Nitrite

The amount of nitrite ($NO_2^-$) in the stream will be increased by the conversion of $NH_4^+$ to $NO_2^-$ and decreased by the conversion of $NO_2^-$ to $NO_3^-$. The conversion of $NO_2^-$ to $NO_3^-$ occurs more rapidly than the conversion of $NH_4^+$ to $NO_2^-$, so the amount of nitrite present in the stream is usually very small. The change in nitrite for a given day is:

$\Delta NO2_{str}=(\beta_{N,1}*NH4_{str}-\beta_{N,2}*NO2_{str})*TT$ 7:3.2.8

where $\Delta NO2_{str}$ is the change in nitrite concentration (mg N/L), $\beta_{N,1}$ is the rate constant for biological oxidation of ammonia nitrogen (day$^{-1}$ or hr$^{-1}$), $NH4_{str}$ is the ammonium concentration at the beginning of the day (mg N/L), $\beta_{N,2}$ is the rate constant for biological oxidation of nitrite to nitrate (day$^{-1}$ or hr$^{-1}$), $NO2_{str}$ is the nitrite concentration at the beginning of the day (mg N/L), and $TT$ is the flow travel time in the reach segment (day or hr). The local rate constant for biological oxidation of ammonia nitrogen is calculated with equation 7:3.2.5. The calculation of travel time is reviewed in Chapter 7:1.

The rate constant for biological oxidation of nitrite to nitrate will vary as a function of in-stream oxygen concentration and temperature. The rate constant is calculated:

$\beta_{N,2}=\beta_{N,2,20}*(1-exp[-0.6*Ox_{str}])*1.047^{(T_{water}-20)}$ 7:3.2.9

where $\beta_{N,2}$ is the rate constant for biological oxidation of nitrite to nitrate (day$^{-1}$ or hr$^{-1}$), $\beta_{N,2,20}$ is the rate constant for biological oxidation of nitrite to nitrate at 20$\degree$C (day$^{-1}$ or hr$^{-1}$),$Ox_{str}$ is the dissolved oxygen concentration in the stream (mg O$_2$/L), and $T_{water}$ is the average water temperature for the day or hour ($\degree$C). The second term on the right side of equation 7:3.2.9, $(1-exp[-0.6*Ox_{str}])$, is a nitrification inhibition correction factor. This factor inhibits nitrification at low dissolved oxygen concentrations.