Exponential First-Order Decay

The modeling of the in-stream concentration below the point of discharge depends on the nature of the pollutant: for example, is the pollutant conservative and is settling a dominant process? For a conservative pollutant with negligible settling, the concentration can be determined simply from the equation above for c0. For pollutants assumed to follow first order decay, the stream velocity and decay parameters must be estimated. For a known cross-sectional area, Ac and flow rate, Q, the stream velocity U can be estimated as follows:

Eqn. 2

Ac is calculated based on user-entered data correlating stage to flow and width. The concentration of the pollutant at some distance downstream, L, from the point of discharge is the concentration as calculated in equation above for c0, multiplied by a first order decay term, based on the decay parameter k (/day), as shown in the following equation.

Eqn. 3