Ground Water Issue


United States 

Environmental Protection 

Agency 

Movement and Longevity of


Viruses in the Subsurface 

Ann Azadpour-Keeley1, Barton R. Faulkner1, and Jin-Song Chen2 

Background 

In order to assist federal and state decision makers, the Applied 

Research and Technical Support Branch (ARTSB) of the Ground 

Water and Ecosystems Restoration Division (GWERD) has, since 

1989, developed a series of over 30 Ground Water Issue Papers 

intended to be brief, state-of-science documents focused on a 

technical issue of expressed interest and prepared in a concise 

and readable format. The purpose of this Issue Paper is to discuss 

some of the conditions under which viral contaminants may 

survive and be transported in the subsurface, identify sources as 

well as indicators of viral contamination, outline the effects of 

hydrogeologic settings on viral movement, and introduce the 

reader to the current state of virus transport modeling along with 

an example of modeling applications. 

The 1986 Safe Drinking Water Act (SDWA) amendments directed 

EPA to develop national requirements for drinking water 

disinfection. The legislation required every public water supply 

system to disinfect unless it fulfills criteria assuring equivalent 

protection (Macler, 1996). To provide direction for the regulations 

associated with “acceptable” health risks to the public (Macler, 

1996), EPA established maximum contaminant level goals 

(MCLGs) for pathogenic microorganisms in drinking water, setting 

a level of zero for viruses (U.S. EPA, 1989a,b). Due to the various 

technical and economic considerations involved in monitoring 

water for these MCLs, a “treatment technique” was proposed to 

reduce or eliminate viruses (Yates et al., 1990). On June 29, 1989, 

a Surface Water Treatment Rule (SWTR) was published addressing 

microbial contamination of drinking water from surface sources, or 

from ground-water sources directly influenced by surface water, 

with strict provisions for filtration and disinfection (U.S. EPA, 

1989a). On January 14, 2002, a SWTR was promulgated with 

special emphasis on the protozoan Cryptosporidium (U.S. EPA, 

2002). 

The development of a corresponding rule for ground water, the 

Ground Water Disinfection Rule (GWDR, later designated as the 

Ground Water Rule), to meet SDWA requirements began in 1987 

and led to a published discussion piece (draft GWDR, U.S. EPA, 

1992). The deadline for the GWDR proposal was dependent upon 

completion of studies of the status of public health with respect to 

the microbial contamination of ground water, based on studies 

(Abbaszadegan et al., 1999a,b; Lieberman et al., 1994, 1999) to 

1 National Risk Management Research Laboratory, ORD, 

U.S. EPA, Ada, Oklahoma. 

2

 Dynamac Corporation, Ada, Oklahoma. 

generate a more careful nationwide picture of the problem. As 

there are significant differences between ground water and surface 

water in terms of the type and degree of treatment, the GWDR 

regulatory workgroup realized that the assessment of vulnerability 

as a function of site specific conditions (i.e., hydrogeology, land 

use pattern) was a key element to be addressed (Macler, 1996). 

Subsequently on May 10, 2000, U.S. EPA proposed “...to require 

a targeted risk-based regulatory strategy for all ground-water 

systems addressing risks through a multiple barrier approach that 

relies on five major components: periodic sanitary surveys of 

ground water systems requiring the evaluation of eight elements 

and the identification of significant deficiencies; hydrogeological 

assessments to identify wells sensitive to fecal contamination; 

source water monitoring for systems drawing from sensitive wells 

without treatment or with other indications of risk; a requirement 

for correction of significant deficiencies and fecal contamination 

(by eliminating the source of contamination, correcting the 

significant deficiency, providing an alternative source water, or 

providing a treatment which achieves at least 99.99 percent 

(4-log) inactivation or removal of viruses), and compliance 

monitoring to insure disinfection treatment is reliably operated 

where it is used.” (U.S. EPA 2000) 

It should be emphasized that this document is not intended for use 

in establishing the finalized GWDR or for the interpretation of the 

results of those investigations. To that end, the reader is referred 

to the Federal Register (U.S. EPA, 2000). 

For further information contact Dr. Ann Azadpour-Keeley (580-

436-8890) at the Ground Water and Ecosystems Restoration 

Division of the National Risk Management Research Laboratory, 

Ada, Oklahoma. 

Introduction 

Over 97 percent of all fresh water on earth is ground water and for 

over 100 million Americans who rely on ground water as their 

principal source of potable water (Bitton and Gerba, 1984), over 

88 million are served by community water systems and 20 million 

by non-community water systems (U.S. EPA, 2000). Historically, 

ground water has been considered a safe source of drinking water 

which required no treatment. It has long been believed that this 

valuable resource was protected from surface contamination 

because the upper soil mantle removed pollutants during 

percolation (Amundson et al., 1988). It was also believed that, 

even if contaminated, ground water would be purified through 

adsorption processes and metabolism of indigenous aquifer 

microflora (Dizer et al., 1984). 

As water demands increase, the possibility of artificially recharging 

ground water with wastewater or surface water will also increase, 
particularly in states like California, where ground water supplies 

half of the state’s fresh water, and Arizona, where ground water 

supplies all of the fresh water demand. These activities may result 

in increasing the concern for waterborne diseases; a concern not 

unwarranted in lieu of the recent worldwide rise in waterborne 

diseases and a report by the American Academy of Microbiology 

(Colwell, 1996) indicating that drinking water is not safe 

microbiologically. 

In the United States alone, the annual number of reported illnesses 

resulting from contact with waterborne pathogens was estimated 

to be as low as one million and as high as seven million; and 

between 1971 and 1982, 51 percent of all waterborne disease 

outbreaks were due to the consumption of contaminated ground 

water (Craun, 1985). Macler (1995) estimated that approximately 

20-25 percent of the United States’ ground-water sources are 

contaminated with microbial pathogens, including more than 100 

types of viruses. A literature review by Craun (1989) indicated that 

approximately one-half of the surface water and ground-water 

sources tested contained enteric viruses. Even nine percent of 

the conventionally treated drinking water (coagulation, 

sedimentation, filtration, post-filtration disinfection using 

chlorine/ozone) tested positive for enteric viruses (Gerba and 

Rose, 1993). 

Ground water serves as a water source for 93 percent of the 

communities in Minnesota, with the most extensive use being from 

karst topography in the southern half of the state. In this type of 

geology, cracks, sinkholes, and macropores allow rapid percolation 

of surface water into ground-water reservoirs. The biological 

contamination of 18 private rural wells during 16 months of 

sampling showed that 17 out of 18 wells contained detectable 

levels of indicator bacteria and coliphages (Amundson et al., 

1988). 

Although water-transmitted human pathogens include various 

bacteria, protozoa, helminths, and viruses (Bull et al., 1990), 

agents of major threat to human health are pathogenic protozoa 

(Cryptosporidium and Giardia) and enteroviruses (Schijven and 

Hassanizadeh, 2000). Despite ample information regarding the 

fate of viruses in the subsurface, research on the persistency of 

pathogenic protozoa through passage in soil and ground water is 

just now emerging (Brush et al., 1999; Harter et al., 2000). In the 

past it was generally believed that the presence of pathogenic 

protozoa was confined to surface water. Contrary to that 

expectation, recent monitoring results from 463 ground-water 

samples collected at 199 sites in 23 of the 48 contiguous states 

suggested that up to 50 percent of the ground-water sites were 

positive for Cryptosporidium, Giardia, or both, depending on the 

parasite and the type of ground-water source (vertical wells, 

springs, infiltration galleries, and horizontal wells) (Hancock et al., 

1998). 

Viruses are small, obligate, intracellular parasites that infect and 

sometimes cause a variety of diseases in animals, plants, bacteria, 

fungi, and algae. Viruses are colloidal particles, negatively charged 

at high pH (pH 7), ranging in size from 20 nm to 350 nm. The 

smallest unit of a mature virus is composed of a core of nucleic 

acid (RNA or DNA) surrounded by a protein coat. With this unique 

feature of viral structure and colloidal physicochemical properties, 

the transport of viruses in soil and ground water can act with a 

combination of characteristics ranging from solutes, colloids, and 

microorganisms. 

Enteroviruses (see Table 1) are a particularly endemic class of 

waterborne microorganisms which cause a number of ubiquitous 

illnesses including diarrhea, gastroenteritis, and meningitis, only 

to name a few. Included in this group are poliovirus, hepatitis type 

A (HAV), coxsackievirus A and B, and rotavirus. Although 

gastroenteritis is the most common disease resulting from these 

microorganisms (Lukasik et al., 1996), other associated illnesses 

include hepatitis, typhoid fever, mycobacteriosis, pneumonia, and 

dermatitis (Bull et al., 1990; Sherris et al., 1990; Levine et al., 1991; 

Payment et al., 1991). Therefore, in addition to the protection of 

ground-water resources by adequate set-back distances between 

the sources of contamination and wells for drinking water, the 

major concern in water treatment facilities is the removal of 

pathogens prior to consumption. 

Since adsorption to soil particles seems to be a significant cause 

of virus removal (Schijven and Rietveld, 1996; Schijven et al., 

1998) and the same processes are applicable to other watertransmitted pathogens to various degrees (Schijven and 

Hassanizadeh, 2000), viruses are often selected as conservative 

models for the transport of major biological contaminants in the 

subsurface. This selection is based on the knowledge that viruses 

generally travel greater distances than bacteria (Scheuerman 

et al., 1987) and protozoa due to their relatively small size 

(see Figure 1), with variations depending on their degree of 

inactivation and adsorption characteristics (Keswick and Gerba, 

1980; Herbold-Paschke, 1991). It should be pointed out that 

although the disease which is caused by polioviruses has 

essentially been eradicated in this country, thereby limiting their 

presence in the environment, much of the historical data is 

available for this virus since it was often used in transport models 

as a marker. 

