7.5 Dispersion Evaluations for Volatile Emissions

1. Information is sufficient to reach a decision regarding volatile emissions.

In this case either:

a. Volatile emissions, after consideration of dispersion, are below

applicable OSHA standards. No further emissions evaluation is

necessary.

b. Volatile emissions, after consideration of dispersion, exceed

applicable OSHA standards, and management actions should be

considered. A decision to implement management actions for

emissions, such as capping or treatment, may require more detailed

information prior to design of such actions. If management actions

are selected, no further emissions evaluation is necessary.

2. Information is not sufficient to reach a decision regarding volatile

emissions. Further evaluation in Tier IV, or management actions as an

alternative to further evaluation, should be considered. A decision to

implement management actions for emissions, such as capping or

treatment, may require more detailed information prior to design of such

actions. If management actions are selected, no further emissions

evaluation is necessary.

7.5 Dispersion Evaluations for Volatile Emissions

Actual contaminant concentrations in the air resulting from sediment

contaminant fluxes are site specific and are affected by atmospheric conditions

such as wind speed, mixing, temperature, as well as the location of the receptor.

To evaluate the impact of sediment contaminant fluxes upon site and near-site air

concentrations, a conservative estimate of actual air concentrations should be

applied for both Tier II and Tier III volatile evaluations. An example scenario to

estimate contaminant air concentrations could incorporate maximum fluxes

obtained from modeling or laboratory testing into calculations that assume a

worst-case, well-mixed set volume of air over the CDF. Contaminant

concentrations can then be estimated for a predetermined period of time to give a

conservative estimate of possible contaminant air concentrations.

The contaminant flux predictions obtained from the models and fluxes

obtained from evaluation of sediment properties or laboratory testing can be

converted to an exposure concentration to evaluate the emission. The

contaminant emission is mixed with the overlying column of air, which is stripped

or entrained into prevailing winds and transported offsite. The resulting

contaminant concentration in the air overlying the site is a function of the

contaminant flux, size of the site, and the air exchange rate with prevailing wind.

The air exchange rate is a function of wind speed and site exposure. As such, the

evaluation should be performed at low, medium, and high wind speed.

A screening model for evaluation of dispersion is included in the Tier II

spreadsheet calculations for volatiles. An additional model developed using data

obtained from testing conducted with the laboratory apparatus described in

7-11

Chapter 7

Guidance for Evaluation of Volatile Emissions