EPA/600/R-15/137 | September 2015 | www.epa.gov/research
United States
Environmental Protection
Agency
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Notice/Disclaimer Statement
The research was supported in part by an appointment to the Research Participation Program for the
U.S. Environmental Protection Agency, Office of Research and Development, administered by the Oak
Ridge Institute for Science and Education through an interagency agreement between the U.S.
Department of Energy and EPA. It has been subjected to the Agency's review and has been approved
for publication. Note that approval does not signify that the contents necessarily reflect the views of the
Agency. Mention of trade names, products, or services does not convey official EPA approval,
endorsement, or recommendation. This report was generated using references (secondary data) that
could not be evaluated for accuracy, precision, representativeness, completeness, or comparability;
therefore no assurance can be made that the data extracted from these publications meet EPA's stringent
quality assurance requirements.
Questions concerning this document or its application should be addressed to:
M. Worth Calfee, Ph.D.
Decontamination and Consequence Management Division
National Homeland Security Research Center
U.S. Environmental Protection Agency (MD-E343-06)
Office of Research and Development
109 T.W. Alexander Drive
Research Triangle Park, NC 27711
Phone: 919-541-7600
Fax: 919-541-0496
E-mail: Calfee.Worth@epa.gov
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Acknowledgements
The U.S. Environmental Protection Agency would like to acknowledge the government employees and
others who worked together to develop this report. The following key contributors and reviewers
provided significant input:
• M. Worth Calfee, U. S. Environmental Protection Agency
• Robert Janke, U. S. Environmental Protection Agency
• Sang Don Lee, U. S. Environmental Protection Agency
• Matthew Magnuson, U. S. Environmental Protection Agency
• Shawn Ryan, U. S. Environmental Protection Agency
• Jeff Szabo, U. S. Environmental Protection Agency
• Jenia Tufts, Oak Ridge Institute of Science and Education
The authors also acknowledge Leroy Mickelsen, Eric Rhodes, and Jacky Rosati Rowe for their
comprehensive review of this report.
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Table of Contents
1.0 Introduction 1
1.1 Objectives 3
1.2 Methods 4
2.0 Factors Affecting Contaminant Fate and Transport in Urban Areas 5
2.1 Urban Persistence and Migration 5
2.2 Urban Particulate Transport Mechanisms 5
2.3 Urban Storm Water Runoff 6
2.4 Urban Storm Water Modeling 7
2.4.1 Models 8
2.4.2 Particle Size Distribution (PSD) 8
2.5 Runoff and Sediment Transport 9
2.6 Overland Pollutant Transport 11
2.6.1 Storm W ater Aggregates 11
2.6.2 Particle-Bound Metals 12
2.6.3 Microbial Aggregation and Transport 13
3.0 Conclusions and Research Needs 15
3.1 Research Questions 16
4.0 References 17
5.0 Search Terms 22
Table of Figures
Figure 1. Scope of report 3
Figure 2. Forces involved with sediment detachment, adapted from Southard, 2006 10
Figure 3. Comparison of sediment particle sizes and a Bacillus spore 11
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Acronyms and Abbreviations
Ba Bacillus anthracis
Bti Bacillus thuringiensis israelensis
Btk Bacillus thuringiensis kurstaki
CBR Chemical, Biological, and/or Radiological
Cp Clostridium perjringens
DHS Department of Homeland Security
EPA U.S. Environmental Protection Agency
FEMA Federal Emergency Management Agency
HSRP Homeland Security Research Program
Ls Lysinibacillus sphaericus
PSD Particle Size Distribution
TOC Total organic carbon
USD A U.S. Department of Agriculture
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Executive Summary
This report supports EPA's mission to address critical needs related to homeland security, which
includes decontamination following a chemical, biological, and/or radiological (CBR) attack or release.
The release of a biological agent, such as Bacillus anthracis (Ba) spores, in an urban area could create
large areas of complex contamination. Because the many transport pathways in an urban environment
create a broad area of study, this report narrowly focuses on the potential for spore transport from urban
surfaces during and following precipitation events.
The main process affecting conservative pollutants (pollutants with no chemical formation or loss) in
water, like spores, is adsorption onto a solid. In storm water, these aggregates are then transported with
the sediment particles in the water. Based upon the literature, most runoff sediment is in the fine particle
size range (i.e. <2.5 jam), increasing the likelihood of spore-sediment interaction.
It is possible that many deposited spores will be removed from urban surfaces during the early phase of
a precipitation event by what is known as the "First Flush phenomena," however this needs further
study. Many research questions need to be addressed in order to inform site characterization and
sampling strategies following an urban release.
As a result of this literature review, the following research questions have been identified that will
inform site characterization and sampling strategies following an urban release of Ba spores:
• What is the potential for spore entrainment in storm water?
• What is the potential for spores to migrate with fine grain sediments in storm water?
• What is the aggregation rate of spores with fine sediments?
• What is the aggregation rate of spores with fine sediments in rapidly moving water (i.e. with
shorter residence times)?
• Does the spore-particle association behavior seen in runoff hold true for overland flow?
• What are the most effective methods to collect particle-bound spores in storm water samples?
• What is the most effective method to collect particle-bound spores directly from active overland
flows?
• Do organic and inorganic aggregates containing spores require different decontamination
approaches?
• Can precipitation event intensity, volume and duration, be parameterized to model the magnitude
of the initial wash off (First Flush) of spores from urban surfaces?
• What are the key parameters needed to predict the extent of spore accumulation and wash off
from snow? Are these parameters different for liquid precipitation?
• What is the potential for spore retention in vegetation buffers?