Although it goes without question that the United States has one 

of the safest public drinking water supplies in the world, current 

and future challenges - like the emergence of new waterborne 

diseases, varying water source quality, and increased 

contamination of ground water - must be met with the best 

available scientific knowledge. 

Sources of Viruses 

As shown in Figure 2, there are a number of avenues available for 

the introduction of viruses to the subsurface (i.e., land disposal of 

untreated and treated wastewater, land spreading of sludge, 

septic tanks and sewer lines, and landfill leachates), as well as a 

number of parameters which affect their migration and survival. 

Among these, septic systems may pose a significant chemical as 

well as biological threat to surface and ground waters. According 

to Canter and Knox (1984), one trillion gallons of septic-tank waste 

is released into the subsurface annually. Although phosphate and 

bacteria are ordinarily removed by soil, nitrate and polioviruses 

(used as the viral marker) may escape these processes and move 

through the soil into the ground water (Alhajjar et al., 1988; 1990). 

The presence of viral particles is even more significant in light of 

studies that indicate they are not necessarily inactivated in septic 

tanks (Stramer, 1984) and may move into the ground water where 

they may survive for long periods of time (Dizer and Hagendorf, 

1991; Yates et al., 1985). Gerba and Bitton (1984) isolated 

viruses from ground-water samples as deep as 30 meters and as 

far as 100 meters from sewage treatment infiltrate basins. Vaughn 

et al. (1983) isolated septic tank viruses which had traveled 3.6 

meters through the unsaturated zone and 67 meters from the 

source through the saturated zone. At several other sites enteric 

viruses have migrated laterally in ground water a few hundred 

meters (Noell, 1992), and Bales et al. (1993) reported that poliovirus 

used as the viral indicator was detected from a deep well located 

more than 1000 meters from the apparent source area. 

Substantial amounts of excess sludge, which may contain viruses 

and other pathogenic microorganisms, are generated from 

wastewater treatment facilities which use activated sludge 

2

Table 1. Water-transmitted Enteroviruses (modified from Bull et al., 1990)


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Rainfall 

Land 

Application Irrigation 

Sludge 

Refuse 

Drinking 

Water 

Well 

Septic Tank 

Sewer Line 

Fault Defective 

Well 

Casing 

Channelization 

Injection 

Well 

Effluent 

Desiccation 

Consolidated Rock 

KEY 

Virus Survival 

Migration 

Soil Factors 

Texture 

Organics 

(humic acids) 

Ionic 

strength 

Adsorption 

Structure 

pH 

Permeability 

Ground Water 

pH 

Temp. 

Microbial antagonism 

(aerobic vs. anaerobic) 

Flow rate 

PATHWAYS OF VIRUS TRANSPORT 

Applied Sludge 

Rain Runoff & 

Sediment 

Infiltration Adsorbed Phase 

Degradation 

(Inactivation) 

Solution Phase 

Degradation 

(Inactivation) 

Leaching 

Saturated Zone 

Water Table 

Figure 1. Migration and survival of viruses in the subsurface (modified from Keswick and Gerba, 1980). 

3 
Typical 

Bacteria 

(2,000 nm) 

Vaccinia 

Virus 

(300 nm) 

Small Animal Cell 

(12,000 nm) 

Epstein-Barr 

Virus 

(130 nm) 

Rhinovirus 

(24 nm) > 400  m

Gravel 

400  m - 12 m 12  m - 0.4 m

5  m - 0.2 m

Sand 

Silt 

Protozoa 

Fungi 

Bacteria 

500 - 200 nm Rickettsiae Chlamydiae 

300-20 nm Viruses 

10-1nm Macromolecules 

1 nm Molecules 

Atoms 70% 

33% 

15% 

33% 

15% 

33% 

LARGE PORES MEDIUM PORES FINE PORES 

Sand 

Loam 

100  m - 10 m

10 mm 

1 mm 

0.01 mm 

1000 nm

100 nm 

100 pm 

10 pm

10 nm 

1 nm

0.001 mm

100

m 

10

m 

1 m

0.1 mm 

  

 

 

Figure 2.	 Relative comparison of sizes of microorganisms and molecules with hydraulic equivalent diameters of pore 

canals (modified from Matthess and Pekdeger, 1981). Note: According to the Soil Science Society of America, 

definitions of the various grain sizes include: gravel, >2 mm; sand, 2 mm - 50 μm; silt, 50 μm - 2 μm; and 

clay, <2 μm. 

processes. Since anaerobic sludge digestion is not sufficient for 

complete viral inactivation, the potential spread of viruses during 

sludge disposal needs to be considered. Therefore, waste 

management practices should take into consideration hydraulic 

loading and contaminant transport characteristics. In this respect, 

taking advantage of the unsaturated zone as the means for the 

retention of viruses and source control may become valuable, as 

was demonstrated by Farrah et al. (1981). It was shown that 

enteroviruses introduced as tracers were efficiently retained by a 

sludge soil mixture and were not detected in deep wells located on 

the sludge disposal site or nearby lagoon. Interestingly, a significant 

diversity among the enteroviruses toward the sludge soil mixture 

was seen since, during the initial part of the examination, poliovirus 

accounted for greater than 90 percent of the viruses in sludge, 

whereas later, it was determined that echoviruses and 

coxsackieviruses were the most common enteroviruses identified 

(Farrah et al., 1981). 

Due to the above considerations, the various drinking water 

standards promulgated since 1975 could be violated by the initial 

release of treated and untreated wastewater into the environment. 

Clearly, the microbial concentration of wastewater applied to the 

land depends on the extent of treatment it receives. For example, 

in the United States a typical raw sewage contains 7x106 plaqueforming units (PFU)/1,000 liters (Gerba et al., 1975). Primary and 

secondary wastewater treatment, followed by disinfection, reduces 

virus concentrations to about 600 PFU/1,000 liters. The application 

of tertiary treatment followed by disinfection which leads to almost 

viral free effluent (6PFU/1000 liters) is not a common practice 

(Vilker et al., 1978). 

Indicators of Viruses in Water 

Prior to recently obtained data, which indicated a significant risk 

associated with a low number of enteroviruses in drinking water 

supplies (Haas, 1983; Haas and Heller, 1990), it was generally 

believed that coliform bacteria were appropriate and reliable 

indicators of the sanitary and biological states of aquatic 

environments. Viral indicators were not used in the past because 

it was believed that: 

•	 viruses were not normal flora of the intestinal tract, and were 

excreted only by infected individuals (with the exception of 

children), usually several orders of magnitude lower than 

those for coliforms; 

•	 there was a lack of viral detection methods for each of the viral 

groups of public concern; 

•	 enteric viruses only multiplied within living susceptible cells, 

and their numbers would be drastically decreased in sewage 

because of the presence of bacteria, and even further 

decreased by sewage treatment, dilution, and natural 

inactivation; and 

•	 although experimentally it had been shown that infection may 

result from the ingestion of only a very few virus particles, 

community risk of infection from low level virus contamination 

has not been determined. 

Marzouk et al. (1980) have questioned the validity of bacterial 

indicators in monitoring the virological quality of water, especially 

for those countries with high incidence of waterborne illnesses 

with viral etiology. It has now been established that the bacterial 

indicator system does not accurately reflect the occurrence of 

viruses in aquatic environments. Bacterial indicators have a much 

higher inactivation rate as compared to enteroviruses. Thus the 

reduction of a bacteria to a safe level by treatment or natural dieoff during self purification in natural waters could leave a large 

number of pathogenic enteric viruses. 

Cliver (1971) has proposed the use of human enteroviruses as 

virological indicators of water and wastewater pollution since they 

retain their infectious properties for a long period of time. During 

4

the earlier studies, the use of specific enteroviruses, such as 

polioviruses and/or HAV, has also been proposed due to the 

frequent isolation of these viruses in sewage contaminated surface 

waters (Goyal, 1983). It was noted by Payment et al. (1985) that 

polioviruses can be used as an indicator of enteroviruses on the 

basis of their persistency. 

In practice, the use of HAV as a surrogate for poliovirus was 

criticized by Metcalf et al. (1978) since hepatitis A is more sensitive 

to chlorination and would be readily inactivated by water and 

wastewater treatment. Currently, since the disease which is 

caused by polioviruses has essentially been eradicated in this 

country, thereby limiting their presence in sewage, they no longer 

serve as a natural indicator of sewage contamination. Poliovirus 

also may not be a suitable index of sewage pollution in those 

countries where live attenuated poliovirus is used for vaccination 

(Katzenelson, 1978). Additionally, if used as a single “marker,” the 

transport of poliovirus may be significantly retarded compared to 

other viruses (Powelson and Gerba, 1993; 1994). 

Recognition of the limitations of enteroviruses as the model of viral 

pollution has led to proposals for using bacteriophages (Stetler, 

1984; Havelaar, 1987; Morinigo, 1992.). The phage index offers 

several advantages because: (1) phage are constant inhabitants 

of the human intestinal tract; (2) phage are non-invasive to 

humans; (3) quantitative phage assays are cheap, easy and rapid 

(Bales et al, 1989; Gerba, 1985); and (4) phage have similar 

physical properties to enteric viruses (Snowdon and Cliver, 1989). 

