vvEPA
United States
Environmental Protection
Agency
Office of
Solid Waste and
Emergency Response
Publication 9285.6-17
EPA-540-R-06-072
July 2008
ECO Update/
Ground Water Forum
Issue Paper
Intermittent Bulletin
Evaluating Ground-Water/Surface-Water Transition Zones in
Ecological Risk Assessments
Joint Document of the Ecological Risk Assessment Forum and the
Ground Water Forum
IN THIS BULLETIN
1 Introduction
1.1 Purpose of This Joint ECO Update/Ground
Water Forum Issue Paper 2
1.2 The Ground-Water/Surface-Water Transition
Zone 4
1.2.1 Definition of the Transition Zone 4
1.2.2 Spatial and Temporal Variations of
Transition Zones 4
1.2.3 Ecological Role of the Transition Zone 4
The ECO Update Bulletin series provides technical information and practices to EPA Regions and States on specific components of the
ecological risk assessment process at Superfund sites and RCRA Corrective Action facilities. This document does not substitute for CERCLA,
RCRA, or EPA regulations, nor is it a regulation. Thus, it cannot impose legally binding requirements on EPA, the States, or the regulated
community and may not apply to a particular situation based on the circumstances. The Ecological Risk Assessment Forum and Ground Water
Forum identify and resolve scientific and technical issues related to risk assessments and remediation of Superfund and RCRA sites. The Forums
are supported by and/or advise OSWER's Technical Support Centers and provide state-of-the-science technical assistance to EPA project
managers.
1.3 Ground-Water and Contaminant Discharges in
Transition Zones 6
1.4 Transport and Fate of Contaminated Ground-
Water in Transition Zones 6
2 Framework for Including the Transition Zone in
Ecological Risk Assessments 7
2.1 The Ecological Risk Assessment Process and
the Integrated Team 7
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2.2 Including the Transition Zone in Designing
and Conducting Ecological Risk Assessments 9
2.2.1 Framework for Incorporating the Transition
Zone into Problem Formulation 9
2.2.2 Hydrologic Regime and Contaminant
Fate and Transport Considerations during Problem
Formulation 12
3 Tools for Characterizing the Hydrogeology and
Ecology of the Transition Zone 13
3.1 Hydrogeological Characterization 13
3.2 Characterization of Ecological Resources,
Their Exposures, and Resulting Effects 15
4 Evaluating Ecological Risks in the Transition
Zone and Associated Ground-Water Discharge
Areas 16
4.1 Evaluation of Ground-Water and Transition
Zone Water Chemistry 16
4.1.1 Evaluating Ground-Water Chemistry in the
Screening-Level Risk Assessment 16
4.1.2 Evaluating Transition Zone Water
Chemistry in the Baseline Risk Assessment 20
4.2 Evaluating Biota Exposure and Effects 21
4.3 Characterizing Risks 22
5 Summary 22
6 Glossary 22
7 References 26
TABLES
1 Examples of Case Studies Where Ground-
Water and Surface-Water Investigations Were
Employed to Answer Site-Specific Questions
Regarding Ground-Water Contaminant
Exposure, Risks, and Management 14
2 Tools That May Aid in the Identification and
Characterization of Areas of Contaminated
Ground-Water Discharge 17
3 Tools That May Aid in the Characterization of
Ecological Resources of the Transition Zone
and in the Evaluation of the Effects of Exposure
of Those Resources to Contaminated Ground-
Water 18
FIGURES
1 Plan View of Ground-Water Flow,
Contaminant Transport, and Ground-Water
Discharge Areas along a Hypothetical Stream
Channel 7
2 Conceptual Model of Different Types of
GW/SW Exchange Conditions at the bed of a
Surface-Water Body That May Affect the
Transport of Contaminated Ground-Water into
an Overlying Water 7
3 Conceptual Site Model Depicting Contaminant
Transport via Ground-Water Flow, Followed
by Discharge Through the Bedded Sediments in
the Transition Zone into Overlying Surface-
Water 9
4 An Example Decision Tree for Evaluating
Ecological Risks Associated with the Discharge
of Contaminated Ground-Water through the
Transition Zone 19
TEXT BOXES
1. The 8-Step Ecological Risk Assessment
Process for Superfund (U.S. EPA 1997) 9
2. Endpoints and Surrogate Receptors 11
3. Using AWQC in GW/SW ERAs 20
1. Introduction
1.1 Purpose of This Joint ECO Update/
Ground Water Forum Issue Paper
Currently, there is a common perception that
the discharge of contaminated ground-water to a
surface-water body does not pose an ecological
risk if contaminant concentrations in surface-water
samples are below analytical detection limits or at
very low concentrations. The transition zone
represents a unique and important ecosystem that
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exists between surface-water and the underlying
ground-water, receiving water from both of these
sources. Biota inhabiting, or otherwise dependent
on, the transition zone may be adversely impacted
by contaminated ground-water discharging
through the transition zone into overlying surface-
waters. Ecological Risk Assessments (ERA)
addressing contaminated ground-water discharge
to surface-waters typically have not evaluated
potential contaminant effects to biota in the
transition zone. However, numerous
hydrogeological and ecological methods and tools
are available for delineating ground-water
discharge areas in a rapid and cost-effective
manner, and for evaluating the effects of
contaminant exposure on transition zone biota.
These tools and approaches, which are commonly
used in hydrogeological and ecological
investigations, can be readily employed within the
existing EPA framework for conducting
screening- and baseline-level ERAs in Superfund
(U.S. EPA 1997) to identify and characterize the
current and potential threats to the environment
from a hazardous substance release.
This document was initially prepared as an
ECO Update/Ground Water Forum Issue Paper to
highlight the need to treat the discharge of ground-
water to surface-water not as a two-dimensional
area with static boundary conditions, but as three-
dimensional volumes with dynamic transition
zones. This ECO Update applies equally to
recharge zones and can be used to evaluate
advancing plumes that have not yet reached the
transition zone. This document encourages project
managers, ecological risk assessors, and
hydrogeologists to expand their focus beyond
shoreline wells and surface sediments and define
and characterize the actual fate of contaminants as
they move from a strictly ground-water
environment (i.e., the commonly used "upland
monitoring well nearest the shoreline") through
the transition zone and into a wholly surface-water
environment. The approach is presented to help
users identify and evaluate potential exposures and
effects to relevant ecological receptors within the
zone where ground-water and surface-water mix.
The transition zone data collected for the ERA
may also supplement data collected for the
evaluation of potential human health risks
associated with the discharge of contaminated
ground-water. Should ground-water remediation
be warranted (as a result of the risk assessment),
the locational, geochemical, and biological aspects
of the transition zone can be considered when
identifying and evaluating remedial options.
This ECO Update builds on the standard
approach to ERA (U.S. EPA 1997), by providing a
framework for incorporating ground-
water/surface-water (GW/SW) interactions into
existing ERAs (see U.S. EPA 1997 and 200la for
an introduction to ecological risk assessment). The
purpose of the ERA within the risk assessment
process is to:
a. Identify and characterize the current and
potential threats to the environment from a
hazardous substance release;
b. Evaluate the ecological impacts of alternative
remediation strategies; and
c. Establish cleanup levels in the selected
remedy that will protect those natural
resources at risk (U.S. EPA 1994a).
This ECO Update focuses on the first of these
by illustrating how one might consider GW/SW
interactions when designing and conducting an
ERA, both in terms of characterizing the
physicochemical environment of the transition
zone and evaluating potential ecological risks that
may be incurred by receptors in the transition
zone. The discharge of contaminated ground-
water to a surface-water body through the
underlying sediments is the principal focus of the
document but other sources of ground-water
contamination are also included that may be
contributing potential risks to the biota of the
transition zone and the overlying surface-waters
(e.g., ground-water moving through contaminated
sediment, NAPL discharge to sediment or surface-
water, the role of downward vertical gradients).
This document also identifies a suite of tools that
can be used by all members of a site team
(especially ecologists and hydrogeologists) to (1)
determine the locations of contaminated ground-
water discharging to surface-water; (2) estimate
exposure point concentrations at these areas for
use in evaluating potential ecological risks; and (3)
evaluate actual and/or potential ecological effects
of contaminants as they discharge to surface-
water. Throughout this document, ecological
resources means habitats, species, populations, and
communities that occur at or utilize the ground-
water discharge areas and the associated transition
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zones, sediments, and surface-waters, as well as
the ecological functions of these entities (e.g.,
productivity, benthic respiration, biodegradation).
1.2 The Ground-Water/Surface-Water
Transition Zone
1.2.1 Definition of the Transition Zone
The GW/SW transition zone represents a
region beneath the bottom of a surface-water body
where conditions change from a ground-water
dominated to surface-water dominated system
within the substrate. It is a region that includes
both the interface between ground-water and
surface-water as well as the broader region in the
substrate (and, on occasion, up into the surface-
water body) where ground-water and surface-
water mix. Transition zones occur in stream, river,
estuarine, marine, lake, and wetland settings, and
may include the mixing of cold and warm waters,
fresh and marine waters, or waters having other
physical or chemical differences. The transition
zone is not only an area where surface and ground-
water mix, but also an ecologically active area
beneath the sediment/water interface where a
variety of important ecological and
physicochemical conditions and processes may
occur. Transition zones beneath streams and rivers
may be termed hyporheic zones (White 1993) and
those beneath lakes and wetlands termed
hypolentic zones. A new discipline that studies
ground-water relationships to surficial ecological
systems is referred to as "ecohydrology" (Wassen
and Grootjans, 1996) and has been the subject of
recent study (Hayashi and Rosenberry 2002).
The existing and potential ecological effects of
contaminated ground-water in the transition zone
can be important considerations in site
characterization and ecological risk assessment. In
the past, ground-water and surface-water were
typically viewed as separate compartments of an
aquatic ecosystem, connected at the
sediment/surface-water boundary. This paradigm
ignored (1) the ecosystem that occurs within the
transition zone, (2) the important geochemical and
biological roles this zone may have in the local
ecosystem (i.e., Gibert et al. 1994), and (3) the
dynamic nature of this zone that results from the
highly variable flow conditions in ground-water
and surface-water. The new paradigm in this ECO
Update/Issue Paper explicitly includes
consideration of the transition zone as a vital
habitat that is interconnected with, and supports
the surface-water ecosystem (Valiela et al. 1990;
Williams 1999).
