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

 

 

THEORETICAL SQGs

 

Theoretically formulated SQGs employ equilibrium partitioning (EqP) to describe the bioavailabilty of contaminants from sediments to benthic organisms.  Using concentrations in sediments along with material properties, the amount of contaminants from sediments that will enter the water and thus benthic population can be estimated.  It is generally assumed for these methods that the entire mass of the contaminant in solution is available for benthos uptake. The certainty in this method relies on the precision of measurement of contaminant concentrations and material constants as well as an accurate description of the system to be evaluated.

 

Aqueous Concentration EqP Example

 

To demonstrate the first application of EqP SQGs, we will consider a simple EqP procedure utilized by the Army Corps of Engineers in the investigation of a confined disposal facility in the Saginaw River for dredged sediments from a Federal navigation channel (Myers, 1991).  The dredged sediments were being investigated for concentrations of PCBs.

 

Assumptions that have been made include equilibrium conditions, no sorption of the contaminant to sediment particles and no contamination in the pore space. All of the contaminant either exists in solution in the water or in the solid phase. Note that while this example is meant to be instructive of the theory behind EqP, more recent and complex formulations have been proposed as stated in the previous section.

 

A material property of organic contaminants, the partitioning constant, Kp, is a measure of the solubility of the contaminant in water and can be used to predict the concentrations of a mass of that contaminant that will be present in the aqueous phase. Kp has been measured for most chemicals and can be found in reference tables or calculated as shown in Equation 1.

 kP    (Eqn. 1)

In the most general equation (Equation 2), assuming equilibrium of the system, Kp is equal to the ratio of the concentration of the contaminant in the aqueous phase to the concentration in the solid phase.  

  (Eqn. 2)

While this equation is quite simple and only assumes equilibrium, both Kp and the current concentration of the contaminant in the solid phase must be assumed or measured, although average Kp values are readily available in tables for most contaminant.

Equations 3 and 4 can also be used in more stipulative scenarios, and equilibrium assumptions still apply.

 

                           Capture2                                                 (Eqn. 3)                           

Equation 3 is only applicable for closed systems which is often an oversimplification for aquatic systems.  However, all values besides Kp can be easily known. In addition, this formula provides the initial value for the contaminant concentration in the sediments.

                                                                                      (Eqn. 4)

Equation 4 is a simplification of Equation 2 that can be made if one assumes large value for both the sediment concentration and partitioning constant.

 

Additional complexities could be added to this system by incorporating the uptake rate of the contaminant from the water to the benthic organisms or the rate of decay of a contaminant.

 

Adorbtion EqP Example

 

The aqueous solution formulation may be modified to capture the sorption of the contaminant to sediment particles. Conditions are still at equilibrium, but this example assumes that the contaminant exists in the aqeous phase, sorbed to particles, and in the solid phase. This describes a three-phase contaminant system which is a more accurate description for sediment contamination.

 

For Adsorbtion, another material property, Kads, will need to be measured or found in a reference table for the contaminant. In most cases, it is best to measure this value in a lab because it will change based on the type of contaminants and sediments.

 

If we assume that the contaminant concentration is small and the surface area of available sediments will not be saturated, we can use Equation 5.

 

sorption (Eqn. 5)

 

As we can see, the mass of the sorbed contaminant is dependent on an accurate description of Kads and the concentration in the aqueous phase. Additionally, the mass of sediments may be hard to estimate depending on the size and mobility of the system.

 

This example does not account for a phenomenon known as sorbtion hysteresis in which contaminants in the pore space of the sediments replace other sorbed contaminant molecules and vice versa, a process called sorption hysteresis.

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