Depth Of Landfill Research Articles

Investigating the characteristics of combustible fraction of legacy waste: A study on energy recovery potential and GHG emission quantification

In India, majority of the generated municipal solid waste (MSW) was dumped in poorly managed landfills and dumpsites over the past decades and is an environmental and health hazard. Landfill mining is a promising solution to reclaim these sites along with the recovery of resources (materials and energy). During landfill mining operations, the combustible fraction is one of the major components recovered and needs proper management for maximizing resource recovery. For the identification of appropriate resource recovery options, knowledge of the physicochemical characteristics is required. The present study aims to assess the depth-wise change in the composition of legacy waste and the physicochemical characteristics of the combustible fraction. Furthermore, a material flow analysis considering the incineration of combustible fraction was performed to estimate the energy generation potential and the associated greenhouse gas (GHG) emissions. The results of the compositional analysis of dry legacy waste revealed that the fine fraction (<4 mm soil-like material) was dominating with a share of 36%. The depth-wise analysis showed a decrease in the calorific value with increasing landfill depth, while no specific trend was observed for the other parameters analyzed, including proximate and ultimate analysis, and chlorine content. The material flow analysis performed for 100 tonnes of wet legacy waste indicated that 52 tonnes of waste is combustible fraction. The GHG emissions through incineration of one tonne of dry combustible fraction would be 1389 kg CO2-eq, with 1125 kWh of electrical energy generation potential.

Numerical simulation to optimize passive aeration strategy for semi-aerobic landfill

Passive aeration has been proven to be efficient for oxygen supply in landfill. The combination of passive aeration and semi-aerobic landfill offers a cost-effective and energy-efficient approach to solid waste (SW) treatment. However, determining the optimal strategy for this combination has remained unclear. This study aimed to investigate the strategy of passive aeration in a semi-aerobic landfill using numerical simulation methods. A model coupled hydrodynamic model and compartment model for degradation of SW was implemented. The accuracy was well validated by comparing measured and simulated results in a pilot-scale landfill. Compared with natural convection, passive aeration by funnel caps could increase air input by 20 %. By simulating volumetric fraction distribution of CO2, CH4 and O2 in landfill, an orthogonal experiment including 4 factors was designed to identify that the diameter of tubes (DT), the spacing between tubes (ST) and the landfill depth (LD) have substantial impacts on aerobic zone ratio (AZR) of landfill. But the diameter of gas ports (DGP) has an indiscernible effect. The optimized factors were determined to be as follows: DT = 0.3 m, ST = 15.0 m, DGP = 0.05 m, and LD = 4.0 m, under which the semi-aerobic landfill could enhance SW degradation. Large diameter and spacing of tubes are favorable to improve the AZR at the top of the landfill, and the aerobic zone mainly exists near the ventilation tubes. These findings contribute to the development of more efficient and sustainable solid waste treatment strategies in semi-aerobic landfill.

A Study on the Impermeability of Nanodispersible Modified Bentonite Based on Colloidal Osmotic Pressure Mechanisms and the Adsorption of Harmful Substances

With the growing demands of human beings, sanitary landfill, along with the increase in landfill depth and leachate water pressure, has put forward new and higher requirements for the impermeable layer. In particular, it is required to have a certain adsorption capacity of harmful substances from the perspective of environmental protection. Hence, the impermeability of polymer bentonite–sand mixtures (PBTS) at different water pressure and the adsorption properties of polymer bentonite (PBT) on contaminants were investigated through the modification of PBT using betaine compounded with sodium polyacrylate (SPA). It was found that the composite modification of betaine and SPA could reduce the average particle size of PBT dispersed in water (reduced to 106 nm from 201 nm) and enhance the swelling properties. As the content of SPA increased, the hydraulic conductivity of PBTS system decreases and the permeability resistance improves, while the resistance to external water pressure increases. It is proposed a concept of the potential of osmotic pressure in a constrained space to explain the impermeability mechanism of PBTS. The potential of osmotic pressure obtained by linear extrapolation of the trendline of colloidal osmotic pressure versus mass content of PBT could represent the external water pressure that the PBT resist. Additionally, the PBT also has a high adsorption capacity for both organic pollutants and heavy metal ions. The adsorption rate of PBT was up to 99.36% for phenol; up to 99.9% for methylene blue; and 99.89%, 99.9%, and 95.7% for low concentrations of Pb2+, Cd2+, and Hg+, respectively. This work is expected to provide strong technical support for the future development in the field of impermeability and removal of hazardous substances (organic and heavy metals).