For example, it has been shown that MS-2 is similar to poliovirus 

in shape and size, 28 nm, and PDR-1 resembles rotavirus in 

shape and size, 62 nm (Powelson et al., 1990). Both phages 

survive for long periods of time in ground water and have a low 

tendency for adsorption to soil surfaces (Yates et al., 1985; 

Powelson et al., 1990). The use of coliphage as an indicator of 

water hygiene has been suggested by many investigators (Niemi, 

1976; Borrego et al., 1987). 

Detection systems have become more specific due to the concern 

for proliferation of some coliphages in sewage water (Borrego and 

Cornax, 1990, Snowdon, 1989: Armon and Kott, 1995), and the 

presence of enteric viruses in the absence of coliphages (Deetz 

et al., 1984). The use of F-specific phages (Kamiko and Ohgaki, 

1992; Nasser and Oman, 1999), and RNA phages of the E 

morphological groups (Havelaar et al., 1993) has been suggested. 

These viruses are similar to enteroviruses in morphological 

characteristics and are only invasive to F-pili carrier bacteria. 

Adapting the F-specific phages, using Salmonella typhimurium 

WG49 strain (Havelaar et al., 1993) or the combination of fecal 

streptococci and E. coli viruses, has also been proposed as the 

most promising indicator of remote pollution (Cornax and Morinigo, 

1991). Furthermore, in determining the efficiency of a drinking 

water treatment system, the use of Clostridium perfringens and 

somatic coliphages as indicators of viruses and protozoa cysts 

has been suggested (Payment and Franco, 1993; Hirata et al., 

1991; Geldenhuys and Pretorius, 1989). 

Viral Transport and Survival 

The ability to determine travel distances and survival times of 

viruses in the subsurface is crucial for regulatory agencies which 

are attempting to maintain sources of contamination at sufficient 

distances from sources of drinking water to protect public health 

(Keswick, 1982a; Yates et al., 1987). It is a general consensus 

that the transport of pathogens in the subsurface depends on the 

extent of their retention on soil particles and their survival. A 

myriad of studies have been conducted to determine viral transport 

rates under various experimental conditions. Table 2 summarizes 

selected studies performed under both laboratory and field 

conditions. Results shown in Table 2 indicate that solid materials 

could generally adsorb/retain as much as 95 %, or even more, of 

the viruses injected into a column. In a column study with 

breakthrough (Dowd and Pillai, 1997), 79% - 100% of viruses were 

removed from solution. As in any environmental field investigation, 

there remains a multiplicity of options with respect to the selection 

of an appropriate tracer (see Table 2). For example, despite the 

claim of Yeager and O’Brien (1979) that phages are unsuitable 

indicators of enterovirus, many others have suggested that phages 

are easier to work with, and may be more accurately evaluated in 

quantitative measurements. A thorough review of the earlier 

literature suggested that polioviruses were used extensively as 

tracers during transport studies; whereas, the more recent works 

are focused on bacteriophages. 

The major factors which affect viral transport characteristics in the 

subsurface are provided in Table 3. Among all the factors, 

temperature appears to be the only well defined parameter 

causing a predictable effect on viral survival (Yates and Gerba, 

1984). A direct relationship between a rise in temperature and 

viral inactivation rates (K= log inactivated/hr) among various 

viruses has been suggested (personal communications, C.P. 

Gerba, 2003). Badawy et al. (1990) stated that during the winter 

(4-10o

C), viral inactivation rates for coliphage, poliovirus, and 

rotavirus were 0.17, 0.06, and 0.10 per hour, respectively. Whereas, 

during the summer (36-41o

C), the inactivation rates for MS-2, 

poliovirus, and rotavirus were 0.45, 0.37, and 0.20 per hour, 

respectively. This study also indicated that viruses may remain 

viable for 3 to 5 weeks on crops irrigated with sewage effluent; 

polio and coxsackievirus up to four months on vegetables during 

commercial and household storage; and up to 30 days on 

vegetables stored at 4o

C. Rhodes et al. (1950), reported a survival 

of 188 days for poliovirus in river water at 4oC. Interestingly, Blanc 

and Nasser (1996) reported that HAV survives longer than other 

enteric viruses at higher temperature. It should be pointed out that 

this information is based on ambient air. A more direct comparison 

would be the correlation with temperatures in the subsurface. In 

this regard the inactivation rates for enteroviruses are 0.06 (10 -

15°C), 0.08 (15 - 20°C), and 0.19 (20 - 25°C). 

Microbial ecology may also play an important role in the inactivation 

of waterborne viruses (Cliver and Herrmann, 1972; Herrmann and 

Cliver, 1973) especially in surface waters. For example, microbial 

activity could affect viral survival by the action of proteolytic 

enzymes of some bacteria (Cliver and Herrmann, 1972) and 

protozoa (Mose et al., 1970) in destroying the viral capsid protein. 

In fact, Deng and Cliver (1995) demonstrated rapid inactivation of 

HAV in the presence of bacteria. 

A report by Wellings et al. (1975) claimed that viruses may survive 

for periods of at least 28 days in ground water. Persistence of 

enteric viruses in ground water beneath land treatment sites and 

septic tank discharges has been well documented in a review by 

Keswick and Gerba (1980) where viral particles were recovered at 

distances of over 1 kilometer from their source. In an important 

study performed to monitor viral movement through the soil, 

Stramer (1984) introduced stool containing poliovirus into septic 

tanks and detected 220 viral particles per milliliter in a well 

53.3 meters away only 12 days after the initial viral introduction. 

The same author found that the viral particles traveled 4.45 meters 

per day and persisted for 100 days in ground water after leaving 

the septic tanks. 

Investigations on the persistence of viruses and indicator bacteria 

in ground water indicate that enteric viruses survive for longer 

periods of time (Keswick et al., 1982b; Yeager and O’Brien, 1979; 

and Niemi, 1976) because they are more resistant to environmental 

conditions (Shuval et al., 1971). To that effect, in an attempt to 

5

Schaub

Table 2. Virus Transport and Attenuation in the Subsurface 

Adsorbent Influent Effluent Experimental Flow Virus Removal 

(Depth) Loading Numbers Conditions Property Capacity Reference 

Virus Attenuation by Laboratory Batch Test 

Activated T4 phage 3.9 x 106 pH = 6.9, Flask reaction

Carbon (108

 PFU/ml) PFU/ml Ionic Strength for 24 hours

0.08 s/l = 1:100

(mg/mL)

T = 23 °C

Sediments, QB, fr, MS2, 102

-108 pH = 7.2 Flask 

Kaolin, and T4 PFU/ml T = 25 °C reaction for

Cellulose, and (104

-109 one hour

Carbon Black PFU/ml)

Virus Transport/Attenuation Through Percolating Laboratory Columns 

5 Soils T2 phage < 106 Distilled water Continuous

(30 - 40 cm) (2 x 107 PFU/ml and traces of (0.078-0.313

PFU/ml) salts cm/min)

pH = 6.3,

T1 phage T = 20 °C

(4.8 x 107 < 106

PFU/ml) PFU/ml

Sandy Forest Polio 1 4.8 x 105 Secondary Continuous

Soil (2 x 107 PFU/ml effluent (pH and

(20 cm) PFU/ml) 7.2) followed intermittent 

by distilled

water

Coarse Sand Polio 1 5.0 x 103 Ground water, Continuous 

(13 cm) (105

 PFU/ml) PFU/ml sewage effluent (0.001cm/

2.5 x 104 pH 8.3, T = 5 °C min)

PFU/ml and 25 °C

Clay Loam Polio 1 <0.1 x 103 Ground water, Continuous 

(13 cm) (105

 PFU/ml) PFU/ml sewage effluent (Avg =0.001

(Not pH 4.3, cm/min) 

detected) T = 5 and 25 °C

Alluvial aquifer PRD-1, MS2 108

-109 Ground water Intermittent 

sediments (109

 PFU/ml) PFU/ml and traces of (a 2 ml-pulse

(20 cm) salts injection

pH = 7.3 flushed with

T = 21 °C 6 pore

volumes) 

Ottawa sand ��-174 5x104 pH = 7.5 flow 

(10-20 cm) bacteriophage PFU/ml T = 6-9 °C (1.6 -3.4

cm/h)

Field Case Studies of Virus Transport/Attenuation 

Sewage f2 phage 47% of Settled sewage Flow 

infiltration site (105

 PFU/ml) initial effluent (Avg = 0.6

soils loading adjusted to 105 cm/min)

(silty sand and dropped PFU/ml

gravel, 18.3 after 7

meters) hrs. 

Flat lands Enteric Polio Secondary Ground water

cypress dome viruses: (52*), effluent spray

soils (sandy with Polio (71*), Coxsackie irrigation

varied clays, 7 Coxsackie (6*),

meters) (75*), Echo (0 )

Echo (30*)

* Number of PFU counted in 500 mL sample water.

96% viral removal, Cookson

removal rate: 0.04 hr-1 - and North

0.8 hr-1 in the 1st 12 hrs. (1967)

and 0.002 hr-1 in the 2nd

12 hrs.

Viral adsorption was Sakoda et

dependent on surface al. (1997)

acidity of the adsorbents 

No virus breakthrough, Drewry

over 95% viral removal, and

the highest numbers Eliasson

remained in the top few (1968)

centimeters of the

column

97% viral removal, Duboise et

Polio 1 retention was al. (1976)

greater under

intermittent flow,

breakthrough was only

observed with distilled

water 

No virus breakthrough Sobsey et

al. (1995)

No virus breakthrough Sobsey et

al. (1995)

79 - 100% removal, Dowd and

breakthrough occurred Pillai

after 1-2 pore volumes (1997)

No breakthrough Jin et al.

