Study On The Effect Of Rainfall On Redox Zones And Pollutants Degradation In Landfill Leachate Polluted Groundwater

Recentaly, it has become a worldwide environmental problem that leachates leaking from municipal solid waste landfill contaminate groundwater. Landfill leachates may be characterized as water composed of dissolved organic matter, inorganic macrocomponents, heavy metals and xenobiotic organic compounds, most of which are toxic and harmful, this poses a great threat to the safe use of groundwater.In an aquifer with a continous leachate relaease, redox zones will be developed. Theoretically,a methanogenic zone(MZ) elvolves close to the landfill, with and downgradient from which are in sequence of sulfate reduction zone(SRZ), iron reduction zone(IRZ), maganese reduction zone(MRZ), nitrate reduction zone(NRZ) and oxygen reduction zone(ORZ). These zones are significant to the study that is to predict polluted groundwater, to control and remediate pollution.A series experiments were conducted to study the redox zones phenomena, to propose a method of assigning redox zone, to study the evolution of different zones with time, dynamic temporal and spatial change of some pollutants, influences laws of different rainfall intensity and different composition of rainfall on the redox zones and degradation of some pollutants.To study the redox zones in the landfill leacates polluted groundwater plume is important to the controlling and remediation of area polluted by landfill leachate. Method and standard of assigning redox zones is the base of the studying. Sampling points taking place multi electron-acceptor processes (EAPs) account for 57.74% of all the sampling points, which indicates that redox process in the polluted groundwater plume is very complicated; some places may take place one EAP, while other places may take places two or much moreEAPs; it is indicated that assigning redox zones based on the concentration range of redox sensitive species has limitations.The method of assigning redox zones proposed in this paper can be divided into two steps. The first step is: to caculate the redox sensitive species concentrations of different samplins points at different time which formula is deduced from the chloride dilution formula, to compare the caculated concentrations with the detecded ones, then to judge the EAPs taking place in the sampling points. The sampling points just take place one EAP is easy to assign redox zone, while the the sampling points take place more EAPs will enter into the second step. The second step is: to calculate the contributions to the degradation of organic matter by different EAPs, then to judge the redox zones according to the caculated contributions.The method proposed in this paper which is based on contributions to the degradation of organic matter by different EAPs is effective and feasible, 83.55% of the sample points could be assigned a redox label.Experiments were carried out to study the evolution of redox zones with time, dynamic temporal and spatial changes of some pollutants, and the results indicated that:(1)Redox zones do exist in the polluted groundwater with a continous leachate relaease, a MZ elvolves close to the landfill, with and downgradient from which are in sequence of IRZ, NRZ, and ORZ. (2) Redox zones tend to develop downwards at first, and then migrate to the downgradient when it reaches to the water-resisting floor.(3)Not all of the theoretical redox zones will appear in the redox zones evolution course simutaneously. NRZ appears at the 30th day, whose shape is narrow, its area increases first and then decreases with time elapsing. IRZ and MZ do exist in the whole course of the experiment, areas of which increase with time elapsing. ORZ tends to become narrow with time, which is predicted that it will become smaller and smaller gradually till it disappears when the experiment is going on. The whole system evolves to the more reductive environment.(4) Chloride, CODCr and ammonium show similar change laws during the course of evolution of the system, which increase first and then decrease or fluctuate; the time that different sampling points take to reach the maximam is related to their positions. Generally speaking, the closer the sampling point closes to the leakeage point in the horizontal and vertical direction, the shorter time that the sampling point takes to reach the maximam.Experiments were carried out to study the effect of different rainfall intensity on redox zones and pollutants degradation in landfill leachate polluted gruondwater, it can be concluded that:(1) Rainfall does affect the redox zones in landfill leachate polluted groundwater. Influence laws are:â‘ after the rainfall, areas of MZ and IRZ decrease, while areas of NRZ and NRZ increase, the whole system develops to the more oxidative environment.â‘¡with the increase of the rainfall intensity, the area impacted by the rain increases too; which is shown as: area decreasing percentages of MZ and IRZ after the light rainfall and strong rainfall increase, area increasing percentages of NRZ and ORZ increase.â‘¢with the increase of the rainfall intensity, the time that redox zones take to recover to the previous situation becomes longer. After rainfall, the whole system evolves to more oxidative environment, which will quicken the polluttants degradation; this is important to naturae remediation of the polluted area by landfill leachate.