Study On The Biogeochemistry And The Influence Factors In The Landfill Pollution Sites

The municipal landfill leachate pollution to the surrounding environment like water, soil has been increasingly serious. The attenuation of the leachate pollutants in the subsurface environment is closely related to the microbial activity. Therefore, it's important to take a deep research on the biochemical process occurring in the leachate pollution sites, which can provide a theoretical basis and reference for control and management of these sites.According to the foreign and domestic research situation, a serial of column and static experiments were conducted to study the rule of microbial activity and organic matter content's temporal and spatial variation both in the liquid phase and the deposit phase in the different leachate contaminated sites; to explore the electron acceptor's consumption rate, the redox zones'turnover, the mutual relations of the redox sensitive compounds in different redox zone; to analyze the influencing factors of the biochemical process occurring in the subsurface pollution environment. Results of the experiments indicate that:(1) For the continuously pollution sites, the microbial activity in the effluent went up as times went on and went down with the distance both in the soil and in the sand medium. The increasing rate was fast in the soil than that in the sand. In the deposit phase the microbial activity was 1.5～2.5 times in the soil than that in the sand. The microbial activity respectively decreased 1.56×10^-3A/cm and 1.94×10^-3A/cm in the liquid phase and deposit phase in the soil, 1.06×10^-3A/cm and 0.92×10^-3A/cm in the sand.For the controlled pollution site without leaching any more, the microbial activity underwent the lag phase, logarithmic phase, stationary phase and decline phase. The stationary phase was longer in the soil. The microbial activity growth rates were respectively 0.081A/d and 0.022A/d in soil and sand. The decline rates were respectively 0.0095A/d and 0.0051A/d in soil and sand. The peak values were respectively 0.259A and 0.197A in soil and sand.(2)For the continuously pollution sites, the concentration of TOC in the effluent increased first and then decrease with the time and decreased with the distance in both the soil medium and the sand medium. In liquid phase the TOC concentration reduced 22.74 mg·L^-1·cm- 1 and 16.18 mg·L^-1·cm^-1 respectively in the soil and the sand. In deposit phase the organic matter decreased 0.115 g·kg^-1·cm^-1 and 0.0625 g·kg^-1·cm^-1 respectively in the soil and the sand. For the controlled pollution site, the organic matter was decreased with the time. It decreased 6.45%, with a rate of 0.195g·kg^-1·d^-1 in the soil and 22.5%, with a rate of 0.135 g·kg^-1·d^-1 in the sand. The decreasing occurred in the stationary and decline phase.(3) In the landfill leachate pollution site, the variation rule of the redox sensitive matters were as follows:The average consumption rates of DO were 0.16 mg·L^-1·d^-1 and 0.14 mg·L^-1·d^-1 respectively in the soil and the sand. The oxygen reduction zone migration rates were about 3.4cm·d^-1 and 4.3cm·d^-1 respectively in the soil and the sand.The nitrate reduction zone was smaller, and migration rate was faster than the oxygen reduction zone. The background value of nitrate in the soil was higher than that in the sand. The average consumption rate of the nitrate were about 2.1mg·kg-1·h-1 and 0.045 mg·kg-1·h-1 respectively in the soil and the sand. During the electron acceptor's reduction process, the intermediate NO₂^- was not found remaining. The concentration declined sharply below the detection limit after reached the peak, where the dissolved oxygen concentration was around 3～4mg·L^-1. This indicated that aerobic denitrifying bacteria in the nitrate reduction zone are rather active. However when the dissolved oxygen concentration was under 1mg·L^-1, the nitrate reduction process stopped, which meant to a certain extant, dissolved oxygen in the low would inhibit the nitrate reduction process. The iron reduction zone was in a wide range, and lasted for a long time. Its migration rate was slower than the oxygen reduction zone.The iron reduction zone came first in the soil. The migration rate was faster in the soil. The average growth rates of Fe²⁺ were 12.7 mg·kg^-1·d^-1 and 2.55mg·kg^-1·d^-1 respectively in the soil and in the sand. The dissolved oxygen concentration was below 0.5mg·L^-1 in the soil and below 0.1mg·L^-1 in the sand. After Fe3+ gradually became the main terminal electron acceptor utilized by the microbes to degrade organic matter, the concentration of TOC/organic matter declined significantly. This illustrate that iron reducing bacteria played an important role in the natural attenuation of organic matter in the landfill leachate pollution sites.The migration rate of Sulfate reduction zone was faster in the sand than that in the soil. In the soil column the sulfate reduction zone was basically behind the iron reduction zone, while in the sand column there were large area of overlap between the sulfate reduction zone and the iron reduction zone(4) impact to the microbial activity and attenuation rate of organic matter from the Environmental factors(nutrient, electron acceptor, temperature, humidity, etc.) were as follows:In the soil the microbial activity peak value increased 0.426A for the addition of the nutrient nitrogen and phosphorus. It made the active area wider and the stationary phase prolong 4 days. Compared with the control group, the concentration of the organic matter in the nutrient group decreased 6.78%, and the decomposition rate increased 0.21 mg·kg^-1·d^-1. In the sand the microbial activity peak value increased 0.391A, and the logarithmic phase prolonged 5 days with the addition of the nutrient. The microbial activity was still in the stationary phase till the end of the experiment without decline, keeping active. Compared with the control group, the concentration of the organic matter in the nutrient group decreased 11.17%, and the decomposition rate increased 0.055 mg·kg^-1·d^-1. With the addition of electron acceptor NO₃^-, the microbial logarithmic phase in the soil was prolonged, and the peak value increased 0.024A. Adding electron acceptor SO₄^2- did not enhance the microbial activity peak value, but extend the stationary phase for 5 days. In general, the microbial activity peak value was higher in the NO3- group than that in the SO₄^2- group; the stationary phase was longer in the SO₄^2- group than that in the NO3- group. The electron acceptor NO₃^- was easy to use rapidly by the denitrifying bacteria, so the organic matter decomposition occurred mostly at the beginning of the experiment. Compared with the control group, the organic matter decreased 3.85% in the NO3- group, and 2.42% in the SO₄^2- group. In the sand, the lag phase was shortened and the logarithmic phase was prolonged with the addition of electron acceptor NO₃^-. The microbial activity peak value increased 0.147A. In the SO₄^2- group the stationary phase prolonged 2～3 days. Compared with the control group, the organic matter decreased 6.4% in the NO₃^- group, and 5.9% in the SO₄^2- group.Low temperature depressed the microbial activity and inhibited the decomposition of the organics. The organic matter decreased 6.45% and 1.77% respectively in 25℃and 4℃. The average decomposition rate were 0.195 mg·kg^-1·d^-1 and 0.055 mg·kg^-1·d^-1 in 25℃and 4℃.Raising humidity made the microbial stationary phase prolonged, and the natural attenuation rate of organic matter increased. When the moisture content were 50% and 60%, the organic matter decreased 6.45% and 9.84%, and the average decomposition rate were 0.195 mg·kg^-1·d^-1 and 0.305 mg·kg^-1·d^-1 respectively.