TRANSPORT OF MICROORGANISMS THROUGH SOIL

JAMAL ABU-ASHOUR 1, DOUGLAS M. JOY 1 , HUNG LEE 2., HUGH R. WHITELEY 1 and SAMUEL ZELIN 1

I School of Engineering and 2 Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1

(Received April 21, 1992; accepted August 4, 1993)

Abstract. 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.

1. Introduction

Environmental and public health problems associated with the spreading of sewage on land have been observed since the dawn of the 20th century. Instances of land application of sewage are increasing because this disposal process removes some of the pollutants from the applied sewage, constitues a possible aquifer recharge source, and increases crop yields by supplying essential nutrients and by improving soil properties (Lance et al., 1982; Tim et al., 1988). However, disadvantages of land application may include degradation of quality of surface and groundwater through chemical and microbial contamination, and accumulation of heavy metals in soil.

Spreading agricultural wastes may constitute a source of pathogens to the groundwater, surface water and soil. The application of these wastes to agricultural lands can cause environmental problems even when the application procedures are within the current guidelines. Problems have been demonstrated in Ontario by Dean and Foran (1990a, b, 1991), Fleming etal . (1990) and Palmateer etal. (1989) where application of liquid manure to agricultural fields have resulted in rapid movement of a tracer bacterium, nalidixic acid-resistant Escherichia coli, through the soil and under drain systems leading to contamination of surface receiving waters. Microbial contamination of water and soil due to land application of liquid manure and other liquid wastes is difficult to treat, because once applied, manure

* Corresponding author. Telephone: (519) 824-4120, Exentension 3828. Fax: (519) 837-0442; Email: hlee@voguelph.ca

Water, Air and Soil Pollution 75: 141-158, 1994. (~) 1994 Kluwer Academic Publishers. Printed in the Netherlands.

142 JAMAL ABU-ASHOUR'~ET AL.

becomes a potential non-point source of pollution, less susceptible to correction than a point source (Crane et al., 1983; Khaleel et al., 1980).

Pathogenic bacteria and viruses known to cause disease have been detected in groundwater. Contaminated groundwater causes almost half of the outbreaks of water-borne diseases each year in the United States (Craun, i979, 1984). The most important pathogenic bacteria and viruses that might be transported to groundwater include Salmonella sp., Shigella sp., Escherichia coli and Vibrio sp., and hepatitis virus, Norwalk virus, echovirus, poliovirus and coxsackievirus (Corapcioglu and Haridas, 1984; Craun, 1984; Gerba and Keswick, 1981).

Land application systems are designed and installed with the assumption that the soil can act as a living filter with the potential for self purification through biological processes that reduce microbial concentrations (Tim et al., 1988). However, both field and laboratory observations have shown that microorganisms can migrate significant distances through soil in both vertical and horizontal directions (Chen, 1988; Keswick et al., 1982; Stewart and Reneau, 1981; Viraraghaven, 1978). Bacterial migration up to 830 m and viral migration up to 408 m have been reported (Gerba et al., 1975; Keswick and Gerba, 1980).

The ability of microorganisms to migrate through soil increases the probabil- ity of water contamination. The chance of contamination will increase further if microorganisms have the ability to survive for long periods of time. In labora- tory studies reported by Gerba et al. (1975), E. coli survived up to 4.5 months in groundwater maintained in darkness. Under the same conditions, Gerba and Keswick (1981) found that a pathogenic strain ofE. coli survived for 4 months and a saprophytic strain of E. coli survived 5.5 months. This survival occurred despite a reduction of 99.9998% of E. coli and 99.9995% of faecal streptococci in 20 days.

In another study, Chandler et al. (1981) assessed the persistence of indicator bacteria (faecal coliforrns and faecal streptococci) on land to which pig manure had been applied. They found the time required for a 90% reduction in number of indicator bacteria in the top 30 mm of soil ranged from 7 to 20 days. The die- off rate constant for E. coli and faecal streptococci in groundwater ranged from 0.16 to 0.36 day -1 and from 0.03 to 0.23 day -1, respectively (Gerba and Bitton, 1984). Survival of viruses was found to vary widely. The experimental data of Gerba and Keswick (1981) in the study above indicated a reduction of 99.9% in viral concentration in 20 days. The die-off rate constant ranged from 0.046 to 0.77 day -1 and from 0.39 to 1.42 day -1 for poliovirus and coliphage f2, respectively (Gerba and Bitton, 1984).