The key role of denitrification and dissimilatory nitrate reduction in nitrogen pollution along vertical landfill profiles from metagenomic perspective

Landfill are persistent sources of nitrogen (N) pollution even in the decades after closure. However, the biological pathways of N-pollution, particularly N2O and NH4+, at different landfill depths have received little attention. In this study, metagenomic analysis was conducted on landfill refuse from vertical reservoir profiles in two closed landfills named XT and MT. NH4+ concentrations were found to be higher in deeper layers of MT, while greater potential for N2O emissions occurred in XT and the shallow layers of MT. Furthermore, the community structure and function of N-metabolizing microbes were more strongly defined by landfill depth than landfill type. Denitrification, involving abundant nirK and norB genes, was identified as the major pathway for N2O production in both XT and MT-shallow, while dissimilatory nitrate reduction with abundant nirBD genes was identified as the major pathway for NH4+ accumulation. Microbes of norB-type and nirBD-type were positively affected by NO3− in XT, whereas negatively affected by contents of organic material and moisture in MT-shallow. The mechanism by which nitrogen fixation, with abundant nifH genes, contributes to NH4+ accumulation in MT-deep should be further elucidated. These findings can provide a theoretical basis for governing scientific N-pollution control strategies throughout the entire landfill process.

A landfill serves as a critical source of microplastic pollution and harbors diverse plastic biodegradation microbial species and enzymes: Study in large-scale landfills, China

Microplastics (MPs) are emerging pollutants. Landfills store up to 42% of worldwide plastic waste and serve as an important source of MPs. However, the study of MPs distribution and the plastic biodegradation potential in landfills is limited. In this study, the distribution of abundance, size, morphology and polymer type of MPs and plastics biodegradation species in refuse samples along landfill depths were extensively investigated within a large-scale landfill in Shenzhen, China. In addition, plastics biodegradation enzymes were evaluated in seven Chinese large-scale landfills leachate. MPs distribution pattern was investigated in all refuse samples. The abundance of MPs in refuse samples varied between 81 and 133 items/g. The size of MPs in all samples varied between 0.03 and 5 mm, and the average sizes were 1.2 mm ± 0.1 mm. The main morphology and polymer type were fragments and cellophane, respectively. Landfill depth was significantly negatively correlated with the relative abundance of MPs size 1–5 mm (p < 0.05) and was positively correlated with the relative abundance of MPs size < 0.2 mm (p < 0.05), suggesting that plastics were broken down during municipal solid waste decomposition. The multiple regression on matrices analysis further showed the landfill depths and plastic morphology significantly impact the MPs distribution. The strains, Lysinibacillus massiliensis (with relative abundance of 1.8%) for low-density polyethylene and polystyrene biodegradation, and Pseudomonas stutzeri (0.1%) for low density polythene and polypropylene biodegradation, were detected on the plastic surface with high relative abundance. Furthermore, 75 plastic degradation species and their associated 31 enzymes (breakdown 24 plastics) were discovered in seven landfills leachate samples.

Microplastics in Landfill Bodies: Abundance, Spatial Distribution and Effect of Landfill Age

Almost a quarter of the plastic produced in Europe still ends up in landfills. In addition to the loss of valuable resources, this leads to the generation and accumulation of microplastics in landfills. The microplastics abundance in the refuse and their spatial distribution in the landfill body have not been practically studied. In the current work, changes in the abundance and characteristics of microplastics in landfill refuse from 3 age sections of the Lapės regional landfill, Lithuania, to a depth of 10–20 m were studied. A microplastics abundance of up to 55 particles/g or 52.8 g/kg was found. The lowest microplastics abundance was found in the old section, while the highest in the young (numerical) and the middle-aged (mass) sections. Moreover, microplastics abundance increased with the age of landfilled waste and depth, which may reflect the fragmentation of microplastics and their transport. Polyethylene and polypropylene were the dominant polymer types in all sections, while films were the dominant shape. The carbonyl index of PE microplastics was calculated to monitor microplastics oxidation. The analysis showed an increase of carbonyl index with landfill depth and landfill age, proving the intensive degradation of microplastics. Thus, landfills are large reservoirs of microplastics and their potential sources.

