Frequently Asked Questions (FAQs) about SQuiRTs

Here you can find answers to common questions about the Screening Quick Reference Tables (SQuiRT) cards.

Screening Quick Reference Tables (SQuiRTs) • top

Q. On page 2, what does the "Increasing" predicted toxicity gradient mean?

A. Each sediment quality guideline is intended to describe a particular—but unique—point in a spectrum of toxicity. These points range from lower thresholds, below which samples are presumably nontoxic, to upper thresholds of toxicity, above which toxicity is predicted. The guidelines have been ordered according to the level of toxicity each is intended to describe, from low to high.

Q. Why don't the individual chemical values always increase across the predicted toxicity gradient?

A. The individual measurement endpoint for a Upper Effects Threshold (UET) or Apparent Effect Thresholds (AET) may be more sensitive than a Probable Effects Levels (PELs) or Effects Range-Median (ERM) value, since PEL and ERM values incorporate several endpoints in their determination. A UET, therefore, may have a lower value than a PEL.

Q. Why are background levels included?

A. Background levels have no development basis in risk characterization or toxicity evaluation. However, for purposes of screening samples which may be indicative of contaminant source areas (the main purpose of the SQuiRT cards is to aid you in doing this screening), background concentrations are useful for distinguishing possible contaminant sources.

Q. Why does NOAA apply a default dilution factor of only 10x for the discharge of ground water to surface water?

A. We prefer to use site-specific information whenever it is available. But because such data have not been derived, we acknowledge that some level of dilution would occur. We chose to use a conservative order-of-magnitude dilution factor for screening purposes to ensure a high degree of confidence that any contaminant source eliminated from further consideration is not likely to pose substantial risk. Conversely, this is not meant to imply that contaminant sources that do not pass this screening do pose risk.

Q. Why isn't the value for total DDTs in the marine TEL/PEL, ERL/ERM, or AET benchmarks simply the sum of the individual three isomers?

A. This is a great example of why an understanding of the derivation of these various benchmarks is key to their proper application. The total DDT benchmark is not simply a mathematical summation of the other three benchmarks. It is derived by the same data evaluation process as any other compound. Therefore, it is responsive to the most potent of the isomers that were present in the samples used to generate the benchmark.

The total DDT benchmark should only be used for screening when there is no more specific information about the constituent composition of “total DDT.” Because each isomer has its own potency, individual concentrations of each should be screened whenever possible.

Q. If I am evaluating organic carbon normalized data, shouldn’t I adjust the sediment benchmarks (e.g., TELs, PELs, ERLs, ERMs etc.)?

A. Essentially, no. These benchmarks (as noted on the cards) were generally developed on a dry weight basis (there are some exceptions, such as the freshwater UETs). Adjusting the benchmarks to a default or assumed level of Total Organic Carbon (TOC) would be inappropriate. It would be better to adjust the raw data back to a dry weight basis. This is typically the way a laboratory originally would have reported the raw data anyway.

SQuiRTs: Sediment Benchmarks FAQ • top

Q. What are T20 & T50 values?

A. Individual chemical logistic regression models were developed based on 10-day survival results from marine amphipod tests (Ampelisca a. and Rhepoxynius a.). Chemical concentrations corresponding to 20% and 50% probability of observing toxicity were then calculated.

Q. What are LEL & SEL values?

A. These benchmarks are based upon chronic, long-term impacts of contamination to benthic organisms. The Lowest Effect Level (LEL) is a level of sediment contamination that can be tolerated by the majority of benthic organisms. The Severe Effect Level (SEL) is that at which pronounced disturbance of the sediment-dwelling community can be expected. This is the concentration that would be detrimental to the majority of the benthic community.

If a single parameter equals or exceeds the LEL, it is anticipated that material represented by that sample may have an adverse effect of some benthic resources. If all analytes (the subjects of your chemical analyses) are below LELs, no significant effects are predicted. If any single analyte equals or exceeds its SEL, the representative material is considered highly contaminated and is anticipated to have significant adverse impacts on benthic resources.