(2) Nitrite is one of the most important intermediate products in the microbial metabolism. It is founded in this study that nitrite changes greatly among redox sensitive species and increases dramatically after different intensity rainfall, which concentration after rainfall can be 72.52 times larger than its before rainfall. Therefore, it is pratical and feasible to use nitrite as the index reflecting the lasting time and depth to which redox zones are impacted by different rainfall intensity. It is concluded after the analysis of nitrite that the depth impacted by the strong rainfall is deeper than the one impacte by the light rainfall and the time impacted by the strong rainfall is longer than the one impacte by the light rainfall.(3) It is concluded that rainfall does stimulate microbial metabolism and organic nitrogen degradation after r of CODCr and statistical datas of nitrate, nitrite, ammonium before and after light rainfall are analysed. With the increase of rainfall intensity, dilution also increases.Experiments were carried out to study the effect of normal rainfall, high electron-acceptor rainfall and acid rainfall on redox zones and pollutant degradation in landfill leachate polluted groundwater. Results indicate that:(1) Areas of redox zones before and after different composition rainfall at the same intensity are analysed, it can be concleded that:â‘ As to the MZ, areas of MZ after normal rainfall and high electron-acceptor rainfall decrease first and then increase, while area of MZ after aciad rainfall increases, which indicate that methanogenic bacteria activity is inhibited by normal rainfall and high electroacceptor rainfall, stimulated by the acid rainfall.â‘¡As to the IRZ, areas of IRZ after three kinds of rainfall decrease first and then increase, which indicates that iron reductive bacteria is inhibited by rainfall. Area decreasing percentage of IRZ after acid rain is the biggest of all the three compositon rainfalls and the time that IRZ recovers to the situation before acid rainfall is the longest of all them too, which indicates that inhibition affected by acid rainfall is the stongest of all the three compositon rainfalls.â‘¢As to the NRZ, areas of NRZ after the three kinds rainfall all increase first and then decrease, Which indicates that nitrifying bacteria activity is enhanced, pollutants degraded by nitrifying bacteria also increase.â‘£As to the ORZ, area of ORZ after normal rainfall increases first and then decreases, while areas of ORZ after high electron-acceptor rainfall and acid rainfall decrease. In the early days of the pollution, rainfall will be beneficial to enhance aerobiosis competition activity and to the pollutants degraded by aerobiosis.(2) Nitrite is a very good index reflecting influence and effect of different composition rainfalls on microbial metabolism.â‘ Microbial metabolism stimulation by the high electron-acceptor rainfall is stronger than normal rainfall and the stimulation time is longer than normal rainfall, which does faver to the pollutants degradation.â‘¡Nitrifying bacteria activity in the polluted groundwater is inhibited by acid rainfall, which is unfavorable to the pollutants degradation.(3)Dilutions of chloride due to three kinds of rainfalls at the same rainfall intensity are very close. Decrease percentage of CODCr after high electron-acceptor rainfall is more than the one after normal rainfall. Ammonium change laws after rainfall is related to the nitrate concentration in the rain, with the nitrate concentration in the rain increase, ammonium concentration in the polluted groundwater will decrease. Increase of some matters concentration in the rain which are terminal electron-acceptors in the microbial metabolism course (such as nitrate) will be beneficial to pollutants degradation.Rainfall is one of the most common phenomena in nature. Oxygen (O2), nitrate (NO3-), and sulfate (SO42-) that are common components in the rain could be introduced to the groundwater system as the more favroble terminal electron acceptors in the microbial metabolism process, which may change the redox environment and affect the nature attenuation of contaminants. Therefore, it has great realistic significance that to study the effect of rainfall on redox zones and pollutants degradation in landfill leachate polluted groundwater. The innovations of this work are that:(1)An effective and feasible method of assigning redox zones was proposed. The method of assigning redox zones proposed and used by other researchers was chosen by conserding redox sensitive matter concentration in leachate and polluted water of the plume, which was subjective; it is also difficult to determine redox zones in the zones transition part. The above problems can be settled by using the method proposed in this paper based on contributions to the degradation of organic matter by different electron acceptor processes, which is more objective and scientific.(2)Dilution and convection were quantificationally distinguished from other attenuations. Generally speaking, dilution and convection were not quantificationally considered in other researchers.(3)Nitrite was proposed to use as an index which can reflect the lasting time and depth to which redox zonation are impacted by different rainfall intensity. Indexes reflecting depth to which redox zones were impacted by rainfall used by other researchers did not change clearly. It was founded in this paper nitrite changed dramatically aftrer different rainfall intensity. Therefore, it is pratical and feasible to use nitrite as the index reflecting the lasting time and depth to which redox zonation are impacted by different rainfall intensity.(4)Effect of different intensity and composition of rainfall on redox zones and pollutants degradation in landfill leachate polluted groundwater was studied. There were some reaesrches on the effect of one recharge event, while there is no research on the effect of different intensity and composition of rainfall on redox zones and pollutants degradation in landfill leachate polluted groundwater; so this study enriches this area.