(1997)

53% removal, 48 hrs. Schau and

breakthrough in 18 Sorber (1977)

meter well

Removal ratio of 27% Wellings et

for Polio, 69% for al. (1975)

Coxsackie, and100%

for Echo viruses found

in 7 meter deep well

after rainfall

6

Table 3. Factors Influencing Virus Fate in Soils


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7 
monitor the survival of pathogenic microorganisms with ground 

water collected from a 145-meter deep well in Florida, it was 

shown that poliovirus type 1 (K=-0.0019) was more stable than E. 

coli or S. faecalis (K=0.0012) while coliphage f2 had the highest 

decay rates. This characteristic is further substantiated by data 

indicating that both rotaviruses and enteroviruses may be more 

resistant to chlorination than indicator bacteria (Melnick et al., 

1978). In terms of their relative susceptibility, some enteroviruses 

such as HAV are more stable under adverse environmental 

conditions than poliovirus 1. The inherent diversity for the longevity 

of this class of viruses toward factors that affect their survival (i.e., 

soil type, pH, temperature) is apparent in Table 4. During this 

assessment, the die-off rate constants were calculated from 

selected literature which were primarily acquired from groundwater investigations. 

The die-off rates in Table 4 represent the time rate of change of the 

concentration of a microorganism in ground water/soil by assuming 

the virus die-off follows first-order kinetics. It is noted that die-off 

rates are also referred to as inactivation, or decay, or survival rates 

in the literature. Inactivation is a process by which viruses lose 

their ability to produce progeny (Bitton, 1980; Bitton et al., 1983). 

Removal rates in solution in batch studies may represent die-off 

rates of viruses, while removal rates in column or chamber studies 

may represent attachment/adsorption rates and/or die-off rates 

(Powelson and Gerba, 1994). 

From the case studies examined (i.e., batch, chamber, column 

and field tests), the following findings were observed. 

•	 Viruses adsorbed on solid surfaces can possess a significantly 

longer time of activity than viruses suspended in solution. 

Different inactivation rates in water and on solids were 

reported. For example, the inactivation at pH 7.2 is 0.055 h-1 

for E. coli phage adsorbed on solids, and is 0.28 h-1 for the 

virus in suspension (Sakoda et al.,1997). However, in many 

publications the inactivation rates on solids and in solution 

were not distinguished. The inactivation rates used in studies 

of virus survival and transport are difficult to interpret. 

•	 Transport of a virus in the subsurface can be controlled by 

multi-processes, such as advection, dispersion, adsorption, 

inactivation/decay, etc. Many case studies usually focus on 

only one or a few processes and ignore others which can be 

of significance in controlling the transport of viruses. 

•	 Parameters used in transport studies are rarely obtained from 

independent experiments, and few experiments have been 

designed to obtain these independent parameters. Examples 

of the studies developing these independent parameters are 

Bales et al. (1991) and Dowd et al. (1998). 

•	 Many column studies have been conducted to examine 

adsorption/inactivation of viruses, but few have been 

conducted to examine their elution/desorption in columns. 

Examples of the studies considering these latter processes 

are Jin et al. (1997), Dowd et al. (1998), and Powelson et al. 

(1993). 

•	 In many cases, equilibrium adsorption is of little significance, 

and kinetic sorption with prevailing attachment/sorption 

appears to control virus removal in the field (e.g., Schijven 

and Hassanizadeh, 2000). 

•	 Many experimental studies in relation to virus transport have 

been published; however, relatively few efforts have been 

made to simulate experimental results. Jin et al. (1997) and 

Dowd et al. (1998) are two examples of such simulation. 

As discussed earlier, viral transport through porous media is 

controlled by sorption and by inactivation (Bales et al., 1993; 1995; 

Bitton, 1975; Murray and Laband, 1979). However, adsorption of 

viruses to soil should not be confused with their inactivation since 

adsorption is not permanent and can be reversed by the ionic 

characteristics of the percolating water (Vilker et al., 1978; Bales 

et al., 1993). Reversible sorption of poliovirus type 1 and coliphage 

T2 from clay resulted in fully infectious particles (Carlson et al., 

1968). Viruses can remain infective after a travel distance of 

67 meters vertically and 408 meters horizontally (Keswick and 

Gerba, 1980). According to Murray and Parks (1980), various 

forces involved in the attachment of viruses to soil particles may 

include hydrogen bonding, electrostatic attraction and repulsion, 

van der Walls forces, and covalent ionic interaction. Bales et al. 

(1991) demonstrated the importance of solution pH and soilsurface hydrophobicity in attachment and detachment of 

bacteriophage from solid surfaces. Bales et al. (1993) have 

shown that low levels of organic matter in porous media can retard 

viral transport. 

Adsorption or release of viruses from soil particles is due to the 

amphoteric nature of the external viral proteins. Thus, both ionic 

strength and pH strongly affect the adsorption process 

(Duboise et al., 1976). Many viruses sorb more strongly in acidic 

water. Any sharp increase in the pH may enhance the detachment 

and, therefore, the mobility of the viruses that are attached to the 

soil matrix. Hydrophobic interactions are also involved in the 

adsorption of viruses to sands (Dizer et al., 1984). Virus adsorption 

is significantly influenced by a number of parameters such as the 

type of virus, soil type, virus load, pH, and salt concentration 

(Gerba and Bitton, 1984). Although viruses including polio, HAV, 

reovirus, and coxsackievirus sorb more strongly to clay rather 

than silt and sand particulate, the extent of sorption of 

coxsackievirus seems to be limited and without any relationship to 

the texture of geologic materials. Batch studies with 28 viruses 

and 9 soil types indicated a wide range of virus adsorption from 

0.01 to 99.9 percent (Goyal and Gerba, 1979). The diversity of 

data reported in the literature makes viral transport modeling 

difficult (Powelson et al., 1990). According to Yates et al., (1987), 

modeling capabilities far exceed our current understanding of the 

behavior of viruses in soil and ground water. 

Hydrophobic interactions are apparently also responsible for 

sorption of viruses at the air-water interface in unsaturated soils 

(Thompson et al., 1998). Some experimental evidence suggests 

viruses sorbed at these sites may undergo accelerated inactivation 

rates (Thompson and Yates, 1999). When viruses are adsorbed 

to the air-water interface they may be considered to be effectively 

removed from the transport process (Chu et al., 2001). This is 

because environmental models have not yet been developed for 

advection at this surface. Just as virus inactivation may be 

accelerated at the air-water interface, some have suggested 

sorption at the solid-water interface may enhance virus longevity 

(Sim and Chrysiopoulos, 2000). These notions have yet to be 

rigorously tested experimentally, and as yet, a physical basis for 

them has not been established. 

It should also be noted that most soils have enormous buffering 

capacity to maintain a pH balance, thereby averting the release of 

viruses. The soil’s organic content can further serve as a retardation 

factor for some viruses. In general, reoviruses sorb strongly to 

organic materials as compared to polioviruses and HAV. Vilker et 

al. (1978) also questioned the results of transport studies based 

on artificially high initial concentrations of viruses and high water 

flow rates as compared to those observed in the field (approximately 

0.01 cm/min). As expected, the behavior of viruses, as with any 

other biotic system in the environment is diverse. For example, 

while Drewry and Eliassen (1968) have shown that percolation 

through a few meters was sufficient for the removal of viral 

8

Table 4. Die-off Rate Constants (day-1) of Pathogens in the Subsurface 

Microorganisms 

Die-off Rate 

(day-1)* Environmental Conditions 

Experimental 

Methods Reference 

Poliovirus 1 a 0.96 

a 0.52 

0.77 

0.21 

b

 0.01 

b

 0.02 

b

 0.03 

0.013 

0.07 

0.016 

0.024 

c

0.51 

c

0.66 

c

 > 1.42 

c

 > 1.42 

SW; pH, 8.3; T, 23-27 °C 

SW; pH, 8.3; T, 4-8 °C 

SW; pH, 7.8; T, 12-20°C 

GW; pH, 7.8; T, 3-15 °C 

GW; pH, 7.4; T, 10 °C 

GW; pH, 7.4; T, 20 °C 

GW; pH, 7.4; T, 30 °C 

GW saturated loamy soil; T, 10 °C 

GW saturated loamy soil; T, 23 °C 

GW saturated sandy soil; T, 10 °C 

GW saturated sandy soil; T, 23 °C 

GW, sandy soils; pH 8.3; T, 5 °C 

GW, sandy soils; pH 4.3; T, 25 °C 

GW, clay loam; pH 8.3; T, 5 °C 

GW, clay loam; pH 8.3; T, 25 °C 

Chamber# 

Chamber 

Batch test 

Batch test 

Column test 

O’Brien & Newman 

(1977) 

Keswick et al. (1982b) 

Nasser & Oman (1999) 

Blanc & Nasser (1996) 

Sobsey et al. (1995) 

Poliovirus 3 1.26 

1.0 

SW; pH, ; T, 23-27 °C 

SW; pH, 7.5; T, 9-12 °C 

Chamber 

Chamber 

O’Brien & Newman 

(1977) 

Keswick et al. (1982b) 

