Evaporation mass transfer coefficient

information to the contrary, is assumed to be 1. The biodiffusion coefficient

and the depth of bioturbation are important factors in the determination of the

required cap thickness, and thus the best possible estimates should be used. The

ranges for Dbio and Lbio are quite large, and an extensive tabulation is presented

by Matisoff (1982). An examination of these data suggests that a depth of bio-

turbation of 2 to 10 cm is typical and that biodiffusion coefficients are generally

in the range of 0.3 to 30 cm2/year. As indicated previously, however, the con-

taminant flux is controlled by transport through the cap and is essentially

insensitive to the bioturbation mass transfer coefficient. The contaminant

concentration in the bioturbated layer, however, is heavily dependent upon the

biodiffusion coefficient.

Evaporation mass transfer coefficient

The overall evaporation mass transfer coefficient is taken as equal to the

water-side mass transfer coefficient. This is generally valid for volatile organic

compounds but less true for many PAHs, which tend to exhibit significant air-

side mass transfer resistances. A water-side mass transfer coefficient for evapo-

rative losses is given by Lunny, Springer, and Thibodeaux (1985) as

2.23

2/3

19.6 Ux

(B39)

where Ux is the wind speed at 10 m (miles/hour), Dw has units of square

centimeters/second, and Ke has units of centimeters/hour. Lyman, Reehl, and

Rosenblatt (1990) provide information on air-side coefficients that may be

important for some compounds, notably low-volatility PAH compounds.

Example

Several design bases are possible for specifying the physico-chemical con-

tainment afforded by a cap. There are at least five quantities that may be of

interest to the cap designer and for which models were presented here. These are

the breakthrough time, the pollutant release rate (as a source term input to other

fate and effects models), concentrations at the sediment-water interface or in the

overlying water column, and the time to approach steady state. The two physico-

chemical properties of the cap material that have the largest effect on the effi-

cacy of the cap are the organic carbon content and the cap thickness. Each of

these calculations will be illustrated given a cap thickness. In general, the

process would be applied iteratively using a guessed cap thickness until the

desired breakthrough times, fluxes, etc, are achieved.

The selected example considers a sediment contaminated with a moderately

hydrophobic polyaromatic hydrocarbon, pyrene. The contaminant is initially

present in the upper 35 cm of sediment at a level of 100 mg/kg. A cap of initial

thickness of 50 cm is placed over this sediment. Both the cap and the sediment

contain 1-percent organic carbon. Consolidation of the cap after placement

B20

Appendix B Model for Chemical Containment by a Cap