Steady-state Saturated Flow Research Articles

Insights into the transport and bio-degradation of dissolved inorganic nitrogen in the biochar-pyrite amended stormwater biofilter using dynamic modeling

The stormwater biofilter is a prevailing green infrastructure for urban stormwater management, but it is less effective in dissolved nitrogen removal, especially for nitrate. The mechanism that governs the nitrate leaching and performance stability of stormwater biofilters is poorly understood. In this study, a water quality model was developed to predict the ammonium and nitrate dynamics in a biochar-pyrite amended stormwater biofilter. The transport of dissolved nitrogen species was described by advection-dispersion models. The kinetics of adsorption and pyrite-based autotrophic denitrification are included in the model and simulated with a steady-state saturated flow. The model was calibrated and validated using eleven storm events. The modeling results reveal that the contribution of pyrite-based autotrophic denitrification to nitrate leaching alleviation improves with the increased drying duration. The nitrate removal efficiency was affected by a series of design parameters. Pyrite filling rate has a minor effect on nitrate removal promotion. Service area ratio and submerged zone depth are the key parameters to prevent nitrate leaching, as they influence the emergence and discharge time of nitrate breakthrough. The high inflow volume (high service area ratio) and small submerged zone can lead to earlier and increased discharge of peak nitrate otherwise the peak nitrate could be retained in the submerged zone and denitrified during the drying period. The developed mechanistic model provides a useful tool to evaluate the treatment ability of stormwater biofilters under varying conditions and offers a guideline for biofilter design optimization.

Unsteady phenomena in saturated flow boiling heat transfer – An experimental study on oscillating refrigerant mass flux

The present work pertains to experimental studies on the unsteady phenomena in the saturated flow boiling of R407C stemming from the oscillatory refrigerant flow rate with the triangle shape in the horizontal slender annular channel. Attention is mainly paid to examine the effects of different parameters, including mean mass flux, amplitude and period of the oscillating mass flux, saturation temperature of R407C, input thermal flux, and gap size on the boiling diagrams and heat transfer coefficients. Firstly, the steady-state saturated flow boiling is studied as baseline cases. It was concluded that beyond the onset of nucleate boiling region, the mass flux of refrigerant has little influences on the boiling diagrams, which indicates that the fully developed nucleate boiling is the major mechanism. The saturated flow boiling heat transfer coefficient is enhanced as the gap size is reduced. Then, the experiment focuses on the temporal changes in the wall temperature Tw and the heat transfer coefficient hr stemming from oscillatory flow conditions. When the mass flux is oscillated periodically with the triangle shape, the Tw and hr are also oscillated temporally and enhanced with increasing in either the amplitude or frequency of oscillating mass flux. At higher input thermal flux, Tw decreases with the decreasing mass flux, which has opposite tendency compared to the single-phase convection with lower thermal flux. For intermediate exerted thermal flux, there is a negligible oscillation in the Tw and hr for most cases; the amplitudes of oscillations are lower than 0.2 °C. Besides, the significant oscillations in the wall temperature and heat transfer coefficient are noted the case with large period of the oscillating mass flux.

Bacterial mobility facilitated by soil depth and intact structure

Pathogenic bacteria, widely present in septic tanks, sewage sludges, and wastewater, can move through the subsurface environment to pollute drinking water. Bacterial transport behaviors vary with soil properties and are subject to soil pore networking. In this study, we investigated the transport of Escherichia coli 652T7 through intact and disturbed soils collected from different soil depths (0−5 cm, 5−10 cm, 10−15 cm, and 15−20 cm). The results obtained under steady-state saturated flow conditions demonstrated significant influences of soil depth and soil structure on the transport of E. coli 652T7. The breakthrough percentages of E. coli 652T7 from the intact soil cores increased with soil depth from 36 % at 0−5 cm to 63 % at 5−10 cm, 83 % at 10−15 cm, and 100 % at 15−20 cm. A similar trend was observed for the breakthrough from the repacked soil cores except with lower percentages (i.e., 16 % at 0−5 cm, 49 % at 5−10 cm, 76 % at 10−15 cm, and 86 % at 15−20 cm). Such an increase with soil depth was attributed to a combined effect of decreases in soil organic matter content (from 3.84 % to 2.47 %), free iron oxides (from 142.25 mg kg−1 to 110.66 mg kg−1), and zeta potential (from −17.6 mV to −29.0 mV) with soil depth. The larger breakthrough percentages of E. coli 652T7 from the intact than the repacked soil cores are speculated due to the existence of larger macropores, lower pore connectivity density, and reduced access to attachment sites in the intact soils than in the disturbed soils. Overall, this study suggests that tillage and organic amendments might be effective measures for reducing bacterial movement in soils.

