The extensive use of titanium dioxide nanoparticles (nano-TiO2) in many consumer products has raised concerns about possible risks to the environment The magnitude of the threat may depend on whether nano-TiO2 remains dispersed in the environment, or forms much larger-sized aggregates or clusters. Currently, limited information is available on the issue. In this context, the purpose of the present article is to report initial measurements of the morphology and rate of formation of nano-TiO2 aggregates in aqueous suspensions as a function of ionic strength and of the nature of the electrolyte in a moderately acid to circumneutral pH range typical of soil and surface water conditions. Dynamic light scattering results show that 4-5 nm titanium dioxide particles readily form stable aggregates with an average diameter of 50-60 nm at pH approximately 4.5 in a NaCl suspension adjusted to an ionic strength of 0.0045 M. Holding the pH constant but increasing the ionic strength to 0.0165 M, leads to the formation of micron-sized aggregates within 15 min. At all other pH values tested (5.8-8.2), micron-sized aggregates form in less than 5 min (minimum detection time), even at low ionic strength (0.0084-0.0099 M with NaCl). In contrast, micron-sized aggregates form within 5 min in an aqueous suspension of CaCl2 at an ionic strength of 0.0128 M and pH of 4.8, which is significantly faster than observed for NaCI suspensions with similar ionic strength and pH. This result indicates that divalent cations may enhance aggregation of nano-TiO2 in soils and surface waters. Optical micrographs show branching aggregates of sizes ranging from the 1 microm optical limit of the microscope to tens of micrometers in diameter.

The remarkable complexity of soil and its importance to a wide range of ecosystem services presents major challenges to the modeling of soil processes. Although major progress in soil models has occurred in the last decades, models of soil processes remain disjointed between disciplines or ecosystem services, with considerable uncertainty remaining in the quality of predictions and several challenges that remain yet to be addressed. First, there is a need to improve exchange of knowledge and experience among the different disciplines in soil science and to reach out to other Earth science communities. Second, the community needs to develop a new generation of soil models based on a systemic approach comprising relevant physical, chemical, and biological processes to address critical knowledge gaps in our understanding of soil processes and their interactions. Overcoming these challenges will facilitate exchanges between soil modeling and climate, plant, and social science modeling communities. It will allow us to contribute to preserve and improve our assessment of ecosystem services and advance our understanding of climate-change feedback mechanisms, among others, thereby facilitating and strengthening communication among scientific disciplines and society. We review the role of modeling soil processes in quantifying key soil processes that shape ecosystem services, with a focus on provisioning and regulating services. We then identify key challenges in modeling soil processes, including the systematic incorporation of heterogeneity and uncertainty, the integration of data and models, and strategies for effective integration of knowledge on physical, chemical, and biological soil processes. We discuss how the soil modeling community could best interface with modern modeling activities in other disciplines, such as climate, ecology, and plant research, and how to weave novel observation and measurement techniques into soil models. We propose the establishment of an international soil modeling consortium to coherently advance soil modeling activities and foster communication with other Earth science disciplines. Such a consortium should promote soil modeling platforms and data repository for model development, calibration and intercomparison essential for addressing contemporary challenges.

The biological clogging of natural porous media, often in conjunction with physical or chemical clogging, is encountered under a wide range of conditions. Wastewater disposal, artificial groundwater recharge, in situ bioremediation of contaminated aquifers, construction of water reservoirs, or secondary oil recovery are all affected by this process. The present review provides an overview of the techniques that are used to study clogging in the laboratory, or to monitor it in field applications. After a brief survey of the clogging patterns most commonly observed in practice, and of a number of physical and chemical causes of clogging, the various mechanisms by which microorganisms clog soils and other natural porous media are analyzed in detail. A critical assessment is also provided of the few mathematical models that have been developed in the last few years to describe the biological clogging process. The overall conclusion of the review is that although information is available on several aspects of the biological clogging of natural porous media, further research is required to predict its extent quantitatively in a given situation. This is particularly true in cases that involve complicating factors such as predation or competition among organisms.