LELs and SELs are based upon the Screening Level Concentration approach using benthic community observations, and are calculated through a two step process:

  • Step 1: Analyte concentrations at all locations where a given benthic species has been observed are plotted in increasing order, and the 90th percentile is calculated as a conservative estimate of the upper tolerance range for that species.
  • Step 2: The 90th percentiles for all species are ranked in increasing order, and the 5th and 95th percentiles are calculated as the LEL and SEL. A minimum of 20 species must be included.

Total Polycyclic Aromatic Hydrocarbons (PAHs) for this benchmark represent the sum of 16 parent compounds.

Q. What are the ARCS Hyalella azteca TELs?

A. The Assessment and Remediation of Contaminated Sediments (ARCS) program in the Great Lakes has sponsored numerous investigations which included bioassays for the amphipod Hyalella a. These results, along with those from other freshwater areas, were used to generate a Threshold Effects Levels (TEL) benchmark value.

Q. What are Effects Range-Low (ERLs) and Effects Range-Median (ERMs)?

A. These benchmarks are based upon a database primarily of synoptic marine sediment chemistry and sediment toxicity bioassay data. For a given analyte, the samples which were categorized as toxic by the original principal investigator is excerpted, and that subset is then ranked by increasing analyte concentration and the 10th and 50th percentiles determined. As such, these benchmarks are not analogous to LC10s or LC50s (lethal concentrations to 10% or 50% of the sample population).

Q. What are AETs?

A. Apparent Effects Thresholds (AETs) are another benchmark based upon empirical relationships between sediment concentrations and observed toxicity bioassay results or observed benthic community impacts. For each analyte, paired observations are ranked in increasing concentrations. The highest concentration associated with a nontoxic sample then sets the AET value, such that only toxic samples are observed at higher concentrations. Note, however, that toxic samples may also have been observed at values below the AET. AETs are applied as a set, such that a single analyte exceeding its AET would be predictive of adverse impacts. Separate AETs are developed for specific bioassay endpoints (species) and for benthic community impacts. AETs were originally developed using marine data; however, freshwater values have subsequently been calculated as well.

Q. What are TECs & PECs?

A. Threshold Effect Concentrations (TECs) and Probable Effect Concentrations (PECs), sometimes referred to as “consensus-based values,” are merely the average of numerous other benchmarks, many of which have been included in the SQuiRT cards, such as SELs and PELs. They combine benchmarks derived from both freshwater and marine data.

Q. What are the Dutch target, intervention, and serious values?

A. The Dutch values are all derived with environmental harmonization in mind, meaning that analyte concentrations in any given matrix are set such that they should not contaminate another matrix above risk-based values for that matrix. These standards allow for the definition of clean (target values), slightly contaminated, or seriously contaminated (intervention values). Recommendations for these risk-based standards are developed within one agency, and then the regulatory standards, usually based on those recommendations, are set by another. When data were not sufficient to support for full intervention values, recommendations of values representing serious contamination levels have been reported. For some analytes, where recommendations have not progressed through the regulatory process, the Serious Risk Concentration (SRCeco) is reported.

The target values indicate the level at which there is a sustainable quality. In terms of curative policy, this means that the target values indicate the level that has to be achieved to fully recover the functional properties for humans and plant and animal life. Besides this, the target values give an indication of the benchmark for environmental quality in the long term on the assumption of negligible risks to the ecosystem.

Q. What are Threshold Effects Levels (TELs) and Probable Effects Levels (PELs)?

A. These benchmarks are similar to ERLs/ERMs but have distinct differences in their derivation. Like ERLs/ERMs, their derivation is based upon a database of synoptic contaminant concentrations and sediment toxicity bioassays or benthic community metrics. The database for TELs/PELs, however, is much larger. Also, the freshwater TELs/PELs in particular are more robust because the database is roughly split between toxicity bioassays and benthic community metrics. Most importantly though, unlike ERLs/ERMs, TEL and PEL calculations do make use of the non-toxic samples: These benchmarks are calculated as a geometric mean using the full suite of information from the database (see their original publication for the full equations).

