Jamal Abu-Ashour

TRANSPORT OF MICROORGANISMS THROUGH SOIL

Microorganisms migrating into and through soil from sources on the land surface may cause a serious threat to both ground and surface waters. It has been estimated that microorganisms can migrate significant distances in the field. Results from various studies suggested that preferential flow through macropores, worm holes, cracks, and fractures is the main reason for such observations. However, a quantitative representation of this phenomenon has not been provided. Microorganisms migrate through soil by advection and dispersion, while being subjected to effects of filtration, adsorption, desorption, growth, decay, sedimentation and chemotaxis. Both laboratory and field investigations have contributed important information on bacterial movement in soils. Qualitative comparisons are generally transferable from laboratory to field situations. Quantitative agreement is much more difficult to establish. Available mathematical modelling of microbial transport is limited in practical application because of the simplifying assumptions used in its development.

MOVEMENT OF BACTERIA IN UNSATURATED SOIL COLUMNS WITH MACROPORES

Rapid movement of bacteria through the soil has been observed after applications of manure to agricultural fields. Preferential flow through macropores has been suggested as the main reasons for these observations. Experiments with repacked soil columns were used to study the effect of artificially created macropores, soil type, soil water content, and simulated rain application on movement of a tracer bacterium, nalidixic acid-resistant Escherichia coli. Results form these experiments showed a significant increase in the number of biotracer cells passing through a soil column when macropores were present and the soil was wet. There was no passage of biotracer cells through a dry soil with macropores. No biotracer cells were eluted from columns without macropores even when the soils were wet. Simulated rainfall applied on the top of the soil columns caused bacteria to travel deeper into the soil. These results confirm the important role that macropores play in the movement of bacteria through heterogenous soils.

Transport of bacteria on sloping soil surfaces by runoff

Pathogenic bacteria exist at soil surfaces as a result of practices as spreading of liquid manure on agricultural lands or use of treated wastewater for irrigation. Rainfall is a major factor affecting vertical and horizontal movement of bacteria in soil. Surface runoff carries bacteria significant distances downstream causing serious threats to ground and surface waters. This study uses a nalidixic acid‐resistant Escherichia coli strain as a biotracer monitoring extent of bacterial migration on sloping soil surfaces by runoff action. Two 10×10‐m plots in two sites having different slopes were sprayed with water containing biotracer. Soil texture at sites was clay loam. Sixteen days after spraying, two heavy rainfalls that caused runoffs were recorded. First rainfall occurred 2 days after spraying plots. Samples were collected from soil and runoff at different distances downstream of the plots. Biotracer was found in soil and runoff samples some 20 m downstream from center point of plot having the milder slope. Biotracer was found in soil and runoff samples further downstream of the second plot with the steeper slope reaching a 35‐ and 30‐m distance respectively. Most soil and runoff samples collected after the second rainfall, occurring 15 days after inoculation, contained no biotracer except small numbers found in soil samples taken from center point of each plot 5 m downstream. Results confirm the important role of runoff in bacterial transport on soil surfaces. They show E. coli survives in semiarid areas for a long time and increases potential of contamination. © 2000 John Wiley & Sons, Inc. Environ Toxicol 15: 149–153, 2000

Effect of Interstitial Velocity on the Adsorption of Bacteria onto Soil

The adsorption of bacteria onto soil is affected by the physical and chemical characteristics of the soil and water, the size and morphology of the bacterial cells, and the water-flow characteristics in the soil. The present study focuses on the latter factor by investigating the effect of the interstitial velocity on the adsorption of bacteria onto soil. Columns of 10 cm diameter and 130 cm height, respectively, were packed with a sandy soil. The columns were saturated with water containing nalidixic acid-resistant Escherichia coli as a bio-tracer at three different pH levels. The columns were maintained at 20°C for 24 h before connecting the column outlet to its inlet by a pump in a closed loop. Water containing the biotracer was re-circulated through the column for another 24 h at three different interstitial velocities. Water samples were taken from a sampling tap connected to the pump at 4-h intervals. These samples were analyzed to determine the biotracer concentration. The results show that more biotracer cells were retained in the soil at the lower interstitial velocity. The higher interstitial velocity resulted in higher shear forces which caused more desorption of the biotracer cells from the surfaces of the soil particles. Bacterial adsorption was higher at the acidic pH value. The survival of the biotracer cells in soil solution was also tested at three different pH levels. The results show that no decline in the biotracer concentration occurred during the test period.

Soil physical properties and infiltration rate as affected by tire dynamic load and inflation pressure.

