54 Global Journal of Earth Science and Engineering, 2018, 5, 54-67 E-ISSN: 2409-5710/18 © 2018 Avanti Publishers Methods of Determining Retardation Coefficients of Organic Compounds in Aquifers Munjed A. Maraqa * Department of Civil and Environmental Engineering, United Arab Emirates University, Al Ain, United Arab Emirates Abstract: Sorption is an important mechanism that affects the mobility of organic compounds in the subsurface environment. Sorbed compounds move slower than that of groundwater, causing retardation in their movement. Accurate determination of retardation coefficients (R) of organic compounds in aquifers is critical for understanding their movement, fate, and remediation. Several methods, including predictive tools, laboratory experiments, and field experiments have been utilized for determining sorption-related retardation. The objective of this paper was to review and compare between the different methods used for the determination of R of organic compounds, with emphasis on predictive- and laboratory-based approaches. Predictive tools are based on the use of quantitative structure-activity relationships (QSARs). Laboratory methods utilize different types of reactors including batch, stirred-flow, circulation- through-column, or miscible displacement through packed columns. In addition, data from the column method have been analyzed in various ways to determine R. Discrepancies between results from different methods or from different analysis approaches have been reported. This create uncertainty about the suitability of some of these methods or the used analysis approaches. This paper highlights the possible causes for the observed discrepancy and establishes the limitations and appropriateness of the used methods and analysis approaches. Keywords: Batch method, Column method, Retardation factor, Sorption, Organic compounds, Sorption equilibrium coefficient, Groundwater contamination. 1. INTRODUCTION Groundwater accounts for approximately 35% of all water withdrawals by humans. Thus, protecting groundwater resources from anthropogenic contamination is important especially in areas with limited surface water resources, such as arid and semi- arid countries. This has been emphasized by the World Health Organization [1] as well. However, contaminants such as synthetic organic compounds are still being released into the subsurface water. The fate of organic contaminants in the subsurface environment has received a lot of attention in the last four decades due to their impact on human health [2, 3]. Organic compounds could be ionic or nonionic, aromatic or aliphatic. Owing to the presence of these compounds in groundwater and their potential to reach underlying aquifers, several studies focused on their movement and methods of treatment. Attention was given, for example, to chlorinated organic compounds [4, 5], aromatic hydrocarbons [6, 7], polynuclear aromatic hydrocarbons [8], polychlorinated biphenyls [9], and pesticides [10]. In recent years, studies have started covering emerging compounds such as pharmaceuticals, hormones, and personal care *Address correspondence to this author at the Department of Civil and Environmental Engineering, United Arab Emirates University, Al Ain, United Arab Emirates; Tel. +971 3 713 5157; Fax: +971 3 713 4997; Email: m.maraqa@uaeu.ac.ae products [11-14]. These contaminants are released in the subsurface water through different sources. The highest reported pollution source in the United States is underground storage tanks [15]. Other sources include leakage from oil pipelines [16], sewage pipelines [17], septic systems [4, 18], leaching from landfills [19, 20], irrigated lands [21], and livestock farming [22]. The mobility of dissolved organic compounds in aquifers could be influenced by several transfer and transform mechanisms. These compounds move within groundwater through advection; a process that depends on the hydraulic conductivity and the hydraulic gradient of the aquifer. These compounds also disperse during their transport due to molecular diffusion and mechanical dispersion. The sum of molecular diffusion and mechanical dispersion is termed as hydrodynamic dispersion. Advection and dispersion are transfer mechanisms that affect both conservative and nonconservative compounds. Additionally, organic compounds could be influenced by reactive transfer mechanisms or could be subject to transformation. Transform mechanisms cause degradation of the compound either through a chemical reaction (hydrolysis, oxidation-reduction, or complexation) or through biological processes. Reactive transfer mechanisms, however, do not alter the structure of the compound, but affect its mobility in the liquid phase by reducing its speed relative to that of groundwater, causing compound retardation.