The potential problems associated with land application of sewage or liquid agricultural wastes are receiving increasing attention from the public and regula- tory agencies. Regulations, recommendations and disposal guidelines have been established to reduce the risks involved in such practices. Morrison and Martin (1977), as reported in Crane et al. (1983), have made recommendations to reduce health risks resulting from the application of manure or slurries to agricultural land. These authors recommended that no direct contact should occur between

TRANSPORT OF MICROORGANISMS THROUGH SOIL 143

the applied wastes and a crop. Further, subsequent application of liquid waste to the same land should be made only after an adequate period of time to maximize the die-off of pathogenic bacteria and to avoid any potential for buildup. They also recommended that populated areas not be irrigated with liquid wastes. Appli- cation of liquid agricultural wastes is not recommended on frozen ground. In a field study conducted in Ontario, Canada, Culley and Phillips (1982) found that manure applications in winter resulted in significantly higher faecal coliform and faecal streptococcus counts in the surface runoff, and faecal streptococcus counts in subsurface discharge when compared with applications during other seasons.

In this review we will focus on factors that affect microbial transport through soil. Knowledge of microbial transport mechanisms is needed to contribute to better land application practices that will minimize health and environmental problems.

2. Microbial Transport Through Soil

Bacteria and viruses have been shown to travel through porous media with the distance travelled being dependent on the type of porous medium. In studies by Gerba et al. (1975), coliforms travelled from 0.6 m in fine sandy loam to 830 m in sand-gravel; bacteriophage T4 travelled up to 1.6 km in a carbonate rock terrain area. Stewart and Reneau (198 l) detected migration of coliforms from septic tank drainfields in both vertical and horizontal directions to monitoring wells of 152- and 305-cm depth located within 30 m of the drainfields. The extent of migration in both directions varied depending on the position of the monitoring well relative to the drainfield. They attributed these differences to variations in water flow.

The movement of microorganisms through soil can be very fast. smith et al. (1985) compared the movement of a streptomycin-resistant E. coli K12 strain and C1- tracer through soils of different texture. With the Huntington silt soil contained in 0.28-m undisturbed columns, about 90% of the E. coli applied initially moved through the column in 17 min, while about 70% of the applied C1- moved through within the same time. The authors suggested that such rapid movement resulted from the presence of continuous macropores. In a study by McCoy and Hagedorn (1979), they found E. coli strains were transported in the subsurface a t an apparent maximum speed of 17 cm min -a. In another study in Humberside, U.K., bacteriophages were injected into an aquifer by boreholes at 366 and 122 m from a pumping well (Skilton and Wheeler, 1988). The results showed that bacteriophages moved rapidly, reaching a maximum speed of 2.8 cm s -1. Many studies of bacterial movement through soil have been conducted in the field and the results generally show a rapid movement and high concentration of bacteria reaching receiving waters. The explanation normally provided is that the observed phenomena are due to preferential flow of microorganisms through macropores, cracks, fractures, worm holes and channels formed by plant roots, or animals in the soil. Preferential flow through macropores has been observed in both laboratory and field studies (Chandler et al., 1981; Thomas and Phillips, 1979; van Elsas et

144 JAMAL ABU-ASHOUR ET AL.

al., 1991). The influence of macropores on transport of dissolved and suspended matter through soft was reviewed by White (1985). He suggested the flow along preferential paths was the cause of rapid movement of dissolved and suspended matter through soil.

Studies of chemical transport in preferential towpaths may give guidance on transport of microorganisms. Rice et al. (1988) studied movement of solutes and herbicides under irrigated fields. They found that preferential flow resulted in solute and herbicide movement velocities of 1.5 to 2.5 times greater than those expected based on water balance considerations. The results from other studies showed also the flow of water and chemicals through macropores is more rapid than that through a soil matrix (Beven and Germann, 1982). Everts and Kanwar (1988) used a hydrograph separation technique to quantify the preferential and matrix flow components to a tile line. They found that preferential flow contributed less than 2% of the total water flow. However, flow of bromide and nitrate account for up to 25% of these tracer chemicals found in the tile line. This large contribution occurred because these chemicals moved through preferential paths at the applied concentrations.

