Simulation of Water and Contaminant Transport Through Vadose Zone - Redistribution System

Movement of water in vadose zone, mainly focusing on infiltration and percolation that involves percolation of water under gravity from soil surface and redistribution which is the capillary rise of water movement upwards, is presented. In the global hydrologic cycle, 76% of the precipitating water enters the soil via percolation-infiltration, which leads to the downward movement of water (L’vovich 1974). The water used by natural processes, can move downwards due to infiltration and lift from groundwater table during natural redistribution process. The forecasting of water movement in unsaturated infiltration redistribution system is linked between soil hydraulic properties and hydrologic condition of natural surface water system. The understanding of water movement processes associated with infiltration and redistribution has a number of practical applications. One such application is to predict the fate and transport of materials through soil including nutrients, organic carbon and microbes under natural processes, which in turn will help in developing appropriate management plans for irrigation, fertilizer application and waste disposal on land

Contaminant transport processes in onsite waste disposal systems

Groundwater contamination is an important environmental problem to the present day. The groundwater pollution caused by pathogens from human excreta, particularly in developing countries is directly associated with the lack of safe drinking water and proper sanitation. A septic system is the simplest and the most economic onsite sanitation unit. It includes a tank and a drainage field. A septic tank is designed to receive domestic wastewater, especially black and grey, or a combination of both. Due to the worldwide spread of septic tank usage, it was estimated that they disposed of the largest volumes of wastewater into the ground through their drainage fields. Poorly discharged effluent often contains high concentrations of organic carbon, nutrients and pathogens. Therefore, there is a need to develop a model for estimating the migration of contaminants in a drainage field. The aim of this study was to develop a model for the transport and fate of pollutants discharged by a septic tank below the ground surface. Firstly, a conceptual model was developed to describe removal mechanisms and the fate of contaminants such as chemical oxygen demand (COD), nitrate, phosphate and Escherichia Coli (E.Coli) in unsaturated soil conditions. The governing equations were formulated to support the established conceptual model. The migration of contaminants depends on the advection-dispersion transport and retardation processes that can be evaluated using a modified form of the Richards equation. The retardations are mainly due to adsorption and biodegradation processes that are traditionally described using the multiplicative Monod’s equations and isotherm equation, respectively. Secondly, a mathematical model was developed by modifying these governing equations. The mathematical model is in the form of hyperbolic/parabolic partial differential equations with strong nonlinearity due to pressure head dependencies in the specific moisture capacity, hydraulic conductivity terms and complex retardation processes. In order to solve the mathematical model, a numerical approach was employed. The developed model was solved numerically using the Galerkin finite element method. The numerical model was coded with MATLAB software. Finally, the developed model was calibrated using the writer’s laboratory data and other data obtained from case studies. The movement of water and wastewater and the retardation of contaminants were investigated experimentally and the results of these experiments were compared with the simulations of the developed model. Two types of porous media were used, sand and topsoil. Sand is a uniform and non-reactive porous medium which provides an effective permeability for infiltration. Topsoil is a non-uniform and reactive porous medium which provides various rates of retardation. The infiltration experiments were conducted using laboratory and pilot scale columns of 20 and 120 cm effective heights, respectively. Laboratory scale sand and soil columns were used to determine the hydraulic properties of sand and soil, the movement of water with various boundary conditions such as gravitational, static equilibrium of capillary and infiltrationredistribution flows. A pilot scale soil column was used to examine the transport of contaminants in field conditions. The computational code used in the advective-dispersive transport (Richards’ equation) was applied to the data obtained from the laboratory scale soil columns. The simulation results for the hydraulic pressure head and moisture content in both sand and soil columns matched the observed data well. The contaminant transport model could satisfactorily estimate the contaminant concentration and retardation zone. The simulation results indicate that all contaminants except E.Coli were reduced significantly across a 15 cm depth (elevation of 105 cm) whereas the E.Coli reduction zone was observed within 10 cm depth (elevation of 110 cm) of the soil column. Eight case studies were used in the model verification processes. The developed model could effectively predict the profiles of pressure head and moisture content observed in infiltration and infiltration-redistribution systems. Furthermore, the developed model could predict the profiles of non-reactive and reactive contaminant concentrations presented in all the case studies. This indicates that the developed model is an effective alternative tool for predicting the migration of contaminants in the ground underneath a septic effluent drainage field

