Prediction and control of elevated temperatures within landfills under aeration and recirculation based on the thermal non-equilibrium model.

Aeration is an effective approach to sustainable landfilling but may lead to elevated temperatures within landfills, resulting in landfill fires or explosions. Therefore, aeration is usually combined with leachate recirculation to control the elevated temperatures within landfills. To predict landfill temperatures during aeration and recirculation, a local thermal non-equilibrium model is developed considering the heat generation of biodegradation, the heat removal due to evaporation and leachate-gas flow, and the effects of the capillary. The solver is implemented in OpenFOAM based on the finite volume method and validated against a waste-column experiment and an in-situ aeration test. The simulation results demonstrate that the assumption of local thermal equilibrium will distinctly overestimate the temperature, maximally by 15 °C in the studied case. The model is then used to simulate a typical aerobic landfill unit to investigate the formation of explosive gas mixtures and elevated temperatures under different operating conditions. The simulation results of gas composition suggest that aeration will not result in explosive gas within landfills. A reasonable recirculation method for temperature control with corresponding operating parameters under a group of values of aeration pressure (2000-4000 Pa) and recirculation rate (0.0001-0.0008 m/s) are proposed, which can provide some guides for the design of an aeration and recirculation combined system. For a given total volume of added leachate, a higher recirculation rate does not always mean better cooling, and the cooling effect of continuous recirculation is better than that of intermittent recirculation.

Modeling the spreading and remediation efficiency of slow-release oxidants in a fractured and contaminated low-permeability stratum.

Traditional remediation technologies cannot well remediate the low permeability contaminated stratums due to the limitation in the transport capacity of solute. The technology that integrates the fracturing and/or slow-released oxidants can be a new alternative, and its remediation efficiency remains unknown. In this study, an explicit dissolution-diffusion solution for the oxidants in control release beads (CRBs) was developed to describe the time-varying release of oxidants. Together with advection, diffusion, dispersion and the reactions with oxidants and natural oxidants, a two-dimensional axisymmetric model of solute transport in a fracture-soil matrix system was established to compare the removal efficiencies of CRB oxidants and liquid oxidants and to identify the main factors that can significantly affect the remediation of fractured low-permeability matrix. The results show that CRB oxidants can achieve a more effective remediation than liquid oxidants under the same condition due to the more uniform distribution of oxidants in the fracture and hence a higher utilization rate. Increasing the dose of the embedded oxidants can benefit the remediation to some extent, while at small doses the release time over 20 d has little impact. For extremely low-permeability contaminated stratums, the remediation effect can be significantly improved if the average permeability of the fractured soil can be enhanced to more than 10-7 m/s. Increasing the injection pressure at a single fracture during the treatment can enlarge the influence distance of the slow-released oxidants above the fracture (e.g., 0.3-0.9 m in this study) rather than below the fracture (e.g., 0.3 m in this study). In general, this work is expected to provide some meaningful guidance for the design of fracturing and remediating low permeability contaminated stratums.

Influence mechanism of thermally enhanced phase change on heat transfer and soil vapour extraction.

Thermal enhanced soil vapour extraction (T-SVE) is a remedial technique involving gas, aqueous, solid and nonaqueous phases along with mass and heat transfer. Interphase mass transfer of contaminants and water evaporation/condensation will cause the redistribution of phase saturation, eventually affecting the performance of T-SVE. In this study, a multiphase, multicompositional and nonisothermal model was developed to simulate the T-SVE treatment of contaminated soil. The model was calibrated using published data from the SVE laboratory and T-SVE field experiments. The temporal and spatial distributions of the contaminant concentrations in the four different phases, the mass transfer rates and the temperatures are presented to reveal the coupling interactions that occur between multiple fields during T-SVE. A series of parametric studies were carried out to investigate the effect of water evaporation and adsorbed/dissolved contaminants on the T-SVE performance. It was found that endothermic evaporation, exothermic condensation and the interaction between different removal paths of a contaminant played critical roles in the thermal enhancement of SVE. Ignoring them can result in significant differences in the removal efficiency values.

Contaminant back-diffusion from layered aquitards subjected to barrier-controlled source zones.