Many factors have been observed to affect the survival and movement of microorganisms in soil. These are mainly related to interactions between soil, water, microorganisms and the surrounding environment (Crane et al., 1983; Ger- ba and Bitton, 1984; Tim et al., 1988). These factors are summarized in Tables I and II. The effect of these factors on movement of microorganisms in soil have been a subject of study by many researchers, van Elsas et al. (1991) studied the influence of soil properties on the vertical movement of a genetically-marked Pseudomonas f luorescens bacterial strain through 50-cm long soil columns of loamy sand. They added bacterial cells at the top of soil columns of 5.3 and 13% moisture content, and measured the concentration of cells translocated to various depths. They found the ratio of cell concentration in the dryer soil to that in the wetter one ranged from 68% at 20 cm depth to 98% at the soil surface. These researchers also studied the effect of soil bulk density on bacterial transport. They observed a trend towards a higher degree of transport to lower soil layers at the lowest bulk density (1 g cm -3) compared to higher bulk densities (1.15 and 1.3 g cm-3). Studies by Huysman and Verstraete (1993a) also demonstrated the strong influence of soil bulk density on bacterial transport. In their work an increase in the bulk density from 1.27 to 1.37 g cm -3 resulted in up to a 60% decrease in the migration of bacteria in laboratory columns.

�9 Soil texture can affect bacterial movement through soils. Smith et al. (1985) compared the movement of streptomycin-resistant E. coli through both undisturbed and repacked soils of different texture. In undisturbed soils, 22% of the applied E. coli passed through a 0.28- m column of Maury silt soil while 44 and 79% of the microorganisms passed through similar columns containing the Crider silt and Bruno silt loam soils, respectively. When columns were repacked with the same soils, at least 93 % of the applied E. coli were retained in the soil core over the same

TRANSPORT OF MICROORGANISMS THROUGH SOIL 145

TABLE I

Factors affecting the survival of enteric bacteria and viruses in soil

Factor Comments

1. Microorganisms and their

physiological state.

2. Physical and chemical nature of

receiving water.

- pH

Soil water content

Organic matter content

Texture and particle size

distribution

- Temperature

- Availability of nutrients

- Adsorption properties

3. Atmospheric conditions

- Sunlight

- Water (vapor and precipitation)

- Temperature

4. Biological interactions

- Competition from indigenous

microflora

- Antibiotics

Toxic substances

5. Application method

- Technique

- Frequency of application

- Organism density in waste material

- Shorter survival time in acidic soils (pH 3-5) than

in alkaline soils.

- longer survival time in wet soils and during

times of high rainfall.

- Increased survival and possible growth when

sufficient amount of organic matter is present.

- Finer soils especially clay minerals and humic

substances increase water retention by soil

which increases survival time.

- Longer survival at lower temperature.

- Increases survival times.

- Microorganisms appear to survive better in

sorbed state.

- Shorter survival time at the soil surface.

- Same as in (2) above.

- Same as in (2) above.

- In sterile soil, survival is increased.

- Many microorganisms cannot survive in the

presence of antibiotics

- Same as antibiotics

Sources: Crane et al. (1983); Gerba and Bitton (1984).

146 JAMAL ABU-ASHOUR ET AL.

TABLE II Factors affecting movement of microorganisms in soil

2.

3.

4.

Soil physical characteristics - Texture - Particle size distribution - Clay type and content - Organic matter type and content - pH - Pore size distribution - Bulk density

Soil environment and chemical factors - Temperature - Soil water content - Soil water flux

Chemical and Microbial factors - Ionic strength of soil solution - pH of infiltrating water - Nature of organic matter in waste effluent solution (concentration and size) - Type of microorganism - Density and dimensions of the microorganism - Presence of larger organisms

Application method - Soil drying between applications - Time of application (winter, spring)

Bitton et al. (1979); Culley and Phillips (1982); Crane et al. (1983); Gerba and Bitton (1984); Opperman et al. (1987); Peterson and Ward (1989); van Elsas et al. (1991).

elution volume. The Size and morphology of microorganisms may affect their transport through

soil. Gerba and Bitton (1984) reported in a study that when E. coli and coliphage f2 were injected together into an aquifer, the larger E. coli were detected 150 m down gradient in an observation well ahead of the smaller coliphage. The reason for this rapid movement of E. coli is not known. On the other hand, Kott (1988) who studied the movement of different types of bacteria in sand columns, showed that bacterial size and morphology did not affect the filtration efficiency of the sand. Fontes et al. (1991) studied the effects of ionic strength of artificial groundwater, cell size, mineral grain size and the presence of heterogeneities within the porous media on bacterial transport. They found the grain size was the most important factor while cell size and ionic strength were about equally important, but of lesser

TRANSPORT OF MICROORGANISMS THROUGH SOIL 147

importance than the grain size in controlling bacterial transport. Gannon et al.

