Experimental Study of Fingering Flow in Porous Hele-Shaw Cells

With the aim of studying the physical process concerning the unstable fingering phenomena in two dimensions, experiments of vertical infiltration through layered sand were carried out in the laboratory using Hele-Shaw cells. We developed a light transmission method to measure the dynamics of water saturation within flow fingers in great detail with high spatial and temporal resolution. The method was calibrated using X-ray absorption. We improved the measured light transmission with correction for scattering effects through deconvolution with a point spread function which allows us to obtain quantitative high spatial resolution measurements. After fingers had fully developed, we added a dye tracer in order to distinguish mobile and immobile water fractions. Fully developed fingers consist of a tip, a core with mobile water, and a hull with immobile water. We analyzed the dynamics of water saturation within the finger tip, along the finger core behind the tip, and within the fringe of the fingers during radial growth. Our results confirm previous findings of saturation overshoot in the finger tips and revealed a saturation minimum behind the tip as a new feature. The finger development was characterized by a gradual increase in water content within the core of the finger behind this minimum and a gradual widening of the fingers to a quasi-stable state which evolves at time scales that are orders of magnitude longer than those of fingers' evolution. In this state, a sharp separation into a core with fast convective flow and a fringe with exceedingly slow flow was detected. All observed phenomena, with the exception of saturation overshoot, could be consistently explained based on the hysteretic behavior of the soil-water characteristic

Examining the Effect of Pore Size Distribution and Shape on Flow through Unsaturated Peat using Computer Tomography

The hydraulic conductivity of unsaturated peat soil is controlled by the air-filled porosity, pore size and geometric distribution as well as other physical properties of peat materials. This study investigates how the size and shape of pores affects the flow of water through peat soils. In this study we used X-ray Computed Tomography (CT), at 45μm resolution under 5 specific soil-water pressure head levels to provide 3-D, high-resolution images that were used to detect the inner pore structure of peat samples under a changing water regime. Pore structure and configuration were found to be irregular, which affected the rate of water transmission through peat soils. The 3-D analysis suggested that pore distribution is dominated by a single large pore-space. At low pressure head, this single large air-filled pore imparted a more effective flowpath compared to smaller pores. Smaller pores were disconnected and the flowpath was more tortuous than in the single large air-filled pore, and their contribution to flow was negligible when the single large pore was active. We quantify the pore structure of peat soil that affects the hydraulic conductivity in the unsaturated condition, and demonstrate the validity of our estimation of peat unsaturated hydraulic conductivity by making a comparison with a standard permeameter-based method. Estimates of unsaturated hydraulic conductivities were made for the purpose of testing the sensitivity of pore shape and geometry parameters on the hydraulic properties of peats and how to evaluate the structure of the peat and its affects on parameterization. We also studied the ability to quantify these factors for different soil moisture contents in order to define how the factors controlling the shape coefficient vary with changes in soil water pressure head. The relation between measured and estimated unsaturated hydraulic conductivity at various heads shows that rapid initial drainage, that changes the air-filled pore properties, creates a sharp decline in hydraulic conductivity. This is because the large pores readily lose water, the peat rapidly becomes less conductive and the flow path among pores, more tortuous

Effects of solid manure particle fractionation on transport, retention, and release of Escherichia coli

Understanding the effect of manure particle fractionation on transport, retention, and release of bacteria plays a critical role in manure management and environmental policies that address soil and water bacterial pollution. Compared to soil particle size, there is less understanding of the importance of solid manure particle size and fractionation on bacterial fate and transport in soils. Four different cow manure particle sizes (0.25, 0.5, 1, and 2 mm) were used to investigate Escherichia coli fate in a saturated loamy sand soil. Leaching experiments were performed for up to 20 pore volumes. Preferential transport of chloride mitigated as manure particle size increased. The larger manure fractions (1 and 2 mm) showed greater heterogeneity in bacteria transport and release; smaller manure fractions (0.25 and 0.5 mm) had a greater bacteria retention with retarded release. Bacteria release was associated with transport and re-entrainment of manure particles through soil columns. The results highlighted the contribution of fine and transported particles as of primary importance for retention near the surface and transporting bacteria in soil. Similar retention shapes (i.e., exponential) for different fractions illustrated the similarity of manure source, where greater retention was observed at 0−3 cm depth for the smallest (0.25 mm) and largest (2 mm) manure fractions. The findings also highlighted the dependency of bacteria transport, retention, and release on manure physical fractionation, which should be considered in managing soil and manure practices in the field. © 2021 The Author

Field validation of DNA-based biosensor for rapid detection of ultra-trace mercury(II) in natural waters

