Transport of bacteria on sloping soil surfaces by runoff

Transport of Bacteria on Sloping
Soil Surfaces by Runoff

Jamal Abu-Ashour,1 Hung Lee2
1Department of Agricultural Engineering and Technology, Jordan University of Science and Technology,Irbid 22110, Jordan 2 Department of Environmental Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada Received 3 March 1999; revised 2 August 1999; accepted 18 August 1999 ABSTRACT: 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.
ᮊ 2000 by John Wiley & Sons, Inc. Environ Toxicol 15: 149᎐153, 2000
Keywords: biotracer; E. coli; runoff; soil; bacterial transport
use of treated wastewater for irrigation are potentialsources for these pathogens. In a field study conducted Pathogenic microorganisms found at the soil surfaces in Ontario, Canada, Culley and Phillips Ž may originate from several sources. Practices such the that manure applications in winter resulted in signifi- spreading of liquid manure on agricultural lands or the cantly higher fecal coliform and fecal streptococcuscounts in the surface runoff, and fecal streptococcus Corresponding to: Jamal Abu-Ashour; e-mail: counts in subsurface discharge when compared with Contract grant sponsor: Jordan University of Science and Tech- applications during other seasons. Stewart and Reneau 1981 observed migration of coliform bacteria from Contract grant sponsor: Natural Sciences and Engineering septic tank drain fields in both vertical and horizontal Contract grant sponsor: Canada Foundation for Innovation’s directions to monitoring wells of 152- and 305-cm depth Institutional Innovation Fund for Infrastructure.
located within 30 m of the drain fields. The extent of ᮊ 2000 by John Wiley & Sons, Inc.
migration in both directions varied depending on the Samples of this cell suspension were used to inoculate position of the monitoring well relative to the drain 500 mL of TSB and grown as above, for use as the field. They attributed these differences to variations in Some authors reported that the adsorption of mi- Test Sites and Experimental Setup
croorganisms onto soil is reversible, as reviewed byAbu-Ashour et al. Ž 1994 . The term ‘‘reversible’’ im- The experiments were conducted at two sites about plies that adsorbed microorganisms may detach from 200 m apart and located in a field 15 km east of Irbid surfaces of soil particles and desorb in water. They may in northern Jordan. Both sites had a bare soil surface subsequently be readsorbed. The phenomenon of des- with an average bulk density of 1350 kgrm3 and an orption was suggested by Wellings et al. Ž initial soil water content of 20% on a mass basis. The observed that previously virus-free wells near a land soil texture at both sites was clay loam. The average application site in Florida, contained viruses after a slope of the first site was 2% while that of the second period of heavy rainfall. They suggested that viruses were initially adsorbed onto the soil particles and hence The experiments were carried out in November 1997.
could not be detected in wells. However, heavy rainfall At that time of the year, the average day light period caused desorption of these viruses to water flowing into was approximately 11 h. During the experimentation period, the skies were mainly partly cloudy. The aver- The present study investigated bacterial movement age minimum and maximum temperatures during the on sloping soil surfaces by the action of runoff. The same period were 7 and 14ЊC, respectively.
experiments were conducted in a field located about At each of the two sites, a 10 =10-m plot was 15 km east of Irbid in northern Jordan. The area is inoculated by spraying with the biotracer-containing considered a part of the semi-arid region with an cell suspension. A 500-mL volume of inoculum was average annual rainfall of 400 mm. However, the rain added to 19.5 L of distilled water in a 20-L carboy and events in arid and semi-arid regions are normally char- shaken vigorously. Prior to inoculation, samples were acterized by high intensity. Such rain events induce taken from the liquid cell suspension to determine the runoff, especially on slopes, which increases erosion biotracer concentration. These samples were serially and the transport of contaminants on the soil surface diluted, spread on tryptic soy agar plates supplemented bated at 44ЊC for 24 h before being enumerated for colony forming units CFU . A total of 2 =10 contained in two 20-L carboys Žfor a total of 40 L. were MATERIALS AND METHODS
used for each plot. The rest of the cell suspension wasthen poured into a plastic watering can, similar to Microorganism and Culture Conditions
those used to irrigate gardens, and distributed evenly The microorganism used as a biotracer in this study over the surface of test field plots.
was a nalidixic acid-resistant Escherichia coli strain Ž E.
