Journal of Hazardous Materials 143 (2007) 690–701 Finite element modeling of contaminant transport in soils including the effect of chemical reactions A.A. Javadi ∗ , M.M. AL-Najjar Department of Engineering, University of Exeter, Exeter, Devon EX4 4QF, UK Available online 9 January 2007 Abstract The movement of chemicals through soils to the groundwater is a major cause of degradation of water resources. In many cases, serious human and stock health implications are associated with this form of pollution. Recent studies have shown that the current models and methods are not able to adequately describe the leaching of nutrients through soils, often underestimating the risk of groundwater contamination by surface-applied chemicals, and overestimating the concentration of resident solutes. Furthermore, the effect of chemical reactions on the fate and transport of contaminants is not included in many of the existing numerical models for contaminant transport. In this paper a numerical model is presented for simulation of the flow of water and air and contaminant transport through unsaturated soils with the main focus being on the effects of chemical reactions. The governing equations of miscible contaminant transport including advection, dispersion–diffusion and adsorption effects together with the effect of chemical reactions are presented. The mathematical framework and the numerical implementation of the model are described in detail. The model is validated by application to a number of test cases from the literature and is then applied to the simulation of a physical model test involving transport of contaminants in a block of soil with particular reference to the effects of chemical reactions. Comparison of the results of the numerical model with the experimental results shows that the model is capable of predicting the effects of chemical reactions with very high accuracy. The importance of consideration of the effects of chemical reactions is highlighted. © 2007 Elsevier B.V. All rights reserved. Keywords: Contaminant transport; Unsaturated soil; Finite element; Chemical reaction 1. Introduction In recent years, interest in understanding the mechanisms and prediction of contaminant transport through soils has dramati- cally increased because of growing evidence and public concern that the quality of the subsurface environment is being adversely affected by industrial, municipal and agricultural activities. Transport phenomena are encountered in almost every aspect of environmental engineering science. In assessing the environ- mental impacts of waste discharges it is important to predicate the impact of emission on contaminant concentration in nearby air and water [25]. Contamination of groundwater is an issue of major concern in residential areas which may occur as a result of spillages of hazardous chemicals, dumping of toxic waste, land- fills, waste water, or industrial discharges [9]. Hazardous waste disposal is increasingly becoming one of the most serious prob- ∗ Corresponding author. Tel.: +44 1392 263640; fax: +44 1392 217965. E-mail addresses: a.a.javadi@ex.ac.uk (A.A. Javadi), m.m.al-najjar@ex.ac.uk (M.M. AL-Najjar). lems confronting health and the environment. The movement of chemicals through the soil to the groundwater represents a degra- dation of these resources. In many cases, serious human and stock health implications are associated with this form of pollu- tion. One of the most challenging problems in modeling of solute transport in soils is how to effectively characterize and quan- tify the geometric, hydraulic, and chemical properties of porous media. During the past two decades, attempts have been made to develop physical, analytical and numerical models for contam- inant transport through soils considering the effects of different transport mechanisms. Abriola and Pinder [1] developed a gen- eral model that addressed the multiphase flow problem and the transport of organic species. Celia and Boluloutas [6] presented a numerical technique for the solution of advection–diffusion type of transport equation. Li et al. [19] developed a numerical model to simulate contaminant transport through soils taking into account the influence of mechanisms of the miscible con- taminant transport including advection, mechanical dispersion, molecular diffusion and adsorption. Karkuri and Molenkamp [17] presented a formulation for groundwater flow and pol- lutant transport through multi-layered saturated soils in one 0304-3894/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2007.01.016