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1.0 Introduction
This report supports EPA's mission to address critical needs related to homeland security, which
includes decontamination following a chemical, biological, and/or radiological (CBR) attack. Part of
EPA's Homeland Security Research Program (HSRP) mission is to conduct threat and consequence
assessment research and deliver products that improve the ability of EPA to assist decision makers in the
preparation for and recovery from public health and environmental emergencies resulting from terrorist
threats and incidents. One specific focus area of HSRP research is on decontamination methods and
technologies that can be used in the recovery efforts resulting from a CBR contamination incident. In
recovering from an incident and decontaminating an area, it is essential to identify and implement
appropriate decontamination technologies.
The release of a biological agent, such as Bacillus anthracis (Ba) spores, in an urban area could create
large areas of complex contamination including building exteriors, streets and sidewalks, vegetation, and
other open spaces (DHS-EPA 2009). The response to a large-scale urban biological release would pose
considerable challenges due to these varied and complex surfaces, but perhaps a greater challenge would
be how to target environmental sampling to characterize the extent and magnitude of the contamination
given the likely dynamic nature of the spatial distribution of the contaminant over time. Understanding
contaminant fate and transport in an urban environment is vital in "predicting" contaminant
concentrations spatially and temporally, which can inform sampling and mitigation plans. This
information can facilitate effective sampling and decontamination technology selection and deployment,
to best achieve a reduction in contaminant levels such that the risk to public health is minimized.
Following a large-scale urban contamination incident, the potential for spores to remain unmitigated,
and therefore a potential public health threat, for weeks to months is high. Even if the release is detected
immediately, site sampling and characterization will not begin until after the first phase of the response
is complete (DHS-EPA 2009, FEMA 2011), a stage during which a characterization strategy is
developed (DHS 2012). Following initial site characterization to support law enforcement and public
health, further characterization supporting environmental remediation will follow. Environmental
sampling will occur to determine the magnitude and extent of contamination, as well as an assessment of
the potential for reaerosolization and contamination relocation from both natural and anthropogenic
processes (DHS 2012).
Site characterization may involve the sampling and analysis of a complex range of surfaces and
matrices, and both the planning and execution of this effort is anticipated to be time consuming. During
this time, the spatial distribution of the contamination may evolve and change. According to current
guidance, formal written plans need to be in place before sampling can begin. Even once a sampling
strategy has been agreed upon and executed, sample analyses, data reduction, and generation of a
comprehensive site characterization map will take significant time. During this time, and later while
mitigation options are being explored and formally planned based upon initial sampling results,
contaminants may migrate to other areas, changing the margins of high concentration zones, potentially
contaminating even previously uncontaminated areas. Because "consequence management for wide
area disasters can escalate quickly into a substantially unmanageable problem" (DHS 2012), it is critical
that the mechanisms behind the relocation of contamination from one area to another in an urban
environment be more completely understood.
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While guidance on indoor biological sampling and decontamination exists (EPA-CDC 2012), guidance
on strategies for outdoor sampling and remediation/mitigation following a release of Ba spores is
lacking (Pottage, Goode et al. 2014). Previous reviews of decontamination methods and strategies have
all pointed to the difficulty (next to impossible) of decontaminating a large outdoor area (AFRL 1999),
and the dearth of tested decontamination strategies for wide-area use (Campbell, Kirvel et al. 2012).
While some authors suggest that many indoor decontamination and sampling strategies would be useful
in the outdoor environment (Krauter, Edwards et al. 2011, Campbell, Kirvel et al. 2012) and some
sporicidal technologies have demonstrated efficacy on outdoor materials in lab-scale tests (EPA 2010,
Calfee, Choi et al. 2011), more research remains to demonstrate how indoor or outdoor strategies can be
made applicable to a wide area event. For example, the variety of surface types, potential for
contamination relocation, and the potentially limitless spatial distribution of contamination poses both
research and operational challenges. For instance, the Census Bureau defines urban areas as densely
developed and populated residential and commercial areas having an abundance of impermeable
surfaces (76FR 2011), some of these impermeable areas can be as much as 17.5% (Nowak and
Greenfield 2012). As such, these areas typically contain concentrated areas of buildings, roads, and
sidewalks, as well as smaller areas of vegetation (76FR 2011). Thus, urban areas do not necessarily
resemble indoor areas, so it is unclear how indoor area decontamination strategies and techniques will be
applicable. Additionally, by definition indoor areas are enclosed whereas outdoor areas are open, which
could impact a decontamination strategy as in the outdoors a decontaminant may disperse more quickly
than it does indoors, potentially reducing contact times.
Although the first step in the remediation of an outdoor area following the detection of a biological
release is to identify the contamination zones (Campbell, Kirvel et al. 2012), tracking the spread of
contamination in such an environment could be difficult and may complicate or delay mitigation and
remediation efforts. Because no one can predict the location, extent or magnitude of a contamination
incident, and all urban areas have unique features, preplanning a sampling and decontamination
response for an outdoor incident is nearly impossible. Current response guidance focuses on casualty
decontamination (Lake, Divarco et al. 2013), response operational and organizational structures (JP
3-41 2012), and developing mitigation strategies. These mitigation strategies focus on areas of high
contamination, followed by areas of lower contamination using unspecified physical methods (chosen
following a cost/benefit and risk/benefit analysis), or rely on natural attenuation (Raber, Kirvel et al.
2011).
Outdoor decontamination approaches are not prescriptive, rather they are incident specific, based upon
many inputs, and the methods are those which have demonstrated efficacy (DHS-EPA 2009; DHS
2012). Much of the current mitigation response strategy focuses on the identification and
characterization of the contaminant, site characterization to determine contaminant distribution, and an
assessment of the threat of human exposure (DHS-EPA 2009). As part of the site characterization,
contaminant fate and transport can be estimated using mathematical modeling (DHS-EPA 2009),
although transport properties will vary based upon spore preparation, potentially making accurate
transport estimates problematic (DHS 2012). In spite of this potential limitation, site specific transport
estimates will be an important facet in the approach to guiding response planning and optimizing the
deployment of resources in order to reduce the overall cost and length required to return to normalcy.
