preferential flow

The objective of this study was to investigate the effect of content and size of gravel on water infiltration in two loam and sandy loam soils. For this purpose, two series of repacked soil samples, with loam and sandy loam textures, and mass contents of 10, 20, 30 and 40 % fine gravel (4-4.75) and coarse gravel (7.9- 15.8) mm, and a treatment with no gravel (CK) were created separately. The prepared soil samples were compacted into 50 cm high cylinders with an inner diameter of 21.5 cm. The cumulative infiltration of both series of soil samples were measured using the Marriott system. Regarding both series of experiments, 10 cylinders were used. The experiments of infiltration were done with no replication. The presence of the gravel increased the cumulative infiltration in both sandy loam and loam soils compared to the (CK). The average amount of water infiltration in loam treatments, combined with fine and coarse gravel were, respectively 25 and 104 percent, higher than the one in sandy loam treatments, combined with fine and coarse gravel. Fine gravel of sandy loam and coarse gravel of loam soils, had the highest effect on the increase of cumulative infiltration. However, some fluctuations were observed under the gravel content of 20 %. The results showed that the gravel does not always increase the amount of soil water infiltration. The results also indicated that the gravel affects the amount of water infiltration in both soils by affecting the total, and the fine earth bulk densities.

The cow manure application is a common way to improve soil properties, however, the uses may cause bacteria contamination in the soil profile as well as the surface water and groundwater. Thus it is crucial to monitor the amount of bacterial transport and retention on the soil surface and deeper under cultivation conditions. The literature review shows that the structure of manure, soil structure, cultivation, soil moisture, soil salinity, and irrigation methods are important in the transport and retention of bacteria in the porous media. The plant’s roots cause cracks, creating preferential flow in the soil profile. Preferential flow accelerates pollutant transportation in the soil. Therefore It is essential to survey the transport of bacteria in the presence of plants. In this research, it has been investigated the effect of grass cultivation conditions and the size of the particles on the retention of Escherichia coli bacteria in the soil profile.

This research was conducted in the greenhouse and column experiments condition with grass cultivation. Cow manure was dried for 72 hours and passed through 0.25, 0.5, 1.0, and 2.0 mm sieves. The grass was prepared and placed uniformly on the surface of the soil columns. Then 14 days were given for the stabilization and settlement of the grass roots into the soil columns. The columns were irrigated once every two days by custom surface method until the soil moisture to the field capacity. The column’s height was divided into six equal layers, in which each layer was sampled to measure the remaining E. coli bacteria. The live counting method was used to determine the bacteria accumulation on the soil profile. For the measurement, a nine cubic centimeter of distilled water was added to one gr of each soil sample and they were put and kept in a shaker for 30 minutes. After preparing the soil samples’ dilutions, 100 µl of each dilution was cultured on the Eosin Methylene Blue (EMB) culture medium. The plates were retained at 37 °C, the temperature suitable for E. coli growth. After 24 hours, the bacteria colonies were counted and reported per ml. Due to the difference in the initial concentration of bacteria in the cow manure samples, the relative concentration index of bacteria was used to compare the concentration of bacteria in the effluent and the soil profile. This is the bacteria concentration ratio to the concentration of input bacteria, at each depth of the soil with the S/C0 parameter, it was shown that C0 and S are the bacteria concentration in the manure columns and the bacterial growth rate in the soil depth respectively.

In this research, the distribution of the soil bacteria is studied at the end of the test period. The results show that in treatments without grass cultivation, it can be seen that the maximum S/C0 for all cow’s manure sizes in the surface layer was 10 cm. With increasing depth, the relative concentration of the bacteria in all treatments has decreased. In the conditions of grass cultivation on the soil surface, the relative concentration of bacteria for the manure particle size of 0.25 mm at lower depths was more than the treatment with a larger particle size. The smaller size of the manure particles of 0.25 mm due to their transfer to the lower layers has led to the observation of a higher amount of bacteria compared to other sizes at lower depths. In other words, due to their small size, cow manure particles with a diameter of 0.25 were more easily transported to the lower layers of the soil column by flowing through the pores created by the plant roots. The reason for the decrease of the bacteria absorption in the soil surface layer compared to the treatments without cultivation can be attributed to the soil structure. The root growth causes the creation of macropores and large pores on the soil surface and increases the transfer of bacteria to the lower layers.

