In mass balance, which variables are tracked to estimate emissions?

• A. Only the ambient temperature.
• B. Color of the effluent.
• C. Only the pH of water.
• D. Inputs, outputs, and accumulation of contaminants in the system.

Answer: (D)

Mass balance hinges on accounting for three things within the system: what enters (inputs), what leaves (outputs), and what remains or changes inside over time (accumulation). By measuring the amounts that come in and go out and tracking how much mass stays in the system, you can determine what is being added or released as emissions. The ambient temperature, color of the effluent, or pH might influence chemical reactions or operator decisions, but they do not provide the direct mass flow information needed to estimate emissions.