Low-permeability aquitards may serve as secondary sources of slow-releasing contaminants into the adjacent aquifer system, creating considerable obstacles to groundwater cleanup. Accurately capturing the exchange of contaminant mass between aquitards and aquifers can facilitate site management and remediation. Previous simulation studies were mainly limited to one-dimensional (1D) back diffusion from aquitards during the remediation of the source zone. In this study, a novel two-dimensional (2D) back-diffusion model is developed to investigate the storage and release of contaminants in aquitards after source isolation. This model coupled the dynamical decay of isolated sources and the diffusion-sorption process of contaminants in the layered aquitards. Exact analytical solutions for the present 2D multilayer model were derived using the finite cosine transform, Duhamel Theorem, separation of variables, and transfer matrix method. Results indicated that the previous 1D model would overestimate the contaminant concentration in the aquitard and the back-diffusion risk when the source zone was isolated. The proposed 2D back-diffusion model enables quantitative prediction of how source zone width, source concentration, and aquitard heterogeneity impact plume trailing time, thus aiding in understanding the mechanisms of aquifer contamination beyond barrier-controlled source zones.

pH-sensitive nano-polyelectrolyte complexes with arthritic macrophage-targeting delivery of triptolide.

Since pro-inflammatory macrophages take on a critical significance in the pathophysiology of rheumatoid arthritis (RA), the therapeutics to affect macrophages may receive distinct anti-RA effects. However, the therapeutic outcomes are still significantly impeded, which is primarily due to the insufficient drug delivery at the arthritic site. In this study, the macrophage-targeting and pH stimuli-responsive nano-polyelectrolyte complexes were designed for the efficient targeted delivery of triptolide (TP/PNPs) on the arthritic site. The anionic and cationic amphiphilic copolymers, i.e., hyaluronic acid-g-vitamin E succinate (HA-VE) and the quaternized poly (ß-amino ester) (QPBAE-C18), were prepared and then characterized. The result indicated that TP/PNPs with the uniform particle size of âˆ¼ 175 nm exhibited the high drug loading capacity and storage stability based on the polymeric charge interaction, in which DLC and DEE of TP/PNPs were obtained as 11.27 ± 0.44 % and 95.23 ± 2.34 %, respectively. Mediated by the "ELVIS" effect of NPs, CD44 receptor-mediated macrophage targeting, and pH-sensitive endo/lysosomal escape under the "proton sponge" effect, TP/PNPs exhibited the enhanced cellular internalization and cytotoxicity while mitigating the inflammation of LPS-activated RAW 264.7 cells. Even after 96-hour after administration, PNPs were preferentially accumulated in the inflammatory joints in a long term. It is noteworthy that after treatment for 14 days with 100 µg/kg of TP, TP/PNPs significantly facilitated arthritic symptom remission, protected cartilage, and mitigated inflammation of antigen-induced arthritis (AIA) rats, whereas the systematic side-effects of TP were reduced. In this study, an effective drug delivery strategy was proposed for the treatment of RA.

The era of low-permeability sites remediation and corresponding technologies: A review.

Rational utilization of soil resources and remediation of contaminated soils are imperative due to the rapidly growing demand for clean soils. Currently, many in-situ remediation technologies are less suitable at low-permeability sites due to the limitations of soil permeability. This work defines a low-permeability site as a site with hydraulic conductivity less than 10-4 cm/s, and summarizes the migration characteristics of representative contaminants at low-permeability sites, and discusses the principles and practical applications of different technologies suitable for the remediation of low-permeability sites, including electrokinetic remediation technology, polymer flushing technology, fracturing technology, and in-situ thermal remediation technology. Enhanced and combined remediation technologies are further described because one remediation technology cannot remediate all contaminants. The prospects for the application of remediation technologies to low-permeability sites are also proposed. This work highlights the necessity of low-permeability sites remediation and the urgent need for new remediation technologies, with the hope to inspire future research on low-permeability sites.

Forward and back diffusion of reactive contaminants through multi-layer low permeability sediments.

