The Gaussian plume model assumes that (1) there is continuous emission from the source and that diffusion in the direction of travel is negligible, (2) diffused material is a stable gas that remains suspended in the air for long periods and therefore no material is deposited from the plume as it moves downwind, (3) at any point in the plume (cross- sectional plane perpendicular to the direction of dispersion), the distribution of pollutant concentration (from the crosswind and vertical directions) is normal, and (4) the spread of the plume can be represented by the standard deviation of the pollutant concentration, which is consistent with the averaging time of the concentration estimate.