg exceeded the standard. The over-standard rate of the Cd and Hg insamples was65.38%and59.62%respectively. The contents of Cd and Hg were above thestandard by85.48%and82.95%, respectively.The range of the potential ecological hazard index in the upper, middle and lowerreaches of the Tangliutang River was31.59-131.14, not exceeding150. The whole river was within the low ecological hazard level. But each river reach varies in potential ecological riskindex. The degree of the potential ecological risk index of the river decreased in the order ofmiddle river reach, lower river reach and upper river reach. The comprehensive potentialecological risk index of the middle reach is high because of the contamination of heavy metalsCd and Hg. Potential ecological risk of individual pollutants of heavy metals Cd and Hg in themiddle river reach was assessed to be somewhere in the middle, and it was the same with theindividual risk assessment of Hg in the lower river reach. In other reaches of the river,individual grades of potential ecological risk assessment of Cd and Hg were low. Individualgrades of potential ecological risk assessment of Cr、As、Pb in the upper, middle, lowerreaches of the Tangliutang River were all low.The contamination level of the Tangliutang River sediments in most upper reachesshould be closely watched while the river sediments in the village section and in most of thelower reaches were only slightly polluted. The heterogeneity in the spatial distribution ofheavy metal contamination showed that in the land reclamation at a later stage, the mixture ofdifferent pollution levels was important to the construction of safe tilled layer.Lastly, a plot of ground,30cm thick, was constructed by using rotary cultivation to mixthe Tangliutang River sediments with the bare sandy land along the band of the Nandu River.And it was divided in four subplots with additive and without additive in the soil. Swampcabbages and bitter gourds were grown in the four sample plots of land. The heavy metalcontents of harvested swamp cabbages and bitter gourds were measured to assess therelationship between heavy metal contamination and the safety of agricultural products.Results showed that it was feasible to till in a rotary way dredged river sediments and riverband sand to construct mixed tilled land to replace the original land soil to grow agriculturalproducts, but there existed a certain degree of risk. The roots of swamp cabbages were moresensitive than the stem leaves to heavy metals Cr、As and Cd, etc. in the soil. Bitter gourdssuffered lower contamination of heavy metals than leaf vegetables. Soil amendment affectedthe activity of the sediments with more content of Cd, resulting in the content of Cd in theagricultural products and therefore meeting the safety standards, but the residue of heavymetals from soil amendment affected the content of heavy metals in agricultural products aswell.