Contaminants stored in the low permeability sediments will continue to threaten the adjacent shallow groundwater system after the aquifer is remediated. Understanding the storage and discharge behavior of contaminants in the aquitards is essential for the efficient remediation of contaminated sites, but most of the previous analytical studies focused on nonreactive solutes in a single homogenous aquitard. This study presents novel analytical solutions for the forward and back diffusion of contaminants through multi-layer low permeability sediments considering abiotic and biotic environmental degradation. Three representative source depletion patterns (i.e., instantaneous, linear, and exponential patterns) were selected to describe the dissolution of dense non-aqueous phase liquids (DNAPL) in the aquifer more realistically. At the forward diffusion stage, the mass storage of contaminants in the aquitards with the instantaneous pattern is the largest, nearly twice that with the exponential pattern. A simple equivalent homogeneous model is generally adopted in the risk assessment. However, relative to the proposed multi-layer model, it will significantly underestimate the onset of the back-diffusion of heterogeneous aquitards and overestimate the persistence of aquifer plumes. The previously-reported semi-infinite boundary assumption is also not applicable, with a maximum error of over 200% in the long-term prediction of back diffusion behavior of a thin aquitard. Moreover, when the degradation half-life is less than 16 years, less than 10% of the contaminants stored in the aquitards will diffuse into the overlying aquifer, suggesting that biostimulation or bioaugmentation can effectively mitigate back-diffusion risk. Overall, the proposed diffusion-reaction coupled model with multi-layer media is of great value and high demand in predicting the back-diffusion behavior of heterogeneous aquitards and guiding the soil bioremediation.

Investigating the roles of advection and degradation in chlorinated solvent back-diffusion from multi-layer aquitards: A novel analytical approach.

Aquitards contaminated by chlorinated solvents may act as a secondary source slowly releasing contaminants into adjacent aquifers, thus severely hampering the remediation of groundwater systems. Accurate predicting the long-term exchange of solvents between aquifers and aquitards can more effectively guide site management and remediation. This study presented a general analytical model for the back-diffusion of chlorinated solvents through multilayer aquitards. This model considers the slow advection and local degradation of dissolved constituents in natural aquitards and the dynamic depletion of dense nonaqueous phase liquid (DNAPL) source zone in aquifers. Transient solutions for the proposed multilayer model were derived using Duhamel's Theorem, the separation of variables method, and the transfer matrix method, verified against experimental and numerical concentration data. Results reveal that advection in aquitards can significantly shorten the trailing time of chlorinated solvent plumes, and highly adsorptive soils may reduce this effect in layered aquitards. The previous no-degradation model is no longer applicable to predict the back-diffusion behavior of chlorinated solvents when the extent and rate of solvent degradation are large, giving a "strong-effect zone". Based on numerous example simulations and data fitting, the forecast functions for the back-diffusion onset time and plume trailing time were proposed, greatly facilitating remediation decisions and risk assessment of chlorinated-solvent contaminated sites.

Approximate analytical model for transient transport and oxygen-limited biodegradation of vapor-phase petroleum hydrocarbon compound in soil.

Volatile organic compounds (VOCs) contamination may occur in subsurface soil due to various reasons and pose great threat to people. Petroleum hydrocarbon compound (PHC) is a typical kind of VOC, which can readily biodegrade in an aerobic environment. The biodegradation of vapor-phase PHC in the vadose zone consumes oxygen in the soil, which leads to the change in aerobic and anaerobic zones but has not been studied by the existing analytical models. In this study, a one-dimensional analytical model is developed to simulate the transient diffusion and oxygen-limited biodegradation of PHC vapor in homogeneous soil. Laplace transformation and Laplace inversion of the Talbot method are adopted to derive the solution. At any given time, the thickness of aerobic zone is determined by the dichotomy method. The analytical model is verified against numerical simulation and experimental results first and parametric study is then conducted. The transient migration of PHC vapor can be divided into three stages including the pure aerobic zone stage (Stage I), aerobic-anaerobic zones co-existence stage (Stage II), and steady-state stage (Stage III). The proposed analytical model should be adopted to accommodate scenarios where the transient effect is significant (Stage II), including high source concentration, deep contaminant source, high biodegradation capacity, and high water saturation. The applicability of this model to determine the breakthrough time for better vapor intrusion assessment is also evaluated. Lower first-order biodegradation rate, higher source concentration, and shallower source depth all lead to smaller breakthrough time.

Enhanced delivery of amendments in fractured clay sites based on multi-point injection: An analytical study.