(1991a) studied the relationship between cell size, cell surface hydrophobicities and surface charges of 19 bacterial strains and their transport through Kendaia loam soil. They found transport was related to cell size, with bacteria shorter than 1 #m being transported in higher percentages through soil than longer ones. There was no correlation between the transport or retention of these strains by soil and their hydrophobicities or net surface charges. They also found no relation between the presence of flagella and the extent of bacterial transport through soil.

Scholl et al. (1990) studied the influence of soil mineral composition, ionic strength and pH on bacterial attachment to aquifer materials. Their results indi- cated that interactions between mineral grains in the aquifer and bacterial cells influenced adhesion of cells to the mineral grains, and hence play an important role in determining the movement of bacteria through saturated porous media. Gannon et al. (199 lc) studied the influence of NaC1 in the carrying solution, cell density and flow velocity on transport of Pseudomonas sp. strain KL2 through 0.3-m columns of aquifer sand under saturated conditions. When 108 bacterial cells in 0.01 M NaC1 were applied to the column at a flux of 10-4m s -1, only 1.5% of the applied bacteria passed through the column within 2 h of application. However, when dis- tilled water was used as the carrying solution, 60% of the applied bacteria passed through a similar column under the same flow conditions. Their results clearly indicated that movement of bacteria added to sandy aquifers may be enhanced or reduced by modifying the chemical composition of the carrying solution.

The presence of plants and large living organisms may affect the persistence and movement of microorganisms in soils. For example, bacteria in soil are subject to competition and predation from other bacteria such as streptomycetes, myxobacter and Bdellovibrio, and larger soil organisms such as protozoa and nematodes (Peter- son and Ward, 1989; Ramadan et al., 1990; Tim et al., 1988). In some instances, the presence of such organisms may enhance mixing of microorganisms within soil. Opperman et al. (1987) studied the effect of the earth worm Eisenia Foetida (Savigny) on movement of cattle slurry through soil. The slurry was obtained from drainage ditches beneath the cattle shed floor. Ten worms were introduced to 17.5- cm sand columns. Their activity was found to mix the slurry with the sandy soil to a depth of 17.5 cm as indicated by the movement of coliform bacteria through the soil.

3. Transport Mechanisms

Several laboratory and field studies showed that average velocity of microorgan- isms moving through soil was greater than that of a chemical tracer such as chloride or bromide (Harvey et al., 1989) or the flow of ambient groundwater without any tracer (Wood and Ehrlich, 1978). In a field study, Harvey et al. (1989) injected a mixed bacterial population, collected from ground water and stained with a flu- orochrome dye DAPI, together with bromide into a sandy aquifer and followed

148 JAMAL ABU-ASHOUR ET AL.

their movement through the aquifer sediment. They discovered that the stained bacteria travelled faster than the bromide tracer, the peak bacterial concentration was reached 1 to 2 h before that of bromide. This suggests the existence of one or more mechanisms that accelerated microbial movement, and these mechanisms may differ from those involved in transport of the chemical tracer.

Several studies investigated mechanisms of microbial transport and attempts have been made to quantify the contributions of each mechanism. Microbial trans- port through soil depends on a complex set of physical and chemical conditions (Harvey, 1989) which are presently not well understood. There are disagreements in the literature as to the relative importance of various mechanisms to microbial transport through soil.

Gray and Williams (1971), as reported in Wollum and Cassel (1978), summa- rized four modes of microbial transport through soil. They are: (1) movement in water films due to motility of the microorganisms; (2) hyphae elongation, a mode whereby microorganisms move from one water film to another; (3) microbial growth which may contribute to microbial transport; and (4) microbial dispersion through soil by water movement. The latter mode is independent of microbial motility or growth. In addition, microorganisms can be carded by the water flow.

Tim et al. (1988) suggested that transport and attenuation mechanisms which affect microbial movement through soil can be divided into physical, geochem- ical and biological processes. We have adopted these sectional headings in the discussion that follows.