1.2.2 Spatial and Temporal Variations of
Transition Zones
The locations and characteristics of transition
zones and associated ground-water discharge areas
vary both spatially and temporally. These spatial
and temporal variations will affect the occurrence
and distribution of habitats dependent on ground-
water discharge, and influence the ecological roles
that the transition zone may have in maintaining
local biotic communities. Not all areas of a
surface-water body receive ground-water
discharge.
The spatial distribution and the rate and
direction of water flow within transition zones will
be influenced by the type of water body into which
the discharge is occurring, the elevation of
surface-water relative to that of ground-water, and
the underlying geological conditions. The rate of
ground-water discharge may vary among the
multiple discharges in direct response to hydraulic
conditions and the varied geological
characteristics in the discharge areas (Fetter 2000;
Winter 1998). When there are large variations
within a transition zone, a few preferential
discharge areas may account for the majority of
the discharge. Ground-water discharge rates also
may vary temporally at individual discharge areas,
reflecting seasonal changes in hydrogeologic
conditions. Precipitation events, surface-water
releases at dams or locks, and tidal fluctuations
(including the reversal of water flow in the
transition zone) also affect the rate of ground-
water discharge to surface-water (Tobias et al.
2001).
1.2.3 Ecological Role of the Transition
Zone
The understanding of the role that transition
zones have in ecosystems directly influenced by
ground-water discharges is increasing (Danielopol
et al, 2003). Benthic and epibenthic communities
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(particularly invertebrate larvae, worms, bivalves,
and fish) are major components of the transition
zone ecosystem and many of these organisms
spend part or all of their life cycle in contact with
the sediments and ground-water that comprise this
zone. These communities are well-known, valued
for their ecological roles, and commonly assessed
in ERAs. Typically, ERAs evaluate the effects of
contaminated sediments on these benthic and
epibenthic organisms because they are linked to
upper-level trophic organisms via the food chain.
However, as discussed in the examples below,
other ground-water-influenced habitats within the
transition zone as well as other transition zone
organisms are ecologically important and therefore
may appropriately be considered in the ERA. This
document provides a framework to allow an ERA
to better evaluate the existing and potential effects
of contaminated ground-water on benthic
ecosystems.
Although water may flow in either direction in
a transition zone (i.e., both ground-water discharge
to surface-water and surface-water recharge to
ground-water), the transport of contaminants by
ground-water discharging to surface-water is the
subject of this document. In some aquatic systems,
areas of ground-water discharge provide important
habitats for a variety of aquatic biota and create
thermal refugia for fish by supplying cooler, well-
oxygenated waters during summer months or
maintaining ice-free habitats in colder climate
streams (Power etal. 1999).
Areas of ground-water discharge can create
conditions capable of supporting spawning,
feeding, and nursery habitats (Dahm and Valett
1996). For example, Geist and Dauble (1998)
showed how nest site selection by salmonids is
strongly influenced by the location of ground-
water discharge zones in streams and estuaries.
Ground-water discharge areas in streams may also
provide important refugia for fish and
invertebrates during the dry phase of intermittent
streams and during stream flood events (Stanford
and Ward 1993; Power et al. 1999). Algal
community structure and recovery following
disturbance have been shown to be influenced by
ground-water discharge to the surface-water
(Grimm 1996). Because of the important
ecological role of the ground-water discharge
areas, the discharge of contaminated ground-water
may result in adverse ecological impacts to biota
utilizing those areas (Carls et al, 2003).
In addition to the habitats at the
sediment/surface-water interface, transition zones
in these discharge areas have been shown to
provide direct habitat for a variety of insect and
fish larvae (Hayashi and Rosenberry 2002). For
example, studies of freshwater hyporheic
ecosystems have shown that some invertebrates
utilizing the transition zone as a refuge may
descend meters into the transition zone on a daily
or seasonal basis.
Furthermore, a healthy, diverse flora and fauna
in the transition zone is beneficial to basic aquatic
ecosystem functioning. The wide array of
organisms within the transition zone are critical to
nutrient, carbon, and energy cycling in aquatic
food webs (Storey et al., 1999; Hayashi and
Rosenberry 2002). For example, up to 65 % of
invertebrate production in a sandy stream was
reported to occur in the hyporheic zone (Smock, et
al. 1992; Boulton 2000). The thickness of the
transition zone directly affects the amount of
habitat available for these organisms. A potential
for adverse impacts exists where contaminants,
degradation by-products, and/or secondary
stressors (such as low dissolved oxygen [DO])
associated with the ground-water come in contact
with these biota in transition zone habitats.
The microbial community of the transition zone
via their function in carbon and nutrient cycling
has been shown to play an important, potentially
beneficial role at some sites in the biodegradation
and attenuation of ground-water contaminants
(Lorah et al. 1997; Ford 2005). For example, at a
site in Angus, Ontario, a detailed hydrogeological
study indicated microbial activity in the thin
transition zone of the Pine River to be responsible
for significant attenuation of a chlorinated solvent
plume (Conant et al. 2004). Microorganisms are
often responsible for the very sharp oxidation-
reduction (redox) gradients that frequently occur
across the transition zone (Fenchel et al. 1988;
Wetzel 2001). These biochemical changes may aid
the degradation and attenuation of organic
contaminants, or may release chemicals (e.g.,
naturally occurring iron and manganese,
degradation products of the organic contaminants)
from the transition zone sediments; and these in
turn can affect aquatic biota. Ground-water
discharge may alter microbial activity in the
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transition zone, reducing DO levels to the point
where habitat quality and biota are adversely
affected (Morse, 1995; Pardue and Patrick, 1995).
1.3 Ground-Water and Contaminant
Discharges in Transition Zones
Critical to the proper evaluation of ecological
risks in the transition zone is an accurate
determination of the location of contaminated
ground-water discharge, which is expected to
occur within a broader discharge zone.
Determining contaminant discharge locations may
be relatively straightforward or quite complicated,
depending on the location of the source(s) of
ground-water contamination with respect to a
surface-water body, the hydrogeologic complexity
of the flow system, the temporal variability in
water table and surface-water levels, and the size
(both vertically and horizontally) of the plume
relative to the general ground-water flow paths.
Plumes of contaminants will flow from
contaminant source areas to points of discharge
along pathways governed by the permeability of
materials, the configuration of the hydraulic
gradient, and density differential with respect to
the surface-water body. One should not assume
that a contaminant plume will discharge at a
location that represents the shortest distance from
a ground-water contaminant source area to the
surface-water (Woessner 2000; Conant 2004). For
example, contaminants originating from a source
located in an upland area adjacent to a highly
permeable stream corridor may be transported by
ground-water for some distance downgradient
(Figure 1, location A), sometimes following
ancient paleochannels in the geology, before
eventually discharging to the stream.
In contrast, ground-water contamination from a
site located directly upgradient and generally in
direct line with the stream channel and ground-
water flow may be transported to the nearest point
in the stream where it may discharge completely
(Figure 1, location B). In some cases, ground-
water transport of some contaminants may
continue on to the next meander, with additional
discharge of these contaminants occurring farther
downstream. A contaminated ground-water plume
may also partially discharge at one location
(Figure 1, location Cl), with the remainder of the
plume discharging at yet another downgradient
location (Figure 1, location C2), or the plume may
pass under the surface-water body without
discharge. Similarly, at any of the discharge
locations several different GW/SW exchange
conditions are possible that could affect the
vertical transport of contaminated ground-water
into overlying waters (Figure 2).
Patterns of ground-water discharge and other
ground-water/surface-water interactions vary over
time. Stream reaches and lakes may change from
being locations of ground-water discharge to
places of surface-water recharge to the underlying
deposits when water levels in the surface-water
body suddenly rise or the water table in the
adjacent deposits decline below the surface-water
level. Daily reversals in flow direction in the
transition zone can occur in tidally influenced
areas. Annual erosion and deposition of sediments
along a riverbed can alter patterns of discharge
(such as those shown in Figure 2) by rearranging
the configuration of low and high permeability
deposits. Even the implementation of remedial
actions can alter ground-water/surface-water
interactions if they change ground-water levels.
For example, pump and treat remedies could cause
drawdown of the water table and change ground-
water discharge zone in an adjacent surface-water
body into areas of induced infiltration (recharge of
surface-water into the subsurface). Ground-
water/surface-water interactions are dynamic but
the transition zone is defined to encompass this
full range of temporal and spatial variability.
1.4 Transport and Fate of Contaminated
Ground-Water in Transition Zones
Many factors influence the transport and fate of
contaminated ground-water as it travels though the
subsurface prior to discharging to a surface-water
body. Conant (2000) summarizes some of the
most important factors in the context of
contaminant plumes that discharge to surface-
water:
Physical and chemical characteristics of the
contaminants;
Geometry and temporal variations in the
contaminant source zone (release area);
Transport mechanisms (advection and
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dispersion); and
Reactions (destructive and non-destructive).
The complexity and dynamic conditions of the
transition zone can considerably alter the plumes
passing through the zone. For example, Conant et
al. (2004) found that a tetrachloroethene (PCE)
ground-water plume changed its size, shape, and
composition as it passed through the transition
zone. Biodegradation in the top 2.5 m of the
transition zone also reduced the PCE
concentrations but created high concentrations of
seven different transformation products thereby
changing the toxicity of the plume. The
biodegradation was spatially variable and
concentrations in the streambed varied by a factor
of 1 to 5000 over distances of less than 4 m
horizontally and 2 m vertically. Widely ranging
concentrations of volatile organic contaminants
have also been observed in plumes discharging to
lakes (Savoie et al, 2000) and wetlands (Lorah et
al , 1997). These studies not only demonstrate the
spatial variability of contaminant concentrations in
the transition zone, but also suggest that aquatic
life within the zone can be exposed to relatively
high concentrations when the contamination has
not yet been diluted by surface-water.
Concentrations in contaminant plume discharges
can change over time. Previous discharges may
have acted as sources of contamination to the
transition zone thus loading the associated
sediment with metals or hydrophobic organic
compounds. Moreover, the pattern of ground-
water flow and contaminated discharge might have
been different in the past such that contaminants in
those sediments may not be at the locations that
current ground-water flow paths would predict.
Direct sampling of the transition zone can help
identify such suspected conditions. It is important
to note that transport and fate factors other than
ground-water flow (e.g., sorption, reaction time)
need to be considered in the conceptual site model
as areas of high ground-water discharge flow may
not necessarily be areas where the highest
concentrations will be found in the transition zone.
Conant et al., (2004) observed that interstitial
water having the highest concentrations of organic
contaminants and degradation products occurred
in low discharge areas of the streambed. This
finding likely reflected sorbed, retarded, or slowly
advecting plume remnants of past high-
concentration discharges that had yet to get all the
way through the lower permeability, organic
carbon-enriched deposits (Conant et al., 2004).