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1.1 Objectives
This report analyzes existing scientific knowledge concerning the fate and transport of contaminants in
the outdoor urban environment. More specifically, it discusses the major factors that determine
contaminant redistribution, and how those factors are used to predict contaminant movement over time
when subject to typical environmental and anthropogenic forces. This information may be used to guide
sampling and mitigation efforts, and to identify knowledge gaps in need of further scientific study. The
scope of this report is limited to the redistribution of contaminants in an urban environment by rain and
storm water, as outlined by Figure 1.
Bounds of this Study
Wash
Down
Overland
Flow
Retention Basin
Storm Water Co ection
waste
Water
Treatment
River
Underground
Sewer Systems
Figure 1. Scope of report.
Formulating mitigation strategies to a biological release in an urban setting requires a thorough
understanding of how such contaminants spread through this unique environment. Sampling and
mitigation plans should account for contaminant fate and transport in order to be comprehensive. In the
absence of existing fate and transport data for Ba spores in the days, weeks and months post-release in a
large urban area, the purpose of this report is to formulate hypotheses regarding the fate and transport of
Ba spores in an outdoor urban environment based upon the reported behavior of similarly sized particles
and other contaminants, and to identify research gaps that can more precisely address these questions.
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1.2 Methods
Information about contaminant fate and transport in an urban environment for this literature review was
considered from unclassified peer reviewed journal articles, conference proceedings and textbooks found by
searching citation databases including the EPA Desktop Library, PubMed, and Google Scholar.
Additionally, published technical and guidance reports, and books were included. Articles referenced
include original research and literature reviews. Reports referenced include research, current state and local
response guidance documents, and government or military response guidance documents. Books referenced
were published within the last five years or are the most recent edition of an established source. Search
terms are included in Section 5 of this report. The search was limited to articles published in the English
language, but there was no restriction on geographic location.
This report was generated using references (secondary data) that could not be evaluated for accuracy,
precision, representativeness, completeness, or comparability and therefore no assurance can be made that
the data extracted from these publications meet the U.S. Environmental Protection Agency (EPA) quality
assurance requirements. However, the sources of secondary data were limited to peer-reviewed documents.
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2.0 Factors Affecting Contaminant Fate and
Transport in Urban Areas
2.1 Urban Persistence and Migration
Field studies examining the fate and transport (via air, water and surface mechanisms) of Ba spores
following an urban release are not publically available. For this reason, preplanning sampling and
mitigation efforts following such an event is difficult because the quality of the preplanning is hampered
by a lack of data. There are data in the literature on the transport of some radionuclides following
nuclear release incidents (Chernobyl) that can be used to inform potential urban fate and transport, but
this would only be applicable to an atmospheric release (because such nuclear accidents often result in
emission of contaminants high into the atmosphere), not to a low/street level release. One long-term
field study on urban persistence of Bacillus thuringiensis kurstaki (Btk) spores, a Ba surrogate, over a 4-
year period showed persistence in soil throughout the study. The surrogate remained detectable to a
lesser extent on surfaces (up to 48 weeks), vegetation (grass up to six weeks and leaves up to 24 weeks),
and in water (up to 48 weeks), but these results were not consistent across all sampling sites. The authors
concluded that in urban environments, some Bacillus species will remain detectable for many years
(Van Cuyk, Deshpande et al. 2011).
Another study tracked spore transport in an urban area following the release of less than 50 g B.
amyloliquefaciens (Group VI Bacillus species) in the 0-10 |im diameter range mixed with other
pesticides as dry powder via a blower from a truck on a road. Seventy two hours after release, the
surrogate was not detected at the release site, but was detected by air samplers outdoors about 2 miles
away, which authors attribute to reaerosolization, which is particle detachment from a surface following
settling. Surface samples at this remote location indicate a surface loading of 102/m2 CFU/wipe, while
surface samples collected at the release site were negative. Air samplers also detected the surrogate in
underground subway stations near the release site and several miles away, although the transport
mechanism was not reported (Garza, Van Cuyk et al. 2014).
2.2 Urban Particulate Transport Mechanisms
Particle transport by any mechanism is primarily a function of particle diameter (Hinds 1999), but many
other variables can impact the fate and transport of particles in an urban environment. Potentially
confounding our understanding of particle transport in an urban environment are some of the unique
environmental features including buildings, numerous impervious surfaces, thermal convection from
paved surfaces, the presence of street canyons (advection from canyons), and microenvironments, to
name a few.
Following a release and initial deposition, other factors potentially impacting transport and confounding
consequence management include surface water runoff to drainage areas/ponds; the formation of
concentrated spore pools; weathering of spore (or other) materials; resuspension (causing redistribution)
from road and foot traffic, wind, and sampling (Van Cuyk, Veal et al. 2011), causing a potentially ever-
changing contamination zone. Spores may also be transported on clothing during sampling. In a large
scale study tracking the movement of Btk following spraying of a slurry in an urban area, the authors
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found contamination in sampler personnel vehicles and hotel rooms, and evidence of reaerosolization in
the field throughout the study (Van Cuyk, Veal et al. 2011).
Because the many transport pathways create a broad area of study, this report narrowly focuses on the
potential for spore transport from above ground urban surfaces (Figure 1) during and following
precipitation events. This transport mechanism is likely to be a major factor in spore relocation in an
urban environment, and covering all potential mechanisms would not be possible in the time frame of
this study. Other transport mechanisms not addressed in this report will be covered in future studies.