3.1. PHYSICAL PROCESSES

The principle physical pro~esses for microbial movement through porous media are convection or advection and hydrodynamic dispersion. In advection, microorgan- isms are carried with bulk water flow (Yates and Yates, 1990) and their movement is governed by the velocity of water (McCoy and Hagedorn, 1979). In a simple model, advection is equal to the average velocity of groundwater as determined from the product of hydraulic conductivity and hydraulic gradient all divided by porosity (Corapcioglu and Haridas, 1984).

Hydrodynamic dispersion is the spreading of microorganisms as they move along the water path as a result of both microscopic and macroscopic effects (Tim et al., 1988). It is measured by determining the concentration vs. time dependence of a tracer with respect to a sampling point in the flow path. It may be described by the general transport equation (Equation (1)) in vectorial form (Matthess et aL, 1988; Pekdeger and Matthess, 1983):

OC ( D ) Vw Ot = d i v RddgradC --Rdd g radC-AC (1)

where

TRANSPORTOF MICROORGANISMSTHROUGHSO~ 149

D = coefficient of hydrodynamic dispersion gradC = concentration gradient v~o = average groundwater velocity Rd = retardation factor A = elimination constant.

Two distinct processes are operative in dispersion: molecular diffusion and mechanical mixing (Yates and Yates, 1990). In addition, the mobility of microor- ganisms may cause some dispersion (Tim et al. 1988). Diffusion is defined as the spreading of microorganisms due to a concentration gradient. It is considered to be of negligible importance in bacterial transport compared to mechanical mixing (Yates and Yates, 1990). However, diffusion is an important transport mechanism when small particles (<1 #m) such as viruses are involved.

Three processes are involved in mechanical mixing (Cunningham et al., 1988; Yates and Yates, 1990). First, mixing may occur due to the fluid velocity distribution within individual pore spaces. Second, mixing may result from variations in true pore velocities among pore channels of different size and surface roughness. Third, mixing may be caused by convergence and divergence of individual pore channels. The existence of fractures, macropores, cracks and worm holes in soil enhances mixing and results in more effective dispersion. Dispersion results in dilution of contaminant pulses, attenuation of concentration peaks and arrival of contaminants well ahead of the time expected on the basis of average velocity of water flow (Mackay et al., 1985).

3.2. GEOCHEMICAL PROCESSES

Geochemical processes are mainly attenuative in nature. They hinder and/or delay microbial transport through soil. They include filtration, adsorption/desorption and sedimentation. Filtration occurs when a microorganism is prevented from flowing through a pore. There are three filtration mechanisms (McDowell-Boyer et al., 1986).

(1) Cake, surface or vacuum filtration. This occurs when microorganisms are too large to penetrate into soil. Microorganisms accumulate on the soil surface and form a mat which affects soil permeability. A cake or a mat is expected to form on a soil surface if the ratio of soil grain diameter to that of microorganism is less than 10.

(2) Straining. This occurs when the ratio of soil grain diameter to that of microorganism is between 10 and 20. Kovenya et al. (1972), as described in McDowell-Boyer et al. (1986), studied the importance of particle size relative to soil grain size in controlling particle migration under saturated and unsaturated flow conditions. They found that particles whose sizes were 1 and 10 #m were removed by both cake and straining mechanisms at the surface.

(3) Physical chemicalfiltration. This occurs when the ratio of soil grain diameter to that of microorganism is greater than 20. In a sand or gravel soil this ratio exceeds 1000 (McDowell-Boyer-et al., 1986). The number of microorganisms retained by

1 5 0 JAMAL ABU-ASHOUR ET AL.

this mechanism depends mainly on the extent of microbe-media collision and attachment mechanisms.

Corapcioglu and Haridas (1984) used an expression given by Herzig el al. (1970) to calculate the percentage of bacteria retained by straining. The expression is given by Equation(2) below. They concluded straining affects bacterial movement and should be included in transport equation formulation.

1

1 r r = l ( 1 - n o ) T r Z -~a 1 + ~ - 1

where O- =

n o

d , d 9 = Z =

(2)

the volumetric percentage of bacteria retained on the medium surface the initial porosity suspended particle and grain mean diameters, respectively the coordination number which indicates the interconnectedness in the network of a porous medium.

Most studies agree that the effect of filtration on viral movement is negligible. However, there is no clear understanding of the effect of filtration on bacterial movement. Several researchers suggested that filtration of bacteria by soil particles was the main limitation to their transport and hence the major mechanisms for their removal (Grunnet and Olesen, 1979; Hagedorrl et al., 1981; Matthes and Pekdeger, 1981). On the other hand, Tim et al. (1988) neglected the effect of filtration on microbial transport in the formulation of their model. Huysman and Verstraete (1993b) have also shown that the degree of filtration is dependent not only on the volume of water applied to laboratory columns but also on the time delay between inoculation and irrigation as well as rate of irrigation.