Influence of dissolved organic matter and heavy metals on the utilization of soil-like material mined from different types of MSW landfills

Soil-like material (SLM) mined from municipal solid waste (MSW) landfills can be used as nursery cultivation soil, landfill cover, and as a building material. However, SLM utilization is restrained by heavy metal (HM) contents whose speciation and migration are influenced by their dissolved organic matter (DOM) content. Therefore, the properties of aged refuse and the correlation between DOM and HM forms were studied using samples from different types of MSW landfills. The dominant components of aged refuse were SLM (18.80%–83.51%) and plastics (11.17%–65.51%). The moisture, organic matter, and pH ranged from 29.55% to 57.92%, 15.70%–57.68%, and 7.84–8.51, respectively. The Zn content was highest (455.48–1379.27 mg/kg) in the SLM, followed by Cu (96.29–428.90 mg/kg), Cr (49.10–236.21 mg/kg), Pb (53.52–222.71 mg/kg), and Ni (20.92–39.10 mg/kg). The SLM cannot be used for agriculture because the HM contamination exceeds the multiple of 0.07–7.99. Zinc in the acid-soluble state and reducible state had the highest mobility in SLM. However, Cu and Pb, mainly in the oxidizable state, and Cr and Ni, in the oxidizable and residual states, were relatively stable. In the sanitary and simple MSW landfills, the average proportion of protein-like materials decreased from 84.44% to 82.61% and from 65.58% to 55.94%, respectively, as the landfill depth increased. Both the acid-soluble and oxidizable HM states and all forms of Zn in the SLM were significantly positively correlated with tyrosine-like materials (r = 0.58*–0.87**). Protein-like materials may enhance the mobility of HMs.

Vertical distribution of antibiotics and antibiotic resistance genes in a representative municipal solid waste landfill, China

The vertical distribution of sulfonamides (SAs), tetracyclines (TCs), macrolides (MLs), and their related antibiotic resistance genes (ARGs) were comprehensively investigated and characterized in a representative municipal solid waste (MSW) landfill in China. The total concentrations of target antibiotics in the MSW landfill were SAs > TCs > MLs. The abundances of mexF (10.78 ± 0.65 log10copies/g) and sul genes (9.15 ± 0.54 log10copies/g) were relatively high, while the tet genes (7.19 ± 0.77 log10copies/g) were the lowest. Both the abundance of antibiotics and genes fluctuated with landfill depth, and the ARGs of the same antibiotics were consistent with depth change. Intl1 and sul genes (sul1, sul2) were tightly connected, and a close relationship also existed between tet genes (tetM, tetQ) and MLs resistance genes (ermB, mefA). High-throughput sequencing showed the dominant genera were Sporosarcina (38%) and Thiobacillus (17%) at sampling points A and C, while the microbial community varied with depth increase at point B were Brevundimonas (20%), Sporosarcina (20%), Pseudomonas (24%), Lysobacter (28%), and Thioalkalimicrobium (14%), respectively. Network analysis further visualized the relationship among antibiotics, genes, and microbial communities and the results indicated the non-random connection among them and the possible host of the target gene. Even at 12.0 m below the landfill surface, the pollution of antibiotics resistance was still serious, which posed difficulties for subsequent landfill remediation and pollution control.

Effects of Environmental Factors on the Spatial Succession of the Bacterial Community in Municipal Solid-Waste Landfills

Secondary pollution control and rapid stabilization of municipal solid waste (MSW) landfills are closely associated with bacterial communities. The spatial variations in bacterial community structure and the environmental factors controlling it were studied by the 16S rRNA gene sequencing of MSW sampled at different depths in a large sanitary landfill (LSL) and a medium non-sanitary landfill (MNSL), respectively. The total nitrogen (TN) and total phosphorus (TP) content in the LSL and MNSL samples displayed a downward trend as the depth increased. However, the organic matter (OM) content followed the opposite trend. Diversity of microbes was highest in the middle layer of both the LSL and MNSL. Firmicutes and Proteobacteria were the dominant phylum both in the LSL and MNSL. In the LSL, the dominant bacteria genera succeeded from Psychrobacter, Pseudomonas to Clostridium, Savagea. However, in the MNSL, the genera succeeded from Uncultured, Pleomorphomonas to Hydrogenispora, Acetomicrobium. The type of landfill and the different environmental factors control the succession of the bacterial community in LSL and MNSL. The pH, OM, TN, and TP will change in the landfill as the landfill depth increased. The impact degree of landfill depth on pH, OM, TN, and TP content in the LSL was 15.3%, 16.1%, 4.9%, and 4.2%, respectively. These values were lower than those in the MNSL. In addition, depth and OM had the strongest correlations but no significance with the dominant bacteria both in the LSL (|Rdepth|average=0.48;|ROM|average=0.36) and MNSL (|Rdepth|average=0.51;|ROM|average=0.36). This indicated that bacterial community succession was mainly dependent on space, time, and MSW characteristics in different types of landfills.

