Plume Dispersion Modeling of Aromatic VOC from the Deep Water Horizon Oil Spill

Environmental impacts of air toxics and criteria pollutants from the 2010 Deep Water Horizon oil spill off the Louisiana coast are highly uncertain due to the sparseness of data. As such, estimates of surface concentrations and integrated exposure rely heavily on model calculations constrained by available information. The limited aircraft and ship-based measurements of aromatic volatile organic compounds (VOC) collected by NOAA during June of 2010 over the Gulf of Mexico are compared against Gaussian plume dispersion calculations to test the consistency between the model and observations. The dispersion model tested is the U.S. EPA's Offshore and Coastal Dispersion Model (OCD), designed primarily for regulatory applications. Reasonable agreement is found between the dispersion calculations and the measurements, despite uncertainties in the atmospheric turbulence parameters used to drive the model. This agreement yields a degree of confidence that the empirically based OCD model can be used to derive upper limit exposure estimates of aromatic VOC at the ocean surface. In order to extend the OCD models to the longer time period of DWH exposure, results from a detailed chemical transport model (WRF/Chem) are used to relate OCD model results to time dependent meteorological forcing. Downscaling of the regional model results allows maximum concentration estimates over the entire spill period to be made for use in exposure assessments. For human exposure to C6-C8 aromatic VOC the combined modeling approach yields median concentrations over the DWH spill site that are at least a factor of 100 less than current permissible exposure limits.