Another attentuation or retardation mechanism in this category is adsorption: It is defined as the process of collecting substances that are in aqueous suspension or solution on a suitable interface (Weber, 1972). Gerba et al. (1975) noted that adsorption plays an important role in removal of bacteria by soils that contain clay. Harvey and Garabedian (1991) reported that bacterial transport is influenced to a great extent by adsorption and in part by filtration. On the other hand, adsorption has been suggested as the primary removal mechanism for viruses due to their small size (Corapcioglu and Haridas, 1984; Gerba and Keswick, 1981; Grunnet and Olesen, 1979; Lance and Gerba, 1984). The normally negatively charged bacterial and viral surfaces are strongly adsorbed by anionic adsorbents (Matthess and Pekdeger, 1981).

Adsorption of microorganisms to soil surfaces is reversible (Pekdeger and Matthess, 1983; Yates et al., 1987). It may be influenced by changes in tem- perature, pH and the presence of cations and/or anions in the surrounding medium. It also changes with varying type and texture of the soil, and depends on the type of bacteria or viruses involved (Matthess et al., 1988). The reversible process of microbial adsorption is best described by the Freundlich isotherm (Burge and

TRANSPORT OF MICROORGANISMS THROUGH SOIL 151

Enkiri, 1978; Gerba and Lance, 1978; Matthess and Pekdeger, 1981; Tim et al., 1988). The general form of the Freundlich isotherm is shown in Equation (3) below (Weber, 1972):

q = k d C 1In (3)

where q = amount (microorganisms) adsorbed per unit mass of adsorbent (soil) C = equilibrium concentration of adsorbate (microorganisms) in solution

after adsorption kd, n = empir ical constants.

The term 'reversible' implies that adsorbed microorganisms may detach from surfaces of soil particles and desorb in water. They may subsequently be readsorbed. The phenomenon of desorption was suggested by Wellings et al. (1975) who observed that previously virus-free wells, near a land application site in Florida, contained viruses after a period of heavy rainfall. They suggested that viruses were initially adsorbed onto the soil particles and hence could not be detected in wells. However, heavy rainfall caused desorption of these viruses to water flowing into the wells where they were detected.

Another mechanism that may facilitate or restrict microbial transport through soil is sedimentation. Sedimentation is the gravitational deposition of particles on soil grain surfaces. It occurs when the density of particles is higher than that of liquid and when the flow properties are such that the tendency for gravitational settling is greater than that for movement with liquid (Matthess and Pekdeger, 1981).

Some researchers suggested the importance of sedimentation of bacteria and viruses has been overestimated (Matthess and Pekdeger, 1981; Pekdeger and Matthess, 1983). ~ Microorganisms are very small (<5#m) and are neutrally buoyant with a density of about 1 g cm -3 (Pekdeger and Matthess, 1983). Therefore, they tend not to settle (Corapcioglu and Haridas, 1984). However, Gerba et al. (1975) suggested that sedimentation may be a mechanism of removal of some bacteria. Yao et al. (1971) noted that gravitational settling plays a significant part only in the removal of relatively large particles (>5#m) and the removal efficiency of such particles is proportional to the square of the particle diameter.

3.3. BIOLOGICAL PROCESSES

Some microbial processes such as growth or death may affect the concentration of microorganisms in soil, thereby affecting microbial transport measurements. Some factors which affect growth and survival of microbial cells are shown in Table I. Growth and survival depend on a number of interrelated factors such as the availability of nutrients, prevailing environmental conditions, and competition with indigenous or other organisms (McFeters and Singh, 1991; Roszak and Colwell, 1987). Changes in these factors may affect the fate of microorganisms (Jackman et al., 1992). The dynamics of microbial populations is complex and difficult

152 JAMAL ABU-ASHOUR ET AL.

to quantify. Tim et al. (1988) used the following first order irreversible reaction equation to describe it.

dC -- k(OC + pq) (4)

dt

where C = t

k 0

P q

concentration of microorganisms in the aqueous phase. = time = specific decay rate constant = volumetric water content = soil bulk density = mass of microorganisms adsorbed per unit mass of soil.