FIGURE 1 Plan View of Ground-Water Flow, Contaminant
Transport, and Ground-Water Discharge Areas along a
Hypothetical Stream Channel (Modified from Woessner
2000).
1 ) Short-Circuit Discharge ( 5 ) Recharge
Springs or Conduits
Cfyj No Discharge
} or Horizontal Flow
/ \ .. tw.
s
2 ) High Discharge
Geologic Window
/
FIGURE 2 Conceptual Model of Different Types of GW/SW
Exchange Conditions at the bed of a Surface-Water Body
That may Affect the Transport of Contaminated Ground-
Water into the Overlying Water (Modified from Conant
2004). (The arrows point in the direction of GW flow, and
the arrow size depicts the relative rate of flow.).
2. Framework for Including the
Transition Zone in Ecological Risk
Assessments
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2.1 The Ecological Risk
Assessment Process and the
Integrated Team
The ERA Guidance identifies an 8-step
framework for designing and conducting
ecological risk assessments for the Superfund
Program (Text Box 1; U.S. EPA 1997). This
framework describes the steps and activities
needed to design and conduct scientifically
defensible risk assessments that will support
management decisions regarding site cleanup
leading to a Record of Decision. Critical aspects of
the framework are problem formulation and the
associated development of a conceptual site model
(CSM). Problem formulation establishes the goals
and focus of the risk assessment, i.e., the
ecological components and processes that are
potentially harmed or at risk, as well as the
assessment endpoints (specific processes, or
populations/communities of organisms to be
protected). The CSM characterizes the
toxicological relationships between the
contaminants and the assessment endpoints, as
well as the exposure pathways by which the two
are potentially linked (i.e., contaminant migration
pathways, chemical alterations, and organism life
histories; see ERA Guidance Steps 1 and 3). The
CSM may also develop the risk questions to be
addressed by the assessment (ERA Guidance Step
3), and identify the endpoints that will be
measured (measurement endpoints) in order to
provide the data necessary to address the risk
questions. Because contaminants will partition
among water, sediment, and organisms, a holistic
CSM that includes all relevant compartments will
be the most useful to guide the ERA and help
determine how the partitioning has occurred or is
occurring within the transition zone. This should
help project managers with decisions about source
control, which media to remediate, the influence
of remedial work on contaminant fate and
transport, and the potential for partitioning to alter
the effectiveness of a proposed remedy (such as a
sediment cap).
In the design and conduct of an ERA that
includes transition zones and areas of ground-
water discharge, it is critical that the project
manager assemble a risk assessment team that is
interdisciplinary and includes ecological risk
assessors and hydrogeologists at a minimum. For
practicality in this paper the term "hydrogeologist"
is used to generically include all the team
members who work mostly on the physical,
hydrologic, and hydrogeologic aspects of site
characterization (i.e., hydrologists,
hydrogeologists,, etc.). Similarly, the term
"ecologist" is used to generically include all the
members who work mostly with the biological
aspects (risk assessors, biologists, benthic
ecologists, ichthyologists, zoologists, botanists,
malacologists, limnologists, microbiologists, etc.).
These disciplines should work closely together
starting as early in the ERA process as possible.
To adequately characterize the hydrogeological
setting of a site, the hydrogeologists need to
understand the local ecosystem, the habitats, the
ecological endpoints to be protected from the
adverse effects of ground-water-associated
contaminants, and the exposure pathways that link
the contamination and the endpoints. Similarly, it
is critical for the ecological risk assessors to
understand the spatial and temporal variability in
the transition zone locations and the potential
mechanisms for transport of contaminants by
ground-water to surface-water. It is important to
remember that the ground-water plume may not
have reached the surface-water at the time of the
assessment, but if it is likely to discharge to the
surface-water in the future, there still is a risk of
release that needs evaluation. Because, the spatial
and temporal variability in ecological systems can
be quite different from the hydrogeological
system, the integrated team will insure data will be
collected on scales useful for all disciplines. This
interdisciplinary focus is most effective when
initiated during problem formulation (U.S. EPA
Guidance Steps 1 and 3). At this stage, the
integrated assessment team will address: (1) the
hydrologic regime of the site and its context in the
watershed, (2) where and when ecological
exposures may be occurring, (3) which organisms
(and ecosystem functions) may be exposed to
contaminants in the ground-water at the transition
zone and associated ground-water discharge area,
(4) which processes are affecting contaminants
during transport (e.g., abiotic transformations,
biodegradation, dispersion, diffusion, adsorption,
dissolution, volatilization), (5) what additional
data may be needed to support the risk assessment,
and (6) the appropriate scope to fit project needs.
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Text Box 1: The 8-Step Ecological Risk
Assessment Process for Superfund (U.S.
EPA 1997)
Step 1: Screening-Level Problem Formulation
and Ecological Effects Evaluation
Step 2: Screening-Level Exposure Estimate
and Risk Calculation
Step 3: Baseline Risk Assessment Problem
Formulation
Step 4: Study Design and Data Quality
Objectives Process
Step 5: Field Verification of Sampling Design
Step 6: Site Investigation and Analysis Phase
Step 7: Risk Characterization
Step 8: Risk Management
2.2 Including the Transition Zone in
Designing and Conducting
Ecological Risk Assessments
It is often difficult to describe complete exposure
pathways when contaminants move among
multiple environmental media and habitats. In
aquatic systems, it is critical to recognize the
static, dynamic, and interactive aspects of different
media and their associated habitats. Currently,
with ERAs that have ground-water and surface-
water interactions, problem formulation and the
CSM typically identify the contaminant source
area, the ground-water flow paths from the
contaminant source area, the surface-waters that
receive discharge of contaminated ground-water,
the media that may be contaminated (e.g., ground-
water, surface-water, and sediment), and the
habitats and ecological receptors that occur in
those surface-waters. While these ERAs often
include some aspects of the transition zone in the
CSM, they more often do not specifically consider
the ecological importance of the transition zone
nor the relationships and interactions among
ground-water flow, surface-water hydrology,
sediment dynamics, and the transition zone biota.
Rather, these ERAs typically evaluate only the
biota associated directly with the sediment/water
interface and/or with the overlying water column
for adverse ecological impacts. In such ERAs,
there is no explicit consideration of a transition
zone, only a boundary line that separates ground-
water and surface-water that is assumed to be the
sediment/surface-water interface. Hence, the biota
and ecological processes associated with this zone
may not be appropriately considered during
problem formulation. Appropriate consideration of
the transition zone means that exposure, pathways,
and potential effects are evaluated in a manner
sufficient to meet the purpose of the ERA set forth
in EPA guidance as indicated in Section 1.1 above.
An effective approach to developing a CSM is
illustrated in Figure 3. This can be adapted to
accommodate a variety of different ground-
water/surface-water settings such as wetlands
(Lorah et al. 1997) and estuaries (Fetter 2000).
FIGURE 3 Conceptual Site Model Depicting Contaminant
Transport via Ground-Water Flow, Followed by Discharge
Through the Bedded Sediments in the Transition Zone into
Overlying Surface-Water
2.2.1 Framework for Incorporating the
Transition Zone into Problem
Formulation
Consideration of the transition zone should
begin as early as possible in the 8-step ERA
process, preferably during problem formulation
and CSM development. It cannot be
overemphasized that problem formulation and the
CSM should be based on the combined knowledge
of the interdisciplinary team approach which
includes hydrogeologists and ecologists on the
team, at a minimum, and preferably should include
the critical review of other team members, such as
the project manager and a toxicologist. The
following 5-step framework has been designed to
incorporate the transition zone into problem
formulation of the ERA process and to help
develop a comprehensive ground-water/transition
zone/surface-water CSM for any aquatic
ecosystem.
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Step 1 Review available site-related chemistry
data to identify known or potential
contamination
Step 2 Identify the hydrogeological regime and
potential fate and transport mechanisms
for ground-water contaminants,
including (a) identification of areas of
contaminated ground-water discharge
and (b) the spatial and temporal
variability in the magnitude and location
of the discharges.
Step 3 Identify ecological resources at areas of
ground-water discharge, including
associated transition zones.
Step 4 Identify ecological endpoints and
surrogate receptors.
Step 5 Develop a dynamic CSM and associated
risk hypotheses and questions.
The activities in these steps usually take place
during the design and conduct of an ERA, and thus
do not necessarily identify activities that would be
in addition to those normally developed when
following the U.S. EPA 8-step process for an ERA
(Text Box 1). In addition, due to the relationship
between the CSM and ecological endpoints, the
risk assessment team may find it useful to revisit
these steps as they refine both the CSM and
selection of endpoints.
Step 1 Review available site-related chemistry
data to identify known or potential
contamination. In this step, the team determines
if there is a potential for the ground-water to be
contaminated, and, if so, whether the contaminants
could be transported through the transition zone
into overlying surface-water. Specifically, the
team will focus on the question: Is there known or
potential (1) ground-water contamination and/or
(2) sediment or surface-water contamination
related to ground-water, and, (3) if so, by what
contaminants? The answer to this question will be
based on a review of the historical site-related
chemistry data regarding the source (i.e., the
nature of the release and the known or suspected
contaminants), potential contaminant migration
pathways, and the affected environmental media
(i.e., evidence of contamination in soil, ground-
water, sediment, biota, and/or surface-water,
including transformation products). This
information will also be used to determine which
contaminants may be encountered by ecological
resources associated with the site. If it is
determined that contamination is present or likely,
the extent of contamination in discharging ground-
water will need to be characterized.
Step 2 Identify the hydrogeological regime and
potential fate and transport mechanisms for
ground-water contaminants, including (a)
identification of areas of contaminated ground-
water discharge and (b) spatial and temporal
variability in the magnitude and location of
ground-water discharge. The nature and extent of
GW/SW interactions at a site and the specific
locations of ground-water discharge areas are
important in the determination of potential
exposure points for ecological receptors. In this
step, the hydrogeologist and ecological risk
assessor delineate contaminated areas and identify
areas of contaminated ground-water discharge
(and associated transition zones). The focus of this
step is to address the question: Where is the
contamination and where is contaminated ground-
water reaching the transition zone and then
discharging to the surface? Potentially
contaminated ground-water discharge areas can be
identified on the basis of:
Available chemical and hydrologic data from
site wells and shoreline work in the area (e.g.,
ground-water chemistry, NAPL presence,
aquifer extent, preferential pathways, hydraulic
conductivity, hydraulic gradients and flow
directions [vertical and horizontal], water table
elevation, and seasonal precipitation patterns);
Physical features indicative of a ground-water
discharge area may be identified during a site
visit including seeps, pools in streams, and
plant species that prefer ground-water
discharge;
Direct investigations during the site visit to
locate and delineate ground-water discharges
(e.g., using simple measurement techniques
such as temperature or conductivity probes,
minipiezometers with manometers or
differential pressure gauges, or seepage meters;
observations of certain plant species, areas of
mineral precipitation, or areas with sheens;
geophysics to map and track plumes);
Direct investigations of contamination in the
transition zone (e.g., sampling interstitial water
using minipiezometers, miniprofilers, passive
diffusion samplers), including temporal
variability.