Coxsackievirus A-13 

Coxsackievirus B-1 

Coxsackievirus B-3 

Coxsackievirus A-9 

Coxsackievirus B-3 

a 3.4 

0.41 

0.19 

b

 2.2 

b

 0.12 

SW; pH, 8.3; T, 23-27 °C 

SW; pH, 8.3; T, 4-8 °C 

GW; pH, 7.8; T, 3-15 °C 

Sand-silty soil; pH 7.8, T, 23 °C 

Sand-silty soil; pH 7.8, T, 23 °C 

Chamber 

Chamber 

Batch test 

O’Brien & Newman 

(1977) 

Keswick et al. (1982b) 

Hurst et al. (1980) 

Fecal streptococcus b

 0.27 

0.23 

GW; pH, 7.5; T, 9-12 °C 

GW; pH, 7.8; T, 3-15 °C 

Chamber 

Chamber 

McFeters et al. (1974) 

Keswick et al. (1982b) 

Fecal Coliforms 

E. coli 

E. coli 

b

 0.45 

0.32 

b

 0.001 

b

 0.018 

b

 0.03 

GW; pH, 7.5; T, 9-12 °C 

GW; pH, 7.8; T, 3-15 °C 

GW; pH, 7.4; T, 10 °C 

GW; pH, 7.4; T, 20 °C 

GW; pH, 7.4; T, 30 °C 

Chamber 

Chamber 

Batch test 

McFeters et al. (1974) 

Keswick et al. (1982b) 

Nasser & Oman (1999) 

Rotavirus SA-11 0.36 

b

 0.20 

GW; pH, 7.8; T, 3-15 °C 

GW; pH, 7.8; T, 23 °C 

Chamber 

Batch test 

Keswick et al. (1982b) 

Hurst et al. 

Coliphage f2 

F+ phage 

0.39 

b

 0.01 

b

 0.02 

b

 0.03 

GW; pH, 7.8; T, 3-15 °C 

GW; pH, 7.4; T, 10 °C 

GW; pH, 7.4; T, 20 °C 

GW; pH, 7.4; T, 30 °C 

Chamber 

Batch test 

Keswick et al. (1982b) 

Nasser & Oman (1999) 

8.3

(1980) 

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�������������������������������� ������������������������������������������������������ � ������������������������������������������������������� �������������������������������������������������������������� ���������������������������������������������������������������� ����������������������������������������������������������

9

Table 4. continued


Hepatitis A virus b 0.06 

b

 0.016 

b

 0.03 

0.001 

0.01 

0.015 

0.023 

c

0.42 

c

0.45 

c

 > 0.94 

c

 > 0.94 

GW; pH, 7.4; T, 10 °C 

GW; pH, 7.4; T, 20 °C 

GW; pH, 7.4; T, 30 °C 

GW saturated loamy soil; T, 10 °C 

GW saturated loamy soil; T, 3 °C 

GW saturated sandy soil; T, 10 °C 

GW saturated sandy soil; T, 23 °C 

GW sandy soils; pH, 8.3; T, 5 °C 

GW sandy soils; pH, 8.3; T, 25 °C 

GW clay loam; pH, 8.3; T, 5 °C 

GW clay loam; pH, 8.3; T, 25 °C 

MS2 eriophage 0.05 

0.16 

0.12 

0.19 

0.028 

0.053 

0.032 

c

 > 1.45 

c

 > 1.45 

2.24 

0.15 

0.28 

5.82 

0.32 

0.57 

GW saturated loamy soil; T, 10 °C 

GW saturated loamy soil; T, 23 °C 

GW saturated sandy soil; T, 10 °C 

GW saturated sandy soil; T, 23 °C 

N. Carolina GW; pH, 7.9; T,12 °C 

Arizona GW; pH, 8.2; T, 12 °C 

New York GW; pH, 7.3; T, 12°C 

Clay loam; pH, 4.3; T, 5 °C 

Clay loam; pH, 4.3; T, 25 °C 

Wetland, 0-3 BGS (m); summer 

Wetland, 3-70 BGS (m); summer 

Wetland, 0-70 BGS (m); summer 

Wetland, 0-3 BGS (m); winter 

Wetland, 3-70 BGS (m); winter 

Wetland, 0-70 BGS (m); winter 

PRD-1 bacteriophage 0.028 

0.026 

0.055 

0.034 

d �5 

GW saturated loamy soil; T,10 °C 

GW saturated loamy soil; T,23 °C 

GW saturated sandy soil; T, 10 °C 

GW saturated sandy soil; T, 23 °C 

Sandy aquifer; pH 5.7; T, 11.5 °C 

�X-174 

bacteriophage 

14.2 - 17.3 Ottawa sand saturated with 

phosphate saline solution (pH = 

7.5); T, 6 - 9 °C 

MS-2 bacteriophage 0.5 

1.8 

GW with fresh soil; T, 25 °C 

GW leached soil; T, 25 °C 

M-1 

PRD-1 

0 

0 

Sand (fine-medium grained) 

Sand (fine-medium grained) 

2

bact

Batch test 

Batch test 

Batch test 

Column test 

Nasser & Oman (1999) 

Blanc & Nasser (1996) 

Blanc & Nasser (1996) 

Sobsey et al. (1995) 

Batch test 

Batch test 

Batch test 

Column test 

Field test 

Field test 

Blanc & Nasser (1996) 

Blanc & Nasser (1996) 

Yates & Gerba (1984) 

Sobsey et al. (1995) 

Chendorain et al. (1998) 

Chendorain et al. (1998) 

Batch test 

Batch test 

Field study 

Blanc & Nasser (1996) 

Blanc & Nasser (1996) 

Bales et al. (1995) 

Column 

study 

Jin et al. (1997) 

Column 

study 

Powelson et al. (1991) 

Field study Bales et al. (1997) 

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10

contamination, poliovirus type II used as a “marker” was isolated 3 x 105 plaque-forming units (PFU) with and without colloid clay


from a 30 meter deep well located 100 meters from a wastewater particles was used in the batch study, and the evolution of the


drain field in Michigan (Mack et al., 1972). Therefore, current rule- amount of bacteriophage was recorded as shown in Figure 3. The


making considerations for the upcoming Ground Water Rule entail inactivation and adsorption mechanisms of viruses in a


a sampling program requirement. Some of the discrepancies in montmorillonite suspension are mathematically described as:


the reported results of these investigations also may be attributed


to physical heterogeneity (Harvey et al., 1993) and the earlier δC δS


methods used for the detection and concentration of enteric θ δt 

+ρ δt = θμ1C −ρμsS (1)


viruses which were usually less than 50 percent efficient (Gerba,


1985). The use of reliable current methodologies (i.e., molecular


techniques) can, however, minimize the variance between reported ρ

δ

δ 

S

t = θkattachC −ρkdetachS (2)
 and actual numbers. Practices designed to ensure compliance


with drinking water standards might more properly rely instead on


a cadre of multidisciplinary approaches including predictive models, where:


geological settings that result in viral retention, as well as sampling C is the number of free viruses per unit volume in the aqueous
 and analysis. To this end, EPA is aiming, by its proposed Ground phase,
 Water Rule, to reduce the public health risk related to the ingestion


of waterborne pathogens from fecal contamination for a large S is the number of viruses per unit mass of solid in the solid


number of people served by ground water. phase,


In an attempt to demonstrate how to obtain parameters from t is time,


laboratory experiments which were designed for investigating θ is the volume fraction of the aqueous phase,
 inactivation and adsorption of viruses, the following case study is


offered. ρ is solid density in the suspension,


A Case Study μ1

 and μs 

the aqueous phase and in the attached solid, respectively, 

are the inactivation rate coefficients for free viruses in 

To investigate the influence of inactivation and adsorption and


mechanisms in water, Rossi and Aragno (1999) presented a batch


agitation technique to examine inactivation-adsorption kinetics kattach and kdetach are the attachment (adsorption ) and detachment


simultaneously. An initial amount of bacteriophage T7 of about (desorption) rate coefficients.


100 

80 

60 

40 

20 

0 

0 50 100 150 200 

Observed 

Simulated 

Time (min) 

Figure 3. Adsorption and inactivation kinetics of phage T7 in 2.5% montmorilllonite suspension (kattach = 0.10 min-1, 

kdetach = 0.073 min-1, μ1 = 0.036 min-1, μs = 0 min-1). Experimental data are obtained from Rossi and Aragno 

(1999). 

Phage T7 (%PFU)


11

The left-hand side of equation (1) is the time rate of change of 

viruses in the aqueous phase and in the solid phase, and the righthand side is the loss of viruses due to inactivation in the aqueous 

phase and in the solid phase, respectively. Equation (2) states 

that the time rate of change of the viruses on a solid phase equals 

the difference between the attachment of viruses from solution to 

solid and the detachment of the viruses from solid phase to 

aqueous phase. The system of equations (equations 1 and 2) is 

solved using a second-order Runge-Kutta algorithm with proper 

initial conditions when the inactivation rate coefficients and 

attachment/detachment rate coefficients are known. When 

experimental data are available, a least-square curve fitting 

technique is applied to estimate the parameters. Results of 

Rossi/Aragno parameter estimation indicate that the inactivation 

rate of viruses on solid phase is not of significance (i.e., μs = 0 min-1). 

The same analyses showed that the other parameters are 

kattach = 0.10 min-1, kdetach = 0.073 min-1, and μ1 = 0.036 min-1. In this 

case,θ is 0.975 and ρ is 0.025 mg/ml. These parameters indicate 

that virus attachment processes (the term θkattachC ) are much 

faster than the detachment processes (the term ρkdetachS). This is 

depicted in Figure 3 where the phage T7 concentration dramatically 

decreases in the early time period. 