Effect of crude oil-induced water repellency on transport of Escherichia coli and bromide through repacked and physically-weathered soil columns

Knowledge of the transport and fate of pathogenic Escherichia coli, especially in the areas contaminated with crude oil, is required to assess contamination of shallow groundwater resources. The present study aims to investigate the effect of crude oil-mediated water repellency on the movement of nalidixic acid-resistant Escherichia coli strain (E. coli NAR) and bromide (Br) as an inert tracer in two soil types. The soils were contaminated at three levels of 0, 0.5 and 1% w/w of total petroleum hydrocarbons (TPHs) using crude oil. Steady-state saturated flow in the soil columns was controlled using a tension infiltrometer. Leaching experiments were conducted through the columns of repacked (un-weathered) and physically-weathered clay loam (CL) and sandy loam (SL) soils. The columns leachate was sampled at specific times for 4 pore volumes. The shape of breakthrough curves for the E. coli NAR and Br depended on soil texture and structure and the TPHs level. Preferential flow in the crude oil-mediated water-repellent soils facilitated the transport of contaminants especially E. coli NAR. Filtration coefficient and relative adsorption index of bacteria were greatest in the repacked CL soils and were lowest in the weathered SL soils. Discontinuity of soil pores and lower flow velocity resulted in greater filtration of E. coli NAR in the repacked CL soil than other treatments. Physical weathering induced the formation of aggregates which reduced soil particle surfaces available for retention of water-repellent oil and contaminants. Movement of both bacteria and Br tracer in the weathered SL soil with 1% TPHs was higher than other treatments. This finding was attributed to low specific surface area, continuity of the pores and water repellency-mediated preferential pathways in the weathered SL soil columns. Our findings implied that shallow groundwater resources could be very sensitive to microbial contamination particularly in the oil-mediated water-repellent soils.

Bacterial mobilization and transport through manure enriched soils: Experiment and modeling

A precise evaluation of bacteria transport and mathematical investigations are useful for best management practices in agroecosystems. In this study, using laboratory experiments and modeling approaches, we assess the transport of bacteria released from three types of manure (cow, sheep, and poultry) to find the importance of the common manures in agricultural activities in soil and water pollution. Thirty six intact soil columns with different textures (sandy, loamy, and silty clay loam) were sampled. Fecal coliform leaching from layers of the manures on the soil surface was conducted under steady-state saturated flow conditions at 20 °C for up to four Pore Volumes (PVs). Separate leaching experiments were conducted to obtain the initial concentrations of bacteria released from the manures (Co). Influent (Co) and effluent (C) bacteria concentrations were measured by the plate-count method and the normalized concentrations (C/C0) were plotted versus PV representing the breakthrough curves (BTCs). Transport parameters were predicted using the attachment/detachment model (two-kinetic site) in HYDRUS-1D. Simulations fitted well the experimental data (R2 = 0.50–0.96). The attachment, detachment, and straining coefficients of bacteria were more influenced by the soils treated with cow manure compared to the sheep and poultry manures. Influent curves of fecal coliforms from the manures (leached without soil) illustrated that the poultry manure had the highest potential to pollute the effluent water from the soils in term of concentration, but the BTCs and simulated data related to the treated soils illustrated that the physical shape of cow manure was more important to both straining and detachment of bacteria back into the soil solution. Detachment trends of bacteria were observed through loam and silty clay loam soils treated with cow manure compared to the cow manure enriched sandy soil. We conclude that management strategies must specifically minimize the effect of fecal coliform concentrations before field application, especially for the combination of poultry and cow manures, which has higher solubility and tailing behavior, respectively. Interestingly, the addition of sheep manure with all three soils had the lowest mobilization of bacteria. We also suggest studying the chemistry of soil solution affected by manures to present all relevant information which affect bacterial movement through soils during leaching.