Applications of hydraulic properties models on microscopic flow in unsaturated porous media

Several existing equations for solving the non-linear soil-hydraulic properties are introduced and validated to field and laboratory measured data. Models for non-linear hydraulic properties of unsaturated porous media arise from statistical and mathematical fit through the measured data and they can be expressed in forms of unsaturated permeability versus either pressure head or volumetric moisture content. This paper presents the difference models: Gardner, Knuze et al., Haverkamp et al., van Genuchten and Saxton et al. for calculation of hydraulic properties coefficients, typicallyunsaturated permeability. The accurate and computational efficiency of these five existing models are evaluated for a series of study cases simulating hydraulic properties of unsaturated porous media. The results indicate that all existing models can be applied to homogenous and heterogenous unsaturated porous media, dry and wet cycles and laboratory and field measuring data. Besides, the statistical fit model is inefficient compared to mathematical fit models. Among the mathematical fit models, van Genuchten model is the most promising model. Gardner model can be competitive with van Genuchten model and Haverkamp et al. model is less efficient than others. The mathematical fit models appear to be attractive alternatives to estimate the unsaturated permeability, although there are concerns regarding the stability behaviour of the occupied air in pores, which need to be resolved. The air movement in unsaturated porous media affected the unsaturated permeability, which gives the difference results between wet and dry cycle. Both of unsaturated permeability and volumetric water content of dry cycle were higher than ones of wet cycle. This suggests that the velocity of air-releasing during a wet process was higher than the velocity of air-entering during a dry process. The infiltration is the most important land applications. So, the wet cycle hydraulic properties test might be concerned.Moreover, most of infiltration fields locate on the mixed grain media. So too, the pore-size distribution could affect the unsaturated permeability of porous media. It was observed that the finer material, the lower unsaturated permeability

Modeling of Contaminant Transport In on-site waste Disposal Systems

One of the most common problems of septic systems is poor drainage in the field. Septic systems release number of organic contaminants that cause groundwater pollution, especially organic substances, nutrients and pathogenic microorganisms. However, there are mechanisms that operate between soil and contaminants, which can purify the septic tank effluent. Below the drainage field, soil is separated into two zones; unsaturated and saturated zone, which provide the water movement mainly in vertical and horizontal direction, respectively. The purpose of this paper is to develop a conceptual model and to define governing equations for the transport of septic tank effluent in the vadose zone. Using the concept of mass balance and chemical kinetic equations, the governing equations have been derived

Influence of dispersion on transport of tracer through unsaturated porous media

The dispersion phenomenon has resulted from the various water flow magnitude and direction in porous media. The dissolved tracer tends to spread due to dispersion and then travel time of tracer through the porous media increases. In unsaturated porous media, dispersion coefficient varies with non-linear Darcy\u27s velocity and the water content. These non-linear dispersions were observed in both the laboratory scale and soil columns (20cm). The unsaturated infiltration column and tracer tests have been used to interpret the relationships between Darcy\u27s velocity and the water content together with the dispersion coefficient. However, the dispersivity coefficient cannot be measured directly; it has to be determined from advection-dispersion equation (ADE), which can be used to model the tracer transport in unsaturated porous media. The simulations have been verified that the dispersion of tracer through soil is higher than that of sand columns and also travel time of tracer through soil is longer than that of sand column. Even though, soil has very low degree of pore velocity, high dispersivity induces the increase of flow path distance and the decrease of pore velocity. The maximum dispersivity was observed when the water content of porous media is relatively low; this leads to the maximum of spreading of tracer

Simulation of water movement through unsaturated infiltration-redistribution system