Wheel traffic of agricultural prime movers is a major contributor to detrimental soil compaction. Soil response to differences in tire inflation pressure and vertical load was determined beneath 18.4 R34 rear drive tires on a two-wheel-drive tractor which had the centerline of each rear tire 520 mm laterally outboard of the centerline of its corresponding front tire. No drawbar load was applied to the tractor. The tires were used with two static loads (12.5 and 21 kN) and three inflation pressures (193, 124, and 76 kPa). Inflation pressures recommended by the tire manufacturer were 40 kPa for the 12.5 kN load and 105 kPa for the 21 kN load. Soil bulk density and cone index generally increased and air-filled porosity generally decreased in the 100 to 500 mm depth range beneath the tire centerline as the static load increased and as inflation pressure increased. Water infiltration rate into soil at the tire track centerline generally decreased as the static load increased and as inflation pressure increased.

Survival of bacteria in soil subsequent to greywater application

Greywater can carry many pathogenic bacteria. Survival of these bacteria in the soil‐greywater environment may pose a health risk associated with greywater reuse. This study investigates the survival of a biotracer bacterium (nalidixic acid‐resistant Escherichia coli ‐ E. coli NAR) in soil irrigated by greywater. Greywater volumes obtained from the showers, sinks, laundry and kitchens of five houses in Irbid, Jordan, were mixed with the biotracer and applied to four 1 m × 1 m plots. Tap water was applied to a fifth plot as a control. For 30 days, soil samples were collected from the five plots and then analysed to determine the biotracer content. The results showed that the biotracer survived for 15 days in the plot irrigated by kitchen greywater. The biotracer cells survived for 10, 10, 10, and 7 days in the other plots irrigated with tap water and greywater from showers, sinks, and laundry, respectively. The longer survival of the cells in the plot irrigated by kitchen greywater may be attributed to the higher BOD and nutrients in this greywater.

WATER INFILTRATION THROUGH DIESEL-CONTAMINATED SOIL

ABSTRACT The water infiltration characteristics through soil contaminated by diesel was investigated. Columns were packed with soil containing 15, 10, 5, 1, and 0 percent diesel on mass basis. Two soil types were used, namely, clay loam and sand. The results showed that the water infiltration rate through clay loam soil containing 1 percent diesel was about 50 percent of its value through diesel free soil. This relative rate dropped to less than 35 and 25 percent when soil contaminated with 5 and 10 percent diesel was used, respectively. Increasing the initial diesel concentration beyond 10 percent does not cause further significant drop in the water infiltration rate. Similar trend was observed when the sandy soil was used. However, the effect of initial diesel concentration on the water infiltration rate was less significant compared to the results obtained using the clay loam soil. The relative infiltration rates through the sandy soil ranged from about 80 percent in the sand containing 1 percent diesel to less than 35 percent in sand containing 15 percent diesel. The variation of the relative infiltration rate with degree of diesel saturation does not appear to be linear. Post experiment dissection of the columns showed that water partially displaced diesel pushing it down to lower soil layers. The percentage of diesel displaced by water in the sandy soil was higher than that in the clay loam soil.

Chloride and atrazine transport through saturated soil columns

A possible contamination of water resources by the application of pesticides is a problem confronting many irrigated areas in arid and semi-arid areas. The best management practices have to be adopted to minimize pesticide transport and leaching under irrigated conditions. Atrazine dissipation in loam and sandy loam soils has been tested in the laboratory using disturbed soil columns under saturated flooding conditions. All the experiments were performed in replicates. The chloride transport was also studied to test its behavior as an inert tracer in both the soils. Atrazine and chloride breakthrough curves were analyzed with the parameter optimization program CXTFIT to determine transport parameters including pore-water velocity (v), retardation coefficient (R), hydrodynamic dispersion coefficient (D), and pulse duration (t o ). The pore-water velocity and pulse duration of the solute were estimated from the experimental conditions and kept constant during the optimization procedure. The results indicated that the R of chloride was not significantly different from 1, indicating that chloride is an inert tracer for the types of soil tested in this study. The average R of atrazine was 4.56 and 3.15 for sandy loam and loam soils, respectively. Results also showed that the hydrodynamic dispersion coefficient was much higher in the case of sandy loam soil compared to the loam soil for the two solutes, thus indicating non-equilibrium transport conditions. In the case of chloride, D increased from 0.4 for the loam soil to 16.2 cm2/min for the sandy loam soil. Similar results were observed in the case of atrazine in which D for the sandy loam soil was 60% higher than that for the loam soil. More atrazine leaching is expected under field conditions due to the presence of soil cracks and macropores.