Colloid transport and remobilization in porous media during infiltration and drainage

Colloids are potential vectors of many contaminants in porous media. Understanding colloid transport is critical for assessing the migration of contaminants (e.g., pathogens) in the vadose zone. In this study, a series of column experiments were conducted to investigate the coupled effects of flow velocity, water content, and solution ionic strength on transport and remobilization of a model colloid (montmorillonite clay) in a model porous medium (Accusand) during transient unsaturated flow and steady-state saturated flow. The unsaturated transport experiments included a series of infiltration and drainage pulses (e.g., infiltration with colloids, followed by drainage of colloid suspensions, followed by infiltration with a colloid-free solution and drainage of the solution). Saturated flow experiments included only the infiltration of the colloid and colloid-free solutions. Tests were repeated for a variety of solution ionic strengths. Results showed that colloid transport was more sensitive to changes in solution ionic strength at low infiltration rates, and the effect of infiltration rate was more significant at high ionic strength. As a result, increased flow velocities and water content, resulting from high infiltration rates, enhanced colloid transport and remobilization under ionic strength conditions (e.g., 100 mM) that would otherwise lead to strong colloid retention. This observation conceptually suggests that chemical threshold values for preventing colloid movement in porous media might be larger for transient flow conditions than for uniform flow conditions. In addition, drainage was found to induce remobilization of the retained colloids, suggesting transport of colloids even after termination of injection. Overall, the study experimentally highlights the complicated interdependence of the effects of water content, flow velocity, and solution chemistry on colloid transport and remobilization.

Virus Transport during Infiltration of a Wetting Front into Initially Unsaturated Sand Columns

We investigated the effect of different flow conditions on the transport of bacteriophage phiX174 in Memphis aquifer sand. Virus transport associated with a wetting front moving into an initially unsaturated horizontal sand column was experimentally compared with that observed under steady-state saturated vertical flow. Results obtained by sectioning the sand columns showthattotal (retained and free) resident virus concentrations decreased approximately exponentially with the travel distance. The rate of decline was similar under both transient unsaturated flow and steady-state saturated flow conditions. Total resident virus concentrations near the inlet were an order of magnitude greater than the virus concentration of the influent solution in both experiments, indicating continuous virus sorption during flow through this zone. Virus retardation was quantified using the ratio of the centroids of the relative saturation and virus concentration versus relative distance functions. The mean retardation factors were 6.43 (coefficient of variation, CV = 14.4%) and 8.22 (CV = 8.22%) for the transient unsaturated and steady-state saturated flow experiments, respectively. Attest indicated no significant difference between these values at P < 0.05. Air-water and air-water-solid interfaces are thought to enhance virus inactivation and sorption to solid particles. The similar retardation factors obtained may be attributable to the reduced presence of these interfaces in the two flow systems investigated as compared to steady-state unsaturated flow experiments in which these interfaces occur throughout the entire column.

Asphalt Overlay and Subsurface Drainage of Broken and Seated Concrete Pavement

This research was conducted to study subsurface drainage issues surrounding an asphalt overlay. The type of pavement that was examined in this study was a Superpave overlay on top of a Broken and Seated Jointed Reinforced Concrete Pavement (B&S JRCP). A finite-element model of the pavement was developed and a number of numerical analyses were performed to evaluate the movement of water in the pavement. The pavement drainage modeling was conducted using the SEEP/W 2002 in GEO-SLOPE program. A steady-state saturated flow analysis was used to obtain the flow path of the infiltrated water and flux quantity through the cross-sectional area in the pavement. This analysis was done for the pavement models with different layer arrangements and different drainage practices. The findings of this research revealed that high water permeability in the asphalt base and Superpave surface layers contributed to water flow into the pavement. This infiltration of water into the pavement structure is a serious issue that must be addressed in pavement drainage design. The broken and seated layer functions as an efficient drainage layer.

