Effects of bacterial activity on the saturated hydraulic conductivity of remolded loess
Graphical abstract
Introduction
Loess, an aeolian silt with engineering geological significance, accounts for approximately 10% of the terrestrial surface of the earth that widely distributed in Asia, Europe, and North America. China's Loess Plateau (CLP) has the deepest and largest loess deposition area in the world, with a total coverage area of about 440,000 km2 and a maximum thickness of more than 300 m (Liu, 1985; Sun, 2005). As the “cradle of Chinese civilization”, the CLP covers 6.7% of the total area of the country and is also home to about 10% of the total population. In a dry state, loess has high strength and small deformation, but it is considered to be one of the most problematic soils because its structure collapses when it is wet (Assallay et al., 1997; Ma et al., 2017). Due to water sensitivity, loess areas often suffer from engineering disasters closely related to water (e.g., land subsidence, ground fissure, landslide, and piping) (Hosseinalizadeh et al., 2018; Smalley and Dijkstra, 1991; Wang et al., 2020; Zhuang et al., 2018). With the large-scale construction of projects in recent years, such as high fill foundation, expressway, high-speed railway, embankment and water conservancy projects, it is becoming more and more important to understand the flow of water in loess for engineering application and geological disaster prevention in loess areas.
Saturated soil hydraulic conductivity (K) is an essential parameter to measure water flow through the soil (Chen et al., 2020; Gómez-Hernández and Gorelick, 1989; Piña et al., 2019). Historically, it is usually regarded as a constant value at given conditions over time (Baveye et al., 1998; Chen and Qian, 2017; Chen et al., 2020; Qian et al., 2020; Sanchez-Vila et al., 2006). There is no exception for loess. However, it has been determined that microorganisms are widely distributed in soil. Microorganisms can develop biofilms in many natural and engineered porous media systems. Growth of bacteria in soil could lead to substantial decrease in porosity and permeability due to the biofilm-induced modifications of pore-space geometry (Baveye et al., 1998; Brovelli et al., 2009; Gowrisankar et al., 2017; Khaleghi and Rowshanzamir, 2019). The resultant clogging may decrease water flux and limit nutrient supply, thereby causing a restriction in microbial activity (Arnon et al., 2005). Bielefeldt et al. (2002) and Seifert and Engesgaard (2007), in sand column experiments, found that microbial growth caused 2–4 orders of magnitude decrease in K. Kirk et al. (2012) confirmed that biomass largely remained intact after acidification and continue to reduce K, even when considerable death occurred. Compared to phosphorous content and temperature, carbon source was mentioned as the major cause for biofilm growth (Calderer et al., 2014; Xia et al., 2014). In this regard, the common assumption of unchanged K may be not appropriate and may increase uncertainty in describing, simulating and predicting water and even solute transports in loess. To date, identifying the anisotropy of hydrological conductivity and microstructural changes for loess has been the target of many studies (Feng et al., 2020; Gao et al., 2018; Li and Li, 2017; Shao et al., 2018; Wang et al., 2020; Wang et al., 2018; Xu et al., 2020), while the effect of microbial activity on the hydraulic conductivity of loess has very limited investigation. The knowledge gap causes that there is still little experience to study influence of biomass growth on water movement and to evaluate consequential risk of geohazards in loess engineering.
In this study, the effects of bacterial activity on the hydraulic properties of loess soil were quantified for the first time. Malan loess (Q3), as the most widely distributed loess on the CLP, is often used as the construction material in engineering projects. The main objectives of this study were to explore the complex interactions between hydraulic conductivity and bacterial activity in Malan loess, with the aim to increase our understanding of the mechanisms of interaction in soil-water-microbe systems. Our findings will enable a new insight into the variation of hydraulic conductivity in loess soil at the experimental scale. It also provides a basis for studying the influence of bacterial activities on water migration and the resultant uncertainty in loess areas.
Section snippets
Soil sample collection
Malan loess sampled from Jingyang County of Shaanxi Province, in the middle of China's Loess Plateau, was used in the experiments described below (Fig. 1). The samples were thoroughly mixed, air-dried, and subsequently sieved according to the specification of the soil test (MWRPRC and the Ministry of Water Resources of the People's Republic of China, 1999). Particle size analysis was carried out via a laser particle size analyzer (Bettersize2000, China). The basic physical soil properties are
Changes in hydraulic conductivity
The reduction of saturated hydraulic conductivity for all the lab-scale columns was observed (Fig. 5). Earliest measured values of the saturation hydraulic conductivities (about 10 h) ranged from around 0.14 to 0.17 m/d. As the experiments proceeded, linear decrease of K was observed and then the values tended to be relatively stable. Similar patterns of K reduction were detected in all columns, although the final K values widely ranged between 0.01 and 0.05 m/d. Obviously, K reduction in the
Mechanism of hydraulic conductivity reduction
The accumulation of bacterial cells and the formation of biofilms (extracellular polymeric substance excretion, EPS) can modify the pore-space geometry, resulting in an increase of the resistance of water flow (Engesgaard et al., 2006; Kim et al., 2010). To explore the impacts of various nutrient supply conditions on EPS production, the EPS production by G- was measured using a fluorescence microscope (Leica DMi8-M, Germany). Fig. 7 shows the distribution of bacterial cells and EPS in the loess
Conclusions
The study evaluated the effect of bacterial activity on hydraulic conductivity for the remodeled loess. The variation of hydraulic conductivity was divided into the unaffected stage, linear reduction stage and stable stage. K reduction was unobvious in the specimens at the beginning of the experiments. Then the pores between the soil particles in the specimens were occupied by the growth of Gram-negative bacteria and the accumulation of EPS, which reduced the porosity. It was the main reason
Author statement
Jie Chen: Conceptualization; Data curation; Formal analysis; Funding acquisition; Methodology; Project administration; Supervision; Validation; Writing - original draft; Writing - review & editing. Hui Qian: Conceptualization; Data curation; Funding acquisition; Methodology; Project administration; Supervision; Writing - original draft; Writing - review & editing. Mi Yang: Investigation; Methodology; Software; Supervision; Validation; Writing - original draft. Jinyi Qin: Investigation;
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research is supported by grants from the National Natural Science Foundation of China (41790441, 41931285, 41761144059 and 41572236), China Postdoctoral Science Foundation (300204000181), Special Fund for Basic Scientific Research of Central Colleges in Chang'a University (300102290102 and 300102299506). Anonymous reviewers and the Editor are sincerely acknowledged for their useful comments.
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