Fracturing technology that can enhance the delivery of amendments has attracted attention in the remediation of low-permeability contaminated sites. However, there are few works on the enhanced delivery of amendments based on multi-point injection in a fracture-matrix system. This study develops a two-dimensional analytical model for enhanced delivery of amendments in a finite-domain low-permeability matrix through multi-point injection in a natural, hydraulic or pneumatic fracture. The mechanisms of advection, diffusion, dispersion, sorption and degradation are considered in the model and any injection form (e.g., pulse injection, periodic injection or slow-release injection) can be embedded to obtain a specified solution. Then, a new linear factor R*, which is the ratio of the peak concentration to the trough concentration on the same plane, is introduced to evaluate the concentration fluctuation in the fracture and matrix. Results show that with a stronger line source formed in the fracture right after injection (corresponding to a small R*), the concentration distribution of amendments in the matrix is more uniform at each depth resulting in a smaller residual rate, i.e., (R*-1) × 100%. If the injection wells have been installed unreasonably (e.g., a too large spacing), the continuous injection time is an effective controllable parameter to compensate for this defect. Moreover, a controlled slow-release system can maintain a more stable concentration distribution in the fracture than continuous injection and periodic injection systems, giving a longer residence time. Overall, this work is expected to provide some interesting guidelines for the design of multi-point injection in the fracturing low-permeability sites to enhance the remediation of contaminated soil.

Transient migration behavior of VOC vapor in layered unsaturated soils subjected to multiple time-dependent point pollution sources: Analytical study.

Predicting the migration behavior of volatile organic compounds (VOCs) vapor is essential for the remediation of subsurface contamination such as soil vapor extraction. Previous analytical prediction models of VOCs migration are mostly limited to constant-concentration nonpoint sources in homogeneous soil. Thus, this study presents a novel analytical model for two-dimensional transport of VOCs vapor subjected to multiple time-dependent point sources involving transient diffusion, sorption and degradation in layered unsaturated soils. Two representative time-dependent sources, i.e., an instantaneous source and a finite pulse source, are used to describe the short-term and long-term leakage. Results reveal that soil heterogeneity can cause pollution accumulation, especially in low-diffusivity capillary fringe. The assumption of an equivalent plane source from multiple point sources would significantly overestimate the vapor concentration and the contaminated range. The previous single point source model is no longer inapplicable when the relative distance and/or the release interval between sources is small, giving a strong interaction between multiple sources. Moreover, a faster vapor degradation rate or a higher groundwater level will reduce the area of vapor plume linearly. Hence, close attention should be paid to the time-variation characteristics of multiple sources, the vapor degradation and the groundwater level fluctuation in practice to facilitate soil remediation. The proposed model is a promising tool for addressing the above issue.

Two-dimensional analytical solution for subsurface volatile organic compounds vapor diffusion from a point source in layered unsaturated zone.

Although migration of subsurface volatile organic compounds (VOCs) from contaminant sources in unsaturated soil widely exists, the related analytical models are quite limited. A two-dimensional analytical solution is hence developed to simulate vapor diffusion from the subsurface contaminant source in the layered unsaturated zone. The contaminant source is simplified as a point source leaking at a constant rate. The influences of several important factors, including thickness of stagnant air layer, depth of groundwater table, source characteristics and soil layering characteristics, on vapor migration in subsurface soil are comprehensively investigated by the present model. Soil type does not affect the normalized vapor concentration profile for homogeneous soil, which is not valid for layered soil. The width and effective diffusivity of the upward diffusion pathway and downward diffusion pathway are favorable indexes to evaluate the intensity of subsurface vapor horizontal diffusion. The single-layer capillary fringe assumption overestimates the vapor plume, the assumption can give acceptable result for coarse soil while it is recommended to divide the soil into several layers based on the water-filled porosity profile for fine soil.

An analytical model for one-dimensional diffusion of degradable contaminant through a composite geomembrane cut-off wall.

A one-dimensional analytical model is proposed to analyze contaminant diffusion through a composite geomembrane cut-off wall (CGCW) composed of a geomembrane (GMB) and a bentonite cut-off wall (BCW). The model considers degradation process of contaminant and time-dependent inlet boundary condition which are common in engineering practices. Moreover, two limiting scenarios of the exit boundary condition (EBC) of CGCW for field conditions are taken into account, including the flushing and non-advective semi-infinite aquifer EBCs. The influence of boundary conditions and performance of CGCW are comprehensively investigated. The results show that the upper and lower limits of the mass flux of the exit face of CGCW can be obtained by the models with flushing EBC and the model with non-advective semi-infinite aquifer EBC, respectively. In addition, degradation has substantial influence on the contaminant migration, and smaller half-life in BCW results in smaller contaminant leakage. The performance of CGCW can be improved by embedding GMB at a proper location which is related to the type of contaminant and EBC. Furthermore, thickening HDPE GMB or adopting a coextruded EVOH GMB is efficient to improve the performance of CGCW. The present model can be used as an applicable tool for rational design of CGCW.