The effect of grass cultivation and different sizes of cow manure particle size (2.0, 1.0, 0.5, and 0.25 mm) were evaluated on the distribution of Escherichia coli bacteria in the soil profile. It shows that the bacteria retention rate decreases with increasing depth in the soil profile. Manure particles with a size of 0.25 mm cause more contaminant transport to the lower depth of the soil profile. Preferential flow in the conditions of grass cultivation caused the transport of bacteria to the lower depths. Grass cultivation causes bacteria retention at lower depths compared to the conditions without cultivation. The average bacterial retention index was 68% for a manure size of 2.0 mm at a depth of 10 cm and 48% in the treatment with a manure size of 0.25 mm. It concluded that the possibility of deep soil contamination with the application of manure with finer particles and grass cultivation is higher than with coarser particles and without grass cultivation.

Manures are used to increase pH and fertility of acidic soils in Gilan province. Although they are useful, but contain coliform bacteria that can reach the groundwater resources and lead contamination. This study aimed to investigate an indicator bacterium; Escherichia coli transport in two acidic soils. Two soil samples with pH vlues of 5.88 and 3.99 were taken from Amlash and Lahijan area respectively. For leaching experiment, air dried soil was freely packed in Polyvinyl chloride sylinders (with diameter of 4.8 and height of 14.92 cm). A 0.1 pore volume (PV) of bacteria (1× 108 CFU mL-1) and bromide (0.008 mol L-1) as a pulse flow was applied on the top of the soil columns after water flow rate reached steady state condition and leaching experiment was followed with distilled water. Leachate sampling was carried out in regular time intervals till 4.5 PV and E. coli and bromide concentrations were measured in the leachate. Resident E. coli number were also determined in each cutted 3 cm section of soil after leaching experiment endup. C/C0 peak of E. coli in the leachate of Amlash and Lahijan soil columns was observed at 0.7 and 0.9 PV repectively, while the C/C0 peak of bromide was occurred at 0.8 and 1.8 PV respectively. Early occurance of E. coli bacteria rather than bromide in the leachate of both soils was attributed to preferential water flow path which was dominant in the Lahijan soil column due to more clay and organic carbon content. The most resident E. coli number was determined in the surface layer of both soils which was greater in Amlash soil and decreased by 0.9 and 1.44 (log unit) in Amlash and Lahijan soil columns respectively. Overall, not only the cumulative number of E. coli bacteria was higher in the leachate of Amlash soil column, but also it contained more resident E. coli bacteria rather than Lahijan soil column due to greater pH value.

The contaminant mass balance in a soil profile is highly necessary in long term agriculture management under different irrigation systems. The objective of this research was assessment of the preferential flow on the travel time and mass balance of NO3 under two irrigation systems.

Nine PVC-columns were employed with a 60 cm length and 16 cm diameter. Each column included 50 cm height of clay loam soil and two cracks with 50 cm length and 1 cm diameter that filled by course sand. The treatments were based on two groups at three levels. The main group was crack: soil with blocked cracks (M); soil with remove lateral infiltration to cracks (MC-S); soil with lateraled and vertical correlation and infiltration with cracks (MC-C). The subgroup was irrigation systems: surface irrigation (SI) with 2 cm water constant height and surface drip irrigation system (DI4 & DI2) with 2 and 4 l/h flux, respectively. Each experiment column was irrigated for 6 hours (1 h with distilled water; 2h with solution consist of 176 (mg) of NO3 and 61.8 (mg) of Cl in 1 liter distilled water and finally 3 h with distilled water). Before start of experiment, each column was irrigated with distilled water for 5 h to reach saturation condition. The drainage water separately collected from soil and cracks every 15, 30 and 60 min.

The results showed the value of mass balance of NO3 and Cl in SI and DI4 irrigation systems were more similarity. Based on the results, the mass of NO3 of drainage water in M inDI2 was approximately half in compare to SI and DI4. Whiles, this mass of Cl that transported under DI2was more in compare to SI and DI4. Also, the travel time of Cl in MC-C treatment, in crack section, was shorter of NO3. On the other hand, the results show the mass balance of these anions that transported from crack section in the most of the treatments was 1.5 times more. It indicates vertical flows in course paths of crack have the main role to move and transport of contaminants such as and Cl toward groundwater.

Over all, under high water velocity as SI, NO3 is transporting more rapidly but under condition with low water velocity like as DI2 with longer time of unsaturation condition, Cl is leaching more. The preferential flows in treatment with full interaction between soil matrix and cracks were happen more. Also, the preferential flow has stronger role to transport of Cl than NO3.