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Step 3 Identify ecological resources in areas
ฐf ground-water discharge, including
associated transition zones. As areas of
ground-water discharge are identified, the
ecological risk assessors will evaluate the
conditions at these locations and in the
overlying surface-water to identify the types of
ecological resources that occur (or could occur)
and be exposed to the ground-water-associated
contaminants. The focus of this step is to
address the question: What are the ecological
resources at risk from exposure to ground-
water contamination at this location? The risk
assessors will make this determination on the
basis of observations made during a site visit
and through a review of available ecological
data for the site. Ecological resources may
include habitats, species, populations, and
communities that occur at or utilize the ground-
water discharge areas, the associated transition
zones and sediments, and the surrounding
surface-waters. These resources may be
exposed directly or indirectly through the food
web.
Step 4 Identify ecological endpoints and
surrogate receptors. The habitats that will be
associated with areas of ground-water discharge
may support a wide variety and diversity of biota
that could be exposed to contaminants in the
ground-water. However, it is not feasible or
practicable to directly evaluate all of these biota.
Instead, a few assessment endpoints (Text Box 2)
are selected to represent risks to all of the
individual components of the ecosystem (U.S.
EPA 1992; 1997). In this step, the ecological risk
assessors will identify appropriate assessment
endpoints on the basis of:
Contaminants and their concentrations,
Potentially complete exposure pathways linking
the contaminants with the endpoints,
Mechanisms of toxicity of the contaminants
and knowledge of the potential susceptibility of
the endpoints to the contaminants, and
Ecological relevance of the endpoint.
Detailed guidance on selecting assessment
endpoints and linking them to risk determinations
may be found in U.S. EPA (1997).
Text Box 2: Endpoints and Surrogate
Receptors
Assessment Endpoint. an explicit expression of the
environmental value(s) to be protected. Individual
assessment endpoints typically encompass a group
of species or populations with some common
characteristic, such as a specific exposure route or
contaminant sensitivity, or the typical structure and
function of biological communities or ecosystems
associated with the site (U.S. EPA 1992, 1997).
Measurement Endpoint. a measurable ecological
characteristic that is related to the valued
characteristic chosen as the assessment endpoint.
The measurement endpoint provides measures of
exposure and/or effects (U.S. EPA 1992, 1997).
Surrogate Species', a species that is considered to
be representative of the assessment endpoint and
for which measurement endpoints may be selected
and on which the risk characterization will focus.
Assessment endpoints for the transition zone
will focus on the protection of (1) the biota that
live within or utilize the transition zone or the
ground-water discharge area (including interstitial
water, sediment, and surface-water), (2) other
biota that may be exposed to the ground-water
contaminants either through direct contact or
indirectly through ingestion of food or sediment
contaminated by the ground-water, and (3) the
ecological functions of these biota (e.g.,
productivity, benthic respiration, biodegradation).
For example, transition zone assessment endpoints
may include the maintenance and sustainability of
the infaunal community of the transition zone,
maintenance and sustainability of conditions that
support fish and other surface-water species that
seek out ground-water discharge zones as habitat
or refugia, or maintenance of the epifaunal
community inhabiting the ground-water discharge
areas. For such assessment endpoints, surrogate
receptors (Text Box 2) for the transition zone may
include microbial functions; infaunal organisms or
communities (e.g., meiofauna, or macrobenthic
invertebrates). Other surrogates may include
epifaunal organisms such as plants and bottom
fish, as well as life stages of various organisms
such as incubating fish eggs.
11
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In the case of a baseline ERA, one or more
measurement endpoints (Text Box 2) will be
selected to evaluate each assessment endpoint.
These measurement endpoints could include
benthic macroinvertebrate abundance and
diversity; the survival, growth, or reproduction of
the surrogate receptors as measured by laboratory
and in situ toxicity tests or microcosms; the
concentration of contaminants in the tissues of
surrogate species (as a result of bioaccumulation
or bioconcentration); sediment or ground-water
concentrations; or concentrations in diffusion
samplers. Because there are currently no methods
available to risk assessors that allow for decision-
based interpretations of changes in transition zone-
associated organisms (especially with regard to the
microbial community), the choice of surrogate
receptors and associated measurement endpoints
used to address the assessment endpoints for the
transition zone may be limited to species and
measurement endpoints for which methods are
available.
Step 5 Develop a CSM and associated risk
hypotheses and questions. In this step, the
information and results of the preceding steps will
be used to develop a CSM that identifies the
known or assumed relationships among the
contaminant source, the environmental fate and
transport of the contaminants in the ground-water,
and the assessment endpoints that may be exposed
to the contaminants (Figure 3). The CSM should
also identify the potential effects that the
assessment endpoints may incur from the
exposure. These relationships represent working
hypotheses of how the ground-water contaminants
are moving or will move through the environment
(i.e., moving through the transition zone
discharging to overlying surface-waters) and
affecting the assessment endpoints (associated
with the transition zone and overlying sediments
and surface-waters). The CSM thus helps to
conceptualize the relationships between
contaminants and assessment endpoints, frames
the questions that need to be addressed by the risk
assessment, and aids in identifying data gaps for
which the collection of environmental data may be
necessary.
Risk questions about the relationships between
the assessment endpoints and their predicted
responses when exposed to contaminated ground-
water discharges can be developed along with the
CSM. These risk questions provide additional
bases for the selection of appropriate measurement
endpoints and study designs. Some examples of
risk questions for the transition zone include (1)
Does contaminant exposure exist at ground-water
discharge points, and, if so, do the exposure
concentrations exceed levels considered "safe" for
the assessment endpoints? (2) Are exposures to
contaminants at ground-water discharge points
associated with deleterious effects to the
assessment endpoints? (3) Does the exposure to
contaminated ground-water pose unacceptable
risks to transition zone, benthic, and/or surface-
water assessment endpoints?
2.2.2 Hydrologic Regime and Contaminant
Fate and Transport Considerations during
Problem Formulation
As in any ground-water setting, the transport
and fate of contaminants will be a function of the
characteristics of the geologic materials through
which ground-water is passing, the chemical and
physical characteristics of the native ground-water,
and the physical and chemical characteristics of
the contaminants. In the transition zone, the
mixing of surface- and ground-waters can create
steep gradients (large changes over relatively short
distances) in water quality parameters such as DO
concentration, salinity/conductivity, oxidation-
reduction potential (ORP), pH or temperature
which can be measured in the field, and hardness,
solids, and Acid Volatile Sulfides which can be
measured in the lab. The characteristics of the
substrate (especially sediments) such as mineral
content, grain size, porosity, and TOC in the
transition zone may also change abruptly over
relatively short distances. Each of these
characteristics can strongly influence contaminant
mobility. Contaminants that have traveled
considerable distances in ground-water with little
alteration may, upon entering and passing through
a transition zone, show rapid attenuation in this
zone due to the dynamic physical and chemical
characteristics of the zone. These changing
conditions, as contaminants move from the
ground-water environment to the transition zone,
can facilitate attenuation processes such as
adsorption, microbial degradation of chlorinated
solvents, and precipitation of some dissolved
metals.
12
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On the basis of these characteristics of the
transition zone, two key hydrogeologic questions
to consider in problem formulation are (1) How
close to the ecological resources are the
contaminants or their degradation or
oxidation/reduction products? and (2) What are the
transport and attenuation processes controlling the
mobilization, movement, flux, mass loading, and
observed distribution of contaminants? In
considering these questions in problem
formulation it may be beneficial to understand the
role of smaller scale changes in permeability,
mobilization (such as ground-water moving
through contaminated sediment, etc.), movement
of contaminants in whatever form they are found
(such as dissolved, NAPL, colloid-bound, etc.),
and where the contaminants ultimately come to
reside.
Various GW/SW exchange conditions are
possible at the bed of any surface-water body
(Figure 2) (Conant 2001, 2004). There may be
situations where no ground-water discharges into
surface-water because the hydraulic gradient is
horizontal (Figure 2, No. 4), the hydraulic gradient
is away from the surface-water body (e.g.,
downward vertical gradient; Figure 2, No. 5), or a
geologic barrier is present that prevents discharge
(Figure 2, No. 4). Alternatively, ground-water
discharge may occur at a low rate due to a low
hydraulic gradient and/or the presence of low to
moderate permeability materials that act to slow
the ground-water flow (Figure 2, No. 3).
In contrast to the above exchange conditions,
the presence of a strong hydraulic gradient and/or
highly permeable substrate may result in a
condition where the ground-water is able to
rapidly discharge with little opportunity for
attenuation. In this instance, contaminants come in
contact with organisms that not only live within
the sediment but also live on or use the sediment
surface or overlying surface-water or even
preferentially seek out these areas for spawning or
as thermal refugia (Figure 2, No. 2). Ground-
water discharge areas exhibiting this last exchange
condition may be viewed either as geologic
windows that are easily detected (Figure 2, No. 2)
or as small "short circuits" in otherwise no- or
low-inflow zones (Figure 2, No. 1) (Conant 2004).
The overall density and distribution of such short
circuits may be key factors in determining whether
or not they drive a significant ecological risk. It is
important to remember that in any setting, ground-
water flow rate and direction are controlled by
hydrologic conditions. These conditions can be
highly variable, and multiple sampling events
conducted over time, or other tools that integrate
exposure or effects over time, may be needed to
characterize the transition zone.
3. Tools for Characterizing the
Hydrogeology and Ecology of the
Transition Zone
A variety of tools are available that can be used
to help locate and characterize areas of
contaminated ground-water discharge and
associated transition zones (EPA 2000; see Table
1 for some site-specific examples). Similarly, there
are a number of tools and approaches available for
characterizing the ecological resources of the
transition zone and for evaluating the exposure of,
and effects on, those resources exposed to
contaminated ground-water. The choice of tools
will depend on the environment, the selected
assessment and measurement endpoints, and use
of the Data Quality Objectives Process will help
the site team avoid sampling method bias. While
Tables 2 and 3 highlight commonly used tools for
characterizing the hydrogeology and ecology of
the transition zone, additional tools are identified
in A Compendium of Chemical, Physical and
Biological Methods for Assessing and Monitoring
the Remediation of Contaminated Sediment Sites
(U.S. EPA, 2003).