Prior to inoculation, samples were also collected NAR , kindly provided by G. Palmateer Žformerly from the top 5 cm of soil at three different locations at at Ontario Ministry of Environment Southwestern Lab- each site to determine the background concentration of oratory, London, ON, Canada. Laboratory studies by indigenous nalidixic acid-resistant E. coli cells prior to 1992 confirmed the suitability of using this inoculation. From each of these samples, 10 g of soil tracer bacterium as an indicator of the soil transport were placed in a graduated cylinder and distilled water characteristics of naturally occurring bacteria under was poured into the cylinder until 100-mL volume was various testing conditions. This strain has also been reached. The graduated cylinders were shaken vigor- ously, then 1-mL samples were removed for enumera- studies ŽJoy et al., 1998; Palmateer et al., 1989; Shad- tion of nalidixic acid-resistant E. coli cells as described earlier. No background nalidixic acid-resistant E. coli E. coli NAR was grown by adding a loopful of cells cells were detected in any of these samples.
from a plate culture to a 125-mL Erlenmeyer flask For runoff sample collection, 1-L plastic bottles per- containing 25-mL tryptic soy broth ŽTSB, Difco Labo- forated at the top were inserted into the ground. Care was taken to keep the perforations at the soil surface 17᎐19 h with gyratory shaking at 200 rpm. The cells open to intercept runoff. The bottles were laid at 5-m were harvested by centrifugation at 5000 =g for 20 intervals along the center line of each site. Two ditches min. Cells were washed twice with 0.1 M phosphate along the down slope, each 1 m long and 3 cm deep, buffer, pH 7.5, and resuspended in a phosphate buffer.
were dug near each bottle in a V-shaped pattern as BACTERIA TRANSPORT IN RUNOFF
was recorded in 24 h. The second rain event occurred15 days after inoculation, and resulted in 21 mm ofprecipitation in 24 h. After each of the two rain events,samples from the soil surface Žtop 5 c .
each bottle were collected. Runoff samples were alsocollected from the water retained in the bottles. Allplastic bottles were replaced with new ones after eachrain storm. Soil and runoff samples were analyzed forbiotracer concentration as described earlier.
Another rain event that recorded less than 2 mm in 24 h occurred 10 days after inoculation of the plots.
This rain event did not cause runoff since no water wasfound in any of the plastic bottles laid along the slopesat both sites, and no samples were collected.
The objective of this study was to determine the effectof surface runoff on the movement of bacteria appliedon the soil surface. The experiments were conducted intwo sites having different slopes to investigate the Fig. 1. Experimental site layout.
effect of surface sloping on bacterial movement.
The biotracer was inoculated over the test plots as shown in Figure 1, to intercept runoff and direct it described in Materials and Methods. After both major toward the bottles. This arrangement minimized the rain events, all the bottles at both sites were found to chance of having some of the bottles remaining empty be filled right to the top with water as well as some soil due to the concentration of runoff in small channels particles. After the first heavy rain event, the biotracer which may have diverted the flow completely away was found in both soil and runoff samples taken down- from the bottles. Also, excess runoff would flow over stream of the plot at Site A, reaching a distance of the ditches and the bottles when they became full.
20 m from the center of the plot ŽTable I. Soil and Precipitation was measured using a rain gauge placed runoff samples taken further downstream did not con- at one site. During a period of 16 days after inoculating tain any biotracer cells. Similarly, at site B, after the the plots, two rain events that caused runoff were first heavy rain event, the biotracer was found in soil recorded. The first rain event occurred 2 days after and runoff samples as far as 35 and 30 m, respectively, inoculation of the test plots. A total rainfall of 26 mm downslope from the center of the plot ŽTable I. Sam- TABLE I. Biotracer concentrations in soil and runoff samples after first rain event
ples collected beyond these distances did not contain ment of biotracer cells was reversible and weak, then one would have expected to find higher biotracer con- The close correspondence in the extent of migration centrations in the soil and runoff samples with peak of the biotracer in the soil and in the bottles at both values further downslope of the plots. Such trend was sites show that the experimental setup was adequate not found in the experimental results.