2.3 Urban Storm Water Runoff
Urban areas are characterized by a high degree of impervious surfaces that decrease water infiltration,
increase peak flows, and the total volume of water runoff. These impervious areas increase runoff
velocities and decrease travel time, transporting runoff more rapidly (USD A 1986). Some factors
impacting runoff travel time include surface roughness, channel and slope flow, and flow over plane
surfaces (USDA 1986). One study showed that runoff volumes could be about 14.5% greater from
urban areas as compared to forested areas (Corbett, Wahl et al. 1997). The highest sediment loads in
storm water are typically seen when discontinuous impervious areas comprise about 35% of the
watershed (Corbett, Wahl et al. 1997). The impact of rain intensity on runoff is important for slow
moving storms, however sensitivity of runoff to storm patterns decrease at high storm speeds (De
Lima and Singh 2002).
It is possible that many deposited spores will be removed from urban surfaces with other pollutants
during the early phase of a precipitation event when high pollutant concentrations are seen in runoff
waters, a phenomenon known as "first flush" (Sansalone and Buchberger 1997; Zoppou 2001). First
flush is related to both the buildup and wash off of pollutants (Zoppou 2001) and applies to both
overland and sewer processes (Zoppou 2001). First flush has been attributed to increased rain intensity
at the start of a storm (Zoppou 2001) and correlated to overall storm intensity (Sansalone, Koran et al.
1998). During high intensity precipitation events, short residence times are seen between rainfall runoff
and "dry deposited" particulate matter (Sansalone and Buchberger 1997; Sansalone, Koran et al. 1998),
resulting in particle instability and a lack of equilibrium between particle aggregation and disaggregation
processes (Blazier 2003).
First flush has been defined as the normalized cumulative mass load m(t) divided by the normalized
cumulative runoff volume v(t) over a given time interval:
Jo c(t)q(t)dt
—^ > 1.0
J0 q(t)dt
V
where c(t) is the incremental (i.e. at a specific time point) concentration and q(t) is the incremental
flow rate, Mis the constituent mass and V i s the total runoff volume. (Sansalone, Hird et al. 2005).
Some research has found that the first flush is likely limited to small quantities of fine particles
(Svensson 1987). Other research has shown that about 40 percent of particles were discharged in the
first 20 percent of runoff volume (Li, Lau et al. 2005) with increased concentrations of settleable
particles and microbes at the beginning of a storm (Krometis, Characklis et al. 2007). In one study, high
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intensity precipitation events associated with high runoff volumes were found to exhibit sustained
flushes discharging about 80 percent of the particle number density (PND) in the first 60 percent of the
runoff volume, while low intensity events resulted in a somewhat restricted flush, where 80 percent of
the PND was not flushed until the storm had nearly concluded (Cristina and Sansalone 2003). This is
consistent with previous work demonstrating weak first flush behavior for low flow precipitation events,
although particles between 2-8 |im rapidly washed off during each rain event studied (Sansalone, Koran
etal. 1998).
Highway construction has been shown to result in up to a six-fold increase in total suspended solids
during first flush in storm water from sites during construction, increasing the percent particles in the
clay fraction (<75 jam) from 76% to 96% in the drainage ditch adjacent to the site (Cleveland and
Fashokun 2006). Other variables impacting particle transport across roadways include both the runoff
rate and duration, and the traffic intensity, and that particles in the 2-8 |im range can be quickly washed
off of pavement during the intense rain events (Sansalone, Koran et al. 1998). Another study found that
only events with an average volumetric flow rate of about 1 Lm"1 of drainage width exhibited a rapid
removal of all particulate matter (Cristina and Sansalone 2003). Another found that while first flush
may occur on sloped, small watersheds, site-specific data should be obtained before it is assumed that
the "entire runoff volume" does not require treatment (Sansalone and Cristina 2004).
The results of these studies imply that many deposited spores may be removed from urban surfaces with
other pollutants during the early phase of a precipitation event. However the magnitude of the removal
may be dependent upon precipitation event intensity and duration. Detailed information about storm
events, including volume and intensity, in the days and weeks following an urban release may aid
sampling and mitigation planning and response activities. Additionally, laboratory studies should be
conducted to identify the key parameters needed to predict the extent of spore wash off from urban
surfaces during precipitation events.
2.4 Urban Storm Water Modeling
Urban storm water management is generally focused on water flow management to protect public
health, property, and to decrease pollution loads in rivers and streams. The hydrological regime of a
catchment area drives water quality (Merritt, Letcher et al. 2003), and, as a result, storm water models
tend to focus on water volumes and the spatial/temporal distribution of precipitation (Zoppou 2001).
Storm water quality models generally focus on sediment load and concentration (Obropta and Kardos
2007), however these models contain a lot of uncertainty in parameter values and model outcomes
stemming from a lack of understanding about the interactions between contaminants and sediments
(Merritt, Letcher et al. 2003). Buildup and wash off models are also frequently used to simulate
overland runoff quality. However, more research is needed to decrease model uncertainties (Obropta
and Kardos 2007) by enhancing our understanding of the physicochemical processes governing how
specific pollutant particles adsorb to sediment particles (Zoppou 2001). Current storm water models
tend to predict water quantity better than quality (Obropta and Kardos 2007; Merritt, Letcher et al.
2003), and uncertainty in water quality predictions is greater than for water quantity predictions (Merritt,
Letcher et al. 2003). Since water quantity models are better understood, they may be better predictors of
spore relocation than current water quality models.
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2.4.1 Models
While there are numerous urban storm water models available in the literature, as outlined by Zoppou et
al (2001), most are deterministic-distributed models that focus on water volume and the spatial-temporal
distribution of precipitation (Zoppou 2001). For these models, precipitation volumes are considered to
be uniformly distributed (Zoppou 2001). The two fundamental elements of urban storm water modeling
are rainfall/runoff modeling, including the not well understood buildup and wash off of pollutants from
impervious surfaces (Zoppou 2001; Chen et al, 2006), and transport modeling of flows and pollutants
through sewer systems (Zoppou 2001).