Design method of a modified layered aerobic waste landfill divided by coarse material.

To overcome the weaknesses of traditional landfills, a modified aerobic landfill concept with intermediate covers of coarse material between waste layers functioning as facilities of drainage and aeration has been proposed recently. In this study, a one-dimensional coupled model, including aerobic biodegradation, oxygen diffusion, and advection, is proposed to describe oxygen distribution in this new type of landfill. Homotopy analysis method and perturbation method are applied to solve this model at passive aeration and active aeration, respectively. The model has six input variables, that is, oxygen diffusion coefficient, gas permeability, maximum oxygen consumption rate, layer thickness of waste, and injection pressure and extraction pressure. A combination of their typical values gives rise to over 700,000 scenarios which can be calculated by the proposed solution. The coupled effect of the above variables on oxygen migration is quantitatively investigated, followed by an estimation formula of the minimum oxygen concentration in waste layer. The maximum waste layer thickness is defined as a function of other variables for a given aeration target of oxygen volume concentration larger than 5%. A generalized design method of waste layer thickness, injection pressure, and extraction pressure is then developed for the newly proposed modified layered aerobic landfill, which can promote its popularization and application.

Analytical model for organic contaminant transport through GMB/CCL composite liner with finite thickness considering adsorption, diffusion and thermodiffusion.

A new analytical model for organic contaminant transport through GMB/CCL (geomembrane and compacted clay liner) composite liner is developed, which can consider adsorption, diffusion and thermodiffusion processes and is applicable for typical bottom boundary conditions. The separation of variables method is adopted to derive the solution. The present model is first verified against experimental results and a numerical model. The influence of thermodiffusion on organic contaminant transport in composite liner is then investigated. Toluene is adopted as the representative organic contaminant. The results reveal that when the Soret coefficient ST is not less than 0.01 K-1, the effect of thermodiffusion should be taken into account on the contaminant transport in GMB/CCL composite liner in wet landfills. When the Soret coefficient ST is 0.03 K-1, the breakthrough time of a GMB + 0.75 m CCL composite liner and a 2 m CCL would be overestimated by 20% to 76% due to omitting of the effect of thermodiffusion. Namely, the barrier performance would be greatly overestimated if the effect of thermodiffusion is neglected in these cases. In other aspects, the thermal conductivity of GMB and CCL has little effect on the contaminants transport in GMB/CCL composite liners, so there is no need to modify the materials for this parameter. The present model is an applicable tool for evaluating the barrier performance of the GMB/CCL composite liner, and can provide valuable advices for improving the liner materials.

Comprehensive overview of numerical modeling of coupled landfill processes.

Landfilling is the primary method used for municipal solid waste (MSW) disposal. To design, optimize, and manage landfills with a life span of several decades, a deeper understanding of long-term MSW behaviors is necessary and worthwhile. These behaviors should be modeled using approaches that account for coupled processes so as to capture the evolutionary mechanisms that are mainly dominated by biochemical, mechanical, hydraulic, and thermal processes, as well as the complex interactions among them. Many mathematical models have been developed over the past three decades to address this issue. However, most of them only emphasize some of these processes, with only few models accounting for all the processes. In this review, we present a comprehensive overview of the mathematical and numerical formulations of this coupled problem. Each process occurring in landfills is interpreted in detail using different sub-models and the corresponding parameter values. Then, the existing coupled models for MSW are reviewed, and the challenges and perspectives related to the modeling of the long-term behaviors of MSW are highlighted. We conclude that more reliable constitutive formulations based not only on well-designed laboratory tests but also on field tests are necessary to improve the modeling of MSW behaviors in future.

Control and estimation of maximum gas pressure below landfill cover with horizontal gas wells: Analytical study.

