Relationship Between Sediment and Pore Water Concentrations

The difference between this concentration and the concentration in the overlying

water defines the driving force for contaminant release to that water. In addition,

it is normally this concentration that defines the sediment quality criteria because

it is this concentration that defines the contaminant levels to which benthic

organisms are exposed. Benthic organisms are generally the most sensitive

organisms in the sediment environment, and any contaminants that they may

accumulate may be transferred higher in the food chain. Isolation of contami-

nants from these benthic organisms is one of the most important motivations for

placement of a cap. The objective is to place a cap of sufficient thickness to

realize this isolation.

Relationship Between Sediment and Pore Water

Concentrations

Equation B1 defines an observed partition coefficient between the sediment and

the adjacent pore water. Use of a measured partition coefficient does not require

linearity or reversibility of the sorption isotherm, nor does it require specifica-

tion of the form of the contaminant in the pore water (e.g., dissolved or bound to

particles). For a compound that sorbs to soil with an observed partition coeffi-

obs

cient of Kd (liters/kilogram), the ratio of the total concentration in the soil to

that in the pore water is given by the retardation factor, Rf,

obs

(B3)

The retardation factor is so named because contaminant migration in the pore

water is slowed by the sorption onto the immobile sediment phase.

obs

The value of Kd for either the sediment or the cap should be determined

directly by evaluating the ratio of sediment or cap loading to pore water concen-

tration. In the absence of direct measurement of pore water concentrations,

obs

however, the value of Kd can be estimated for hydrophobic organic com-

pounds that tend to sorb reversibly and nonselectively upon organic matter in the

sediment or pore water. For these compounds, the observed partition coefficient

can be normalized by the amount of organic carbon present in the sediment or

pore water to define a "universal" partition coefficient, Koc, that should be con-

stant for a particular compound. Given such a contaminant at concentration sed

in the sediment, the concentration dissolved in the pore water is given by

7sed

Cdiss

(B4)

Koc foc

Here foc is the fraction organic carbon in the sediment in mass organic carbon per

mass dry sediment. The same relation applies to the capping material if the con-

centrations and properties are characteristic of the cap rather than the underlying

sediment.

Appendix B Model for Chemical Containment by a Cap