The buildup equation presented by Zoppou et al (2001) is:
dPB
~dF = ' _
where Pb(t) is the mass of surface pollutant buildup at time i, I is mass of pollutant accumulation
between storms and ka is the pollutant buildup coefficient (Zoppou 2001).
The wash off equation presented by Zoppou et al (2001) assumes the wash off rate is proportional to the
surface concentration of pollutant and is:
dPyv
= ~kwrPw(t)
where Pwit) is the pollutant mass at time i, kw is the coefficient of pollutant removal, and r is the runoff
flow rate (Zoppou 2001). For both equations, the coefficients are empirically derived (Zoppou 2001).
Both equations are similar to those presented by Chen et al, 2006, and are representative of those
typically found in the literature (Zoppou 2001; Chen and Adams 2006). The models could be evaluated
experimentally in laboratory tests using spores, the results of which could help inform sampling and
other consequence management decisions.
Current models that rely on storm water quantity are not as good at predicting storm water quality
(Obropta and Kardos 2007). Due to the complexity of the many required input parameters needed to
accurately model storm water quality, most models do not produce reliable predictions (Obropta and
Kardos 2007), although the magnitude of the uncertainty has not been quantified. One significant
parameter not typically included in storm water quality models is the particle size distribution (PSD)
(Obropta and Kardos 2007).
2.4.2 Particle Size Distribution (PSD)
Sediment particle size is important because smaller sediment particles adsorb a higher percent of
pollutants per unit mass (Obropta and Kardos 2007). Several studies have shown that urban runoff
contains a large fraction of fine particles having a high surface to volume ratio, increasing adsorption of
pollutants into particle pores and on surfaces (Characklis and Wiesner 1997; Sansalone, Koran et al.
1998; Roger, Montrejaud-Vignoles et al. 1998). Given the size (~1 |im) and density (~1 g/cm3) of a
single Ba spore, it seems likely they will behave similarly to other pollutants by adhering to clay or silt
and being transported along with these particles in runoff.
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Knowledge of PSD in runoff is key to understanding the fate and transport of particulate contaminants
and how those contaminants partition in the different size fractions (Kim and Sansalone 2008). Wide
particle size ranges are seen in runoff during different precipitation events depending upon the event
intensity and duration as well as spatial variations during the same event (Kim and Sansalone 2008).
One study showed greater than 90 percent of the sediment collected from highway runoff to be less than
10 |im (Li, Lau et al. 2006), the mass of which made up less than 10% of total mass collected (Li, Lau et
al. 2005). A one year study of runoff from an urban French highway with about 30,000 vehicles per day
showed that ninety percent of the solid matter by weight was less than 100 |im, and of those less than 50
|im, fifty six percent were clays (Roger, Montrejaud-Vignoles et al. 1998). Another study found about
fifty v/v percent particles with diameter less than 15.2 |im (Andral, Roger et al. 1999) in highway
runoff. A fourth study reported 25-80% on mass basis of fine particulate matter in urban runoff was less
than 75|im (suspended settleable fraction) (Kim and Sansalone 2008). Collectively, it has been found
that much of the road particulate matter can be attributed to tire abrasion of the road surface (Sansalone
and Tribouillard 1999). Those abraded road particles which are less than 10 |im can remain entrained in
storm water and thus are very difficult to remove (Sansalone and Tribouillard 1999).
Given that PSD varies both spatially and with event characteristics (Kim and Sansalone 2008), having
an understanding of PSD for a particular event and location may prove difficult. Additionally, there is
currently no available research in the literature that identifies a correlation between pathogenic
organisms and PSD (DeGroot and Weiss 2008). This gap, however, may be due to sampling issues,
since it is difficult to accurately sample particles and associated contamination in storm water because
they are stratified in the water (buoyant, suspended, settled or settling) (DeGroot and Weiss 2008).
Another study reported particle aggregation in samples occurring in less than six hours, with the fine
particle concentration decreasing and an increase of larger particles in stored samples (Li, Lau et al.
2005).
Based upon the literature, most runoff sediment is in the fine particle size range, increasing the
likelihood of spore-sediment interaction. This suggests contaminants entrained in storm water,
including spores, will likely migrate where fine grain sediments go. For this reason, these areas may be
ideal locations for sampling and mitigation activities following a release. This hypothesis should be
tested in laboratory studies for confirmation.
2.5 Runoff and Sediment Transport
Surface runoff, or overland flow, occurs when precipitation volumes surpass infiltration and depression
storage capacities (Merritt, Letcher et al. 2003). During precipitation events, sediments are suspended
by two types of processes: rainfall splash detachment, and entrainment via overland flow shear stress
(Svensson 1987; Merritt, Letcher et al. 2003). When shear stress is greater than the cohesive strength,
sediment detachment occurs (Merritt, Letcher et al. 2003). (Figure 2 provides an overview of the forces
related to sediment detachment.) The sediment transport rate is governed by flow conditions and
particle properties, with smaller particles "always" transported, and the water transport capacity
dependent upon the particle size distribution (Svensson 1987).
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Flow
~
W=pV
Figure 2. Forces involved with sediment detachment, adapted from Southard, 2006 (Southard 2006).
In Figure 2, Ff is the fluid force, Fc is the contact force, Fl is the lift force, Fd is the drag force, W is the
weight of the particle, is the viscous sheer stress, and + and - are the high and low pressures,
respectively.
In surface runoff, contaminants and sediments are dislodged and transported in four phases: suspended
solids or sediment; dissolved or fine particulate; near-bed layer; and bed-load layer (Obropta and Kardos
2007). Sediment transport generally occurs in the suspended load or bed load, depending on the
properties of the sediment particle and the storm water. The suspended sediment load is primarily silt
and sand moving through the water column, remaining suspended when the upwards velocity is
approximately equal to the settling velocity (Hickin 2009). When the upwards velocity is less than the
settling velocity, the particle will move as bed load by rolling, sliding, and saltating (briefly carried by
the fluid before settling). If the upwards velocity is greater than the settling velocity, the particle will be
transported near the surface in the wash load (Hickin 2009).