Effect of Hydrogeologic Settings on Viral Movement 

The concentration and loading of viruses and the hydrogeologic 

setting through which they move will control the potential for viral 

migration to wells to a much greater extent than biological 

survivability. A hydrogeologic setting often consists of a soil 

underlain by unconsolidated deposits of sand, silt, and clay 

mixtures over rock. The setting further incorporates unsaturated 

and saturated zones. For purposes of this discussion, the amount 

of precipitation available to transport the virus through the 

subsurface will not be considered, although it is recognized that 

infiltration acts as a transport mechanism as well as a dilution 

factor. 

All other factors being equal, the persistence of viruses at a well, 

or other source of water, is most likely where saturated flow 

transports large concentrations of the particles along short flow 

paths through media which contribute little to attenuation. Although 

the interrelated processes that control viral movement and 

persistence in the subsurface are not completely understood 

(Cadmus Group, et. al., 2000), some of the major hydrogeological 

factors that can be used to evaluate the potential for viral presence 

in ground-water wells include: 

•	 transport mechanisms (unsaturated versus saturated flow 

conditions); 

•	 type of media through which the virus will travel (clays versus 

sands versus fractured media); 

• length of flow path to the extraction point (well); and 

• time of travel. 

Hydrogeologic settings with shallow water tables are more 

susceptible to viral transport. Viruses are attenuated or immobilized 

by processes such as dessication, microbial activity, and 

stagnation. Further, viruses commonly bind to soil particles, finegrained materials, and organic matter. The lower transport 

velocities associated with unsaturated conditions (e.g., move, 

stop, move cycle) allow these processes more time to occur. If 

viruses are introduced directly into the water table (such as from 

leaching tile fields associated with onsite sewage disposal) or if 

the volume of contaminants can maintain saturated flow conditions 

(such as in some artificial recharge situations), the potential for 

contamination is increased (Aller et. al., 1987). Where the viral 

concentration is high, the probability of contaminant migration is 

increased regardless of the hydrogeologic setting. Therefore, in 

hydrogeologic settings with deeper water tables and where 

contaminants are not introduced into the aquifer through saturated 

flow conditions, viruses are much less likely to survive being 

transported to a well. 

Hydrogeologic settings with interconnected fractures or large 

interconnected void spaces that lack fine-grained materials have 

a greater potential for viral transport and well contamination. Karst 

aquifers, fractured bedrock and gravel aquifers have been identified 

in the proposed Ground-Water Rule as sensitive hydrogeologic 

settings (U.S. EPA, 2000). In these settings, fractures and large 

void spaces allow rapid transport through the aquifer, thereby 

reducing the amount of time and particulate contact available for 

attenuation. Potential interaction with rock walls along fractures 

is reduced, and contact with fine-grained materials for potential 

sorption sites is minimal. 

Similar to fractured rock aquifers, gravel aquifers with only a small, 

fine-grained fraction have little potential for viral sorption. However, 

as the amount of fine-grained material increases, effective grain 

size decreases, the potential for sorption increases, and travel 

times decrease. Finer-grained aquifers and aquifers where void 

spaces are less interconnected or smaller are, therefore, less 

likely to transport viruses significant distances. 

The potential for physical viral removal by filtration also appears 

to increase as grain size becomes smaller, although the filtration 

processes are not well understood due to their size. However, 

filtration of bacteria, which are larger than viruses, has been 

shown to be an effective removal mechanism. 

Hydrogeologic settings where fractures are not as interconnected 

or where more tortuous flow paths must be followed to reach a well 

also allow for greater viral removal. For example, in many rock 

aquifers, ground-water flow follows bedding planes that may 

result in an elongated, indirect pathway to a well. In other rock 

aquifers, flow must travel around and through cemented portions 

of the matrix thereby increasing the flow path. Similarly, sand and 

gravel aquifers with fine-grained materials in the matrix will have 

less direct flow paths as the water flows around the finer-grained 

materials. Generally, it can be stated that tortuosity increases the 

length of the flow path and decreases the hydraulic conductivity, 

thus decreasing viral survival. Where finer-grained materials are 

present or fractures are less interconnected, flow paths are also 

longer, thereby offering some protection to wells in more permeable 

units. 

Hydrogeologic settings where time of travel is short have a greater 

potential for viral contamination. Where less permeable units 

(called aquitards) restrict or reduce vertical flow to underlying 

aquifers, time of travel is increased. Although inactivation rates 

have been shown to be extremely variable, time is a major factor 

affecting virus viability. 

Due to the importance of hydrogeologic settings, the proposed 

Ground Water Rule thoroughly addresses this issue to identify 

wells that are sensitive to fecal contamination. A component of 

the proposed Ground Water Rule requires states to perform 

hydrogeologic assessments for the systems that distribute ground 

water that is not disinfected (source waters that are not treated to 

provide 99.99% removal or inactivation of viruses). The states are 

required to identify sensitive hydrogeologic settings and to perform 

monitoring for indicators of fecal contamination from sensitive 

hydrogeologic settings (see U.S. EPA, 2000, for the complete 

proposed strategy). 

12

Virus Transport Modeling 

One method of addressing regulations associated with virus 

exposure, such as ground-water disinfection, the application of 

liquid and solid waste to the land, and wellhead protection zones, 

is the application of predictive virus transport models. 

The states may choose to employ fate-and-transport models as 

screening tools to identify hydrogeologic barriers for a particular 

water supply aquifer. (U.S. EPA defines a hydrogeologic barrier 

as the physical, biological, and chemical factors, singularly or in 

combination, that prevent the movement of viable pathogens from 

a contaminated source to a public supply well.) To this end, the 

subject of modeling will become pertinent and will be discussed 

herein. Like most predictive modeling efforts, the results depend 

on the conceptual basis of the model as well as the quality and 

availability of input data (Corapcioglu and Haridas, 1984). Clearly, 

a thorough understanding of the processes and parameters 

associated with virus transport are essential elements in their 

application. 

As shown in considerable detail in Table 3, some of the more 

important subsurface virus transport factors include soil water 

content and temperature, sorption and desorption, pH, salt content, 

organic content of the soil and ground-water matrix, virus type and 

activity, and hydraulic stresses. Berger (1994) indicated that the 

inactivation rate of viruses is probably the single most important 

parameter governing virus fate and transport in ground water. 

Some of the existing models require only a few of these parameters 

which limit their use to screening level activities, while others 

require input information which is rarely available at field scale and 

is usually applied in a research setting. One limitation of most 

models is that they have been developed for use in the saturated 

zone. It has been shown, however, that the potential for virus 

removal is greater in the unsaturated zone than in ground water 

(Gelhar, 1992). 

Despite the number of models developed at present, tests of the 

models against field data are not abundant. Simulation results of 

the developed models were either compared to the analytical 

solutions or fitted to data obtained for laboratory experiments. 

Even though some models were developed to handle the complex 

processes involved in virus transport, only simplified simulation 

results were compared against ideally controlled experimental 

conditions (Vilker and Burge, 1980; Matthess and Pekdeger, 

1981; Tim and Mostaghimi, 1991; Teutsch et al., 1991). 

The existing codes for virus transport can be placed into two 

categories. As shown in Table 5, the first group contains computer 

codes which are readily available to the public and which have 

user’s manuals. The second group, shown in Table 6, contain 

computer codes which were developed for research purposes. 

Better understanding of virus transport mechanisms was the main 

motivation in developing these codes, rather than public 

dissemination. As a result, further discussions herein will be 

limited to the models in Table 5. 

VIRALT, developed for EPA’s Office of Drinking Water, is a 

modular, semi-analytical and numerical code that simulates the 

transport and fate of viruses in ground water. The code computes 

viral concentrations in extracted water describing both steadystate and transient transport including advection and dispersion in 

the vertical direction in the unsaturated zone. Along ground-water 

flow lines in the saturated zone it handles adsorption and 

inactivation. 

CANVAS was developed in order to improve on its predecessor, 

VIRALT. The major enhancements implemented in CANVAS are: 

•	 CANVAS can simulate multiple contaminant sources in the 

unsaturated or saturated zones whereas VIRALT is limited to 

a single source; 

•	 transverse, as well as longitudinal and horizontal dispersion 

in the saturated zone is simulated by CANVAS whereas 

VIRALT is limited to longitudinal dispersion; 

•	 a colloidal filtration term is designed to simulate the facilitated 

transport of viral particles through the unsaturated and 

saturated zones; and 

Table 5. Publicly Available Virus Transport Codes: Group I


VIRULO 

v. 1.0 

2002 Faulkner et al. 

@ 

U.S. EPA-ORD 

A Monte Carlo-based screening model for predicting total 

virus mass atten uation in the unsaturated zone. 

Processes Considered: advection, dispersion, sorption, 

inactivati on, a nd u ncertainty. 

Developed 

by EPAORD 

VIRALT 

v. 3.0 

1994 Park et al. 

@ 

Hydro-Geologic 

A modular semi-analytic and numerical code for transport 

and fate of viruses in the unsaturated zones. 

Processes Considered: advection, dispersion, sorption, 

and inactivation. 

Developed 

for EPA 

CANVAS 

v. 2.0 

1994 Park et al. 

@ 

Hydro-Geologic 

A modular semi-analytical and numerical code for 

transport and fate of viruses in the unsaturated and 

saturated zones. 

Processes Considered: advection, dispersion, sorption, 

inactivation, and colloidal filtration. 

Descendant 

of VIRALT 

VIRTUS 

v. 1.0 

1991 Yates et al. 

@ 

U.S. Salinity Laboratory 

A numerical code for transport and fate of viruses in the 

unsaturated zone. he virus transport is coupled with the 

flow of water and heat through soil. 

