The Soil and Water Assessment Tool (SWAT) model is a continuation of nearly 30 years of modeling efforts conducted by the USDA Agricultural Research Service (ARS). SWAT has gained international acceptance as a robust interdisciplinary watershed modeling tool as evidenced by international SWAT conferences, hundreds of SWAT-related papers presented at numerous other scientific meetings, and dozens of articles published in peer-reviewed journals. The model has also been adopted as part of the U.S. Environmental Protection Agency (USEPA) Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) software package and is being used by many U.S. federal and state agencies, including the USDA within the Conservation Effects Assessment Project (CEAP). At present, over 250 peer-reviewed published articles have been identified that report SWAT applications, reviews of SWAT components, or other research that includes SWAT. Many of these peer-reviewed articles are summarized here according to relevant application categories such as streamflow calibration and related hydrologic analyses, climate change impacts on hydrology, pollutant load assessments, comparisons with other models, and sensitivity analyses and calibration techniques. Strengths and weaknesses of the model are presented, and recommended research needs for SWAT are also provided.

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The Soil and Water Assessment Tool: Historical Development, Applications, and Future Research Directions

The Soil and Water Assessment Tool (SWAT) model is a continuation of nearly 30 years of modeling efforts conducted by the U.S. Department of Agriculture (USDA), Agricultural Research Service. SWAT has gained international acceptance as a robust interdisciplinary watershed modeling tool, as evidenced by international SWAT conferences, hundreds of SWAT-related papers presented at numerous scientific meetings, and dozens of articles published in peer-reviewed journals. The model has also been adopted as part of the U.S. Environmental Protection Agency's BASINS (Better Assessment Science Integrating Point & Nonpoint Sources) software package and is being used by many U.S. federal and state agencies, including the USDA within the Conservation Effects Assessment Project. At present, over 250 peer-reviewed, published articles have been identified that report SWAT applications, reviews of SWAT components, or other research that includes SWAT. Many of these peer-reviewed articles are summarized here according to relevant application categories such as streamflow calibration and related hydrologic analyses, climate change impacts on hydrology, pollutant load assessments, comparisons with other models, and sensitivity analyses and calibration techniques. Strengths and weaknesses of the model are presented, and recommended research needs for SWAT are provided.

Soil and Water Assessment Tool: Historical Development, Applications, and Future Research Directions, The

Intermolecular And Surface Forces

Emerging organic contaminants in groundwater: A review of sources, fate and occurrence

Plastic litter is widely acknowledged as a global environmental threat, and poor management and disposal lead to increasing levels in the environment. Of recent concern is the degradation of plastics from macro- to micro- and even to nanosized particles smaller than 100 nm in size. At the nanoscale, plastics are difficult to detect and can be transported in air, soil, and water compartments. While the impact of plastic debris on marine and fresh waters and organisms has been studied, the loads, transformations, transport, and fate of plastics in terrestrial and subsurface environments are largely overlooked. In this Critical Review, we first present estimated loads of plastics in different environmental compartments. We also provide a critical review of the current knowledge vis-a-vis nanoplastic (NP) and microplastic (MP) aggregation, deposition, and contaminant cotransport in the environment. Important factors that affect aggregation and deposition in natural subsurface environments are identified and c...

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Microplastics and Nanoplastics in Aquatic Environments: Aggregation, Deposition, and Enhanced Contaminant Transport

A conceptual model for colloid transport is developed that accounts for colloid attachment, straining, and exclusion. Colloid attachment and detachment is modeled using first-order rate expressions, whereas straining is described using an irreversible first-order straining term that is depth dependent. Exclusion is modeled by adjusting transport parameters for colloid-accessible pore space. Fitting attachment and detachment model parameters to colloid transport data provided a reasonable description of effluent concentration curves, but the spatial distribution of retained colloids at the column inlet was severely underestimated for systems that exhibited significant colloid mass removal. A more physically realistic description of the colloid transport data was obtained by simulating both colloid attachment and straining. Fitted straining coefficients were found to systematically increase with increasing colloid size and decreasing median grain size. A correlation was developed to predict the straining co...

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Modeling colloid attachment, straining, and exclusion in saturated porous media.

