Effect of a nonwoven geotextile on solute and colloid transport in porous media under both saturated and unsaturated conditions
Introduction
Urbanization increases stormwater runoff leading to an increase of water flows in classical sewer systems and affecting the water quality. Therefore new best management practices (BMPs) have been implemented in urban water management. Among these, infiltration basins are designed to reduce volumes and peak flows from stormwater runoff instead of collecting and redirecting them to classical sewer systems (Chocat et al., 1999). By filtrating particulate contaminants derived from stormwater runoff, BMPs can reduce or prevent impacts on the quality of soils and groundwater resources (Mikkelsen et al., 1994). The presence of various natural and artificial soil layers in the vadose zone induces structural and chemical heterogeneities. Soil heterogeneities are key parameters that control both flow and pollutant transport (Köhne et al., 2009a) in the vadose zone, in particular underneath infiltration basins (Winiarski et al., 2006). Classical infiltration systems consist of different porous materials separated by geotextiles. This artificial barrier can be used to remove pollutants from stormwater runoff prior to infiltration into the ground to reduce the risks of soil and groundwater contamination. The effect of geotextiles on flow and fate of pollutants must be clearly established to optimize their use in infiltration basins and more generally for BMPs.
Geotextiles are thin polymeric materials widely used in geotechnical, environmental and hydraulic applications (Bouazza et al., 2006). The four most common applications of these materials are filtration, separation, reinforcement and drainage. There are two types of geotextiles: woven and nonwoven. Woven geotextiles are manufactured using traditional weaving methods and are extensively used for reinforcement purposes (Lekha, 2004; Sarsby, 2007). Nonwoven geotextiles are manufactured by needle-punching or melt-bonding and are extensively used for drainage (Muthukumaran and Ilamparuthi, 2006), filtration (Leverenz et al., 2000), protection (Nagahara et al., 2004), and separation (Vaitkus et al., 2007). Geotextiles used in filtration and drainage processes permit the flow of liquids, gases and fine soil particles but most of them prevent the passage of soil particles larger than one hundred micrometres in diameter. Using nonwoven geotextiles for filtration and drainage instead of coarse-grained soils is very attractive in geo-environmental applications because of their relatively easy installation and the gain in space (Bouazza et al., 2006).
Before using them in infiltration basins, it is necessary to investigate geotextiles regarding their solute and water transfer properties. Iryo and Rowe (2003) reported that geotextiles can be used to promote water flow as well as to prevent it. Depending on their specific hydraulic properties, geotextiles can be used either as drainage materials or act as drainage barriers. Bouazza et al. (2006) suggested that embedding geotextiles, which are in most cases hydrophobic due to their manufacturing process, in soils can influence significantly the movement of water and give rise to a redistribution of the water content profile. The effect of geotextiles also requires further investigation regarding pollutant transfer. Geotextiles can interact directly with pollutants. For example, Boutron et al. (2009) suggested that geotextile fibres could retain a significant amount of pesticides by adsorption process. Microbial contaminants may biologically interact with geotextiles fibres. As an example Rowe (2005) reported that needle-punched nonwoven geotextiles provide large surface area for biofilm development resulting in clogging of geotextiles. Wetzel et al. (2011) even reported the colonization of benthic organisms on woven and non-woven geotextile materials in the Elbe estuary during 2 years. Geotextiles can also affect flow pathways and thus contaminant transport (Lassabatere et al., 2004).
Contaminant transport in soils is a complex phenomenon involving several basic processes such as advection, dispersion, diffusion, and physico-chemical interactions. Mathematical models describing contaminant transport are usually based on one dimension, namely the Advection–Reaction–Dispersion equation (ARD). The classical ARD has been applied successfully to analyse contaminant transport experiments in homogeneous porous media. Natural or artificial soils, such as topsoils underneath infiltration basins, often exhibit a variety of small-scale physical heterogeneities such as aggregates, cracks, macropores, etc. This physical pore scale heterogeneity results in micro-variations of fluid velocity. A common approach for dealing with this difficulty is to apply the two-region flow concept, taking into account physical non-equilibrium transport (van Genuchten and Wieranga, 1976). This approach, also referred to as MIM for Mobile–IMmobile water fractionation, assumes that the liquid phase can be partitioned between mobile and immobile regions, as convective-dispersive transport is only restricted to the mobile region and while solute exchange by diffusion process takes place between the two regions. Many studies have focused on the specific case of interactions between geotextiles and dissolved pollutants in engineered containment barriers constructed for solid waste management (Edil, 2003; Rowe et al., 2005; Lange et al., 2010; Rowe, 2011). This mode of interaction typically occurs under low flow rate conditions leading to the diffusive transport of contaminants, which induce long contact times between geotextiles and contaminants and better retention by the geotextile. The problem is that the high flow infiltration rates typically encountered in infiltration basins impose short contact times between geotextiles and contaminants, leading to fewer interactions and more convection and dispersion than diffusive transport. Advective–dispersive theory (contaminants migrating with the flow of water through the soil) must be considered to better describe contaminant transport. The literature includes very few works on the effect of geotextiles on solute and contaminant transport under high infiltration rates. Lassabatere et al., 2004, Lassabatere et al., 2005 showed, for example, that geotextiles impact flow homogeneity and pollutant removal in the surrounding soil. These results are valid for three heavy metals in a specific calcareous deposit and have to be confirmed for other kind of pollutants and filtration media.
