Simultaneous In‐Stream Nitrogen and D.O. Balancing

Abstract

A one‐dimensional, pseudo unsteady state, water quality model (SNOAP) was developed to simultaneously solve both nitrogen and dissolved oxygen mass balance equations. Expressions are derived to estimate ammonia‐N exsolution, bacterial ammonia‐N uptake, and aquatic plant ammonia‐N assimilation in terms of standard water quality indices. The SNOAP model calculates hourly values for all nitrogen species reaction rates (including nitrification and denitrification) based on observed in‐stream variation of organic‐N, ammonia‐N, nitrite‐N, nitrate‐N, dissolved oxygen, temperature, and pH. An iterative solution algorithm is required to simultaneously solve the nitrogen and dissolved oxygen mass balance equations. This iterative approach is stable and results in relatively quick convergence. Erroneous values of observed nitrogen species or dissolved oxygen variation, or both, may result in an impossible situation for simultaneous nitrogen and dissolved oxygen mass balance solution. This nonconvergence feature is a significant attribute, in that it allows for a systematic screening of measured water quality data.