[1] Saturated soil column experiments were conducted to explore the influence of colloid size and soil grain size distribution characteristics on the transport and fate of colloid particles in saturated porous media. Stable monodispersed colloids and porous media that are negatively charged were employed in these studies. Effluent colloid concentration curves and the final spatial distribution of retained colloids by the porous media were found to be highly dependent on the colloid size and soil grain size distribution. Relative peak effluent concentrations decreased and surface mass removal by the soil increased when the colloid size increased and the soil median grain size decreased. These observations were attributed to increased straining of the colloids; i.e., blocked pores act as dead ends for the colloids. When the colloid size is small relative to the soil pore sizes, straining becomes a less significant mechanism of colloid removal and attachment becomes more important. Mathematical modeling of the colloid transport experiments using traditional colloid attachment theory was conducted to highlight differences in colloid attachment and straining behavior and to identify parameter ranges that are applicable for attachment models. Simulated colloid effluent curves using fitted first-order attachment and detachment parameters were able to describe much of the effluent concentration data. The model was, however, less adequate at describing systems which exhibited a gradual approach to the peak effluent concentration and the spatial distribution of colloids when significant mass was retained in the soil. Current colloid xfiltration theory did not adequately predict the fitted first-order attachment coefficients, presumably due to straining in these systems. INDEX TERMS: 1831 Hydrology: Groundwater quality; 1832 Hydrology: Groundwater transport

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Physical factors affecting the transport and fate of colloids in saturated porous media

https://www.ars.usda.gov/ARSUserFiles/20360500/pdf_pubs/P2148.pdf

Coupling of physical and chemical mechanisms of colloid straining in saturated porous media.

Our ability to accurately simulate the transport and retention of colloids in the vadose zone is currently limited by our lack of basic understanding of colloid retention processes that occur at the pore scale. This review discusses our current knowledge of physical and chemical mechanisms, factors, and models of colloid transport and retention at the interface, collector, and pore scales. The interface scale is well suited for studying the interaction energy and hydrodynamic forces and torques that act on colloids near interfaces. Solid surface roughness is reported to have a significant influence on both adhesive and applied hydrodynamic forces and torques, whereas non-Derjaguin–Landau–Verwey–Overbeek (DLVO) forces such as hydrophobic and capillary forces are likely to play a significant role in colloid interactions with the air–water interface. The flow field can be solved and mass transfer processes can be quantified at the collector scale. Here the potential for colloid attachment in the presence of hydrodynamic forces is determined from a balance of applied and adhesive torques. The fraction of the collector surface that contributes to attachment has been demonstrated to depend on both physical and chemical conditions. Processes of colloid mass transfer and retention can also be calculated at the pore scale. Differences in collector- and pore-scale studies occur as a result of the presence of small pore spaces that are associated with multiple interfaces and zones of relative flow stagnation. Here a variety of straining processes may occur in saturated and unsaturated systems, as well as colloid size exclusion. Our current knowledge of straining processes is still incomplete, but recent research indicates a strong coupling of hydrodynamics, solution chemistry, and colloid concentration on these processes, as well as a dependency on the size of the colloid, the solid grain, and the water content.

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Colloid Transport and Retention in Unsaturated Porous Media: A Review of Interface-, Collector-, and Pore-Scale Processes and Models

Transport of colloidal particles in porous media is governed by the rate at which the colloids strike and stick to collector surfaces. Classic filtration theory has considered the influence of system hydrodynamics on determining the rate at which colloids strike collector surfaces, but has neglected the influence of hydrodynamic forces in the calculation of the collision efficiency. Computational simulations based on the sphere-in-cell model were conducted that considered the influence of hydrodynamic and Derjaguin-Landau-Verwey-Overbeek (DLVO) forces on colloid attachment to collectors of various shape and size. Our analysis indicated that hydrodynamic and DLVO forces and collector shape and size significantly influenced the colloid collision efficiency. Colloid attachment was only possible on regions of the collector where the torque from hydrodynamic shear acting on colloids adjacent to collector surfaces was less than the adhesive (DLVO) torque that resists detachment. The fraction of the collector surface area on which attachment was possible increased with solution ionic strength, collector size, and decreasing flow velocity. Simulations demonstrated that quantitative evaluation of colloid transport through porous media will require nontraditional approaches that account for hydrodynamic and DLVO forces as well as collector shape and size.

Scott A. Bradford

Papers

Modeling colloid attachment, straining, and exclusion in saturated porous media.

Physical factors affecting the transport and fate of colloids in saturated porous media

Coupling of physical and chemical mechanisms of colloid straining in saturated porous media.

Colloid Transport and Retention in Unsaturated Porous Media: A Review of Interface-, Collector-, and Pore-Scale Processes and Models

Resolving the coupled effects of hydrodynamics and DLVO forces on colloid attachment in porous media.