This paper deals with the movement of water in a natural unsaturated zone, focusing on infiltration-redistribution system. Infiltration refers to the downward movement of water due to the gravitational force and redistribution defines the upward movement of water due to the capillary rise. Under natural conditions, the movement of water through an infiltration-redistribution depended upon the relations among water content, hydraulic conductivity and tension of soil pore. Various combinations of water balance concepts, Richards\u27 equation, soil-physics theory and capillary height concepts were applied to mathematically model the movement of water through infiltration-redistribution system. The accuracy and computational efficiency of the developed model were evaluated for the case study. Besides the laboratory scale sand/soil columns with the inner diameter of 10.4 cm were investigated in order to provide the supporting data for model calibration. Sand/soil layers were packed with a bulk density of 1.80 and 1.25 g/cm3, respectively. The infiltration was sprayed uniformly at the soil surface with the constant rate of 66.1 and 7.18 cm3/h for sand and soil columns, respectively. The redistribution process was developed by which water arriving at the column base enter to the sand/soil column due to capillary rise. The laboratory observations were simulated using the developed model. The results indicate that the developed model could well estimate the movement of water in the infiltration-redistribution system that observed in the case study and the experiments

Application of biomathematical model for Pb(II) biosorption and bioaccumulation

This study investigated the reduction of Pb(II)) by bioaccumulation and biosorption in an artificial biosorbent. The biosorbent was prepared by mixing grounded manure and fine-grain sand at a ratio of 1:1 (w/w) with the media packed into a laboratory scale column (diameter of 4.7 cm). The sweet soy sauce (food grade) solution with pH ranged of 5.0-5.5 and the concentration of 15 g COD L-1 was fed at a rate of 500 mL d-1 to the column surface for a period of 101 d. The active biofilm was then acclimatised for another 79 d, by adding 0.2 mg L-1 of Pb(NO3)2 into an acidic substrate solution. After 180 d, biofilm was matured as the removal efficiencies of COD and Pb(II) were constant at 30 and 60%, respectively. The rate of bioaccumulation was evaluated using the modified Gompertz model. The living microbes can gradually consume Pb(II) and the bioaccumulation efficiency is 14.0 mg Pb g-1 organic matter (OM). The Pb(II) ions act as the trace element, which can enhance the growth of microbes in the biosorbent. The biosorption can be described using Freundlich model. Under the acidic Pb(II) solution (pH 4.0), the highest distribution coefficients of biosorption is 6.3 mg Pb g-1 OM, and the sorbed Pb is biochemically fixed onto OM. The acidic Pb(II) solution can prevent the deposition of Pb(II) prior to biosorption. Pb(II) can be stored in accordance with the microbial cell activities by 56% of total Pb(II) removal. Approximately, 44% of Pb(II) is adhered on OM. At the active zone of biosorbent, the free Pb(II) species in the forms of soluble and exchangeable Pb(II) are well sorbed. The biosorbent could present the high benefit to bound the toxic Pb(II) even the contaminated water is highly acidic (pH = 4.0)

Numerical modelling of tracer transport in unsaturated porous media

Many types of chemical substances have been used as tracers to estimate the migration of contaminant in a porous media. Inorganic ionic compounds have been applied extensively as hydrogeologic tracers. Sodium chloride is generally used as a tracersince this common salt does not degrade or get removed from the system. Movement of tracer could be described as migration of a non-reactive constituent. A tracer transport numerical model was developed according to the advective-dispersive contaminant transport equation in unsaturated porous media. The governing equation was solved numerically and coded in MATLAB program. The objectives of this study were to develop a model for estimating the non-reactive constituent transport in the unsaturated porous media and to determine the impact of ionic strength of tracer and the effect of the thickness of porous media. The experiments were conducted with two different sodium chloride tracer concentrations (low strength-200 mg/L and high strength-500 mg/L) and for two different soil depths (5 and 20 cm). The observation and simulation data indicate that the interference from soil background concentration is significant, provided that the high strength tracer is applied. As expected, the tracer transport in the thick layer took longer elapse time than in the thin layer. The simulation results using the developed model corresponded very well with the observed data