Effect of depth of landfill on the characteristics of soil-like material of aged waste: a case study of Bhalswa dumpsite, India

Effect of depth of landfill on the characteristics of soil-like material of aged waste: a case study of Bhalswa dumpsite, India

Microplastics spatiotemporal distribution and plastic-degrading bacteria identification in the sanitary and non-sanitary municipal solid waste landfills

The municipal solid waste landfill (MSWL) is an important source of microplastics (MPs) and a huge bioreactor for plastic-degrading microorganisms (PDM). However, the spatiotemporal distribution and degradation mechanisms of MPs in MSWLs are unclear. Therefore, they were studied using the samples drilled in a sanitary landfill (SL) and an non-sanitary landfill (NSL). The results showed that there were a lot of polyethylene (PE), polypropylene (PP), polystyrene (PS), polyurethane (PU), Polyamide (PA), Polyethylene terephthalate (PET) and Polyvinyl chloride (PVC) in the landfill, and their abundance ranged from 0 to 80 items/g. The MPs surface gradually faded, became rough and even yielded cracks and holes with the landfill depth and age increase. The tiny-size MPs (< 100 µm) were the most abundant and their amount significantly increased from 28.14% to 49.13% in SL and from 24.54% to 59.51% in NSL, respectively, while large-size MPs were significantly reduced from the top to the bottom. Lysinibacillus (0.21%~67.87%) and Bacillus (0.10%~67.00%) were the dominate PDMs in SL and Candidatus_Caldatribacterium (5.06%~73.48%) was the dominate in NSL. The PE degradation was closely related to Candidatus_Cloacimonas (r = 0.688*) and Candidatus_Caldatribacterium (r = 0.680*); PS and PA were closely related to Candidatus_Contubernalis (r = 0.595*~0.705*) and PVC was closely related to Candidatus_Caldatribacterium (r = 0.547*). In addition to physical and chemical effects, biological effects can also promote the MPs formation in MSWLs.

매립깊이에 따른 폐기물매립시설 안정화 평가

In this study, the stabilization of the waste landfill in the sanitary landfill currently in operation was evaluated according to the landfill depth. The results of property survey of landfill waste ratio of soil is the highest according to the landfill depth, vinyls from 23.6 %~37.5 %, 22.4 %~23.3 %, 15.2 %~19.3 % and 15.8 %~13.2 % for each landfill depth. The ratio of C according to the landfill depth was the highest at 35.53 %, the landfill period 3~5 years, landfill depth 15 m and 5 m at 5~10 years and 10~15 years, showing a ration of 26.21 % and 18.29 %. The CH₄ concentration decreased depending on landfill period(3~5 years, 5~10 years, 10~15 years, more than 15 years), as investigated in 52.5 %, 38.3 %, 29.4 % and 20.1 %. Estimate the stabilization by landfill depth (5 m, 10 m and 15 m) according to organic matter and C/N ratio as a logarithmic function, about 40 years, 43.6 years and 36.6 years. Linear function was found to take 26 years, 26.6 years and 23.9 years.

Effect of gas pressure on municipal solid waste landfill slope stability.

Slope failure in municipal solid waste (MSW) landfills is a common environmental disaster that poses serious ecological and health risks. Landfill slope stability (SS) is sensitive to leachate levels and gas pressure (GP) caused by the degradation of organic material, but the extent of these combined effects remains poorly understood. In this study, a simplified landfill GP calculation method is presented and a circular slide method that considers the combined effects of leachate and GP is established. The results show that the landfill GP is mainly affected by the gas production rate, gas conductivity of the solid waste (SW), and landfill depth. The safety factor of landfill SS is also significantly lower when GP is considered. The distribution of GP is affected by the depth of the failure circle and SW. Landfill slope instability can be explained by localized damage caused by GP breakthrough of the filled SW. This study probably provides important guidance for the design, operation, and management of MSW landfills.