for runoff collection by the bottles. The results confirm The second rain event which occurred 15 days after that runoff was the main medium that carried the inoculation of the test plots with the biotracer also led biotracer downstream at both sites. After spraying, the to runoff. The results summarized in Table II show that biotracer cells initially interacted with some of the soil small concentrations of biotracer cells were recovered particles via adsorption. Runoff likely caused detach- from soil samples taken from the center point of each ment of some of the adsorbed biotracer cells. The plot about 5 m downstream of that point. No biotracer detached cells were then carried downstream by the was found in other soil samples. All samples taken water flow where they were found in soil and runoff from the bottles at both sites were free of biotracer samples. The other likely mechanism for the transport of these cells is by advection in the sorbed phase.
Several reasons may be responsible for the large Runoff caused erosion of some soil particles to which reduction in biotracer concentration in the soil samples biotracer cells were attached. Some soil particles were taken after the second rain event. First, the biotracers observed in the sampling bottles which may indicate may have died off. As reviewed by Abu-Ashour et al.
the occurrence of erosion. The amount of soil found in 1994 , sun light may adversely affect bacterial survival the bottles was not measured, hence the relative contri- in soil. The biotracer cells on the soil surface were bution by erosion could not be adequately assessed. A exposed to direct sunlight during the experimentation combination of these two mechanisms may be responsi- period which may have affected their survival. Die-off ble for finding the biotracer cells downstream of the may also have occurred through competition with in- plots at both sites. The results also show that bacteria digenous microorganisms. Second, biotracer cells may migrated to a greater extent in the steeper site Žsite have moved vertically deeper into the soil, as infiltrat- ing rain water may have carried some of the cells to expected. Likely, the higher water velocity at the steeper layers below the 5-cm sampling depth.
slope resulted in higher shear force which caused more It was remarkable that some biotracer cells re- erosion of soil particles. It may also cause greater mained sufficiently viable for at least 16 days in a detachment of biotracer cells from the soil surface.
semi-arid environment to allow their transport and The presence of small numbers of biotracer cells enumeration. This capability to survive increases the downslope of the plots may indicate that adsorption of the cells onto soil was strong and far from being In conclusion, the present study shows that runoff is instantaneously reversible. This may also be the reason an important mechanism by which microorganisms ap- for finding the greatest number of cells in the plots plied at the soil surface may be transported down- where the biotracer was initially applied. If the attach- stream to areas where they may pose a serious threat TABLE II. Biotracer concentrations in soil and runoff samples after second rain event
to the quality of our water resources. Pathogenic bacte- in Soils With and Without Macropores; Proceedings of ria and viruses applied at the soil surface will cause the Technology Transfer Conference; Sponsored by the more danger if they have the ability to survive for long Ontario Ministry of Environment and Energy: Toronto, Joy, D. M.; Lee, H.; Reaume, C. M.; Whiteley, H. R.; Zelin, S. Can Agric Eng 1998, 40, 153᎐160.
Palmateer, G. A.; McLean, D. E.; Walsh, M. J.; Kutes, W. I.; Janzen, E. M.; Hocking, D. E. Toxicol Assess 1989, 4, Abu-Ashour, J.; Joy, D. M.; Lee, H.; Whiteley, H. R.; Zelin, S. Water Air Soil Pollut 1994, 75, 141᎐158.
Abu-Ashour, J.; Joy, D. M.; Whiteley, H. R.; Zelin, S. Trans Shadford, C. B.; Joy, D. M.; Lee, H.; Whiteley, H. R.; Zelin, Am Soc Agric Eng 1998, 41, 1043᎐1050.
S. J Contamin Hydrol 1997, 28, 227᎐246.
Culley, J. L. B.; Phillips, P. A. J Environ Qual 1982, 11, Stewart, L. W.; Reneau, R. B. J Environ Qual 1981, 10, Joy, D. M.; Abu-Ashour, J.; Botari, J. L.; Etches, C.; Lee, H.; Wellings, F. M.; Lewis, A. L.; Mountain, C. W.; Pierce, L. V.
Sopher, C.; Whiteley, H. R.; Zelin, S. Microbial Transport



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