Attachment 1-2
Guidance for Developing Ecological
Soil Screening Levels (Eco-SSLs)
Assessment of Whether to Develop Ecological Soil Screening Levels
for Microbes and Microbial Processes
OSWER Directive 92857-55
November 2003
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Assessment of Whether to Develop Ecological Soil Screening Levels for Microbes and
Microbial Processes
Executive Summary
Bacteria and fungi (i.e., microbes) are essential components of soil systems and the importance
of microbial processes in terrestrial systems is well recognized. The Eco-SSL workgroup had
lengthy discussions as to the merits of deriving microbial Eco-SSLs, and the potential
significance of either having or not having Eco-SSLs for microbes in this guidance. As an
outcome of these discussions, the Eco-SSL Steering Committee decided that Eco-SSLs for
microbes would not be developed at this time for this guidance. The decision to not include
microbial Eco-SSLs in this guidance should not be interpreted as any indication as to the
perceived importance (or lack of) of microbes to soil ecosystems, or that microbes and microbial
processes do not merit protection. Instead, it is the Steering Committee's recommendation that
evaluation of risks to microbes and microbial processes at a Superfund site not be conducted at
the screening level, but rather decisions as to the appropriate actions be discussed and decided
among the decision makers (e.g., risk managers, risk assessors, responsible parties) as part of the
baseline risk assessment. If determinations are made that microbes are appropriate assessment
endpoints for a Superfund site, problem formulation and workplans should be developed to
specifically address issues associated with scale and uncertainty for these sites.
Introduction
Microbes are essential components of soil systems. Microbes comprises nearly 90% of global
biomass and biodiversity in ecosystems. In soil, microbial decomposition of organic material
governs the rate of mineralization and the cycling of essential nutrients, and is an important
determinant of soil fertility. Without the microbial biomass and the functions it performs, plants
are not self-sustaining. The diversity of microbial taxa involved in various nutrient cycling
processes can be related to the ubiquitous nature of certain substances. For example, relatively
large numbers of microbial taxa can catabolize simple sugars and simple proteins, but
substantially fewer taxa catabolize complex molecules such as lignin, keratin, or chlorinated
biphenyls.
In non-agricultural settings, the importance of associations between plant roots and microbes
(mycorrhizae) has gained recognition over the past twenty years. In particular, mutualism
achieved between a host plant species and one or more fungal species is essential for certain
plant species to flourish. Indeed, estimates are that more than 95% of all plant species and 99%
of all plant individuals are mycorrhizal dependent - that is to say that elimination of mycorrhizal
associations leads to dramatic shifts in the composition of the plant community. Even so, most
associations are facultative and because there is great taxonomic diversity among fungal
symbiots, the loss of one mycorrhizal species would not translate to a meaningful shift in
primary production or structural shift in plant community composition. This is because
Guidance for Developing Eco-SSLs Attachment 1-2 1 November 2003
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individual plants often are colonized simultaneously by a few to several genera of mycorrhizae.
Although microbes are recognized as essential components of soil ecosystems, Eco-SSLs for
microbes and microbial processes have not been included in this EPA Eco-SSL guidance. The
decision not to provide these Eco-SSLs in this guidance was made following lengthy discussions
among workgroup members that included both scientific and non-technical (e.g., policy)
considerations. In support of these discussions, draft position papers on the subject were
developed by the workgroup; one paper represented the merits for microbial Eco-SSLs for
inclusion in this guidance, and the other paper presented reasoning for not deriving microbial
Eco-SSLs for this guidance. In addition, a debate on this issue was conducted at the 20th Annual
Meeting of the Society of Environmental Toxicology and Chemistry (SET AC 1999).
Considerations and Rationale for Not Deriving Microbial Eco-SSLs for this Guidance
The considerations and rationale for the Eco-SSL Steering Committee's decision not to derive
EcoSSLs for microbes or microbial processes can be summarized as:
• This Eco-SSL guidance has been developed to support risk-management decisions for
Superfund sites. As Superfund sites, these sites are identified as having significant
contamination potentially present for many years or even decades. Contaminant risks of
significance to the environment can be reasonably expected to be manifested to higher
organisms (i.e., plants, invertebrates, wildlife). Therefore, the risk assessment should
quantify risks for higher organisms. The absence of unacceptable risk to higher
organisms would indicate that risks to microbes are not of a magnitude or significance to
the ecosystem to warrant separate risk management considerations. There was a
consensus within the Steering Committee that it is unlikely that site conditions would
pose unacceptable risk only to microbes and not be reflected as unacceptable risk to
plants, soil invertebrates, or wildlife. However, it may be important to develop endpoints
in the baseline risk assessment to evaluate microbial processes. In the baseline risk
assessment, attention can be focused on microbial processes as a further explanation of
expected risks for plants, soil invertebrates, or wildlife, to address those situations where
sustained dysfunction of microbial functions can be documented.
• There are a considerable number of published laboratory investigations that demonstrate
contaminant effects to microbes or microbial processes, however, the actual significance
and relevance of much of these data to Superfund sites is uncertain. Uncertainties exist
as to whether laboratory microbial ecotoxicological studies are good surrogates for
natural environments due to differences in factors such as scale, functional redundancies,
and complexities. Based on the potential uncertainties associated with these data, the
Steering Committee concludes that risk management would be better served by not
making screening-level decisions based on these data; instead, assessments of potential
risk to microbial communities and functions should be based on the baseline risk
assessment that utilizes site-specific information rather than generic, screening-level data.
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The scale and dynamic nature of microbes presents a challenge to defining conditions
that indicate actual or potential adverse conditions for microbes. Measurements of
microbial function and community structure are subject to great spatial (across millimeter
distances) and temporal (within minutes to hours) variation, which makes an evaluation
of ecological consequence of any measured change challenging. Of the endpoints typical
used in microbial ecology and toxicology, (e.g., population, diversity, respiration,
enzyme activity, substrate use, mutation rate) each is modified substantially during
testing. Microbial endpoints tend to be highly responsive to fluxes of temperature,
moisture, oxygen and many other non-contaminant factors. While threshold response
levels for plants, soil invertebrates, and terrestrial wildlife among different high quality
studies are generally in good agreement (i.e., typically within an order of magnitude),
microbial responses which can be dramatically influenced by testing conditions often
range across several orders of magnitude among studies (e.g., from <10 ppm to >1,000
ppm) for a given chemical with no apparent explanation for the differences. Functional
redundancy across broad taxonomic groups enables swings in community composition
without remarkable change in rates of decomposition or community respiration.
Consequently, it is exceedingly difficult to relate specific microbial activities with
indications of adverse and unacceptable environmental conditions. For the most common
microbial activities, functional redundancy easily masks population shifts.
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