Dynamic modeling to assess worker exposure to gas-phase volatile organic compounds in collector sewers

A dynamic model was developed to simulate the behavior of volatile organic compounds (VOCs) in collector sewers when released to the sewer as a slug. The model incorporates liquid-gas mass transfer in sewer reaches and drop s tructures. The model was Configured to reflect the structure and flows of a full-scale collector sewer that had been previously characterized by a tra cer study. Selected release scenarios were simulated to reflect potential s pills into the sewer system. In the scenarios, the response of benzene, tol uene, and tetrachloroethylene were predicted and the effect of sewer struct ures and flow conditions on headspace concentrations was assessed. It was f ound that the presence of inverted siphons can substantially reduce gas-pha se concentrations immediately downstream of the siphon. Tributary sewers th at contribute contaminant-free air to the collector can substantially reduc e gas-phase concentrations. Drop structures that enhance liquid-gas mass tr ansfer can create zones of elevated.-as-phase concentrations of VOCs. Conta minants that enter a sewer in the gas phase can be scrubbed into the aqueou s phase, thereby reducing gasphase concentrations. Application of the model to selected spill scenarios suggests that spills of gasoline into sewer sy stems may result in worker exposures to benzene that exceed short-term expo sure limits for this substance. Excessive exposures were observed irrespect ive of whether the compounds entered the collector sewer at the upstream en d that had relatively low flow rates or part way along the collector sewer where flowrates were well established. Spills of other common solvents such as toluene and tetrachloroethylene did not seem to be likely to cause unac ceptable exposures.