JWSS - Journal of Water and Soil Science

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In this paper, a numerical solution is presented for one-dimensional unsaturated flows in the subsurface. Water flow in the subsurface, however, is highly nonlinear and in most cases, exact analytical solutions are impossible. The method of reference-operators has been used to formulate a discrete model of the continuum physical system. Many of the standard finite difference methods and also the finite volume method are special cases of the method of reference-operators. Unlike elementary finite difference methods, the method of reference-operators may by used to construct finite difference schemes on grids of arbitrary structure. A one-dimensional model was developed to predict the soil-water suction (negative pressure head) and water content in a vertical column of a layered soil. The model was verified against some available analytical solutions and experimental results and, in all cases, it showed good agreement.

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Transport and filtering of pathogenic bacteria through porous media and groundwater resources are important and، therefore the effect of various factors on bacteria transport and filtering has been given a great attention nowadays. In this study، effects of calcium sulfate and carbonate on Psedomonas fluorescens filtration and filtering parameters were investigated in saturated sand columns under steady-state flow. The calcium carbonate levels included 0، 5، 10 and 20 %w/w and calcium sulfate levels consisted of 0، 5 and 10 % w/w which were thoroughly mixed with sand (0.15-0.25 mm). The experiment was considered factorial in completely randomized design with three replicates. The treated sands were poured into pyrex cylinders with length of 20 cm and diameter of 7 cm. Then، steady-state saturated flow with constant flux was applied to the columns. When the steady-state flow was established، the bacteria suspension with concentration of 10⁶ CFU cm^-3 (C0) was injected as step input into the columns. The leaching then was continued up to 5 times of pore volume (PV). The effluent concentration of the bacteria (C) was measured at 0.25 PV intervals. Then، the sand columns were divided into 0-5، 5-10، 10-15 and 15-20 cm layers in order to measure the filtered bacteria in each layer. The results showed that the effects of calcium carbonate on retaining of the bacteria in the 5-10 and 10-15 cm layers were significant at 1% level. It was significant at 5% for the 15-20 cm layer. The effect of calcium sulfate was also significant at 5% for the 10-15 cm and 15-20 cm layers. The interactive effects of treatments on bacteria adsorption was significant for the 5-10، 10-15 and 15-20 cm layers. The retained concentration profile and the filtration coefficient were significantly affected by the treatments، showing higher bacteria adsorption at lower depths and predominance of physical filtering. The results showed the high filtering capacity of carbonate and sulfate minerals which could ultimately reduce bacteria transport in saturated porous media towards groundwater resources.

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Microbial transport in soil is critical in different ways, especially in groundwater contamination and bioremediation of groundwater or soil. The main objectives of this research were quantitative study of bacterial transport and deposition under saturated conditions in calcareous soils. A series of column leaching experiments were conducted. Breakthrough curves (BTCs) of Pseudomonas fluorescens and Cl- were measured. After leaching experiment the bacteria was measured in difference layers of the soil columns. The HYDRUS-1D one- and two-site kinetic attachment-detachment models were used to fit and predict transport and deposition of bacteria in soil columns experiments. The results indicated that two-site kinetic model leads to better prediction breakthrough curves and bacteria retention in the calcareous soil in comparison with one-site kinetic model. Interaction with kinetic site 1 was characterized by relatively fast attachment and slow detachment, whereas attachment to and detachment from kinetic site 2 was fast. Fast attachment and slow detachment of site 1 was attributed to soil calcium carbonate that has favorable attachment site for bacteria. The detachment rate was less than 0.01 of the attachment rate, indicating irreversible attachment of bacteria. Most of the cells were retained close to the soil column inlet, and the rate of deposition decreased with depth. Microbial reduction rate for the soil was 4.02-4.88 log m-1. High reduction rate of bacteria was also attributed to soil calcium carbonate that has favorable attachment site for bacteria.

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Macrospore created by decaying plant root provides pathways for rapid transport of pollutants in soil profile. The main objective of this study was quantitative analysis of the effect of plant root (Zea mays L.) on bacterial and chloride transport through soil. Experiments were conducted in 9 soil columns packed uniformly with loamy sand. The treatments were bare soil, bare soil with corn (Zea mays L.) root and bare soil after decaying the corn root. The Breakthrough curves of Chloride were measured. Breakthrough curve (BTCs) of Escherichia coli and chloride were measured, too. The HYDRUS-1D one and two site kinetic attachment–detachment models were used to fit and forecast transport and retention of bacteria in soil columns experiment. The results indicated that the difference between soil hydraulic properties (saturated hydraulic conductivity and flow velocity) of the treatment was significant (p < 0.05). The result also showed that the two-site kinetic model leads to better prediction of breakthrough curves and bacteria retention in the soil in comparison with one-site kinetic model. Interaction with kinetic site 1 was characterized by relatively fast attachment and slow detachment, whereas attachment to and detachment from kinetic site 2 was fast. Most of the cells showed retention close to the soil column inlet, and the rate of deposition decreased with depth. Low reduction rate of bacteria of the soil columns with plant root and with void root channel indicated the presence of macrospores in the soil created by deep corn root system.