3.1 Hydrogeological Characterization
The identification and characterization of
contaminated ground-water may occur during the
screening ERA (Steps 1 and 2 of the 5-step
transition zone framework) and continue during
the baseline ERA. During the screening ERA, this
hydrological characterization may be based, in
part, on
Examination of existing maps of surficial and
bedrock geology and the local hydrology;
Examination of water chemistry data from
existing wells, piezometers, and surface-water;
Examination of boring logs and other geologic
data;
Evaluation of ground-water migration and
preferential pathways;
Collection and examination of remotely sensed
thermal data;
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TABLE 1 Examples of Case Studies Where Ground-Water and Surface-Water Investigations Were
Employed to Answer Site-Specific Questions Regarding Ground-Water Contaminant Exposure, Risks,
and Management
Site
Environmental Setting/Issue
Ground-Water Contaminant
Concern/Question
Nature of Ground-Water/Surface-Water
Investigation
ASARCO Tacoma
Smelter, Tacoma, WA
Metal smelting with arsenic in
ground-water adjacent to Puget
Sound.
Is the arsenic, in parts per
thousand, in ground-water
discharges to the shoreline and
subtidal zones likely to cause an
adverse impact.
Arsenic speciation and electron probe analysis
show pH and redox increase when ground-water
goes through the transition zone results in
precipitation and the arsenic does not enter the
marine environment
Eagle Harbor, WA
Marine habitat, Puget Sound.
Identify zones of discharge to
harbor floor.
Towed temperature and conductivity probe linked
ground-water in the uplands with discharges to
harbor sediment.
Eastland Woolen Mill,
East Sebasticook River,
ME
River system impacted by
chlorinated solvents from
former woolen mill.
Is contaminated ground-water
contributing to sediment toxicity?
In situ and laboratory toxicity tests, nested
multilevel minipiezometers demonstrated spatial
pattern of chlorobenzene transport and toxicity
(Greenberg et al.,2002). Microbial and meiofaunal
analyses documented changes in those
communities.
Leviathan Mine, CA
Open-pit sulfur mine at 7,000 ft
in Sierra Nevada Mountains,
with acidic discharge into
Leviathan Creek.
In highly mineralized geologic
setting, what is relative
contribution of acid mine
drainage and natural acidic
discharge to water quality of the
watershed?
Investigation of Leviathan Creek using a hand-
held combined conductivity, pH, and temperature
meter revealed a single small natural seep,
compared to large inputs from the mine.
McCormick & Baxter
Creosoting Co.,
Portland, OR
http://www.deq.state.or.
us/nwr/mccormick.htm
Site adjacent to Willamette
River. Site used creosote,
pentachlorophenol, and metals
for wood treatment.
Is there seepage of creosote or
other contaminants to the river
via ground-water?
Working with divers collecting sediment samples
and installing minipiezometers and seepage meters
within river, documented non-aqueous phase
liquid (NAPL) discharges from just below
sediment surface and ground-water discharge at
the shoreline and deeper in the river.
St. Joseph, MI
Chlorinated solvent ground-
water plume migrating toward
Lake Michigan.
Is natural attenuation sufficient to
keep contaminants from reaching
the lake?
Geoprobes with slotted screens were used to
identify an offshore solvent plume discharge zone,
demonstrating that natural attenuation was not
completely effective at this site (Lendvay et al.
1998). In 1999, pore water sampling of the near
shore sediments identified the main plume
discharge (MDEQ 2005).
Treasure Island Naval
Station, San Francisco,
CA
Chlorinated solvent plume
migrating toward/into San
Francisco Bay.
Location of ground-water control
monitoring points(water column
measurements or wells and
location of wells, if chosen).
The Navy agreed to place monitoring wells at
locations where a study of tidal mixing in the
ground-water revealed a 20% influence of
seawater; this made the GW/SW transition zone
the remedial compliance point.
Western Processing,
Kent, WA
Small stream (Mill Creek)
along site boundary.
Contaminated ground-water
discharging to stream.
Are stream sediments
contaminated with solvents and
metals, and, if so, what is the
source of the contamination?
Could a simple removal of the
contaminated sediments address
the ecological risks?
Standpipes in the creek indicated artesian flow.
Solvent contamination was found to originate from
surface input, while the metals contamination was
due to the discharge of contaminated ground-
water.
Chevron Mining Inc.
(CMI) (formerly
Molycorp, Inc.),
Questa, NM
Molybdenum mine near the
Red River which is a tributary
to the Rio Grande. Metal and
low pH loads to the river
system from ground-water
upwelling.
Do the concentrations of COPCs
in discharging ground-water,
surface water, and/or sediments
in upwelling exposure areas pose
unacceptable risks to aquatic life?
Laboratory and in situ toxicity tests, multilevel
minipiezometers, exposure chemistry, benthic and
fish community analyses were used to identify two
specific discharge points along the study area as
requiring evaluation during the Feasibility Study.
14
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Site walkovers for visible signs of discharge
(such as areas of differing sediment grain size
and structure or obvious seeps observable
under the low-river stage or tide conditions);
and
Site walkovers using portable (hand-held)
monitoring instruments such as
salinity/conductivity, pH, DO meters, and/or
temperature probes;
Geophysical survey to characterize the
underlying geology and directly or indirectly
detect contaminated ground-water.
The use of "standard" monitoring wells and
piezometers to characterize conditions within the
transition zone may not be feasible, as these tools
will typically be too large to use in a transition
zone environment. A number of relatively
inexpensive and simple portable instruments are
available that may be used to locate areas of
contaminated ground-water discharge. These
instruments include:
Passive Diffusion Samplers
Peepers,
Miniprofilers,
Pushpoint pore-water samplers,
Minipoint samplers,
Sippers,
Hydraulic potentiomanometers
Seepage meters.
For the baseline ERA, additional
hydrogeological characterization data may be
needed to evaluate the assessment and
measurement endpoints and address the risk
hypotheses and questions (see Step 4 of the
transition zone CSM framework). Portable
instruments can be used to (1) rapidly and
inexpensively identify and characterize ground-
water discharge areas, (2) support a screening-
level risk assessment, and (3) yield quantitative
contaminant data of sufficient quality to support
the needs of a baseline ERA. The instruments that
could be implemented at a specific site will be
based on the CSM and the capabilities and metrics
of the individual tools. Because different tools
may have quite different metrics, site
characterization will benefit greatly from early
consideration of how the data will be evaluated,
interpreted, and integrated. When tools cannot
effectively sample the zone of primary interest,
consideration can be given to sampling in adjacent
zones, provided agreements are reached how the
data will be interpreted in the ERA. Brief
descriptions of tools for hydrological
characterization are presented in Table 2.
Additional information regarding the sampling of
ground-water and interstitial water can be found
at:
http: //clu-i n .o rg/techdrct/,
http://www.cpa.gov/tio/tsp/issuc.htm
* hjtpj_/^ywjv_.crt.orgA
3.2 Characterization of Ecological
Resources, Their Exposures, and
Resulting Effects
Numerous tools and approaches are available
for characterizing the ecological resources of a
transition zone and for evaluating the effects of
exposure to ground-water contamination
(Williams 1999). These include survey protocols
using a variety of devices to sample and/or analyze
periphyton, benthic invertebrates, and fish (e.g.,
Barbour et al. 1999) and the microbial community
(e.g., Adamus 1995; Hendricks et al. 1996;
Williams 1999) (Table 3). These tools may be
used to identify the types and abundances of
species, characterize the structure of the ecological
communities, and evaluate microbial processes of
the transition zone and associated ground-water
discharge areas.
Exposure of transition zone biota may be
inferred from survey data by spatially linking
survey habitats with the presence of contaminated
ground-water (as determined using the previously
described hydrogeological characterization tools).
Uptake of ground-water contamination by biota
may be estimated, and exposures characterized,
using in situ approaches such as the direct analysis
of ground-water-associated contaminants in biota
that inhabit the transition zone and associated
areas, or through the chemical analysis of test
organisms following controlled exposure in areas
of contaminated ground-water. Exposure of
transition zone biota may be estimated using
semipermeable membrane devices (SPMDs) to
estimate potential uptake of ground-water
contamination by exposed biota (limitations can be
minimized by field calibration at the site of
15
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interestsee Section 4.2). Exposure levels may
also be inferred through the use of contaminant
uptake factors (such as bioconcentration factors
[BCFs]) that are available in the scientific
literature for many chemicals. Effects can be
inferred from traditional tools applied to the
transition zone (e.g., in-situ toxicity tests,
comparison with criteria or risk-based
concentrations for various media).
4. Evaluating Ecological Risks in the
Transition Zone and Associated
Ground-water Discharge Areas
Ecological risks to most biota in the transition
zone and discharge area from exposure to
contaminated ground-water can be effectively
predicted by (1) evaluating ground-water
chemistry at the transition zone and (2) estimating
the resulting direct and indirect ecological effects
from that exposure. Other approaches can be very
useful when needed to reduce uncertainty
regarding effects on the selected assessment
endpoints. These evaluations may be directly
incorporated into the 8-step process for designing
and conducting ERAs (U.S. EPA 1997; see
Section 2.1). Decisions regarding risk
acceptability and subsequent risk-management
decisions can be made based on the outcomes of
these evaluations. Figure 4 presents an example of
a decision tree for assessing ecological risks
associated with the discharge of contaminated
ground-water through the transition zone. If
unacceptable risks are identified and remediation
is appropriate, the ERA should ultimately provide
risk-based preliminary remediation goals (PRGs)
and will assist in the identification and evaluation
of remedial alternatives and in the evaluation of
remedial success (U.S. EPA 1994a, 1997).
4.1 Evaluation of Ground-water and
Transition Zone Water Chemistry
The concentrations of chemicals in the ground-
water and transition zone waters can be evaluated
in the screening and baseline ERAs (Figure 4).
These evaluations compare measured chemical
concentrations to benchmark values that represent
water concentrations considered protective of
exposed aquatic biota. Chemicals present at
concentrations below the benchmark values are
assumed to pose acceptable risks to the transition
zone biota. The baseline ERA may also employ
evaluations of exposure and effects to support a
risk characterization.
