Question

Which of the following are valid assumptions of the Gaussian plume dispersion model used for estimating ground-level pollutant concentrations from a point source?

• A. The pollutant is conservative, i.e., it does not undergo any chemical reaction, decay, or deposition during transport
• B. Wind speed and direction are constant (steady-state) over the travel time being modeled
• C. The terrain is highly irregular with complex topographic features
• D. Emission rate from the source varies significantly with time during the modeled period

Hint:

Recall the Gaussian plume model assumes a conservative pollutant, steady-state wind and constant emission rate, and flat/uniform terrain - the opposite of options C and D.

Answer 1

The Correct Option is A, B

Step 1: Recall the core assumptions of the Gaussian plume model. The Gaussian plume dispersion model, used to estimate downwind ground-level concentrations from a continuous point source (e.g. a stack), is built on several simplifying assumptions to make the diffusion equation tractable in closed form.

Step 2: Examine option A. The model assumes the pollutant is conservative - it is not created or destroyed by chemical reaction, radioactive decay, or physical deposition during atmospheric transport, so that mass is simply dispersed, not lost, by turbulent diffusion. This is a valid, standard assumption of the model, so option (A) is correct.

Step 3: Examine option B. The model assumes steady-state conditions, meaning wind speed, wind direction, and the atmospheric stability class remain constant over the time period and downwind distance being modeled, allowing the plume centerline to be treated as a straight line. This is also a valid, standard assumption, so option (B) is correct.

Step 4: Examine option C. The basic Gaussian plume model assumes flat, homogeneous, and unobstructed terrain; it does NOT account for highly irregular or complex topography (hills, valleys, buildings), since such features cause channeling, turbulence, and recirculation zones the basic Gaussian formulation cannot capture without extra correction terms. So option (C) is a false/invalid assumption for the basic model.

Step 5: Examine option D. The model assumes a constant, steady emission rate from the source (matching the steady-state assumption in option B); it does NOT assume the emission rate varies significantly with time, since a strongly time-varying source would violate the steady-state plume formulation. So option (D) is also false.

Step 6: Conclude. The valid assumptions are (A), conservative pollutant, and (B), steady-state constant wind; options (C) and (D) describe conditions the basic Gaussian model explicitly assumes AGAINST (complex terrain, and time-varying emissions).