Mercury (Hg) remains a significant pollutant of global concern. In particular, contamination of surface water and groundwater by Hg poses severe threats, unrecognized in many cases, to the drinking water safety of numerous, often economically challenged, communities in the world. While the speciation of aqueous Hg varies depending upon the environmental factors, inorganic Hg(II) represents a primary regulator of its fate and bioavailability in natural waters. In this regard, both the public sectors and private families are soliciting water quality sensors able to detect aqueous Hg(II) down to the ultra-trace level (below the drinking water limit) sensitively, reliably and quickly. Here, we present a summary of the development of DNA-based biosensors for Hg(II) that incorporates recent advance in the field deployment of two forms of the DNA functionalized biosensing tools. The first form is the DNAfunctionalized hydrogel sensor that can be readily applied through direct immersion in solution and water. The second setup is the DNA-DGT sensor that integrates the DNA-functionalized hydrogel with the diffusive gradients in thin films (DGT) technique to unlock more versatile applications in water, soils and aquatic sediments. These two types of Hg(II) sensors were tested with hydrochemically diverse ground and surface waters from the Datong Basin, northern China and the Great Lakes region, North America. The results indicate that the DNA-functionalized hydrogel sensor was able to measure total dissolved Hg(II) quickly (within few hours), yet inapplicable to Hg(II) concentrations below 10 nM especially in the presence of interfering components (e.g., Cl- and natural dissolved organic matter). In contrast, the DNA-DGT sensor could detect variably ultra-trace Hg(II) (even <1 nM) depending upon the deployment time. In combination with equilibrium species calculations, the DNA-DGT sensor shows the capacity to differentiate the partitioning of Hg(II) between various aqueous species and to calibrate the interferences by water temperature and natural dissolved organic matter. It reveals that the bioavailability of Hg(II), even at the ultra-trace levels, to the water organisms varies significantly depending upon the environmental conditions. Furthermore, the sensor measurements together with results of hydrochemical analyses suggest that the transformation of Hg(II) is linked to the biogeochemical cycling of sulfur in the groundwaters. Overall, our DNA-based sensors represent ultrasensitive, field-deployable detection methods that can unravel the mobility of Hg(II) in natural waters and early warning of Hg pollution to the drinking water.This research was undertaken thanks, in part, with support from the Global Water Futures Program funded by the Canada First Research Excellence Fund (CFREF). Additional funding funding was also provided by the NSERC SPG biosensor project awarded to Juewen Liu., the GWF Winter Soil Processes in Transition project awarded to Fereidoun Rezanezhad., and the CERC program awarded to Philippe Van Cappellen

Particle fractionation controls Escherichia coli release from solid manure

Bacteria transport through soil is a complex process particularly when the cells are released from solid manures and co-transported with particles. This study focuses on understanding of the Escherichia coli release from different particle fractions (0.25-, 0.5-, 1-, and 2-mm) of solid manure and evaluating different influent boundary conditions during cell release from manure and when a solid manure is applied to the soil. The 0.25-mm and 2-mm particle sizes resulted a greater cell release compared to 0.5-mm and 1-mm fractions (p < 0.05). The shape and magnitude of the cell release curves (CRCs) from the original manure bulk were mainly influenced by the two 0.25-mm and 2-mm fractions, respectively. The arithmetic mean for normalizing the CRCs and the time variable- based normalized CRCs for the manure-treated soil were the robust variables in evaluation of the experimental data. However, a single maximum bacteria concentration could provide the realistic dataset for the modeling process. Evaluation of the root-mean-squared-error and Akaike criterion showed that the two- and three-parametric models are recommended for simulating the cell release from solid manure in comparison with one parametric models. This study also suggests considering separate microbial release evaluations, with regards to influent concentration, for manure and manure-treated soils to propose best management practices for controlling bacteria pollution. Further research will reveal the key roles of different woody components and soluble material ratios for the various solid manures in bacteria release

Particle fractionation controls Escherichia coli release from solid manure

Funding Information: This work was supported by Shahrekord University . Nasrollah Sepehrnia was supported by Alexander von Humboldt Foundation and Postdoctoral Fellowship at Leibniz University of Hannover, Germany. Publisher Copyright: © 2021 The Author(s)Peer reviewedPublisher PD

Biodegradation Kinetics of Benzene and Naphthalene in the Vadose and Saturated Zones of a (Semi)-Arid Saline Coastal Soil Environment