Processes Considered: advection, dispersion, sorption, 

and inactivation. 

Researchoriented 

code 

Program Name Year Authors Description Rema rks 

T

13

Table 6. Other Virus Transport Codes (Developed for Research Purposes): Group II


Authors 

Chu et al., 

2001 

Sim & 

Chrysikopoulos, 

2000. 

Lindqvist et al., 

1994 

Tan et al., 

1994 

Hornberger et al., 

1992 

Tan et al., 

1992 

Harvey & 

Garabedian, 

1991 

Lindqvist & 

Bengtsson, 

1991 

Tim & 

Mostaghimi, 

1991 

Taylor & 

Jaffe, 

1990 

Matthess et al., 

1988 

Corapcioglu & 

Haridas, 

1985 

Matthess & 

Pekdeger, 

1981 

Vilker & Burge 

1980 

Vilker et al., 

1978 

Title of Research Paper 

Mechanisms of virus removal during 

transport in unsaturated porous media 

Virus transport in unsaturated porous 

media 

A kinetic model for cell density 

dependent bacterial transport in porous 

media. 

Transport of bacteria in an aquifer 

sand: Experiments and model 

simulations. 

Bacterial transport in porous media: 

Evaluation of a model using laboratory 

observations. 

Transport of bacteria during 

unsaturated soil water flow. 

modeling the movement of bacteria 

through a contaminated sandy aquifer. 

Dispersal dynamics of ground-water 

bacteria. 

Model for predicting virus movement 

through soils. 

Saturated and biomass transport in a 

porous medium. 

Persistence and transport of bacteria 

and virus in ground water - A 

conceptual evaluation. 

Microbial transport in soils and ground 

water: A numerical model. 

Concepts of a survival and transport 

model of pathogenic bacteria and 

viruses in ground water. 

Adsorption mass transfer model for 

virus transport in soils. 

Application of ion 

exchange/adsorption models to virus 

transport in percolating beds. 

Use of colloid filtration theory in 

Journal 

Solution 

Method Processes Considered Medium 

WRR 

37(2) 

FDM Advection, dispersion, masstransfer, adsorption, and 

blocking 

Unsaturated 

1-D 

WRR 

36(1) 

FDM Advection, dispersion, 

adsorption, and mass-transfer 

Unsaturated 

1-D 

WRR 

30(12) 

FDM & 

ANAL 

Advection, dispersion, and 

non-equilibrium sorption 

Saturated 

1-D 

WRR 

30(2) 

FDM Advection, dispersion, and 

sorption (max retention 

capacity included) 

Saturated 

1-D 

WWR 

28(3) 

ANAL Advection, dispersion, and 

clogging/declogging 

Saturated 

1-D 

SSSAJ 

56(5) 

QuasiANAL Dispersion and sorption Unsaturated 

1-D 

ES&T ANAL Advection, dispersion, 

sorption, and filtration 

Saturated 

1-D 

ME ANAL Advection, dispersion, nonequilibrium sorption, and 

decay 

Saturated 

Sand 

Column 

Ground 

Water 

29(2) 

FEM Advection, dispersion, linear 

equilibrium, sorption, and first 

order decay. 

Program Name: VIROTRANS 

Unsaturated 

Soil 

WRR 

26(9) 

FEM Advection, dispersion, 

sorption, growth/decay, and 

shear/filtration. he change in 

parameter values due to 

biofilm clogging was 

included. 

Saturated 

Column 

JCH ANAL Advection, dispersion, 

sorption, and filtration. 

Saturated 

1-D 

AWR ANAL, 

FEM 

Advection, dispersion, 

sorption, decay/growth, and 

clogging/declogging. 

Transport equation is coupled 

with nutrient concentration. 

Saturated 

1-D & 

2-D 

STE Not Clear Discussion on controlling 

factors for bacteria/virus 

transport. 

Saturated 

medium 

WR ANAL Adsorption mass transfer 

model. 

Batch & 

Column 

AIChE ANAL Ion exchange/adsorption Saturated 

Column 

25 (1) 

21(1) 

2(2) 

8(188) 

21(149) 

14(783) 

178(84) 

T

WRR: Water Resources Research SSSJ: Soil Sciences Society of America Jour. ANAL: Analytical 

ME: Microbial Ecology WR: Water Research FEM: Finite Element Method 

JCH: Jour. of Contaminant Hydrol. AWR: Advances in Water Resources FDM: Finite Difference Method 

ES&T: Environ. Sci. & Tech. STE: Science of the Total Environment AIChE: Am. Inst. for Chem. Eng. Symp. Ser. 

14

•	 allows the virus inactivation coefficient to be either 

temperature-dependent or given as a user-specific value. 

VIRTUS is a finite difference model for virus fate and transport in 

unsaturated soil. The model allows the virus inactivation rate to 

vary based on soil temperature. It supports unsteady flow, 

transport in layered soils, different inactivation rates for adsorbed 

versus freely suspended viral particles, and the flow of heat 

through soil. It assumes that viruses are introduced at the soil 

surface. VIRTUS is based on mass conservation of a contaminant 

in porous media and couples the flow of water, viruses, and heat 

through the soil. The model can be used to estimate the number 

of viruses that reach ground water after traveling through soil from 

a contamination source. 

VIRULO was developed to fill the need for a predictive screening 

model. It uses the assumption of gravity flow infiltration and time 

averaging to solve governing equations for advection, dispersion, 

adsorption, and mass-transfer developed by Sim and 

Chrysikopoulos (2000). This model was produced with the idea 

that in many cases very little information may be known about a 

particular site. It supplies a small database of default parameters 

for water flow and virus transport. At a minimum, the user needs 

only to select one of the twelve USDA soil categories, a virus 

whose attenuation will be predicted, and a thickness of a soil bed 

of interest. The Monte Carlo method is employed with known or 

assumed distribution functions for the input parameters. Random 

number generators are used internally, and a graphical display of 

the histogram of predicted attenuations is produced, along with 

the number of times a user-specified level of attenuation was not 

achieved in a given number of simulations (1,000,000 by default). 

Virus transport modeling is inherently fraught with uncertainty. It 

has been suggested that models have a tendency to under-predict 

virus transport, and hence their use as a sole criterion for purposes 

of determining regulatory compliance is questionable (Yates and 

Jury, 1995). Current rule-making considerations for the forthcoming 

Ground Water Rule consider modeling as a potential tool which 

can be useful, but a sampling program will always be a requirement. 

Example Application of the VIRULO Screening Model 

The use of municipal sewage effluent for irrigation is a growing 

trend in urban and suburban areas. It represents an attractive 

means by which water that has undergone treatment, but not to a 

level making it suitable for open distribution, can fulfill a need at 

greatly reduced costs. This is because the soil above the groundwater table through which irrigation water percolates can be 

viewed as a natural means of filtration. In a completely engineered 

system, the final stages of treatment to a level suitable for human 

consumption are very costly. Depending on the degree of 

disinfection applied to the effluent, viruses may have undergone 

little or no attenuation prior to irrigation. Therefore, there is a 

potential for viral contamination to underlying aquifers if natural 

filtration above the ground-water table is not sufficient to remove 

viable viruses. U.S. EPA Region VI recently conducted pilot 

Comprehensive Performance Evaluations for ground water for 

selected water supply systems in Texas and New Mexico (e.g., 

U.S. EPA, 2000). In these evaluations, the ground-water system 

itself is considered as a component of the overall water supply 

system performance. The evaluations frequently cite a goal of 

achieving “99.99% virus inactivation,” in order for the groundwater system to be considered acceptable. 

These notions have implications for planning. Parks and golf 

courses are the most common sites of irrigation with municipal 

sewage effluent. In a gross sense, information from soil surveys, 

along with a screening model, can provide an indication of the 

level of risk of viral contamination (to the ground-water table) a 

particular site may have. Figure 4 shows a map of a portion of 

Wake County, North Carolina, which contains the city of Raleigh 

and its suburbs. The areas shown outlined in black represent 

locations of park lands and open space which might be suitable for 

irrigation with municipal sewage effluent. 

The VIRULO screening model treats the total cumulative mass 

attenuation of viruses probabilistically. It contains a small database 

of input parameters describing soil properties that control rate of 

water percolation (Figure 5) and virus properties that control 

sorption, equilibrium partitioning between suspension and 

adsorption to soil particles, and inactivation rates (Figure 6). As 

shown in Figures 5 and 6, the parameters are grouped by default 

according to USDA soil type and virus of interest. The database 

was built from information in the USDA’s UNSODA database, 

managed by the U.S. Agricultural Research Service, and from an 

extensive literature search of experiments for virus behavior in 

soils. All parameters were assumed to be either normally or lognormally distributed, as determined by examination of histograms 

of experimental outcomes. VIRULO uses the Monte Carlo method 

with computer generated random numbers conditioned on the 

parameters to produce outputs of total time-integrated mass 

attenuation, defined as the average total amount of viable viruses 

leaving the bottom of a soil bed divided by the total amount arriving 

at the top of the soil bed (Figure 7). VIRULO presents the 

outcomes in a histogram of values of minus the base-10 logarithm 

of attenuation (Figure 8). 

The soil survey for Wake County includes listings of the seasonal 

high water table, which depends on soil type as well as geographic 

location (proximity to streams and topography). The soil survey 

data can be used as input to the VIRULO model with the default 

parameters to get a spatially explicit representation of the possible 

level of risk associated with irrigating in a particular park or openspace area, as shown in Figure 9. The information produced is far 

from being definitive because of seasonal variability and soil inhomogeneity, but it can serve as a guide to highlight areas of 

concern. The assumptions used in VIRULO and the conditions 

under which large error may be incurred from employing them are 

listed in Table 7. 

