The International Information Center for Geotechnical Engineers

Sediment Quality Guidelines (SQGs): A Review and Their Use in Practice

 

 

EMPIRICAL SQGs

 

Empirically-based SQGs are based on field and laboratory data of benthic organism responses to exposure to metal-contaminated sediments.  These relationships are determined through compilation of data from many different sites with different types and concentrations of contaminants.

 

The following subsections provide examples on the formulation and use of two emprical SQGs approaches.

 

Threshold Effect Limit (TEL) and Probable Effect Limit (PEL) Approach

The TEL is a sediment contamination concentration at which a toxic response has started to be observed in benthic organisms. The PEL is the concentration at which a large percentage of the benthic population shows a toxic response. Using engineering judgement, the consequences of sediment contamination and need and types of applications for remediation can be inferred.

The state of Florida developed equations to determine TEL and PEL based on the concentrations at which benthic organisms from coastal waters showed toxic responses in the lab. Which are shown in Equations 1 and 2, respectively.

 

 TEL

 

PEL 1

 

It is assumed that sediment contamination concentrations below the TEL are acceptable and concentration above the PEL are unacceptable. The grey region in between the TEL and PEL requires further study and judgement to determine the likelihood of environmental consequences.

 

There have been many developments of these types of effects levels by different agencies for different species of heavy metals. Burton references the available limits, as of 2001, for concentrations developed which are similar to the TEL and PEL, although with different names depending on the developer. These are shown in Tables 1 and 2 as a reference to the available SQGs and magnitude of the concentrations that induce toxic responses.

 

TEL Table

Table 1. Concentrations analogous to the TEL developed by different agencies, listed down the rows, for different types of heavy metals, listed as the columns, in milligrams/kilogram (Burton, 2001).

 

 

PEL Table

 

Table 2. Concentrations analogous to the PEL developed by different agencies, listed down the rows, for different types of heavy metals, listed as the columns, in milligrams/kilogram (Burton, 2001).

 

Tables 3 and 4 show threshold effects and probable effects concentrations for organic compounds, also arranged by Burton.

 

TEL Table Organics

 

Table 3. Concentrations analogous to the TEL developed by different agencies, listed as the columns, for different types of organic compounds, listed down the rows, in micrograms/kilogram (Burton, 2001).

 

PEL Table Organics

 

Table 4. Concentrations analogous to the PEL developed by different agencies, listed as the columns, for different types of organic compounds, listed down the rows, in micrograms/kilogram (Burton, 2001).

 

Consensus-Based Empricial SQG Approach

 

The Wisconsin Department of Natural Resource (WI DNR) uses the consensus-based SQG approach for sediment quality assessment to try and limit the uncertainty associated with concentrations that lie between the threshold-effects and probable-effects concentrations.

This method includes a Threshold Effect Concentration (TEC), the contaminant concentration below which a toxic response is not expected and formulated using a TEL-type approach, a Midpoint Effect Concentration (MEC), and a Probable Effect Concentration (PEC), the concentration above which a toxic response is expected and formulated using a PEL-type approach,.  Table 2 shows the approaches recommended by the WI DNR in determing the TEC and PEC. It is also possible to formulate a site-specific TEC and PEC based on laboratory test data for the contaminants and benthos present at a site. Next, TEC and PEC are selected based on allowable risk and level of conservatism deemed appropriate for the project, as either the average result from a suite of SQGs in Table 5 or concentrations at which chosen percentiles of toxic responses are observed in the lab.

 

Table 5. SQGs used by the WI DNR to develop the TEC and PEC.

 

Once a TEC and PEC have been established, they can be used to develop the MEC by taking the average of the TEC and PEC.  Now, contaminant concentrations obtained from field tests can be compared to the three thresholds to indicate the level of concern the contaminants concentrations merit, 1, 2, 3, or 4, with Level 1 being the least concerning and Level 4 the most.  This methodology and a numerical example for arsenic are shown in Table 6.

 

Table 6. An example of the use of the TEC and PEC by the WI DNR.

 

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