Transport of microorganisms may also depend on the intrinsic characteristics of microorganisms. In one laboratory study using 5-cm long columns containing Kendaia loam soil from Ithaca, New York, 19 different bacterial strains belonging to Pseudomonas, Achromobacter, Bacillus, Flavobacterium and Enterobacter spp. were found to exhibit vastly different degrees of transport (Gannon et al., 1991b). After 4 pore volumes, passage of the applied strains ranged from 0.01 to 15%. Such large differences may have been related in part to the size of microorganisms. These researchers also tried to recover microorganisms from both soil and effluent at the end of the experiments. The number of applied cells recoverable ranged from 4.3 to 107%. They suggested that low recovery of some cells was due to strong adsorption of the cells to soil particles. They noted that Flavobacterium DF3 and Bacillus CB3 which gave the lowest recoveries also exhibited the least transport.

In some studies, motility of bacteria was found to contribute to transport veloc- ities under static conditions in nutrient-saturated sand and sandstone. Velocities of up to 0.4 cm/h have been observed (Jenneman et al., 1985; Reynolds et al., 1989). Motile microorganisms can migrate in response to chemical gradients. Such movement, known as chemotactic migration, is a directional motion towards or away from a higher chemical concentration (Berg, 1975; Corapcioglu and Haridas, 1984; Harvey, 1989; Peterson and Ward, 1989; Tim et al., 1988).

Some microorganisms move towards attractive substances such as nutrients using flagellar motion to propel themselves through the medium. In this motion, they normally alternate between smooth swimming and tumbling. Tumbling is characterized by uncoordinated, chaotic motion which randomly reorients the bac- terium for the next swim (Corapcioglu and Haridas, 1984). Berg (1975) reported that most of the movement by rotating flagella is random in occurrence and direc- tion, roughly analogous to Brownian motion. However, Harvey (1989) reported that the speed of bacteria travelling through porous media due to chemotaxis was faster than would be expected from random Brownian motion. For example, Jenne- man et al. (1985) found the rate of penetration of the motile bacterium Enterobacte aerogenes through Berea sandstone cores was 3 to 8 times faster than that of the non-motile bacterium Klebsiella pneumoniae.

TRANSPORT OF M~ROORGANISMSTHROUGHSO~ 153

Chemotactic migration may be an important transport mechanism for some bacteria (Harvey, 1989). However, most studies involving chemotaxis have been conducted in the laboratory. It is not known how motile bacteria can be in a natural soil environment, or if such motility offers a competitive advantage (Peterson and Ward, 1989). Chemotactic migration is not considered a part of viral transport, since viruses do not exhibit chemotaxis (Corapcioglu and Haridas, 1984).

4. Approaches to Microbial Transport Studies

4.1. LABORATORY STUDIES

Laboratory studies have increased our knowledge of microbial transport through soils. Laboratory studies may include the use of both soil containers (batch) or columns. Batch experiments can examine adsorption phenomena such as the adsorption potential of a certain compound without interference from other vari- ables such as filtration (McDowell-Boyer eta! . , 1986; Zsolnay, 1991). However, these experiments may give misleading results, especially in the presence of non- settling particles. In addition, such experiments do not take into consideration the structure of soil (Zsolnay, 1991).

Most studies on microbial transport through soil in the laboratory employ soil columns. Bitton et al. (1979) suggested that laboratory experiments using soil columns may be confusing and results misleading. Numerous variations in exper- imental conditions make it difficult to separate variables that influence microbial transport through soil. Nevertheless, despite the lack of standardization in experi- mental conditions, soil column studies have been instrumental in providing valu- able information on transport characteristics of microorganisms through soils. For example, van Elsas et al. (1991) found the use of relatively homogeneous laborato- ry soil columns permits a more precise study of the effect of individual soil factors on bacterial movement through soil.

Two types of soil columns, undisturbed or repacked, have been used to simulate field situations. Trevors et al. (1990) argued that repacked columns provide repro- ducibility and homogeneity, making it possible to study different factors affecting bacterial movement in soils. At the same time, they acknowledged that undisturbed soil cores have better predictive value for field situations than repacked ones. In contrast, Brown et al. (1979) claimed that studies conducted in repacked columns may not provide information that is applicable to field situations. Despite the con- troversy in the literature regarding the applicability of results from laboratory soil column experiments to field situations, they do provide valuable information on movement of microorganisms through soil.

