Role of macropore flow in the transport of Escherichia coli cells in undisturbed cores of a brown leached soil

Abstract

The objective of this work was to evaluate the transport of Escherichia coli cells in undisturbed cores of a brown leached soil collected at La Côte St André (France). Two undisturbed soil cores subjected to repeated injections of bacterial cells and/or bromide tracer were used to investigate the effect of soil hydrodynamics and ionic strength on cell mobility. Under the tested experimental conditions, E. coli cells were shown to be transported at the water velocity (retardation factor close to 1) and their retention appeared almost insensitive to water flow and ionic strength variations, both factors being known to control bacterial transport in model saturated porous media. In contrast, E. coli breakthrough curves evolved significantly along with the repetition of the cell injections in each soil core, with a progressive acceleration of their transport. The evolution of E. coli cells BTCs was shown to be due to the evolution of the structure of soil hydraulic pathways caused by the repeated water infiltrations and drainage as may occur in the field. This evolution was demonstrated through mercury intrusion porosimetry (MIP) performed on soil aggregates before and after the repeated infiltrations of bacteria. MIP revealed a progressive and important reduction of the soil aggregate porosity, n, that decreased from approximately 0.5 to 0.3, along with a decrease of the soil percolating step from 27 to 2 μm. From this result a clear compaction of soil aggregates was evidenced that concerned preferentially the pores larger than 2 μm equivalent diameter, i.e. those allowing bacterial cell passage. Since no significant reduction of the global soil volume was observed at the core scale, this aggregate compaction was accompanied by macropore formation that became progressively the preferential hydraulic pathway in the soil cores, leading to transiently bi-modal bacterial BTCs. The evolution of the soil pore structure induced a modification of the main hydrodynamic processes, evolving from a matrix-dominant transfer of water and bacteria to a macropore-dominant transfer. This work points out the importance of using undisturbed natural soils to evaluate the mobility of bacteria in the field, since the evolving hydrodynamic properties of soils appeared to dominate most physicochemical factors.

Application of the microbial community coalescence concept to riverine networks

“…clay-dominated geology), given the same rainfall event, a proportionally greater volume of water is likely to flow over the surrounding floodplain soil, picking up soil particles and associated microorganisms before reaching a receiving stream or river. Microbial size (Gannon, Manilal, & Alexander, 1991) and cell surface characteristics, as well as soil properties (Huysman & Verstraete, 1993) and the presence of preferential flow pathways in the soil such as macropores (Martins et al, 2013) could affect the size and composition of the transported community. In addition to geology, magnitude and duration of rainfall event, and time of year are also important; greater volumes of runoff will enter streams following high-intensity and/or long-duration storm events especially when antecedent soil moisture deficit is low (Heppell et al, 2002).…”

Water Transport, Retention, and Survival ofEscherichia coliin Unsaturated Porous Media: A Comprehensive Review of Processes, Models, and Factors

“…It was notable that intermittent spray irrigations in the autumn were generally more effective for retention than single drip irrigation events, indicating that intermittent irrigation does not have a negative impact on the filter capacity, as compared to a single irrigation event. Contradictory findings have been reported (section 5.3.3), for example by Martins et al (2013), who found that preferential transport of E. coli K-12 became more pronounced as a result of repeated injection/infiltration and drainage cycles. Further, results suggested that intermittent and drip irrigation was more effective for bacterial attenuation than steady-state flux (Table 2).…”

“…Coherently, Jiang (2010) found that the soil saturation decreased and that the hydraulic conductivity increased after cycles of imbibitions and drainage, indicating that preferential flow paths had been formed as a result of the irrigation. In undisturbed cores Martins et al (2013) reported that E. coli K-12 breakthrough curves became bi-modal with time as a result of repeated injection/infiltration and drainage cycles. The flow regime had thus gone from uniform flow to preferential flow dominated, and porosimetry showed that this was related to local soil compaction and the formation of macropores; further, it was argued that the most important influencing factor of bacterial transport is the soil structure and its hydraulic properties (Martins et al, 2013).…”

“…Processes in homogeneous soil systems with uniform flow provides a valuable starting point for the understanding of E. coli water transport in porous media; however, in the vadose zone bacterial transport rates are strongly dependent on flow processes, and non-equilibrium, or preferential flow, occurs in most types of soil as an effect of heterogeneities at various scales (Jarvis and Dubois, 2006;Martins et al, 2013). Such flow is considered particularly important in the unsaturated zone, where the transport pathways are dependent on the varying moisture content (Cey et al, 2009).…”

Modeling Microorganism Transport and Survival in the Subsurface

“…For example, breakthrough curves for solutes and microbes in soils frequently exhibit early arrival and concentration tailing or multiple peaks (e.g., Wang et al, 2013). Transport models of varying complexity have been developed to simulate nonequilibrium flow and transport processes (Dhawan et al, 1993; McGechan and Vinten, 2003, 2004; Gharasoo et al, 2012; Guzman and Fox, 2011; Martins et al, 2013). Detailed reviews on nonequilibrium flow and transport models are available in the literature (Jarvis, 2007; Šimůnek and van Genuchten, 2008; Köhne et al, 2009) and are only briefly outlined below.…”