Geotextiles may interact with contaminants associated with particulate, colloidal and dissolved phases. There is abundance in the existing literature of studies carried out to understand interactions between geotextiles and various contaminants associated with the particulate and the dissolved phase. Faure et al. (2006) pointed out that geotextiles can be clogged by fine suspended particles (median diameter of particles ranging from 7 to 50 μm) due to mechanical filtration. Palmeira et al. (2008) examined the biological clogging of nonwoven geotextiles in long-term permittivity tests and Mulligan et al. (2009) used laboratory filtration tests to demonstrate the excellent efficiency of a non-woven geotextile to remove contaminated suspended solids from surface water. Others authors investigated the interaction between geotextiles and contaminants associated to the dissolved phase (Sangam and Rowe, 2001; Athanasiadis et al., 2005; Rowe et al., 2005; Kalinovich et al., 2008; Boutron et al., 2009). However, pollutants are also present as colloidal particles in urban stormwater (Tuccillo, 2006; Durin et al., 2007). These small particles may be captured in porous media by physical, electrostatic and chemical processes (Ginn, 2002). Under unfavourable conditions for electrostatic and chemical interaction, the behaviour of colloids may differ from that of larger particles in that they are not expected to be mechanically filtered by geotextiles and may continue along flow pathways. Their large surface area makes these particles efficient for pollutant transport (McCarthy and Zachara, 1989). Understanding the influence of geotextiles on the transfer of colloids represents a real challenge as assessing the potential risk of soil and groundwater contamination is crucial. Lamy et al. (2008) carried out column experiments to investigate the effect of a geotextile on flow and colloid transport within homogenous sand and heterogeneous gravel under saturated conditions. They concluded that geotextiles impact flow homogeneity and colloid removal. However, the influence of the geotextile may depend also on the saturation degree of the soil – geotextile system. The decreasing water content may affect flow homogeneity and colloid retention processes. The objective of the present paper is to study the influence of a nonwoven geotextile on a non reactive solute and colloidal transport through porous sandy and gravel media under high flow rates. The previous work of Lamy et al. (2008) was extended to the unsaturated conditions in the current paper, to take into account the fact that infiltration basins are generally unsaturated systems. Laboratory column devices were used to inject a tracer solution and a suspension of colloidal particles at a constant flow rate in the porous media with and without a nonwoven geotextile. The experimental tracer elution curves through the columns were inversely modelled using a Mobile–IMmobile (MIM) transport approach implemented using the HYDRUS-1D code. The role of the geotextile in solute and colloid transport will be discussed as a function of porous media (homogeneous sand versus heterogeneous gravel) and the degree of water saturation of the matrix.
Section snippets
Porous media
The porous media used in this work was a homogenous Hostun sand and a heterogeneous gravel. The preparation of the porous materials, prior to transport experiments was described in the previous work of Lamy et al. (2008). Hostun quartz sand showed in Fig. 1a has a narrow particle size distribution, ranging from 0.5 to 2 mm with a median grain size (d50) of 1 mm and a reported saturated hydraulic conductivity (Ks) of 2.7 × 10−3 m s−1. The particle size distribution of the calcareous gravel is
Results and discussion
Tracer and colloid experimental elutions are presented for the sandy and gravel columns under saturated and unsaturated conditions. Experimental findings and numerical modelling are then discussed in relation to the potential role of the geotextiles in solute and colloid transport as a function of porous media heterogeneity and degree of saturation.
Conclusion
The influence of a nonwoven geotextile on solute and colloidal transport was studied on the basis of laboratory column experiments. Analyses of breakthrough curves and modelling tracer experiments indicated that using a geotextile increased the degree of flow homogeneity, as reported by Lassabatere et al. (2004). However, we demonstrated that the contribution of geotextiles to flow homogenization was dependent on the type of porous media and degree of saturation in the soil-geotextile system.
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