SQuiRTs: Soil Benchmarks FAQ • top

Q. What are Eco-SSLs

A. The Ecological Soil Screening Level (Eco-SSL) derivation process represents the collaborative effort of a multi-stakeholder workgroup consisting of federal, state, consulting, industry, and academic participants led by the U.S. Environmental Protection Agency (EPA). It is important to note that the Eco-SSLs are soil screening numbers, and as such, are not appropriate for use as cleanup levels. Screening eco-toxicity values are derived to avoid underestimating risk. Requiring a cleanup based solely on Eco-SSL values would not be technically defensible.

Ecological Soil Screening Levels (Eco-SSLs) are concentrations of soil contaminants set at a level which protects ecological receptors that commonly interact with biota living in or on soil. Eco-SSLs are derived separately for four groups of ecological receptors: plants, soil invertebrates, birds, and mammals. As such, these values are presumed to adequately protect terrestrial ecosystems. Eco-SSLs are derived to be conservative in their estimates of the exposure and effects on species and are intended to be applied at the screening stage of an ecological risk assessment.

The Eco-SSL documents provide an overview for each contaminant. Separate discussions are provided for each receptor group, including a comprehensive list of literature evaluated under the effort and a summary of data used in deriving Eco-SSL values.

SQuiRTs: Water Benchmarks FAQ • top

Q. What are MCLs?

A. Maximum Contaminant Levels (MCLs), or Treatment Techniques, are legally enforceable standards that apply to public water systems. Primary standards protect drinking water quality by limiting the levels of specific contaminants that can adversely affect public health and are known or anticipated to occur in water. MCLs are set by the U.S. Environmental Protection Agency (EPA) under the Safe Drinking Water Act authorities.

MCLs are listed under "groundwater" because they are a common standard applied to this source of drinking water.

Q. What are secondary standards?

A. These are non-enforceable guidelines regarding contaminants that may cause cosmetic effects (such as skin or tooth discoloration) or aesthetic effects (such as taste, odor, or color) in drinking water. The Environmental Protection Agency (EPA) recommends secondary standards to water systems but does not require systems to comply. However, they are included in the SQuiRT cards because states may choose to adopt them as enforceable standards.

Q. What is a LOEL?

A. A Lowest Observable Effect Level (LOEL) is the lowest concentration or amount of a substance, found by experiment or observation, which causes any measurement endpoint (alteration in morphology, functional capacity, growth, development, or life span) of target organisms to be distinguishable from control organisms of the same species and induces strain under the same defined conditions of exposure.

LOELs previously reported by the Environmental Protection Agency (EPA) are included in the SQuiRT cards because many states have chosen to promulgate standards based on these values.

Q. What is CMC?

A. The Criterion Maximum Concentration (CMC) is synonymous with the "acute" Ambient Water Quality Criterion. CMC is an aqueous contaminant concentration not to be exceeded by 1-hour average concentration once every three years. It is calculated as one-half of the final acute value (FAV), which, in turn, is the fifth percentile of the set of the genus mean acute values (GMAV) for species representatives of at least eight animal families: two families in the phylum Chordata, one family in a phylum other than Arthropoda or Chordata, either the Mysidae or Penaeidae family, and four other families.

Q. What is CCC?

A. The Criterion Continuous Concentration is synonymous with the "chronic" Ambient Water Quality Criterion. CCC is an aqueous concentration not to be exceeded by the 96-hour average concentration once every three years. It is a minimal value out of the final chronic (FCV), plant (PCV), and residue (FRV) values.