Bacillus cmthracis spores have a diameter of approximately 1 |im (Carrera, Zandomeni et al. 2007), clay
particles are < 2 |im; silt ranges from 2-50 |im; and sand is in the range of 50 - 2000 |im (USDA
2015). Figure 3 provides a visual comparison of these particle sizes for perspective. On a mass basis,
the surface area of clay particles is typically several orders of magnitude higher than silt particles, and
almost six orders of magnitude more than coarse sand particles (USDA 2015). Sediments with high
surface to mass ratios adsorb and transport particulate and other substances in water systems (Lick
2008). Conservative substances can be adsorbed to sediments and transported with these solids by
advection, the dominant pollutant transport mechanism in runoff (Zoppou 2001). Fine silt is very easily
suspended, which results in more evenly distributed silt in the water column than coarser materials that
tend to settle out faster (Hickin 2009). The wash load is the part of suspended sediment in the clay range
(i.e. < 2 |im), which is finer than silt and remains suspended in the water without the force of turbulence;
it is kept uniformly distributed in suspension via Brownian motion (Hickin 2009). It is possible that
individual Ba spores may adsorb to silt and clay particles forming aggregates, as suggested by some
literature discussed below, but this is not known.
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Bacillus spore: ~ 1 (j.m
Clay particle: < 2
Sand grain: 50 - 2000 |im
Figure 3. Comparison of sediment particle sizes and a Bacillus spore.
2.6 Overland Pollutant Transport
Runoff is available for overland flow, depression storage (low points that store precipitation), and
infiltration in to the soil (Zoppou 2001). Because most urban surfaces are impervious, most models do
not account for subsurface water in the unsaturated zone (Zoppou 2001). Conservative pollutants, like
spores, are inert and not chemically changed during transport (Zoppou 2001). The main process
affecting conservative pollutant transport is adsorption/adhesion onto a solid (typically a sediment)
(Zoppou 2001). These aggregates are then transported by advection (diffusion not believed to be
significant) with the sediment particles in the water (Zoppou 2001). For optimum model accuracy, both
adsorption and advection should be considered, however most models lack these parameters (Zoppou
2001). Adding these parameters to existing storm water models would allow the prediction of spore
transport in a city of interest. The magnitude of pollutant adsorption is a function of pollution
concentration, surface area of the sediment (Sansalone, Koran et al. 1998), and temperature (Zoppou
2001). It is possible that microorganisms attached to high surface area sediments may be protected from
disinfectants, making them less vulnerable to decontamination (Zoppou 2001).
2.6.1 Storm Water Aggregates
Much of the literature on storm water aggregates pertains to holding ponds and tanks and are not
specific to overland flow, however it is not unreasonable to relate these data to overland flow. Although
the residence time of the particles in storm water moving across impervious surfaces is much different
than that of pooling water, many of the same mechanisms apply.
Storm water aggregates are usually comprised of inorganics, microbes and biota (plant matter). As the
particle aggregates increase in size, their porosity also increases, causing the aggregate density to
decrease to near the density of water (Droppo 2001). Additionally, the organic content of particles plays
a significant role for the aggregate, contributing to particle density, settling velocity and pollutant
binding tendencies (DeGroot and Weiss 2008). A range of street sediment particle densities reported in
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the literature are from 2.2 - 2.8 g/m3 (Zanders 2005). In storm water, microbes may remain as singlets
or become aggregates of microbes and particles, potentially becoming more transportable (Characklis,
Dilts et al. 2005).
Three transport processes that foster particle contact and increase the likelihood of aggregation in storm
water are Brownian motion, fluid shear, and differential settling (Lick 2008). Brownian motion, or
thermal diffusion, is applicable to particles less than 0.1 |im (Lick 2008) and in the context of this report,
is significant in terms of collisions between very fine clay particles with single B.a. spores (about 1 |im).
Increased collisions could increase the probability of agglomeration with other particles. Fluid shear is a
collision mechanism applicable to particles between 0.1 and 50 |im (Lick 2008), and would apply to
collisions between a 1 |im spore and clay and silt particles. Differential settling, which is due to gravity,
is applicable to particles greater than 50 |im (Lick 2008), like sand.
2.6.2 Particle-Bound Metals
While particulate metals are likely not a good surrogate for spores, this review would be incomplete
without at least considering what is widely known in the literature about the fate of these particles in an
urban setting. Metals have shown to be the most prevalent sediment-bound contaminant in roadway
runoff (Sansalone, Buchberger et al. 1996) and the particle-bound metals may be an indicator of the fate
of spores in urban runoff. However due to density and surface chemistry differences, correlations
between metal particulate and spore transport should be viewed critically.
Particle-bound metals from roadways appear to persist in the environment for some time. One study
demonstrated that lead from vehicle exhaust remains in the top 15 cm of soil, bound to organic matter
until it is mechanically redistributed or becomes incorporated in surface runoff (Turer, Maynard et al.
2001). The authors estimate that 40% of deposited lead from leaded fuel exhaust still remains in the
soil, bound to organic matter (Turer, Maynard et al. 2001). Other studies have found that surface runoff
of particle bound metals is a function of rainfall intensity (Sansalone, Buchberger et al. 1996; Sansalone
and Buchberger 1997; De Lima and Singh 2002). Additionally, snow wash-off contained more
particle-bound metals and solid particulate than the rainfall runoff, which was attributed to longer
pollutant accumulation times and the high surface area of the snow banks (De Lima and Singh 2002).
Further studies showed that some metals, Zn and Cu, sorbed to rainfall solids more than in snow, but
lead had a higher sorption rate onto solids for first snow (Sansalone and Buchberger 1997).