4.2. FIELD STUDIES

Field studies have been conducted both to explain general observations and to find solutions to specific problems, such as the contamination of a well due to seepage from a nearby septic tank, or groundwater pollution because of land application of

154 JAMAL ABU-ASHOUR ET AL.

sewage. Although field experiments are difficult to carry out, they are necessary to verify laboratory studies and to test conditions that cannot be simulated easily in the laboratory.

An important issue is the compatibility of field results with those obtained from the laboratory. Kuhn et al. (1985) studied microbial transformations of dimethyl- and dichlorobenzenes during infiltration of fiver water to groundwater using soil columns. They found all test compounds showed the same qualitative behaviour in the column as was observed at a field site. A quantitative comparison was difficult because microbial activity in the column was different from that at the field site. McDowell-Boyer et al. (1986) recommended the use of field experiments instead of laboratory soil columns to study microbial transport; they claimed that transport data determined from soil column experiments do not duplicate actual soil structure in the field.

Bitton et al. (1979) compared the findings from 4 field studies that monitored viruses and their removal by soil with results of laboratory soil column experiments designed to simulate the same conditions. In each instance, there was a close correspondence between field and laboratory results. This suggests that carefully controlled laboratory studies can be used to provide information applicable to microbial transport in field situations.

4.3. MODEL STUDIES

Mathematical modelling of microbial transport through soils could be an important tool in evaluation of the long term risk of pathogens entering soil and groundwater and in identifying optimal sites for wastewater disposal (MacKay et al., 1985; Tim et al., 1988; Yates and Yates, 1990). A full summary of modelling techniques is given in Hurst (1991). Many mathematical models of microbial transport through soils have been presented on the basis of the convection-dispersion equation (Cun- ningham et al., 1988; Khan and Jury, 1990; Matthess et al., 1988; Peterson and Ward, 1989; Tim and Mostaghimi, 1991). These models and others such as colloid filtration models, are too simple to describe the complex nature of field sites (Yates and Yates, 1990). They do not adequately describe transport through structured soil columns or macropores because the model parameters are not constant as a function of distance (Harvey, 1989; Khan and Jury, 1990). Many assumptions are usually made to simplify the formulation and solution of the model. The following are some of the assumptions typically found in these models (Tim et al., 1988):

(1) the soil is homogeneous and isotropic,

(2) the flow is steady,

(3) Darcy's law is valid,

(4) the flow is in one dimension,

(5) one or more of the following mechanisms may be considered negligible: microbial growth, predation, microbial diffusion, filtration, sedimentation and chemotaxis, and,

TRANSPORT OF MICROORGANISMS THROUGH SOIL 155

(6) the microorganisms sorbed to soil and those remaining in solution are in equilibrium and their relative amount remains unchanged temporarily.

Models which incorporate most of the assumptions listed above are unrepresen- tative of actual conditions and may produce misleading results. Consideration of all parameters involved in microbial transport would require a more comprehensive numerical model than has been attempted to date.

5. Summary

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 esti- mated 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 were found to travel in groundwater faster than a chemical trac- er under the same flow conditions. This demonstrates that some mechanism(s) exist to allow preferential movement of microorganisms in addition to those that trans- port chemical tracers. Various transport and attenuation processes may modulate and control microbial transport through soil. These include advection, dispersion, filtration, adsorption/desorption, sedimentation, growth, death, and chemotaxis.

Advection when present is always important. Similarly the significance of dis- persion is well established. The process of filtration, adsorption or desorption are reasonably well understood but there is disagrgement on the extent to which they influence bacterial movement. The populatio~a d~namics of growth and death are important but have not been well quantified due to the complex nature of the pro- cesses involved. Sedimentation and chemotaxis have not been demonstrated to be important factors in field investigations.

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. Laboratory column studies do not fully represent conditions found in nature.

Available mathematical modelling of microbial transport is limited in practical application because of the simplifying assumptions used in their development. Most of these models consider only simple particle transport and removal. Results from such models should be used with caution. There is considerable scope for the development of more comprehensive models; special attention should be paid to mechanisms of preferential flow.

156 JAMAL ABU-ASHOUR ET AL.

Acknowledgements

This research was supported by a grant from Ontario Ministry of the Environment. The views and ideas expressed in this paper are those of the authors and do not necessarily reflect the views and policies of the Ministry of the Environment, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. J. Abu-Ashour was supported by a scholarship from the Jordan University of Science and Technology

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