One reason predictive modeling of viral fate and transport is 

especially difficult is the fact that only one or two virus particles can 

infect any human who ingests them. Thus the margin for error is 

very small, and even very low probabilities of virus persistence 

may still represent cause for concern. 

Setback Distances 

Traditionally, state and county regulators have established fixed 

setback distances for all geologic settings in their jurisdictions. 

For example, the distance between a septic tank and a private well 

would, in many instances, be as little as 50 feet and would apply 

for tight clays as well as fractured rock. It would apply to areas 

where the water table was near the surface as well as at 

considerable depth. As discussed in this document, the travel 

time or transport distance of viral particles depends on a number 

of factors including moisture content, geological setting, type and 

depth of the soil overburden, and source loading, only to name a 

few. 

Frequently, guidelines established as minimum distances became 

so standard that a well was often positioned precisely 50 feet from 

the septic tank. In the survey conducted as part of the proposed 

Ground-Water Rule, setback distances were found to be quite 

variable (U.S. EPA, 2000). Some of the distances were presumably 

based on scientific principles, while others were holdovers from 

past practices. 

15

Figure 4. Map of portion of Wake County, North Carolina.


Figure 5. Flow parameters input panel in VIRULO.


16

Figure 6. Virus parameters input panel in VIRULO. 

CW(in) 

C max 

C W (out)

k a 

kS 

time 

L 

Z 

soil 

water table 

air 

water 

  

solid 

time


Figure 7. Conceptual depiction of soil bed in VIRULO. 

17

18

Figure 9. Map of average number of failures per 100,000 Monte Carlo runs based on soil type and seasonal

high water table.

Avg. Number of Failures

per

100,000

runs

0

3

6

43

100

Figure 8. Histogram output panel in VIRULO.
Table 7. Assumptions Made in VIRULO and Possible Consequences


Assumption Potential Sources of Large Error in Assumption 

Soil is homogeneous Macropores, aggregates, not considered, though they may greatly affect hydraulic 

conductivity if present 

Soil does not induce preferential flow Roots, dessication cracks, worm holes and burrows may be present, providing a ready 

conduit for preferential flow to the water table 

Soil and soil water are free of dramatic 

hydrophobic effects 

Organic matter, recent fires can render predicted sorption, mass transfer, completely 

erroneous 

Water percolation is due to gravity only Abrupt rainfalls, floods, or periods of extended dryness can produce hydraulic pressure 

gradients in soils not considered in VIRULO’s hydrau lic conductivity model 

Soil is geochemically homogeneous Microbial activity, decomposition, and mineral weathering can produce conditions that 

affect virus longevity 

Microb ial predation not considered Protozoans present in near-surface can prey on viruses 

Temperatu re not considered Temperature can influence viral survivability and longevity 

Irrigation water contains no surfactants Municipal effluent that has undergone little treatment may contain surfactants, such as 

detergent residues, that can affect both viral sorption and hydraulic conductivity 

One approach in determining setback distances for septic tanks 

in wellhead protection areas and bank filtration sites is to determine 

travel times using ground-water flow characteristics (Yates and 

Yates, 1987). This approach has been implemented in the 

Federal Republic of Germany, for example, where three concentric 

zones protect each drinking-water well. The zone immediately 

surrounding the well is faced with the most restrictive regulations 

which are founded on the belief that a 50-day residence time is 

adequate for inactivation of any pathogen present in contaminated 

water (Dizer et al., 1984). However, a comprehensive study by 

Matthess et al. (1988) involving the evaluation of “50 days zone” 

concluded that the reduction of viruses by 7 log units (current 

regulations) requires a much longer residence time. Matthess et 

al., indicated that a reduction of 7 log units occurred in “about 

270 days (Haltern and Segeberger Forest) in one study, and 

about 160-170 days (Dornach)” would be required according to 

another study. 

Another approach to this important issue is to consider the 

vulnerability to virus transport in the subsurface for portions of a 

state or county or for individual aquifers. Although there are a 

number of approaches to rank vulnerability, DRASTIC is one 

assessment methodology that utilizes hydrogeologic setting 

descriptions and a numerical ranking system to evaluate the 

ground-water pollution potential (Aller et. al., 1987). DRASTIC 

assumes that a potential contaminant will be introduced at the 

ground surface, have the mobility of water and be flushed toward 

the aquifer by infiltration. Utilizing existing information at variable 

scales, the methodology was designed to evaluate areas of 

100 acres or larger. 

DRASTIC is an acronym representing seven reasonably-available 

factors that are used to develop a numerical score. They are: 

Depth to water, net Recharge, Aquifer media, Soil, Topography 

(slope), Impact of the vadose zone media, and hydraulic 

Conductivity of the aquifer. DRASTIC uses a weighting system 

to create a relative pollution potential index that varies between 

65 and 223 with the higher numbers expressing greater 

vulnerability. 

Although DRASTIC was not designed specifically to evaluate the 

movement of viruses in the subsurface, the major transport 

mechanisms and flow paths for viral transport are considered and 

the flexibility of the systems’ rating scheme allows many of these 

factors to be taken into account. For example, Depth to Water 

addresses saturated versus unsaturated flow conditions and their 

importance. Aquifer Media, Soil, and Impact of the Vadose Zone 

Media all are based on descriptive soil and rock terms that allow 

for variation due to fracturing, grain size, attenuation mechanisms 

and overall characteristics that affect flow. Topography addresses 

the tendency of viruses to be introduced into the subsurface or to 

be carried away by runoff. Hydraulic Conductivity addresses the 

relative ease of a contaminant to move with the velocity of water 

through the aquifer. 

Clearly, meaningful setback distances can only be developed by 

using scientific principles that allow for the use of available 

knowledge. The establishment of setback distances from sources 

of viral contamination to points of extraction (wells) can be 

established using DRASTIC if both the hydrogeologic setting and 

sensitivity rankings are considered. For example, high pollution 

potential indices signal the need for greater setback distances. 

However, the hydrogeologic factors that control viral movement 

must be evaluated within this context in order to establish 

reasonable numbers for setback distances. A matrix that 

incorporates the important DRASTIC factors can be utilized to 

establish setback distances that include the vulnerability concept. 

Setback distances must incorporate the knowledge of saturated 

flow, transport pathway length, transport velocities, media 

interaction and potential attenuation mechanisms. These setback 

distances can be used on a regional scale, but can be modified if 

site-specific information is available. The beauty of DRASTIC is 

that its rationale and sensitivity factors are easily displayed, so 

that modification can be readily made. 

Summary 

Existing legislation addresses the protection of ground-water 

sources of drinking water with respect to pathogenic 

19

microorganisms. It is a particularly salient issue since about half 

of the drinking water supplies in this country are obtained from 

aquifers, and between 1989 and 1990 in the United States 13 of 

26 drinking water outbreaks were attributed to contaminated 

ground water with viruses being the main etiologic agents (CDC, 

1991). Therefore, the transmission and survival of human 

pathogens, particularly human enteric viruses, through the soil to 

underground sources of drinking water are a serious risk to public 

health. Among the diverse sources of ground-water contamination, 

septic tank effluents, sludge disposal, and the application of waste 

water to the land are most pervasive. 

The transport of pathogens in the subsurface depends on their 

retention to soil and aquifer materials and their survival. Some of 

the more important factors affecting virus transport include soil 

water content, temperature, sorption and desorption, pH, salt 

content, type of virus, and hydraulic stresses. There are indications 

that the inactivation rate of viruses is the single most important 

factor governing virus transport and fate in the subsurface. 

There continues to be considerable controversy over the use of 

appropriate indicators for sanitary and biological states in 

environmental investigations. Since adsorption and inactivation 

are strongly virus dependent, it is important to realize that there is 

no single virus for which its transport characteristics can be used 

as a model to adequately describe the transport of all enteroviruses. 

One solution may be to use a cocktail of viruses with a range of soil 

passage characteristics. The selection of indicators is often 

influenced by the cost and time required for analyses as well as the 

efficiency of the assay method selected. 

The concentration and loading of viruses and the hydrogeologic 

setting through which they move are major factors influencing 

their transport. Included among the most important hydrogeological 

factors that can be used to evaluate viral transport are the flux of 

moisture in the unsaturated and saturated zones, the media 

through which the particles travel, length of flow path, and time of 

travel. One tool which can be used to evaluate virus exposure is 

the application of predictive virus transport models (Hurst, 1997). 

Like most predictive modeling, the results depend on the conceptual 

basis of the model as well as the quality and availability of input 

data. Clearly, the success of predictive modeling depends on a 

thorough understanding of the processes and parameters involved 

in viral transport. 

Notice 

The U.S. Environmental Protection Agency through its Office of 

Research and Development partially funded and collaborated in 

the research described here under Contract No. 68-C4-0031 to 

Dynamac Corporation. It has been subjected to the Agency’s peer 

and administrative review and has been approved for publication 

as an EPA document. Mention of trade names or commercial 

products does not constitute endorsement or recommendation for 

use. 

Quality Assurance Statement 

All research projects making conclusions or recommendations 

based on environmental data and funded by the U.S. Environmental 

Protection Agency are required to participate in the Agency 

Quality Assurance Program. This project did not involve the 

collection or use of environmental data and, as such, did not 

require a Quality Assurance Project Plan. 

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