Ammonia Modeling for Assessing Potential Toxicity to Fish Species in the Rio Grande, 1989-2002

Increasing volumes of treated and untreated human sewage discharged into rivers around the world are likely to be leading to high aquatic concentrations of toxic, unionized ammonia (NH₃), with negative impacts on species and ecosystems. Tools and approaches are needed for assessing the dynamics of NH₃. This paper describes a modeling approach for first-order assessment of potential NH₃ toxicity in urban rivers. In this study daily dissolved NH₃ concentrations in the Rio Grande of central New Mexico, USA, at the city of Albuquerque's treated sewage outfall were modeled for 1989-2002. Data for ammonium (${\rm{NH}}_4 ^ + $) concentrations in the sewage and data for discharge, temperature, and pH for both sewage effluent and the river were used. We used State of New Mexico acute and chronic NH₃- N concentration values (0.30 and 0.05 mg/L NH₃-N, respectively) and other reported standards as benchmarks for determining NH₃ toxicity in the river and for assessing potential impact on population dynamics for fish species. A critical species of concern is the Rio Grande silvery minnow (Hybognathus amarus), an endangered species in the river near Albuquerque. Results show that NH₃ concentrations matched or exceeded acute levels 13%, 3%, and 4% of the time in 1989, 1991, and 1992, respectively. Modeled NH₃ concentrations matched or exceeded chronic values 97%, 74%, 78%, and 11% of the time in 1989, 1991, 1992, and 1997, respectively. Exceedences ranged from 0% to 1% in later years after enhancements to the wastewater treatment plant. Modeled NH₃ concentrations may differ from actual concentrations because of NH₃ and ${\rm{NH}}_4 ^ + $ loss terms and additive terms such as mixing processes, volatilization, nitrification, sorbtion, and ${\rm{NH}}_4 ^ - $ uptake. We conclude that NH₃ toxicity must be considered seriously for its potential ecological impacts on the Rio Grande and as a mechanism contributing to the decline of the Rio Grande fish community in general and the. Rio Grande silvery minnow specifically. Conclusions drawn for the Rio Grande suggest that NH₃ concentrations may be high in rivers around the world where alkaline pH values are prevalent and sewage treatment capabilities are poorly developed or absent.