Preferential flow is of great importance in the environment and the human health. So, rapid water transportation and consequently, pollutants and pesticides leak out and get into the groundwater, making it very difficult to measure and quantify. To quantify and describe the preferential flow, two gravity-driven models were used: 1) kinematic wave model (KW) introduced by Germann in 1985), and 2) kinematic dispersive wave (KDW) model developed by applying a second-order correction to the Germann’s model by Di Pietro et al. in 2003. So, the experimental data was obtained using the laboratory mini-rainfall-simulator over cylindrical soil samples at the laboratory. Their parameters were obtained using Solver add-ins in the Excel software. Then, the results were compared using the root-mean-square error (RMSE). The results showed that the KDW model could better predict the preferential flow (with lower RMSE). Also, the regression results showed 1) there was no significant relation between the preferential flow and the total porosity, and 2) there is a significant relation between the preferential flow and the macrospores.

One of the mechanisms of solute transport in soil is preferential flow or flow in macropores. In this study, to investigate the influence of macropore paths on solute transport, three soil bulks composed of 3% clay, 4.2% silt and 92.8% sand in a box with 200 cm length, 100 cm width and 45 cm height were used. The first soil bulk was homogeneous (without macrpore path), the second one contained soil and layers of 5 cm thickness, composed of gravels with 2-4 mm diameter, from surface to bottom of the box (with continuous macropore layers), and the third one contained also contained soil and layers of same thickness and material, but the layers extended only to a depth of 35 cm (with non-continuous macropore layers). The transport of NaCl solution in these three soil bulks were experimented and then simulated using GeoStudio. In the soil bulk having continuous macropore layers, in comparison with the one with no macropore layer, the solute traveled the distance between the surface and the bottom in a shorter time (about 27%) and the peak concentrations were sooner observed (10-20 min). However, the non-continuous layers had no significant impact on the speed of solute transport. The GeoStudio software satisfactorily simulated the solute transport with the coefficient of determination more than 0.970 and the values of the root mean square error less than 0.25.

In this study، the effect of soil structure under saturated and unsaturated flow conditions on nonreactive bromide (BR) transport was investigated. The soil structure treatments consisted of undisturbed columns (prismatic and granular structures)، and disturbed columns (single- grain structure). A constant concentration (C0= 0. 005 M) of bromide was supplied on the surface of the columns in a steady-state flow condition. For the saturated flow condition، a flux equal to the highest saturated hydraulic conductivity (Ks) of the columns was applied on all of the columns. To create the unsaturated flow condition، a flux equal to the half of the lowest Ks of the columns was imposed on all of the columns. The leaching of the columns was followed for five pore volumes (5PV) and the bromide concentration of the effluent was measured at 0. 2PV intervals using bromide selective electrode. The breakthrough curve (BTC) of single- grain structure was sigmoidal (S-shaped) and similar to piston-capillry flow form. In contrast، BTCs of the granular and prismatic structures had a steep initial part and later gradual tailed part. The preferential pathways caused the early appearance of bromide in the leachate of columns of these two structures. Tailing of the BTCs might be due to dispersion and diffusion between mobile and immobile water fractions. In saturated condition، the bromide plume appeared earlier than that in the unsaturated condition because of domination of mass flow and rapid macroporous stream. The results demonstrated the importance of soil structure، preferential pathways، and flow conditions in solute and pollutant transport.

Increasing soil contamination by chemicals has become an issue of increasing environmental concern. Leaching of chemicals into and through the vadose zone creats serious problems due to the contamination of the soil matrix, soil solution and groundwater. Therefore, in order to study the effect of the preferential flow, macropores and organic matter on mobility and leaching of the metals such as cadmium lead, and zinc, an experiment was conducted as a factorial-split plot based on the completely randomized design with three replications. Three treatments of the undisturbed soil (U), the disturbed soil (D) and the disturbed soil containing 3 percent organic matter (O) were leached by the solutions with the concentration of 20 mg.L-1 of Cd, Pb, and Zn for a month. Then the concentrations of Cd, Pb and Zn in the leachate were measured at different time intervals. The ANOVA results indicated that the metals had a significant difference in the leachate at 1% and the order of their mobility was: Zn>Pb>Cd. Also, there was a significant difference between different soil treatments at 1% and the concentration of the three metals in U and O treatments was more than their concentrations in D treatment. Furthermore, a significant difference between the time intervals of leaching (pore volumes) was observed at 1%. So that, Cd in leachate of U, O and D treatments indicated a significant difference after leaching for 3, 3 and 5 days, respectively (1%). But, Pb in the leachate of the three soil treatments after leaching for 11 days had a significant difference. Zn concentration only in O treatment had a clear trend at different time intervals of leaching and a significant difference was observed after leaching for 8 days.