Bacterial reductions of the saturated hydraulic conductivity, Ks, of natural porous media have been traditionally associated with the development of anaerobic conditions and the production of large amounts of extracellular polymers by the bacteria. Various researchers have also reported that these reductions occur predominantly at or very near the surfaces of injection of nutrients within the porous media. Attempts to describe mathematically the resulting clogging process have, in the past, been based on the assumption that bacterial cells form impermeable biofilms uniformly covering pore walls. A series of percolation experiments was carried out to determine the extent to which an obligately aerobic bacterial strain, Arthrobacter sp., is able to clog permeameters filled with fine sand. A second objective was to elucidate the mechanism(s) responsible for this process. The experimental results indicate that strictly aerobic bacteria are able to reduce Ks by up to four orders of magnitude. Rapid reductions in Ks are associated with the formation of a bacterial mat at the inlet boundary of the sand columns. When the colonization of the inlet is prevented, clogging proceeds within the bulk of the sand at a noticeably slower rate. Under O2‐ or glucose‐limited growth conditions, this decrease in Ks within the sand does not appear on scanning electron micrographs to be caused by exopolymers, which are entirely absent, but rather seems to be due to the presence of large aggregates of cells that form local plugs within the pores. Under conditions of severe N limitation, the same mechanism seems to be largely responsible for the observed clogging, in spite of the production by the cells of extracellular substances, visible under light microscopy and on scanning electron micrographs. In all cases, the coverage of the solid surfaces by the bacterial cells is sparse and heterogeneous, contrary to the basic tenets of the biofilm model.

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For many years, controversy has surrounded the use of biofilm models to describe the distribution of microbial biomass in natural or artificial porous media. This use is often advocated on the basis of the relative mathematical simplicity of the biofilm concept, and of the widespread availability of analytical solutions or numerical implementations. However, microscopic observations consistently point to a patchy, rather than homogeneous, distribution of microorganisms at the pore scale in many porous media of interest, even under conditions of severe bioclogging. Also, bioclogging models involving biofilms tend to underpredict the extent of permeability reductions in all be the coarse-textured materials. In this context, computer simulations described in the present article show that some of the limitations of biofilm models to describe the bioclogging of porous media are linked to the common constitutive assumption that biofilms are impermeable, that is, that nutrient transport occurs through the biofilms only by molecular diffusion. When this restriction is alleviated and liquid flow is allowed in the biofilms, the level of bioclogging achievable by a given biomass is very significantly increased and is comparable to that observed in experiments. In addition, the distribution of microorganisms becomes patchy and exhibits a self-organized periodic pattern with pores either entirely filled with biomass or without any biomass at all, again similar to published microscopic observations. These results suggest that biofilm models should not be ruled out a priori for the quantitative description of bioclogging in porous media, as long as biofilms are allowed to be permeable.

Experiments were carried out to determine the breakthrough of bacteria through a saturated aquifer sand at three flow velocities and three cell concentrations. Bacteria were either suspended in deionized water or 0.01 mol L -• NaCI solution. Bacterial transport was found to increase with flow velocity and cell concentration but was significantly retarded in the presence of 0.01 mol L -• NaC1. A mathematical model based on the advection-dispersion equation was formulated to describe bacterial transport and retention in porous media. The transport equations for bacteria were solved using the finite difference Crank-Nicolson scheme combined with Newton-Raphson iterations. The best fit of the numerical model to the experimental data was obtained using the downhill simplex optimization technique to minimize the sum of the squares of deviations between model predictions and experimental data by varying three parameters. This numerical model was found to describe the experimental data very well under all the experimental conditions tested. An alternative model (also based on the advection-dispersion equation) was tested against all the experimental data sets, but it did not represent the experimental data as well as the model proposed in this paper. face materials [e.g., Bitton et al., 1974; Wollum and Cassel, 1978; Smith et al., 1985; Parke et al., !986; Tan et al., 1991]. Many environmental factors such as ionic strength and flow velocity of the soil solution and properties of the porous materials have been identified to affect microbial transport in porous media in qualitative terms [e.g., Goldshrnid et al., 1973; Bitton et al., !974; Smith et al., !978; Wollum and Cassel, 1978; Gerba and Bitton, !984; McDowell-Boyer et al., 1986; Fontes et al., 1991; Gannon et al., 1991b; Gammack et al., 1992]. Complex mathematical models have also been developed to describe bacterial transport in porous media [e.g., Corapcioglu and Haridas, 1984, !985; Taylor and Jaffe, 1990]. Despite the experimental and modeling 1Now at Centre for Environmental Mechanics, CSIRO, Canben'a, Australia. efforts, few experimental studies have been designed to test those theoretical and conceptual approaches and to describe the aforementioned environmental factors quantitatively in relation to the microbial transport models developed. Nevertheless, it is generally accepted that bacterial transport can be described by the advection-dispersion equation with modifications to account for growth, decay, attachment, and detachment [e.g., Corapcioglu and Haridas, 1984, 1985; Taylor and Jaffe, 1990; Harvey and Garabedian, 1991; Hornberger et al., 1992; Tan et al., 1992]. Attachment refers to processes such as adsorption and straining that can cause retention of bacteria in a porous medium, and detachment refers to the subsequent dislodgment of bacteria from the surfaces. The attachment and detachment of bacteria are the most important and complex processes affecting bacterial transport and are arguably the least understood. Broadbased attempts have been made t...