4.1.1 Evaluating Ground-Water Chemistry
in the Screening-Level Risk
Assessment
In the screening-level ERA, the maximum
chemical concentration detected in ground-water
is compared to applicable benchmark values (Step
2 of the Superfund ERA process [U.S. EPA
1997]). Use of maximum detected concentrations
of the contaminants is consistent with the use of
conservative assumptions in the screening-level
ERA. The benchmark values used in the screening
ERA are the Ambient Water Quality Criteria
(AWQC) (U.S. EPA 2002a), which identify
concentrations of selected chemicals that are
considered protective of aquatic biota under
chronic exposures in fresh and marine waters (see
Text Box 3). Because the AWQC are considered
protective of benthic organisms, they are suitable
for evaluating transition zone organisms. When an
AWQC is not available for a specific chemical
(e.g., many volatile organic compounds), an
alternative screening value may be selected (U.S.
EPA 1997), or the chemical is carried forward into
the baseline ERA for further analysis by another
approach. The ground-water concentrations should
be compared with the lowest appropriate chronic
criteria. In brackish systems, both freshwater and
marine chronic criteria should be considered. The
assumptions regarding the applicability of AWQC
or other benchmarks for evaluating potential
ecological risks to transition zone biota should be
discussed in the uncertainty analysis that is part of
the risk assessment (U.S. EPA 1997).
Chemicals with maximum ground-water
concentrations below the AWQC are assumed to
pose negligible ecological risk and that chemical-
specific ground-water pathway can be removed
from further consideration in the ERA (Figure 4),
while those with concentrations exceeding
benchmark levels are further evaluated in the
baseline ERA. Depending on the potentially
complete exposure pathways identified in the
CSM, chemicals may need to be evaluated in other
media such as sediment or tissue.
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TABLE 2 Tools That May Aid in the Identification and Characterization of Areas of Contaminated
Ground-Water Discharge
Tool
Direct Push Technology
Geologic and topographic
maps
Hydraulic potentiomanometer
Minipoint sampler
"Mini" Profiler
Passive diffusion sampler
(PDS)
Peepers
PushPoint interstitial water
sampler
Radiologic analyses
Remotely sensed thermal data
Sediment probe
Seepage meter
Sippers
Site walkovers with handheld
meters
Description
Vibracores and Geoprobes are examples of direct push sampling tools that can be used in the sediments to obtain
sediment cores and samples, and, with adaptations, to obtain water samples at depth below the sediment surface.
Surficial and, in some settings, bedrock geologic maps of the stream and near-stream environment may indicate which
zones are most likely to have significant interchange between ground-water and surface-water.
Winter et al. (1988) present a device that consists of a stainless steel probe with a screened section near the tip that is
connected by a tube to a manometer whose other tube can be placed within a surface-water to measure the head difference
between ground and surface-water at a sampling station. The device can also be used to obtain ground-water samples by
detaching the probe from the manometer and withdrawing a sample with a hand pump.
Duff et al. (1998) present a sampler that has six small-diameter stainless steel tubes set in a 10-cm-diameter array preset
to drive depths of 2.5, 5.0. 7.5, 10.0, 12.5, and 15.0 cm. Ground-water samples from all depths are withdrawn
simultaneously by a peristaltic pump.
Conanat et al. (2004) modified a soil vapor probe by Hughes et al. (1992), creating a miniature hand-driven version of a
profiler that can be used to recover interstitial water samples from multiple depths in the same hole to a depth of 1.5m.
The mini Profiler is a thin-walled tube (0.64 mm OD) with a drive point that contains small-screened ports. Pumping
distilled water down the device and through the ports during driving keeps the ports free of material. In sampling mode, a
pump purges the device of distilled water and draws a formation water sample up to the surface. The full-size Waterloo
Profiler can be used to depths of 10s of meters (Pitkin et al., 1999).
Vroblesky and Hyde (1997) and Vroblesky et al. (1996, 1999) present development of an inexpensive sampler that
collects volatile organic compounds (VOCs) by diffusion and has been successfully used at a number of sites to detect
where VOC plumes are discharging to surface-water. Results provide an estimate of average concentration in the
sampled water. Independent data are needed to determine flow direction past the sampler (i.e., if the sampler is collecting
ground-water or surface-water). For additional information, see: http://nia.water.usgs.gov/publications/WTir/
\vri024186/report.htm. PDSs have been developed for other contaminants (e.g. metals).
Hesslein (1976) and Mayer (1976) first developed diffusive equilibration samplers in which the sampler consists of a
vertical array of deionized water-filled chambers separated from interstitial water by a dialysis membrane. A number of
modifications to this basic sampler now exist (USEPA 2001b; Burton et al. 2005). Results and limitations are similar to
those encountered with PDSs above.
MDEQ (2006, in review) presents a sampler that consists of a thin-walled metal tube with a chisel-pointed tip and a 4-cm
screened interval above this tip. A retractable stainless-steel plug prevents clogging of the screen during driving into the
sediment. At the desired depth, an interstitial water sample can be removed by a syringe or peristaltic pump attached to
the top of the device. For additional information on push-point sampling, see Zimmerman et al. (2005).
Krest and Harvey (2003) describe a method using radioactive tracers (which can be quantified much more precisely than
most organic chemicals), best used in areas with very low hydraulic gradient without the potential confounding factors
such as salinity change.
Airborne forward-looking infrared radiometry (FLIR) thermal-imagery equipment. Helicopter-mounted FLIR equipment
takes infrared photographs of the rivers to provide visual images of surface-water temperatures. Areas of ground- water
discharge may be indicated if there is sufficient temperature contrast between the discharging ground-water and
surrounding surface-water temperatures. For additional information, go to: http://geopubs.WT.usgs.gov/open-file/ off)2-
367/of02-367.pdf and http://www.ecy.wa.gov/pubs/01 10041.pdf.
Lee (1985) developed a sediment probe that is towed in contact with bottom sediments and detects zones of plume
discharge by detection of conductivity anomalies. Other researchers have also used conductivity or resistivity
measurements successfully but with more traditional, labor-intensive devices
Unlike the devices discussed above, the seepage meter can give a discharge rate and flow direction through a stream bed.
The basic seepage meter design originally presented by Lee (1977) and Lee and Cherry (1978), consists of the top section
of a steel drum with a plastic bag attached as a sample collector. A variation on this design is the UltraSeep, system
which is instrumented to monitor conductivity, temperature and fluid seepage rate (http://clu-
in.org/programs/21m2/navytools/gsw/). A basic seepage device is driven into the sediment, and natural seepage is
allowed to fill the sample bag. The volume obtained during deployment can be sampled for analysis as well as used to
calculate a seepage rate. If it is known that seepage is into the streambed, the bag can be pre-filled with a known volume
of water to allow seepage into the sediment and calculation of the seepage rate. While there are a number of uncertainties
associated with the use of seepage meters, these meters can provide a measure of what is coming through the sediment
and into surface-water that no other device can provide.
Zimmerman et al. (1978) and Montgomery et al. (1979) present a sampler that consists of a hollow PVC stake with a
porous Teflonฎ collar. The device has a sampling tube that runs its full length and a gas port at the top. The device is
driven into the sediment and evacuated with a hand pump. Interstitial water then seeps into the device. The sample is
removed by displacement with argon gas pumped in through the gas port. The initial filling of the device through
application of a vacuum may limit its utility in sampling VOCs.
Wading a shallow site with appropriate field sampling devices (e.g., temperature, pH, or conductivity meters) may be
useful to preliminarily delineate some contaminant plumes. This may be especially useful in settings with ground-water
discharge through discrete seeps where the measured parameters have steep gradients.
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TABLE 3 Tools That May Aid in the Characterization of Ecological Resources of the Transition Zone and
in the Evaluation of the Effects of Exposure of Those Resources to Contaminated Ground-Water
Tool
Invertebrate community
survey protocols
Laboratory interstitial
water and sediment
toxicity tests
Microbial community
survey protocols
Tissue analysis of resident
biota (bioaccumulation
measures)
Description
These protocols may include sampling devices such as sediment cores and colonization samplers (e.g., rock
baskets, trays of sediment) to collect invertebrates of the infaunal communities at the ground- water
discharge area. The transition zone community can be considered a simple extension of the infaunal
communities. Sediment core samples are taken from the biologically active zone, which may be fairly deep
(ca. 1 m) or fairly shallow (a few cm), or targeted to reach specific macroinvertebrates such as burrowing
shrimp or bivalves (perhaps >1 m). Colonization samplers can be placed on the bottom of a water body as a
means of collecting macroinvertebrate fauna. Following sampling, the collected biota can be analyzed using
well-established bioassessment methods (e.g., as described in Barbour et al. 1999). The use of invertebrate
surveys has proven effective in evaluating contaminated ground- water (Malard et al. 1996). When compared
to uncontaminated sites, the results can reveal whether the invertebrate community has been affected by the
exposure.
These are traditional toxicity tests (U.S. EPA 1994b,e) that can be conducted on samples obtained from
various locations in the transition zone. However, care must be taken to maintain the chemistry (redox, pH)
and physical structure of the sample, and to prevent volatilization of contaminants.
There are well-established methods for investigating microbial communities at the GW/SW transition zone
(e.g., Hendricks 1996). The results of the survey may be useful to show whether there are differences
between the microbial communities in contaminated and uncontaminated ground- water discharge zones.
Biota are collected from the transition zone and/or areas of ground- water discharge and associated surface-
waters and analyzed for the ground-water contaminants.
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SCREENING
ECOLOGICAL
RISK
ASSESSMENT
Compare maximum chemical concentration in ground
water to appropriate screening benchmark value
BASELINE
ECOLOGICAL
RISK
ASSESSMENT
.
f
Are screening
benchmark values
exceeded?
Exit further evaluation
of the ground water -
surface water pathway
in the ERA
Identify ground-water discharge areas and sample
transition zone ground water
Develop exposure point concentrations that reflect a
"reasonable maximum exposure" in the transition zone
Compare maximum and reasonable maximum
exposure concentrations from the transition zone to
appropriate screening benchmark values
Exit further evaluation
of the ground water -
surface water pathway
in the ERA
Evaluate transition zone biota for exposure to and
effects of contaminated ground water in the transition
Benthic
community
analyses
Bioaccumulation
evaluations
Characterize risks
Risk Management
FIGURE 4 An Example Decision Tree for Evaluating Ecological Risks Associated with
the Discharge of Contaminated Ground-Water through the Transition Zone.