Prediction of Dispersivity for Undisturbed Soil Columns from Water Retention Parameters

Dispersivity (α) is a required input parameter in solute-transport models based on the advection-dispersion equation (ADE). Normally α is obtained from miscible-displacement experiments. This dependency on inverse procedures imposes a severe limitation on our predictive capability. If solute breakthrough curves and soil hydraulic properties were measured simultaneously, pedotransfer functions could be developed to predict α from independent measurements. In this study, short (6 cm long) undisturbed columns were employed to investigate the relationship between α and the water-retention curve as parameterized by the air-entry value (ψa) and Campbell exponent (b). We worked with 69 columns from six soil types ranging in texture from loamy sand to silty clay, conventional-till and no-till management practices, steady-state saturated flow conditions, and a step decrease in CaCl2 concentration from 0.009 to 0.001 M Breakthrough curves were measured by monitoring changes in effluent electrical conductivity using a computerized data acquisition system. Estimates of α (calculated using the method of moments) ranged from 1 to 192 mm for the six soil types. Stepwise multiple-regression analysis explained ∼50% of the total variation in α, and indicated that dispersion increased as ψa and b increased. Since both ψa and b increase with increasing clay content, α also increases moving from coarse- to fine-textured soils. Our regression equation can be used as a pedotransfer function to predict α from existing databases of soil hydraulic properties. Further research is needed to independently validate its predictive capability, and to develop strategies for upscaling the model predictions.

Modeling flow and transport in heterogeneous, dual-porosity drained soils

A finite element solution of the equations for coupled flow of water and transport of chemicals in slowly permeable soils containing macropores is presented. Two example solutions are presented for the condition of a horizontal soil profile with a drainage ditch. The first is for steady state saturated flow while the second is for transient water flow produced by time varying rainfall. Through these examples it is found that the characteristic leaching time of a chemical from the soil matrix is determined by the rate of transfer of chemical mass between the pore domains. When the rate of transfer is zero, the rate of leaching is greatly retarded compared to the case where the rate of transfer is nonzero. The chemical outflow from the macropore domain is very rapid when the rate of transfer is zero, while the chemical outflow is greatly delayed, but increased in magnitude when the rate of chemical transfer is nonzero.

Saturated Flow Above a Terraced Barrier

Steady-state saturated flow through a permeable surface layer of uniform depth (verticle thickness) overlying a terraced barrier has been analyzed by the Gram-Schmidt method. The geometry and boundry condition concepts of the problem originated in the reclamation of a surface mine where, after reclamation, a soil layer of relatively high permeability overlies impermeable spoil. The soil surface and the soil-spoil interface both have a terraced contour, and the soil is of uniform thickness. The terraces consist of a series of level benches, each approximately 4 m higher in elevation than the next lower terrace. The terraces are joined by 10 m of 40% slope. The soil is 3 m in depth and it is assumed to be saturated. A new iterative technique involving separating the flow region of concern into two overlapping regions is used to solve the problem. The results show a significant water flow in the soil layer only in the sloped region and in the immediately adjacent region. There is negligible flow in regions where the horizontal distance form the sloped portion of the soil exceeds the soil depth.

Sensitivity analysis of input parameters in numerical modeling of steady state regional groundwater flow

A finite element model was used to examine the sensitivity of computed hydraulic head values to changes in the hydraulic conductivity of subsurface flow systems. Steady state saturated flow in nonuniform anisotropic cross sections was considered. From an examination of several hypothetical cross sections it was apparent that the sensitivity analysis procedure could be useful in locating those areas of the flow system that are most sensitive to changes in conductivity and in determining those components of conductivity that have the greatest effect on the hydraulic head distribution. Consequently, a sensitivity analysis could be very useful in designing efficient hydraulic conductivity measurement programs. One method that has frequently been used to determine conductivities is to adjust the conductivity components in a numerical model until the computed head values agree with field measurements. Information is presented that indicates that this procedure could lead to erroneous conclusions. By using the sensitivity analyses, a method was developed for determining the quantitative value of ratios of conductivity components from field measurements of hydraulic head. In principle, the number of head measurements must be equal to the number of unknown conductivity ratios. The accuracy of the results suggests that the method should be evaluated in a field situation.