Particle-bound metals are mostly found in the less than 125 |im range, and are not easily caught by
roadside vegetation buffers (Zanders 2005). Vegetation buffers are not good at retaining particles less
than 60 |im and poor for 6-30 |im particles (Zanders 2005). And, although their weight was only about
ten percent of the total suspended solids load, another study found particles less than 100 |im accounted
for greater than fifty percent of lead, zinc, and copper pollutants (Furumai, Balmer et al. 2002). For zinc
and copper, the amount of metals sorbed to solids was higher in runoff samples as compared to road
dust, but was about equal for lead (Furumai, Balmer et al. 2002).
While density and surface chemistry differences between spores and particulate metals preclude any
direct comparisons, some of the results outlined above may be useful in identifying laboratory studies on
spore fate in urban environments. For example, studies on the impact of snow on spore to solids
sorption rates, accumulation and wash off would be useful to inform response efforts in a winter release.
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Additionally, studies determining if spores are retained by vegetation buffers would enhance our
understanding of spore transport from road surfaces.
2.6.3 Microbial Aggregation and Transport
When microbes are part of an aggregate, they attach to dense inorganic particles, and are likely to settle
more quickly and survive longer than single microbes, while most planktonic organisms and those
attached to organic particles will remain suspended and transportable (Characklis, Dilts et al. 2005). A
study investigating the persistence of B. sphaericus spores in a pond following mosquito treatment saw
rapid spore settling into the mud, with larger clumps settling within two hours of treatment while fine
particles remained suspended; the spores remained detectable in the mud layer for the duration of the 21
day study (Davidson, Urbina et al. 1984). In another study, the fate of Bacillus thuringiensis israelensis
{Bti) in deionized water naturally seeded with soil dust comprised of 21 percent clay, 59 percent sand
and 20 percent silt and less than 1% organic matter was examined (Ohana, Margalit et al. 1987). The
spores quickly adsorbed to soil particles, with 99.8 percent settling into the mud fraction within 45
minutes (Ohana, Margalit et al. 1987).
The results of these studies suggest that single spore contaminants may quickly become part of
aggregates in urban settings, although research needs to be conducted to determine the aggregation rate
of spores and sediments. Some studies have shown that high surface area suspended sediments with
attached/adsorbed microorganisms may be more difficult to disinfect as there may be areas where the
microbes do not come into contact with the disinfectant (Berman, Rice et al. 1988; Zoppou 2001). This
could be an important consideration for mitigation strategies following a release. There is a research
need to determine if organic and inorganic aggregates containing virulent spores are more difficult to
decontaminate, and if so, what steps can be taken to mitigate this issue following a large-scale urban
release.
Not much is known about microbes and particle association in storm water transport (Krometis, Dillaha
et al. 2009), although a few studies have looked at Clostridiumperfringens (Cp) spores in urban storm
water. They have shown positive partitioning behavior/affinity to associate with settleable particles
(clay, silicates) in urban storm water at rate of greater than 50 percent (Krometis, Characklis et al. 2007;
Characklis, Dilts et al. 2005; Cizek, Characklis et al. 2008) and can persist in sediments for years
(Mueller-Spitz, Stewart et al. 2010). Similar physically to Ba, Cp spores are in the size range of 0.8 -
1 |im (Novak, Juneja et al. 2003), have an exosporium (Hoeniger, Stuart et al. 1968; Henriques and
Moran 2007), and a density of about 1.2-1.3 g/cm3 (Characklis, Dilts et al. 2005).
While the number of settleable particles in storm water increases after a storm water event (Characklis,
Dilts et al. 2005), for one study, the highest concentration of both particle bound and free Cp spores
were seen during the early storm stages (Cizek, Characklis et al. 2008), and for another study
partitioning behavior did not change over the course of the storm (Krometis, Characklis et al. 2007),
although the average settleable fraction of Cp and total suspended solids decreased as the storm
continued (Krometis, Characklis et al. 2007). No significant relationship was seen between microbial
associations with settling particles and particle concentration, total organic carbon (TOC) or temperature
(Cizek, Characklis et al. 2008), or PSD, total suspended solids, and TOC (Characklis, Dilts et al. 2005).
In another study, the authors looked at the long-term persistence of Bti and Lysinibacillus sphaericus
(Ls) after direct application to urban catchment basins (for mosquito control). Two days post treatment,
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most of the applied spores were found in the bottom sludge where they remained at consistent high
concentrations for the 275 day study (Guidi, Lehner et al. 2013). This is consistent with other studies
that show high removal by sedimentation for Cp in suburban detention ponds (Krometis, Characklis et
al. 2007) but does raise questions about spore persistence in sludge and how partitioning may effect fate
and transport and the duration a microbe remains a public health threat (Characklis, Dilts et al. 2005).
While these studies suggest spores will be removed from storm water via their association with fine
particles, research needs to be performed to determine if this holds on urban surfaces during overland
transport. Given the potential short residence times between spores and storm water sediments as they
travel rapidly across surfaces, different partitioning behavior may result.
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3.0Conclusions and Research Needs
The fate of spores following a large-scale urban release is unknown. It is possible that the highest risk to
public health is the primary aerosol during initial release (CDC 2013). While the literature contains
some information about the fate of other contaminants, including clay, silts, microbes, and particle-
bound metals, many questions remain unanswered that need to be addressed in order to inform site
characterization and sampling strategies following such a release. For longer term mitigation strategies,
further research is needed to gain a better understanding of the consequences of spores released in the
urban environment. These consequences include spore transport in air and water over time and the fate
of spores following the application of decontaminants (NSTC 2013).
