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Microplastics residence times in lakes are currently poorly understood. In this work, settling experiments with pristine and biofilm-colonized microplastic particles were combined with model calculations to evaluate settling velocities, particle distributions, and residence times in the epilimnion, metalimnion, and hypolimnion of a hypothetical stratified lake broadly based on Upper Lake Constance. Settling velocities of various biodegradable and nonbiodegradable polymers of various shapes, sizes, and biofilm colonization were measured in a settling column. The settling velocities ranged between ~0.30 and ~50 mm s À1 . Particle sizes and polymer densities were identified as primary controls on settling rates. Microplastic particles that had been exposed to a lake environment for up to 30 weeks were colonized by a range of biofilms and associated extracellular polymeric substances; surprisingly, however, the settling velocity did not vary significantly between pristine and colonized microplastic particles. Simulated microplastic residence times in the model lake varied over a wide range of time scales (10 À1 to 10 5 d) and depended mainly on the size of the particles and depth of the lake layer. Long residence times on the order of 10 5 d (for 1-μm microplastic particles) imply that for small microplastic particles there is a high probability that they will be taken up at some stage by lake organisms. As the lake retention time (~4.5 years) is considerably shorter than the residence time of small microplastics, negligible quantities of these microplastic particles should be found in the lake sediment unless some other process increases their settling velocity.

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The in-depth knowledge of reservoir heterogeneity is imperative for identifying the location of production and injection wells. The present study aimed at experimentally investigating the process of water flooding in the viscous oil-saturated glass micromodels, which contain layers with different permeability where the fractures were placed in different locations. Computational Fluid Dynamics (CFD) simulations of flooding were also conducted to study the impact of different water flow rates and wettability states. The results showed that the fractures, which have a deviation with the trend of maximum pressure gradient line, would widen the water path and vice versa. The performance of injection wells would increase the recovery factor by 18% if these would be located in the zones with high permeability for low flow rates of water. With changes in wettability state from water to oil wet conditions, the oil production will increase by 11%. Computational Fluid Dynamics results also indicated that an increase in the capillary number from 0.8 × 10−6 to 1.6 × 10−5, would cause the recovery factor to decrease as much as 14.34% while further increase from 1.6 × 10−5 to 2.24 × 10−5, the oil production will increase by 9.5%. Comparison between the obtained oil recoveries indicates that the maximum oil recoveries will happen when the injector well is located in the zone where ascending permeability, capillary number greater than 4.81 × 10−6 and also fracture with the most deviation with pressure gradient line (i.e. angular pattern) are gathered in an area between the injection and production wells.

Microplastic (MP) particles are commonly found in freshwater environments such as rivers and lakes, negatively affecting aquatic organisms and potentially causing water quality issues. Understanding the transport and fate of MP particles in these environments is a key prerequisite to mitigate the problem. For standing water bodies (lakes, ponds) the terminal settling velocity (TSV) is a key parameter, which determines particle residence times and exposure times of organisms to MP in lakes. Here we systematically investigate the effects of the physical parameters density, volume, shape and roundness, surface roughness and hydrophobicity and lake water temperature on the TSV of a large number of particles with regular and irregular shapes (equivalent diameters: 0.5–2.5 mm) and different polymer densities using computational fluid dynamics (CFD) simulations. Simulation results are compared to laboratory settling experiments and used to evaluate existing, semi-empirical relationships to estimate TSV. The semi-empirical relationships were generally found to be in reasonable agreement with the CFD simulations (R2 > 0.92). Deviations were attributed to simplifications in their descriptions of particle shapes. Overall the CFD simulations also matched the TSVs from the experiments quite well, (R2 > 0.82), but experimental TSVs were generally slower than model TSVs with the largest differences for the irregular particles made from biodegradable polymers. The deviations of up to 58% were found to be related to the attachment of air bubbles on irregularities in the particle surfaces caused by the hydrophobicity of the MP particles. Overall, density was the most decisive parameter for TSV with increases in TSV of up to 400% followed by volume (200%), water temperature (47%) and particle roundness (45%). Our simulation results provide a frame of reference for an improved evaluation of the relative effects of different particle characteristics on their TSV in lakes. This will in turn allow a more robust estimation of particle residence times and potential exposure times of organism to MP in the different compartments of a lake.