Q. What are AWQC?

A. Ambient Water Quality Criteria serve as the scientific foundation for state surface water quality standards. Separate AWQC are derived to protect aquatic life and human health. The Environmental Protection Agency’s (EPA) Criteria Table lists aquatic life, human health, and nutrient criteria for 158 pollutants, 15 of which were last revised in December 2003. The target goal of the aquatic life AWQC is to protect 95% of aquatic species.

Section 304(a)(1) of the Clean Water Act (CWA) requires EPA to develop, publish, and, from time to time, revise criteria for water accurately reflecting the latest scientific knowledge. Water quality criteria developed under section 304(a) are based solely on data and scientific judgments. They do not consider economic impacts or the technological feasibility of meeting the criteria in ambient water. Section 304(a) criteria provide guidance to states and tribes in adopting water quality standards. The criteria also provide a scientific basis for EPA to develop federally promulgated water quality standards under section 303(c) of the CWA.

Q. What are EcoTox Thresholds?

A. According to the EPA website, “The Superfund program has initiated a project to develop media-specific benchmark values for those chemicals commonly found in surface water, sediment, and soil samples at sites (values for soil are still being developed). The values are referred to as Ecotox Thresholds (ETs), and are defined as media-specific contaminant concentrations above which there is sufficient concern regarding adverse ecological effects to warrant further site investigation. ETs are designed to provide Superfund site managers with a tool to efficiently identify contaminants that may pose a threat to ecological receptors and focus further site activities on those contaminants and the media in which they are found. ETs are meant to be used for screening purposes only; they are not regulatory criteria, site-specific cleanup standards, or remediation goals. For those chemicals with the potential to bioaccumulate to toxic levels in upper trophic wildlife (e.g., methyl mercury, PCBs, DDT, dioxins, and lead), these benchmarks may not be low enough at some sites. The maximum concentration of each chemical at a site is compared to the medium-specific ET to evaluate whether further risk assessment for the chemical is warranted.”

The ETs are essentially a composite of benchmarks derived from other programs, such as AWQC (Ambient Water Quality Criteria) or Tier II for water and ERLs for sediment. Sediment values used for the SQuiRT cards are those based upon EPA’s Equilibrium Partitioning (EqP) approach, assuming 1% Total Organic Carbon (TOC) (EPA 1995). EqP applies chemical partitioning theory to predict the sorbed sediment concentration that would result if water concentrations were set at some benchmark, such as AWQC. It employs numerous assumptions, such as that toxicity for benthic sediment organisms matches that are used to derive AWQC; that water is the only uptake route; that conditions are in equilibrium; that partitioning is adequately represented by octanol:water partition coefficients; and so on.

Q. What is the basis for Canadian Water Quality Guidelines for the Protection of Aquatic Life?

A. Canadian Water Quality Guidelines for the Protection of Aquatic Life help to protect all plants and animals that live in lakes, rivers, and oceans by establishing acceptable levels for substances that affect water quality. As long as conditions are within the levels established by the guidelines, one would not expect to see negative effects in the environment. The guidelines are based on toxicity data for the most sensitive species of plants and animals found in Canadian waters and act as science-based benchmarks for the protection of 100% of the aquatic life species in Canada, 100% of the time.

Q. What is the basis for British Columbia water quality standards?

A. Provincial water quality guidelines are safe levels of substances for the protection of a given water use, including drinking water, aquatic life, recreation, and agricultural uses. In aquatic environments, water quality includes the physical, chemical, and biological quality of the water, sediment, and biota. These guidelines are being developed by the province substance by substance, starting with those most urgently needed for water quality assessments and objectives.

Q. What are the British Columbia working guidelines?

A. According to the Ministry of Environment for British Columbia, Canada, the Working Guidelines are those “guidelines that have not yet been approved by the Province—hence, called the working guidelines. These guidelines were obtained from various Canadian (primarily the Canadian Council of the Ministers of the Environment, or CCME), and other North American jurisdictions. These working guidelines provide benchmarks for those substances that have not yet been fully assessed and formally endorsed by the Ministry. They will be reviewed by the Ministry on a priority basis for their formal approval and use in British Columbia.