Horizontal spacing of horizontal extraction gas wells can be designed to achieve a 90% pumping rate of the total generated landfill gas (LFG) from given waste properties (viz: gas permeability, landfill gas generation and non-homogeneity with depth), cover characteristics and vacuum pressure. However, cover characteristics and vacuum pressure are also important design parameters and different combinations of them result in different distributions of gas pressure in the waste, some of which would induce problematic air intrusion while others might pose threat to cover stability. This paper uses the maximum gas pressure directly below cover to distinguish these combinations, and provides the first study of the effects of the above parameters on potential outcomes. The ability of the overlying cover to resist LFG emission from the landfilled waste is suggested not to exceed a critical value, otherwise the maximum gas pressure below it would become at least 1 kPa larger than atmospheric pressure. A design formula for this critical value is proposed with respect to typical values of waste properties, vacuum pressure and the buried depth of horizontal wells in wide ranges. Together with consideration of recovery efficiency, the proposed method can be used to design a horizontal extraction gas collection system and a cover system in better working condition, and to evaluate the maximum gas pressure below cover. These applications are illustrated by a worked example.

Two-dimensional analytical solution for VOC vapor migration through layered soil laterally away from the edge of contaminant source.

A two-dimensional analytical solution is developed to simulate vapor migration in layered soil laterally away from the edge of contaminant source and has advantages in considering the vapor concentration profile in a functional form near the source edge. The analytical solution is validated against existing analytical solution, numerical model and experimental results. It has also proved to be an alternative screening tool to evaluate the vapor intrusion (VI) risk by compared with existing VI assessment tools. The influence of the characteristics of contaminant source and soil layer on the VI risk are investigated. The existence of capillary fringe effectively reduces VI risk. Among all the single-layer-soil cases, the lateral inclusion zone for sand is the widest due to the thinnest capillary fringe and the lowest effective diffusivity ratio between soil and capillary fringe. For layered soil, the lower effective diffusivity layer overlying the higher one enhances the horizontal diffusion and extends the lateral inclusion zone. The width of lateral inclusion zone increases logarithmically with increasing source concentration while it increases linearly with increasing source depth. Based on the calculation results, a simplified formula is proposed to preliminarily estimate the width of lateral inclusion zone for the typical single-layer-soil cases considering the capillary fringe.

Effect of LCRS clogging on leachate recirculation and landfill slope stability.

Vertical wells are commonly used for recirculating leachate into a landfill which can offer significant environmental and economic benefits. However, in some cases, the leachate collection and removal system (LCRS) at the bottom is overloaded and clogged due to biological and chemical processes. This results in a relatively high leachate level which could pose a threat to landfill slope stability. This study develops a three-dimensional landfill slope model with vertical recirculation wells and then investigates the effect of LCRS clogging on leachate recirculation and slope stability in terms of leachate saturation, pore water pressure, and factor of safety (FS) of a landfill slope. The results show that with an increase in clogging level that is characterized by an increased leachate level, the pore water pressure below the well injection screen is significantly increased by leachate recirculation, giving rise to a decreased slope FS value. In such conditions, the landfill slope formed by highly anisotropic waste is more likely to suffer instability. To prevent this kind of slope failure, a safe injection pressure of vertical recirculation wells is proposed for a wide range of parameter combinations involving waste anisotropy, clogging level, and the setback distance from the slope surface. This design guideline can be used to control the injection pressure in leachate recirculation applications and contributes to a better understanding of the slope stability of a bioreactor landfill.

Design of horizontal landfill gas collection wells in non-homogeneous landfills.

Considering exponential decreases in gas permeability and gas generation of waste with depth, a two-dimensional analytical model is developed to describe the landfill gas (LFG) recovery using horizontal wells. This model is used to simulate more than 680,000 scenarios involving typical values of waste properties, cover characteristics and design parameters for horizontal wells (seven variables in total). The coupled effect of these seven variables on air intrusion and the gas recovery efficiency of horizontal wells are investigated. It is shown that all the variables examined, except for the two variables defining waste non-homogeneity, could be integrated into three dimensionless variables. The horizontal spacing and buried depth of horizontal wells are examined as a function of cover characteristics, waste properties, and vacuum pressure to allow the development of a generalized design method for horizontal wells. An upper limit of horizontal well spacing is defined (for an 85% recovery rate) and a simple formula is provided which can be used to estimate the corresponding level of air intrusion. The upper limit spacing is shown to be affected by the non-homogeneity in gas permeability of waste, cover characteristics, and buried well depth. Using a worked example, the proposed method is shown to be capable of estimating air intrusion into existing horizontal gas collection wells and to optimize the design of horizontal wells considering waste non-homogeneity.