Biodegradation is a key process for the remediation of sites contaminated by petroleum hydrocarbons (PHCs), but this process is not well known for the (semi)-Arid coastal environments where saline conditions and continuous water level fluctuations are common. This study differs from the limited previous studies on the biodegradation of PHCs in Qatari coastal soils mainly by its findings on the biodegradation kinetics of the selected PHCs of benzene and naphthalene by indigenous bacteria. Soil samples were collected above, across, and below the groundwater table at the eastern coast of Qatar within a depth of 0 to-40 cm. Environmental conditions combining low oxygen and high sulfate concentrations were considered in this study which could favor either or both aerobic and anaerobic bacteria including sulfate-reducing bacteria (SRB). The consideration of SRB was motivated by previously reported high sulfate concentrations in Qatari soil and groundwater. Low-and high-salinity conditions were applied in the experiments, and the results showed the sorption of the two PHCs on the soil samples. Sorption was dominant for naphthalene whereas the biodegradation process contributed the most for the removal of benzene from water. Losses of nitrate observed in the biodegradation experiments were attributed to the activity of nitrate-reducing bacteria (NRB). The results suggested that aerobic, NRB, and most likely SRB biodegraded the two PHCs, where the combined contribution of sorption and biodegradation in biotic microcosms led to considerable concentration losses of the two PHCs in the aqueous phase (31 to 58% after 21 to 35 days). Although benzene was degraded faster than naphthalene, the biodegradation of these two PHCs was in general very slow with rate coefficients in the order of 10-3 to 10-2 day-1 and the applied kinetic models fitted the experimental results very well. It is relevant to mention that these rate coefficients are the contribution from all the microbial groups in the soil and not from just one.Scopu

Bioretention cells under cold climate conditions: Effects of freezing and thawing on water infiltration, soil structure, and nutrient removal

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.scitotenv.2018.08.366 © 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/Bioretention cells are a popular control strategy for stormwater volume and quality, but their efficiency for water infiltration and nutrient removal under cold climate conditions has been poorly studied. In this work, soil cores were collected from an active bioretention cell containing engineered soil material amended with a phosphate sorbent medium. The cores were used in laboratory column experiments conducted to obtain a detailed characterization of the soil's bioretention performance during six consecutive freeze–thaw cycles (FTCs, from −10 to +10 °C). At the start of each FTC, the experimental column undergoing the FTCs and a control column kept at room temperature were supplied with a solution containing 25 mg/L of bromide, nitrate and phosphate. Water saturated conditions were established to mimic the presence of an internal water storage zone to support anaerobic nitrate removal. At the end of each FTC, the pore solution was allowed to drain from the columns. The results indicate that the FTCs enhanced the infiltration efficiency of the soil: with each successive cycle the drainage rate increased in the experimental column. Freezing and thawing also increased the saturated hydraulic conductivity of the bioretention soil. X-ray tomography imaging identified a key role of macro-pore formation in maintaining high infiltration rates. Both aqueous nitrate and phosphate supplied to the columns were nearly completely removed from solution. Sufficiently long retention times and the presence of the internal water storage zone promoted anaerobic nitrate elimination despite the low temperatures. Dissolved phosphate was efficiently trapped at all depths in the soil columns, with ≤2% of the added stormwater phosphate recovered in the drainage effluent. These findings imply that, when designed properly, bioretention cells can support high infiltration rates and mitigate nutrient pollution in cold climates.University of TorontoCanada Excellence Research Chairs, Government of CanadaNatural Science and Engineering Research Council ["485238-15","479034-15","950-230892"]NSERC Discovery [RGPIN-2015-03801

Transport of chloride and deuterated water in peat: The role of anion exclusion, diffusion, and anion adsorption in a dual porosity organic media

The final publication is available at Elsevier via https://doi.org/10.1016/j.jconhyd.2019.103497. © 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The dual-porosity structure of peat and the extremely high organic matter content give rise to a complex medium that typically generates prolonged tailing and early 50% concentration breakthrough in the breakthrough curves (BTCs) of chloride (Cl−) and other anions. Untangling whether these observations are due to rate-limited (physical) diffusion into inactive pores, (chemical) adsorption or anion exclusion remains a critical question in peat hydrogeochemistry. This study aimed to elucidate whether Cl− is truly conservative in peat, as usually assumed, and whether the prolonged tailing and early 50% concentration breakthrough of Cl− observed is due to diffusion, adsorption, anion exclusion or a combination of all three. The mobile-immobile (MiM) dual-porosity model was fit to BTCs of Cl− and deuterated water measured on undisturbed cores of the same peat soils, and equilibrium Cl− adsorption batch experiments were conducted. Adsorption of Cl− to undecomposed and decomposed peat samples in batch experiments followed Freundlich isotherms but did not exhibit any trends with the degree of peat decomposition and sorption became negligible below aqueous Cl− concentrations of ~310 mg L−1. The dispersivity determined by fitting the Cl− BTCs whether assuming adsorption or no adsorption were significantly different than determined by the deuterated water (p < .0001). However, no statistical differences in dispersivity (p = .27) or immobile water content (p = .97) was observed between deuterated water and Cl− when accounting for anion exclusion. A higher degree of decomposition significantly increased anion exclusion (p < .0001) but did not influence the diffusion of either tracer into the immobile porosity. Contrary to previous assumptions, Cl− is not truly conservative in peat due to anion exclusion, and adsorption at higher aqueous concentrations, but the overall effect of anion exclusion on transport is likely minimal.The authors acknowledge funding through the Canada Excellence Research Chair (CERC) program to PVC