Multivariate analysis of the effects of age, particle size and landfill depth on heavy metals pollution content of closed and active landfill precursors

Multivariate analysis of a heavy metal pollution survey of closed and active landfill precursors was carried out in order to compare environmental risk levels in relation to age, particle size and depth of the precursors. Landfill precursors (77) were collected and analyzed for 15 USEPA toxic heavy metals using ICP-MS. Heavy metals concentrations in closed landfill precursors were significantly higher than those in the active landfill for 11 of 15 heavy metals investigated (closed landfill order: Fe > Al > Mn > Cu > Pb > Ba > Co > Cr > Ni > Cd > As > Se > Ti). Cluster analysis and correlation studies indicated the distribution of the metals was more influenced by landfill precursor size than by depth of the sample. Principal component analysis (PCA) showed that 10 of 15 of heavy metals of both landfill precursors were from similar anthropogenic sources. Heavy metals pollution indices (Igeo > 5, EF > 40 and CF > 7) of both active and closed landfill precursors exceeded limits in the order of Zn > Cd > Pb > Cu > Ag, indicating a major potential health risk influenced by age and particle size of precursor. Zn, Cd, Cu and Pb of both landfill precursors exceeded the USEPA set standard for assessment of human health risk for each of the metals (1 × 10−4 to 1 × 10−3). This study highlights the need for the integration of a clean-up process for precursors from both types of landfill to reduce possible environmental pollution during a reuse process.

Anisotropy estimation of compacted municipal solid waste using pressurized vertical well liquids injection.

Waste hydraulic conductivity and anisotropy represent two important parameters controlling fluid movement in landfills, and thus are the key inputs in design methods where predictions of moisture movement are necessary. Although municipal waste hydraulic conductivity has been estimated in multiple laboratory and field studies, measurements of anisotropy, particularly at full scale, are rare, even though landfilled municipal waste is generally understood to be anisotropic. Measurements from a buried liquids injection well surrounded by pressure transducers at a full-scale landfill in Florida were collected and examined to provide an estimate of in-situ waste anisotropy. Liquids injection was performed at a constant pressure and the resulting pore pressures in the surrounding waste were monitored. Numerical fluid flow modeling was employed to simulate the pore pressures expected to occur under the conditions operated. Nine different simulations were performed at three different lateral hydraulic conductivity values and three different anisotropy values. Measured flowrate and pore pressures collected from conditions of approximate steady state were compared with the simulation results to assess the range of anisotropies. The results support that compacted municipal waste in landfills is anisotropic, provide anisotropy estimates greater than previous measurements, and suggest that anisotropy decreases with landfill depth.

Evolution of unsaturated hydraulic properties of municipal solid waste with landfill depth and age

Successful modeling of liquid and air flow and hence designing of liquid and air addition systems in the landfills are constrained by the lack of key parameters of unsaturated hydraulic properties of municipal solid waste (MSW), which are strongly dependent on the depth of burial and the degree of decomposition. In this study, water retention curves (WRC) of MSW are measured using pressure plate method on samples repacked according to the in situ unit weight measured during borehole sampling, representing the MSW in shallow, middle, and deep layers. The measured WRC of MSW is well-reproduced by the van Genuchten–Mualem model, and is used to predict the unsaturated hydraulic properties of MSW, including water retention characteristics and unsaturated hydraulic conductivity. The estimated model parameters are consistent with other studies, suggesting that the pressure plate method yields reproducible results. As the landfill depth and age increase, the overburden pressure, the highly decomposed organic matter and finer pore space increase, hence the capillary pressure increases, causing increases in air-entry values, field capacity and residual water content, and decreases in steepness of WRC and saturated water content. The unsaturated hydraulic properties of MSW undergo changes with landfill depth and age, showing more silt loam-like properties as the landfill age increases.