Based upon the available literature reviewed for this report, there are many research questions to be
addressed regarding the fate and transport of spores in urban runoff following a large scale urban
release. Research suggests most runoff sediment is in the fine particle size range, increasing the
likelihood of spore-sediment interaction. This suggests the potential for spore entrainment in storm
water, where they will likely migrate with fine grain sediments. These areas may be unique in each
urban environment, based upon the catchment features, however these areas may be ideal locations for
sampling and mitigation activities following a release. This hypothesis should be tested in laboratory
studies for confirmation.
The literature also suggests that single spores entrained in storm water may quickly become part of
aggregates, changing their susceptibility to decontaminants. The aggregation rate of spores with
sediments and the resulting impacts on sampling, detection, and decontamination efforts, all of which
are important considerations for mitigation strategies following a release, need to be studied. Data are
needed to determine if organic and inorganic aggregates containing spores are more difficult to
decontaminate, and whether steps can be taken to mitigate this issue following a large-scale urban
release. Sampling studies should also be performed to determine the most appropriate and effective
methods to collect particle-bound contaminants in storm water samples as there are currently no
standardized or approved methods for the collection of these samples (Grant, Rekhi et al. 2003).
Developing methods to collect samples directly from overland flows may better inform wash off model
development.
Given that the magnitude of initial spore removal from urban surfaces may be dependent upon
precipitation event intensity, volume and duration, research identifying any correlation between these
parameters and the strength of the initial wash off are needed. Studies of the dominant parameters in
first flush phenomena will help inform a response to prepare effective mitigation approaches following
the first rain event after wide area release. In addition, parameterized studies will help predict the extent
of spore accumulation and wash off from snow and the potential for spore retention in vegetation
buffers.
Finally, given the likely short residence times between spores and storm water sediments in overland
flow during a precipitation event, partitioning behavior needs to be studied to confirm that particle
association behavior seen in runoff holds true for overland flow. Since many current studies focus on
runoff captured in holding ponds, it is important to determine the aggregation behavior of spores only
briefly exposed to sediments that are then left behind on surfaces following a precipitation event. While
many studies suggest that spores and other contaminants are ultimately removed from storm water
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through their association with fine particles, research needs to be performed to determine if this holds
true on urban surfaces during overland transport and what happens to the spores left behind on surfaces
following a precipitation event in order to inform sampling and mitigation planning following such a
release. For example, having an understanding of whether the spores have attached to other sediments
and are no longer singlets that could potentially be inhaled would be important to know for response
planning.
3.1 Research Questions
As a result of this literature review, the following research questions emerge that are relevant to site
characterization, decontamination, and sampling strategies following an urban release of Ba spores:
• What is the potential for spore entrainment in storm water?
• What is the potential for spores to migrate with fine grain sediments in storm water?
• What is the aggregation rate of spores with fine sediments?
• What is the aggregation rate of spores with fine sediments in rapidly moving water (i.e. with
shorter residence times)?
• Does the spore-particle the association behavior seen in runoff hold true for overland flow?
• What are the most effective methods to collect particle-bound spores in storm water samples?
• What is the most effective method to collect particle-bound spores directly from active overland
flows?
• Do organic and inorganic aggregates containing spores require a different decontamination
approaches?
• Can precipitation event intensity, volume, and duration, be parameterized to model the
magnitude of the initial wash off (First Flush) of spores from urban surfaces?
• What are the key parameters needed to predict the extent of spore accumulation and wash off
from snow? Are these parameters different for precipitation?
• What is the potential for spore retention in vegetation buffers?
• What mitigation measures can be taken in the early stages of an incident (before it rains) that
would enhance the containment of spores from a wide area release once it does rain?
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5.0 Search Terms
allintitle: street dust runoff OR flushing OR migration OR transport OR urban
spread OR migration OR containment "bacillus" "outdoor"
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allintitle: street runoff OR flushing OR migration OR transport
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allintitle: road resuspension
allintitle: fate and transport particle
allintitle: fate and transport urban
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allintitle: urban bacillus
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allintitle: anthracis field OR transport OR resuspension OR fate OR spore OR properties OR water OR
soil OR wind OR rain OR UV OR solar -genome -genetic -plasmid -virulence -vaccine -molecular -mice
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allintitle: aerosol generation
allintitle: aerosol generation rain OR soil OR urban OR raindrop OR wind
Particle, metals, and water quality in runoff from large urban watershed
a non-equilibrium relationship between particle aggregation and disaggregation
A Study of The 2001 Anthrax Terror Attacks and the History of Biological Warfare
Aggregation rate of time-particle size resolved
Aggregation rate of time-particle size resolved runoff
Aggregation rate of time-particle size resolved tss
allintitle: bacillus fate OR transport
allintitle: bacillus metals
allintitle: cytology author:hoeniger
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allintitle: genome sequence
allintitle: metal contamination author:VIGNOLES
allintitle: particle fate transport
allintitle: urban fate transport
allintitle: urban particle fate transport
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c perfringens spore |im
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dlvo theory
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floccular sediment transport urban
genome sequence
hydrodynamic forces generated on a spherical sediment particle during entrainment
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hydrograph
Initial adhesion of Bacillus subtilis on soil mineralsas related to their surface properties
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microbes Assessing microbial pollution of rural surface waters: a review of current watershed scale
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microbial fate and transport
microbial fate and transport Clostridium OR spores
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microbial partitioning
Microbial partitioning stormwater
Microbial partitioning stormwater perfringens
Microbial partitioning to settleable particles in stormwater
motion of body in fluid
overland flow
particle size suspended author: slattery
perfringens "spore density" g/cm3
perfringens author: Yolton
perfringens spore density g/cm3
perfringens spore morphology
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perfringins author: Yol ton
pollution follows stormwater
pollution follows water
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rouse number sediment
rouse number sediment urban
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shear stress "first flush"
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STRUCTURE ASSembly function spore surface layers
The Shields Diagram
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ZETA BACILLUS
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