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Text Box 3: Using AWQC in GW/SW
ERAs
As done for any ecological risk assessment, the
assessor should determine whether the specific
AWQC are appropriately protective of benthic
infaunal and epifaunal organisms exposed to
discharging contaminants. This determination,
although difficult if AWQC are not available
for certain contaminants, may be important
where volatile contaminants are discharged. In
these cases, reviewing the derivation of the
AWQC may help determine an appropriate site-
specific screening level, help select
investigatory tools in the baseline ERA, or help
with the uncertainty analysis.
Typically, screening-level ERAs rely on
previously available data. Thus, the equipment and
methods used to provide the ground-water data
(see Table 2) may have been selected and
implemented prior to the involvement of the
ecological risk assessor. In some cases, the
available ground-water data may be from wells
screened below the aquifer that is discharging to
surface-water. Therefore, the risk assessor should
confirm that the ground-water data are acceptable
and that the samples are appropriately
representative for their intended use in the
screening-level risk assessment. Additional
information on ground-water sampling is
presented in a Ground Water Forum Issue Paper
(U.S. EPA 2002b). The ecological risk assessor
should also determine whether the detection limits
for the ground-water data will support a
meaningful comparison to the benchmark values
(e.g., whether the detection limits are at or below
the screening values). If the ground-water data are
not appropriate with regard to sampling issues and
detection limits, they may have reduced value for
the screening ERA.
4.1.2 Evaluating Transition Zone Water
Chemistry in the Baseline Risk
Assessment
In the baseline ERA (U.S. EPA 1997),
chemical concentrations in ground-water at the
transition zone are compared to AWQC (U.S. EPA
2002a) or other benchmark values for protection
of aquatic life, but using more realistic exposure-
point concentrations than those evaluated in the
screening ERA. These new comparisons will not
use maximum detected ground-water
concentrations as in the screening ERA, but rather
use exposure-point concentrations that are
reasonably anticipated or expected to exist or
occur at a site (the reasonable maximum
exposure). Reasonable exposure point
concentrations can be determined, in consultation
with the site hydrogeologist, from a particular well
or set of wells along the flow path(s) from the
source to the discharge zone in the surface-water.
However, it may be preferable to determine this
more realistic exposure-point concentration from
available or new data from transition zone
samples. When new data are to be collected, the
risk assessment team should jointly develop the
sampling design. Similarly, if there are concerns
for human health impacts, usually from foodweb
magnification, then the sampling design should
also be coordinated with the appropriate human
health risk assessors.
Sampling-design considerations for the baseline
ERA should include both hydrogeologic and
ecological factors. Hydrogeologic factors may
include ground-water and surface-water dynamics
and seasonal variability, water table elevation,
surface-water level and flow rates, bed material,
locations of paleochannels, preferential ground-
water flow paths, and contaminant concentrations
in interstitial water from the transition zone.
Ecological factors may include the types and
distributions of biota associated with the transition
zone and ground-water discharge areas, their
contribution to the food web, and life history
aspects of the biota such as seasonal occurrence
and the vertical distribution and movement of the
biota within the sediment. The collection of new
ground-water data for use in the ERA may utilize
one or more of the sampling tools identified in
Table 2 for characterizing hydrologic conditions.
Generally, these sampling tools fall into two broad
categories: (1) tools that actively collect a sample
at a specific time period (e.g., piezometers,
pushpoint samplers) for instantaneous
concentrations and (2) tools that passively collect
samples over time (e.g., peepers, seepage meters,
and PDSs) for more integrated concentrations or
contaminant mass.
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4.2 Evaluating Biota Exposure and
Effects
Baseline ERAs of other ecosystems typically
employ evaluations of exposure and effects to
provide multiple lines of evidence for
characterizing risks. The methods typically
employed in evaluating exposure and effects to
benthic biota can be readily extended to transition
zone biota exposed to contaminated ground-water
discharges. These methods include benthic
community analyses, toxicity testing, and
bioaccumulation evaluations. In selecting these
methods to evaluate exposure and effects to
transition zone biota, the risk assessor must
consider the same issues that are typically
addressed during benthic ecosystem risk
assessments. These issues include, but may not be
limited to, the use of reference sites to address
natural variability and background conditions
(U.S. EPA 1994d), confounding factors that could
affect toxicity results, toxicity testing using media
collected along contamination gradients in order to
develop dose-response relationships, and
uncertainties associated with many of the input
parameters of uptake models. These issues are
typically addressed during the problem
formulation and study design portions of ERA
development (Steps 3 and 4, respectively, of the
Superfund ERA process).
Community analysis of transition zone organisms
can be used to identify differences in community
structure, biomass, species richness and density,
relative abundance, and other parameters (U.S.
EPA 1994c), and a variety of methods are
available for sampling and evaluating transition
zone biota (i.e., Hendricks 1996; Williams 1999).
However, evaluating alterations in transition zone
communities is challenging, and shares exactly the
same issues and considerations as benthic
community analyses or other field studies. These
issues include natural variability (e.g., associated
with ground-water discharge/recharge), the need
for concurrent community analyses at appropriate
reference sites (see Barbour et al. 1999), and the
overarching need for synoptic sampling of
exposures and effects.
Toxicity testing and bioaccumulation
evaluations have been used at several sites to
evaluate the effects of ground-water contamination
on transition zone biota. Toxicity testing, which
involves the exposure of organisms to
contaminated media, provides direct evidence of
contaminant effects on transition zone biota (U.S.
EPA 1994e). A wide variety of toxicity tests have
been developed for use in ecological risk
assessments (U.S. EPA 1994b), and many of these
may be directly applicable to evaluating
contaminant effects on transition zone biota.
While these types of studies are often conducted in
the laboratory using media collected from the site,
in situ studies have also been used and may be
preferable because they provide more realistic
exposures than do laboratory studies (U.S. EPA
1994e; Greenberg et al. 2002; Burton et al. 2005).
Bioaccumulation evaluations examine the uptake
of contaminants by exposed biota and can be used
to infer potential effects to transition zone biota
when concentrations exceed tissue levels
considered adverse to the organisms or their
predators. Bioaccumulation may be measured by
(1) tissue analysis of indigenous biota, (2) analysis
of cultured test organisms (e.g., fish,
macroinvertebrates) exposed in situ (US EPA
2004), (3) the use of SPMDs, and (4) the use of
contaminant-uptake models. Tissue analysis
provides a direct estimate of contaminant uptake
and bioaccumulation under site-specific
conditions. Semipermeable membrane devices
may also provide a site-specific estimate of
passive uptake and bioaccumulation. However,
because SPMDs serve as surrogates for biota and
involve no sampling or analysis of biota, their use
for estimating bioaccumulation should be
approached with caution. Unless a quantitative
relationship has been established between the
bioaccumulation estimated by the SPMD and that
measured in biota exposed at the site, the use of
SPMDs is not recommended for evaluating
bioaccumulation. These devices may, however, be
useful for delineating areas of contaminated
ground-water discharge (as in Step 2 of the
transition zone problem formulation framework)
or monitoring these areas (Huckins et al. 1993).
Because contaminants partition among water,
sediment, and organisms (recall that partitioning
will have been evaluated during problem
formulation and CSM development), sediment
analysis may be necessary to interpret
bioaccumulation results for decision-making.
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While there currently are no examples of
quantitative contaminant uptake models for
transition zone biota, existing approaches used to
estimate contaminant uptake by aquatic biota may
be applicable for use in transition zone
ecosystems. For aquatic biota, contaminant uptake
models employing laboratory-derived BCFs or
field-derived bioaccumulation factors (BAFs) are
commonly used to estimate biota tissue
concentrations from contaminant concentrations
measured in aquatic media (e.g., see Suter et al.
2000). While such models may be used for
estimating tissue concentrations in transition zone
biota, the risk assessor should address many of the
typical modeling issues (such as nonlinearity
between BCFs and ambient contaminant
concentrations when selecting a BCF; and the
potential for deviations from equilibrium
assumptions) in the interpretation of model results.
4.3 Characterizing Risks
Ecological risks to the transition zone are
characterized after the collection and analysis of
physical, chemical, and ecological data have been
completed (Figure 4). The risks can be
characterized using the lines-of-evidence approach
commonly used in ecological risk assessments
(U.S. EPA 1997, 1998). The characterization
includes uncertainty analysis to assist in risk
management. Incorporating the transition zone
leads to improved decision-making in the overall
ERA by reducing uncertainty in the conclusions of
which receptors/assessment endpoints are
significantly impacted, determining which
stressors dominate, and from which compartments
(e.g., surface-water, bedded sediments, upwelling
ground-water) those stressors originate.
5. Summary
The transition zone represents a unique and
important ecosystem that exists between surface-
water and the underlying ground-water, receiving
water from both of these sources. Biota inhabiting,
or otherwise dependent on, the transition zone may
be adversely impacted by contaminated ground-
water discharging through the transition zone into
overlying surface-waters. ERAs addressing
contaminated ground-water discharge to surface-
waters typically have not evaluated potential
contaminant effects to biota in the transition zone.
However, numerous hydrogeological and
ecological methods and tools are available for
delineating ground-water discharge areas in a
rapid and cost-effective manner, and for
evaluating the effects of contaminant exposure on
transition zone biota. These tools and approaches,
which are commonly used in hydrogeological and
ecological investigations, can be readily employed
within the existing EPA framework for conducting
screening- and baseline-level ERAs in Superfund
(U.S. EPA 1997) and satisfy the requirement to
identify and characterize the current and potential
threats to the environment from a hazardous
substance release.
6. Glossary
Abiotic: Characterized by absence of life; abiotic
materials include the nonliving portions of
environmental media (e.g., water, air, soil,
sediment), including light, temperature, pH,
humidity, current velocity, and other physical and
chemical parameters. Abiotic chemical reactions
are not biologically mediated (i.e., do not involve
microbes).
Acute: Having a sudden onset or lasting a short
time. An acute stimulus to a contaminant is severe
enough to induce a rapid response. With regard to
ground-water contamination, the term acute can be
used to define either exposure to a chemical (short
term) or the response to such an exposure (effect).
Aquifer: A body of geological materials such as
sand and gravel or sandstone, that is sufficiently
permeable to transmit ground-water and yield
economically significant quantities of water to
wells or springs
Assessment Endpoint: An explicit expression of
the environmental value that is to be protected,
such as specific ecological processes, or
populations/communities of organisms to be
protected (e.g., a sustainable population of insect
larvae important as fish food)
Baseline Ecological Risk Assessment: An
ecological risk assessment that evaluates the
exposure and effects of a contaminant to
ecological resources under site-specific exposure
scenarios and using site-specific physical,
chemical, and biological data.
