Study On The Natural Attenuation Of Landfill Leachate Pollutants In The Underground Environment And Enhanced Remediation Technology Of Polluted Arquifer

In recent years, along with the economical development, the quantity of solid waste increased year by year, and the landfill pollution to the underground environment has become more and more serious. Landfill leachate is one kind of wastewater in the landfill which including highly concentrated organic matter, ammonium nitrogen and heavy metals. Once seeping it can cause serious pollution to the underground environment. There are sorts of methods to control and remediate the pollution, such as, the"pump and treat", the permeable reaction barrier and so on. However, these methods are not only expensive but also difficult to apply. Considering our country's present conditions, they are not feasible. At the beginning of the 90's, some researches indicated that, the natural underground environment has potential ability to degrade pollutants. From then on, natural attenuation regarded as one of remediation methods is studied further.In the underground environment the effect which can attenuate the pollutants mainly including: geochemistry reaction, biodegradation and so on. Through these effects, the pollutants'concentration and quantity decrease gradually, and the harm to environment and people is reduced, either. But the natural attenuation process is slow, testifying its validity before applying it in the pollution site is necessary. Therefore, it has practical significance for adopting the suitable measurements to control and remediate underground environment, and simultaneously it provides the theory basis to landfill pollution in underground environment risk assessment, to study the landfill leachate natural attenuation and its influence factors in underground environment, and to analyses its natural attenuation mechanism,.According to the domestic and foreign researches situation, a series of experiments were conducted to study six aspects as follows: the migration transformation and the nature attenuation mechanism of landfill leachate pollutants in different mediums of unsaturated zone, the first order dynamics of organic biodegradation in different unsaturated zone, the nature attenuation mechanisms of landfill leachate pollutants in aquifer, the influences of aquifer types in nature attenuation of pollutants, the influences of. landfill leachate types in nature attenuation of pollutants, as well as the enhanced nature attenuation of landfill leachate in aquifer. Accorrding to the experiment results analysis,we study how biodegradation of unsaturated zone contribute to the pollutants natural attenuation, and make a quantitative analysis of biodegradation in the natural attenuation.In the experiment of study on process and factor of leacheat pollutants natural attenuation in arquifer, we analyze the process and character that how does the leachate pollute groundwater,and also study on the main factor that how leachate pollutants affects natural attenuation in arquifer. Besides, base on the above, the enhanced remediation technology of polluted arquifer is discussed. Compare with the prevenient study, the innovations of this work are: (1) it studies the function of biodegradation in leachate natural attenuation by inhibitor of biological effect, analyzes the biodegradation kinetics of leachate pollutants, and calculates the biodegradation coefficient of CODcr. It offers theories and data basis which are about leachate polluted site control and remediation, risk assessment, self-purification ability assessment; (2) based on leachate natural attenuation theory, it studies the enhanced natural attenuatin of leachate pollutant by addition of electron acceptor (FeCl3 and K2SO4) and nutrition (NaH2PO4).With leaking, the landfill leachate first passes through the unsaturated zone. Study on the natural attenuation of landfill leachate pollutants in the unsaturaed zone is the premise of the research on groundwater pollution by landfill leachate. The natural attenuation of pollutant in unsaturated zone is mainly influenced by the absorption and biodegradation. The results of the experiment which simulate the pollutants natural attenuation in unsaturated zone has indicated that: The absorption ability of unsaturated zone medium is limited, while the biodegradation has the main effect on the pollutants decreasing. We can calculate theoretic absorption quantity to CODcr and NH4+ respectively in sand and silty clay is: 52.36mg/kg, 35.34mg/kg and 133.33mg/kg, 96.15mg/kg. In sands and silty clay, the CODcr first order biodegradation dynamic equation is: CODcr = 64221e-0.0017t and CODcr = 65495e-0.0033t.When landfill leachate enters the aquifer, the function which affects the attenuation of the pollutants has changed. The tests of attenuation of landfill leachate pollutants in aquifer indicated that: (1) in the place which is far away from the leaking point, the pollutant concentration is influenced by dilution mainly, in the place nearby the leaking point, the biodegradation of pollutant is more and more important; (2) the landfill leachate will sink continuously after it enters the groundwater, when it meets the aquitard it would accumulate on aquitard and become the second pollution source; (3) when landfill leachate enters the aquifer, it not only polluted the groundwate but also enhanced the redox capability of the aquifer by adding NO3- and Fe3+ into the groundwater, however, the duration of nitrate reduction reaction was very short, after the leachate seeped 40 days, the NO3- concentration in the groundwater dropped to 1mg/L, the duration of iron reduction reaction is long, until seeped 100 days , the content of Fe3+ in aquifer still maintained about 20mg/L.The attenuation experiments indicated that the effect of attenuation is closely related to aquifer medium and the components of leachate. Therefore, they become the main factors in the natural attenuation.The effects of aquifer types on the pollutants natural attenuation are mainly in the following aspects: (1) the retardation of CODcr and NH4+ are different, the CODcr, NH4+ retardation factors of grit and sand are: 1.221, 1.176 and 1.307, 1.252,respectively; (2) the natural attenuation rate are also different,the average natural attenuation rate of CODcr in grit and sand are: 272.96mg/(L·d) and 336.42mg/(L·d); (3) the influence on the leachate NO3- is different, the nitrate reduction rate is faster in the sand than the grit; (4) the effect on the NH4+ is different, for the organic natural attenuation rate is faster in the sand than in the grit, the degradatin of organic nitrogen into NH4+ leads the higher NH4+ concentration in the sand than in the grit. (5) the removal efficiency of heavy metal in the sand is higher than grit, at the end of the experiment, the removal efficiency of Cu, Cd, Pb and Zn in the grit and sand are 23.91%,62.5%,45.45%,94.89% and 51.09%,87.50%,84.85%,97.14%.The effects of leachate types on the pollutants natural attenuation are as follow: (1) the retardation of aquifer media on the leachate pollutant was stronger in the acidification phase leachate than in the methane generation phase leachate. The retardation factors of grit on the CODcr and NH4+ of these two types of leachate were 1.221, 1.176 and 1.307, 1.252; (2) the velocity of organic pollutant attenuation in acidification phase leachate is higher than that in the methane phase leachate, and in the risk assessment of groundwater pollution by landfill leachate, we can not only consider of the concentration of the organic substances but also the attenuation speed; (3) with the natural attenuation of the pollutants, reaction of nitrate reduction in acidification phase leachate is more rapid than that in methane phase leachate, in the end of experiment, both NO3- concentrations decrease by 63.16% and 43.24%; (4) the rules of NH4+ attenuation in acidification and methane phase leachate have huge differences, the concentration of NH4+ in acidification phase leachate increases firstly and then decreases, the concentration of NH4+ in methane phase leachate declines with the time; (5) the Fe3+ concentration in acidification phase is far more than that in methane phase leachate, that is to say the potential ability of organic substances attenuation in acidification phase leachate is far more than that in methane phase. After well known of the mechanism and the factors of leachate natural attenuation in the aquifer, this essay proposed a measurement which added electron acceptor and the nutriment into the aquifer to enhance attenuation efficiency. The electronic acceptor is Fe³⁺ and SO₄^2-; the nutriment is H₂PO₄^-. The experimental results indicated that, adding the electronic acceptor or the nutriment both can effectively enhance the efficiency of the organic pollutants attenuation, but the former is better, and the addition of Fe³⁺ is better than SO₄^2- . When the experiment ended, compared with the natural condition, adding Fe³⁺, SO₄^2- and H₂PO₄^- makes the pollutants removal efficiency increase by: 32.12%, 25.62% and 14.47%.