Contaminant transport from landfills

Abstract

Point sources such as landfills, can release high concentrations of contaminants into the groundwater because of migration of leachate from its bottom, which is generated primarily as a result of precipitation falling on an active landfill surface, leaching out the potential organic and inorganic contaminants from landfilled waste and discharging the same to groundwater in underlying aquifer. To protect the groundwater from contamination, it is quite essential to provide the bottom barrier of suitable thickness. The present study was undertaken to determine the rate of movement of potential contaminants from the bottom of a landfill, so as to evolve a rational method for the determination of thickness of landfill liner. A mathematical model was formulated to express the mass transport of contaminants from a landfill due to the migration of leachate. Various mechanisms of contaminant migration from the bottom of landfill were taken into account. The solution of the model in the form of concentration profile of potential contaminants below landfill was obtained using the explicit Finite Difference Method implemented in MatLab 7.0. The model developed was validated for two cases of field data. The simulations of model were run for a range of permeability values, equivalent height of leachate, and retardation factors. The results of such simulations in the form of maximum relative concentration at various depths, at the end of design period of landfill were used for preparing the design charts. For the conservative contaminant, design charts have been prepared for a range of equivalent height of leachate to represent the mass of landfilled waste.For the non‐conservative contaminant, design charts have been prepared for a range of retardation factors. For the purpose of design charts the simulation of the model was run with finite mass of the contaminant as the upper boundary. Initial concentration (background) was assumed to be zero for a fully flushed boundary. The model gives the spatial and temporal variation of concentration along the thickness of the barrier and the maximum concentration recorded in each depth for the entire domain (50 Years). The simulation was run for time domain of 50 years, and the time step was determined after satisfying the stability criteria of solution. The design charts so prepared can be used for determination of minimum thickness of landfill liner on the basis of maximum permissible relative concentration of contaminant species of interest at the bottom of landfill liner. Design charts were prepared for both conservative and non conservative contaminant. The contaminant transport model developed in this study was applied for determining the impact of Bhalaswa landfill at Delhi on the groundwater in its vicinity. For determining the impact of landfill on groundwater of the region, simulations of the model were run for tracer contaminant Chloride, and the results of such simulations were compared with the observed values of Chloride concentration in the groundwater of the region. Groundwater samples were collected at varying radial distances from the landfill and in the direction of groundwater flow and analyzed for key contaminants. The results of such sampling and analysis show gradually decreasing concentration of the contaminants away from the landfill. Gas generation from Bhalaswa landfill has been estimated using public domain software based on first order kinetics. Analysis of the model results was carried out to determine the impact of model parameters viz. time period of simulation, equivalent height of leachate, depth on the transport of contaminants from Bhalaswa landfill. The results of simulation show that Equivalent height of the leachate will have only marginal effect on the transport of chloride or DOC from Bhalaswa Landfill. The expected gas generation from Bhalaswa Landfill site were computed using public domain simulation software based on first order kinetics. The results of simulations show a considerable potential of gas generation form such landfills which can be collected and utilized for energy recovery. Experimental studies were carried out on eight admixtures (five prepared using synthesized organoclay and three prepared using manufactured organoclay with bentonite and natural soil as remaining constituents) for the determination of their hydraulic and sorption characteristics These admixtures were subjected to laboratory investigations with the objective of investigation the potential use of organoclays with soil‐bentonite admixtures in retarding the movement of contaminants while maintaining required permeability specifications. Such amendment is likely to have impact on the minimum thickness required. In order to evaluate the potential effectiveness of sorptive materials as amendments to clay liners, sorption and permeability tests were performed on admixtures obtained by adding varying amount of the organoclay to the natural soil, and purified bentonite. The results of such studies have been used for transport modeling simulations to determine the effectiveness of each of these admixtures with regard to retardation of organic contaminants from landfill leachate and consequent impact on minimum thickness of liner. Design charts were drawn for the determination of minimum liner thickness based on organic contaminant, and using these admixtures. Retardation factor of the admixtures was found to increase with increase of organoclay content. The sorption behavior of all the admixtures have been found to be consistent and scalable.