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Benchmark Value: In ecological risk assessment,
a media-specific environmental concentration or a
receptor-specific dose concentration that
represents a threshold for adverse ecological
effects (a maximum "safe" chemical concentration
or dose). Media or dose concentrations at or below
a benchmark value are considered unlikely to
cause adverse ecological effects.
Benthos: The community of organisms (plants,
invertebrates, and vertebrates) dwelling on the
bottom of a body of surface-water (e.g., pond,
lake, stream, river, wetland, estuary, ocean).
Bioaccumulation: The process by which
chemicals are taken up and incorporated by an
organism either directly from exposure to a
contaminated medium or by consumption of food
or water containing the contaminant.
Bioaccumulation Factor (BAF): The ratio of the
concentration of a contaminant in an organism to
the concentration in the ambient environment at
steady state, where the organisms can take in the
contaminant through ingestion with its food and
water as well as through direct contact.
Bioconcentration: The process by which there is
net accumulation of a chemical directly from an
exposure medium into an organism.
Bioconcentration Factor (BCF): The ratio of the
concentration of a contaminant in an organism to
the concentration in the exposure medium, where
the organisms can take in the contaminant through
direct contact with the medium.
Biodegradation: The process by which chemical
compounds are degraded into more elementary
compounds by the action of living organisms;
usually refers to microorganisms such as bacteria.
Biomass: Any quantitative estimate of the total
mass of organisms comprising all or part of a
population or any other specified unit, or within a
given area at a given time; typically measured as a
volume or mass (weight).
Biome: A biogeographical region or formation; a
major regional ecological community
characterized by distinctive life forms and
principal plant or animal species.
Biotic: The living portion of the environment;
pertaining to life or living organisms; caused by,
produced by, or comprising living organisms.
Chronic: Involving a stimulus that is lingering or
continues for a long time; often signifies periods
of time associated with the reproductive life cycle
of a species. Can be used to define either exposure
to a chemical or the response to such an exposure
(effect). Chronic exposures to chemicals typically
induce a biological response of relatively slow
progress and long duration.
Community: Any group of organisms comprising
a number of different species that co-occur in the
same habitat or area and interact through trophic
and spatial relationships.
Community Analysis: An analysis of a
community within a specified location and time.
Community analyses may focus on the number of
different species present, the types of species
present, or the relative abundance of the species
that are present in the community.
Community Structure: Refers to the species
composition and abundance and the relationships
between species in a community.
Conceptual Site Model: Describes a series of
working hypotheses of how a stressor (chemical
contaminant) might reach and affect a biological
assessment endpoint; describes the assessment
endpoint potentially at risk from exposure to a
chemical, the exposure scenario for the receptor,
and the relationship between the assessment and
measurement endpoints and the exposure
scenarios.
Diffusion: The process by which both ionic and
molecular species dissolved in water move from
areas of higher concentration to areas of lower
concentration.
DNAPL: dissolved non-aqueous phase liquid
Downwelling: The movement of surface-water
down into or through the underlying porous media
(e.g., recharge to ground-water).
Ecohydrology: An emerging discipline linking
ecology with hydrology through the entire water
cycle over scales ranging from plant community
relationships with ground-water to watershed-level
23
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processes.
Ecological Risk Assessment: The process that
evaluates the likelihood that adverse ecological
effects may occur as a result of exposure to one or
more stressors.
Ecosystem: The biotic and abiotic environment
within a specified location and time, including the
physical, chemical, and biological relationships
among the biotic and abiotic components.
Ecotone: The boundary or transition zone between
adjacent communities or biomes.
Electrical Conductivity: A measure of the ability
of a solution to carry an electrical current.
Conductivity is dependent on the total
concentration of ions dissolved in the water
Environmental Value: (See Assessment
Endpoint). Environmental values include specific
ecological processes or populations/communities
of organisms to be protected (e.g., a sustainable
population of insect larvae important as fish food).
Epifauna: Biota that live on the surface of
sediment, as distinguished from infauna, which
live in the sediment.
Exposure Pathway: The course a chemical or
physical agent takes from a source to an exposed
organism. Each exposure pathway includes a
source or release from a source, an exposure point,
and an exposure route (including respiration [e.g.
via gills], ingestion, etc.). If the exposure point
location differs from the source,
transport/exposure media (i.e., air, water) are also
included.
Exposure Point Concentration: The
concentration of a contaminant at an exposure
point.
Food Web: The pattern of interconnected energy
(food) transport among plants and animals in an
ecosystem, where energy is transferred from plants
to herbivores and then to carnivores by feeding.
Ground-Water Discharge Zone: An area where
ground-water exits the subsurface as a spring or a
seep, as baseflow into a stream, or directly into an
overlying surface-water body (pond, lake, ocean).
Ground-Water/Surface-Water Interface: The
boundary between ground-water and surface-water
that occurs in the substrate beneath the surface-
water body. It is usually defined by examining and
mapping interstitial water quality to determine the
origin of the water. It may be very diffuse and
dynamic and difficult to define (compare with:
Transition Zone).
Habitat: The local environment occupied by an
organism with characteristics beneficial to the
organism. The habitat may be used only during a
certain life stage or season
Hydraulic Conductivity: The capacity of a rock
to transmit water. It is expressed as the volume of
water at the existing kinematic viscosity that will
move in unit time under a unit hydraulic gradient
through a unit area measured at right angles to the
direction of flow.
Hydraulic Gradient: The change of hydraulic
head per unit of distance in a given direction.
Hydraulic head: The height of the free surface of
a body of water above a given point beneath the
surface.
Hypolentic Zone: The zone of ground-water and
surface-water mixing that occurs in the sediments
beneath a lake or wetlands (not beneath moving
waters, see Hyporheic Zone).
Hyporheic Zone: Latticework of underground
habitats through the sediments associated with the
interstitial waters in the substrate beneath and
adjacent to moving surface-waters. The hyporheos
is the community of organisms adapted to living in
this zone. The zone is defined based on biological,
hydrological, and chemical characteristics.
Infauna: Biota that live within or burrow through
the substrate (sediment), as distinguished from
epifauna, which live upon the substrate
Infiltration: Process by which water moves from
the earth's surface or from surface-water down
into the ground-water system.
In Situ: Refers to a condition or investigation
(such as a toxicity test) in the environment (in the
field at a site).
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Interstitial Water: The water filling the spaces
between grains of sediment. Often used
interchangeably with "pore water." The term
indicates only the presence of water, not its origin.
Macroinvertebrate: An invertebrate animal large
enough to be seen without magnification and
retained by a 0.595-mm (U.S. #30) screen.
Measurement Endpoint: A measurable
ecological characteristic that is related to the
valued characteristic chosen as the assessment
endpoint; often expressed as the statistical or
arithmetic summaries of observations that make up
the measurement.
Meiofauna: The small biota (<1 mm diameter)
that inhabit the interstitial spaces in sediment.
Natural Attenuation: The natural dilution,
dispersion, (bio)degradation, irreversible sorption,
and/or radioactive decay of contaminants in soils
and ground-water.
Periphyton: Attached microflora growing on the
bottom of a water body, or on other submerged
substrates, including higher plants.
Permeability: The capacity of a rock for
transmitting a fluid; a measure of the relative ease
with which a porous medium can transmit a liquid.
Piezometer: A small-diameter, nonpumping tube,
pipe, or well used to measure the elevation of the
water table or potentiometric surface. A
piezometer may also be used to collect ground-
water samples.
Pore Water: The water filling the spaces between
grains of sediment. Often used interchangeably
with "interstitial water."
Potentiometric Surface: A surface that represents
the level to which water will rise in tightly cased
wells. The water table is the potentiometric surface
of an unconfined, or the uppermost, aquifer.
Problem Formulation: Problem formulation
establishes the goals, breadth, and focus for an
assessment. In a baseline ecological risk
assessment, problem formulation establishes the
assessment endpoints, identifies exposure
pathways and routes, and develops a conceptual
site model with working hypotheses and questions
that the site investigation will address.
Productivity: (1) The rate of formation of new
tissue or organisms, or energy use, by one or more
organisms. (2) Capacity or ability of an
environmental unit to produce organic material.
(3) Recruitment ability of a population from
natural reproduction.
Refuge (refugia): An area to which an organism
may escape to avoid a physical (e.g., temperature,
water current), chemical (e.g., low dissolved
oxygen, a high contaminant concentration), or
biologic stressor (e.g., a predator).
Risk: The expected frequency or probability of
undesirable effects resulting from known or
expected exposure to a contaminant.
Risk Characterization: A phase of an ecological
risk assessment in which the results of the
assessment are integrated to evaluate the
likelihood of adverse ecological effects associated
with exposure to a contaminant.
Risk Question: Questions developed during the
problem formulation phase of a baseline risk
assessment, about the relationships among the
assessment endpoints, exposure pathways, and
potential effects of the exposure. These questions
provide the basis for developing the risk
assessment study design and the subsequent
evaluation of the results.
Screening Ecological Risk Assessment: An
ecological risk assessment that evaluates the
potential for adverse ecological effects to
ecological resources under very conservative site-
specific exposure scenarios (e.g., maximum
documented exposure concentrations) and using
screening benchmark values.
Species Richness: The absolute number of species
in a community.
Stressor: Any physical, chemical, or biological
entity that can induce an adverse ecological
response (e.g., reduced reproduction, increased
mortality, habitat avoidance).
Surrogate Species: A species selected to be
representative of an assessment endpoint and on
which a risk characterization will focus.
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Total Organic Carbon (TOC): Estimated
concentration of the sum of all organic carbon
compounds in a water or sediment sample by
various methods. It can influence bioavailability
because some contaminants adsorb to organic
carbon.
Toxicity Test: An evaluation of the toxicity of a
chemical or other test material (environmental
media) conducted by exposing a test organism to a
specific level of the chemical or environmental
media and measuring the degree of response
(mortality, reduced growth, reduced egg
production) associated with the specific exposure
level.
Transition Zone: The zone of transition from a
ground-water dominated system to a surface-water
dominated system. It includes, but is not limited to
the zone where the ground-water and surface-
water mix as well as any Ground-Water/Surface-
Water Interface that may be present.
Unconfined Aquifer: An aquifer in which there
are no confining beds between the zone of
saturation and the surface.
Upwelling: The movement of water in an
underlying porous medium up into the surface-
water (e.g., ground-water discharge).
Water table: The elevation of the water surface
in a well screened in the uppermost zone of
saturation (ground-water), i.e., in an unconfmed
aquifer.
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