Field air permeability and hydraulic conductivity of landfilled municipal solid waste in China

The successful design and operation of in situ treatment systems using air and water additions for sustainable landfilling are constrained by a lack of knowledge of the key parameters, such as field air permeability and hydraulic conductivity of landfilled municipal solid waste (MSW). This work provides data on the field air permeability k(a) and hydraulic conductivity K(w) of MSW obtained by conducting short-term air and water injection tests at a landfill in Beijing, China. The k(a) and K(w) values are found to in the range of 1.2 × 10(-13)-1.9 × 10(-12) m(2) and 5.9 × 10(-7)-7.2 × 10(-6) m s(-1), respectively. Both k(a) and K(w) decreased significantly with landfill depth due to the increase in overburden pressure and the finer particles of the waste in deeper layers, leading to a lower porosity of waste. The higher moisture saturation in the deeper layers also contributed to the decrease in k(a). To compare the permeability with respect to air and water, the water permeability k(w) was calculated based on the estimated K(w) and was found to be approximately three orders of magnitude smaller than the corresponding k(a) for waste at the same layer. The differences in k(w) and k(a) may be due to the relative air permeability, the potential short-circuiting of air and active production of biogas, which undermine the relationship between k(w) and k(a). Therefore, to successfully design and operate air and water addition systems in a landfill, in situ measurements of the air permeability and hydraulic conductivity are essential.

Temporal and spatial pore water pressure distribution surrounding a vertical landfill leachate recirculation well.

Addition of liquids into landfilled waste can result in an increase in pore water pressure, and this in turn may increase concerns with respect to geotechnical stability of the landfilled waste mass. While the impact of vertical well leachate recirculation on landfill pore water pressures has been mathematically modeled, measurements of these systems in operating landfills have not been reported. Pressure readings from vibrating wire piezometers placed in the waste surrounding a liquids addition well at a full-scale operating landfill in Florida were recorded over a 2-year period. Prior to the addition of liquids, measured pore pressures were found to increase with landfill depth, an indication of gas pressure increase and decreasing waste permeability with depth. When liquid addition commenced, piezometers located closer to either the leachate injection well or the landfill surface responded more rapidly to leachate addition relative to those far from the well and those at deeper locations. After liquid addition stopped, measured pore pressures did not immediately drop, but slowly decreased with time. Despite the large pressures present at the bottom of the liquid addition well, much smaller pressures were measured in the surrounding waste. The spatial variation of the pressures recorded in this study suggests that waste permeability is anisotropic and decreases with depth.

Solid waste characteristics and their relationship to gas production in tropical landfill

Solid waste characteristics and landfill gas emission rate in tropical landfill was investigated in this study. The experiment was conducted at a pilot landfill cell in Thailand where fresh and two-year-old wastes in the cell were characterized at various depths of 1.5, 3, 4.5 and 6 m. Incoming solid wastes to the landfill were mainly composed of plastic and foam (24.05%). Other major components were food wastes (16.8%) and paper (13.3%). The determination of material components in disposed wastes has shown that the major identifiable components in the wastes were plastic and foam which are resistant to biodegradation. The density of solid waste increased along the depth of the landfill from 240 kg m(-3) at the top to 1,260 kg m(-3) at the bottom. Reduction of volatile solids content in waste samples along the depth of landfill suggests that biodegradation of solid waste has taken place to a greater extent at the bottom of the landfill. Gas production rates obtained from anaerobic batch experiment were in agreement with field measurements showing that the rates increased along the depth of the landfill cell. They were found in range between 0.05 and 0.89 l kg(-1) volatile solids day(-1). Average emission rate of methane through the final cover soil layer was estimated as 23.95 g(-2)day(-1) and 1.17 g(-2)day(-1) during the dry and rainy seasons, respectively.

A model to estimate the methane generation rate constant in sanitary landfills using fuzzy synthetic evaluation

This paper presents a model using fuzzy synthetic evaluation to estimate the methane generation rate constant, k, for landfills. Four major parameters, precipitation, temperature, waste composition and landfill depth were used as inputs to the model. Whereas, these parameters are known to impact the methane generation, mathematical relationships between them and the methane generation rate constant required to estimate methane generation in landfills, are not known. In addition, the spatial variations of k within a landfill combined with the necessity of site-specific information to estimate its value, makes k one of the most elusive parameters in the accurate prediction of methane generation within a landfill. In this paper, a fuzzy technique was used to develop a model to predict the methane generation rate constant. The model was calibrated and verified using k values from 42 locations. Data from 10 sites were used to calibrate the model and the rest were used to verify it. The model predictions are reasonably accurate. A sensitivity analysis was also conducted to investigate the effect of uncertainty in the input parameters on the generation rate constant.