| Landfill leachate (hereinafter referred to as "leachate") was found with higher concentration organic pollutants, being generated from the operation process in the municipal sanitary landfill. Most of the early landfill was built without liner protection or with simply compacted soil as a natural liner. They were not accordance with the standard requirements of the sanitary landfill and can easily lead to leachate leakage. Some new landfill made cohesive soil or artificial complex impermeable as liner protection. Some landfill used special leachate collection and excretory system. But these facilities can also lead to seepage leakage because of the mechanical and natural corrosion as time goes on. Therefore, it is inevitable that the leachate leakage occured when the operating and managing processes of the landfill, which could cause serious pollution to the surrounding ecological environment.Up to the present much works was centered on the chemical analysis of leachate composition and the toxicological study of raw leachate. It was not comprehensive and enough impressive for the risk assessment of the landfill leachate. This was mainly because the penetrating process through the underground environment was often accompanied by transportation and transformation of leachate contaminants. Tracking changes of sensitive redox material during penetrating process, the underground penetrating process of leachate may pass through the different redox zone, namely aerobic respiration, nitrate reduction, iron and manganese reduction, sulfate reduction and methanogenesis etc. This reminded us that the environmental behavior of contaminants in the soil layer may affect leachate ecotoxicity change. As a result, ecological toxicity studies of leachate can not ignored the attenuation of pollutants in the soil layer and we should do some works to more objectively reflect the ecotoxicological effects of leachate in addition to studies on the raw landfill leachate.This dissertation was carried out for the above propose. Lots of work was taken on the changes of complex eco-toxic effects during the penetrating process in the loess soil column. The contaminants attenuation was also determined with the same column. Three different redox zones were observed in the penetrating process via monitoring the changes of sensitive redox material in the loess soil. The correlation analysis between the above two results were performed to explore the mechanism of eco-toxic effects of landfill leachate in the underground environment.1. A loess soil column was established to simulate the underground environment and leachate leakage happened on this soil column. There were several out-flowing ports set on the different penetrating distance (12cm,24cm,36cm,48cm,60cm,72cm, and84cm). The out-flowing leachates were sampled from the different ports and collected for the analysis of physical and chemical characteristics. The results showed that the pH, conductivity, COD and ammonia nitrogen decreased with the penetrating distance. The content of heavy metals such as Cu, Zn, Ni, Mn, Pb and Cd, etc. displayed an upward trend before48cm port, reached the highest value at12-24cm ports, and quickly declined after the36-48cm ports. GC-MS analysis and element analysis of leachate sample on the neighboring48cm port suggested that some heavy metals and organic compounds (especially oxygen-, sulfur-or nitrogen-containing phenyl compounds) should be responsible for the changes of complex ecotoxic effects in the penetrating process.2. The phytotoxic effects of landfill leachate were investigated on the barley seedlings. The results showed that the complex phytotoxic effects of leachate samples changed with penetrating process in loess soil column, demonstrated that:(1) Landfill leachate inhibited the growth and chlorophyll levels, elevated the levels of lipid peroxidation and protein oxidation, and stimulated the antioxidant enzymes’activities of barley seedlings. The effects generally displayed a peak value at12-24cm, slowly declined at36-48cm, and then rapidly decreased with penetrating distance in column.(2) Via statistical correlation analysis between the properties of leachate and the biotoxic effects observed, COD, conductivity and heavy metals (esp. Ni, Mn, Cd) were considered to be responsibly correlated to the variation of biotoxicity, instead of pH and ammonia.(3) The analysis of the organic matter composition found the phytotoxic differences during penetrating process may be related to the changes of nitrogen/sulfur/oxygen-containing phenyls, and the majority of these pollutants were confirmed with endocrine disrupting effects.3. Endocrine disrupters were founded within the landfill leachate, and sexual differences were also displayed on the acute and chronic toxicity of zebrafish. The developmental toxicity of landfill leachate were investigated on the Ohpf and the24hpf zebrafish embryos. The results showed that the condensation blood flow cycle, hatching, edema, and spinal deformity effects of other toxic endpoints were observed when Ohpf and 24hpf embryos were exposed to leachate samples with penetrating process in loess soil column. The detailed results were demonstrated:(1) Landfill leachate can significantly increase zebra fish embryos condensation rate, and inhibit the development of embryonic development process (eye point initiative activities, blood circulation and incubators). The toxicity gradually decreased with a time-dependent and penetrating distance-dependent manner.(2) The out-flowing leachates even after84cm port all can significantly induced teratogenic effects such as pericardial edema and spinal deformity. The visible blastocysts toxicity on Ohpf and24hpf embryos was seen strong and downward with the penetrating process in soil layer.(3) Via statistical correlation analysis between the properties of leachate and embryonic toxcity, COD, ammonia, Ni, Cd, Mn and conductivity were considered to be responsibly correlated to the variation of embryotoxicity. The microbial activity of outflowing leachate sampled from48cm port was significantly higher than the activity from the later ports, and types of contaminants also increased in the outflowing leachate from the same port. This implied that the change of leachate toxicity may be related to biogeochemical role in the penetrating process.4. Only three distinct redox zones (methanogenesis, iron and nitrate reduction zone) were found within the loess soil column. To further explore the correlation between the transportation and transformation of leachate pollutants and its complex toxic, we established the strengthen redox models via adding the electron acceptors to the loess soil column. The leachate were sampled from the different zones and collected for chemical analysis and embryo-toxic endpoint (LC50, meaning as IC50/EC50). The results showed that:(1) The major pollutants of leachate were distributed in the methanogenesis, and then in the iron and nitrate reduction zone. The attenuation in the methanogenesis and iron reduction zones were slower than that in the nitrate reduction zone, indicating that the degradation in nitrate reduction zone could play a major role during penetrating process.(2) The out-flowing leachates from three redox zones all can affect the developmental abnormalities of zebrafish embryos. The toxicity in the nitrate reduction zone was lower than that in the other two redox zones. Chemical analysis suggested that the embryonic teratogenic effects may be associated with phthalate esters, thiazoles, pyrethroids and other endocrine disruptors.(3) The correlation analysis between the properties of leachate and embryonic toxcity showed that the toxic effects of out-flowinig samples from different redox zones was significantly associated which COD, ammonia, conductivity, Ni, Mn. It can be inferred that the changes of teratogenic effects of leachate in penetrating process may be induced by the joint action of endocrine disruptors and heavy metals.The leachate used in this paper were sampled and collected from the ravine-like sanitary landfill in the Loess Plateau. And the soil columns were all established with loess soil to simulating the field conditions. Main contribution of this paper was the illustration on the change of complex eco-toxic effects of leachate during the penetrating process and their possible mechanisms in that. The toxicity of leachate on barley seedlings and zebrafish embryos decreased in a non-linear distance-dependent manner. The transportation and transformation of leachate pollutants in the soil column could be observed to be responsible to the eco-toxicity changes via correlation analyses between them. The further studies found the attenuation process of leachate pollutants in soil column may be divided in to three redox zones according to tracking the sensitive redox materials in the leachate. The above findings of this paper clarify the polluting characteristics for landfill leachate when penetrating in soil column and provided guidance for polluting control and risk assessment of landfill leachate. |
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