Aquatic Life Water Quality Standards Technical Support Document for Nitrate

Nitrate is formed as part of the breakdown of organic wastes, production by nitrogen-fixing plants, and through industrial production. Sources of excess nitrate in the environment can be linked to human activities on the landscape that result in the release of nitrogen to surface and ground waters. Nitrogen cycling in the environment results in nitrogenous compounds such as ammonia denitrifying into the more stable and conservative nitrate ion (NO3). Concern regarding the toxicity of nitrate to aquatic organisms was brought to the attention of the MPCA from comments made during the preceding 2005-2008 rules revision by the Minnesota Center for Environmental Advocacy and concerns raised by the Minnesota Department of Natural Resources. The scientific literature has documented nitrate toxicity at concentrations that are environmentally relevant (Camargo and Alonso, 2006) to concentrations reported from Minnesota surface waters. In addition, the Minnesota State Legislature in 2010 approved funding for the MPCA to develop aquatic life standards for nitrogen and nitrate. Development of a nitrate standard is part of the effort to address these concerns. The MPCA is also engaged in developing a nitrogen budget for the state that focuses on total nitrogen in surface waters. Natural sources of nitrate to surface waters in the state vary; however, when nitrate concentrations in surface water samples from "reference" areas (i.e., areas with relatively little human impact) are compared to samples from areas of greater human impact, the reference areas exhibit much lower nitrate concentrations. Nitrate concentrations in these reference areas are typically below 1 mg/L (Heiskary and Wilson, 2005). Still, elevated concentrations of nitrate have been documented in surface waters throughout the state. A comprehensive assessment of these data is beyond the scope of this document, but current trends in the data suggest that increased nitrate concentrations are associated with areas of higher human activity on the landscape. In the surface water quality standards for Minnesota's Class 1 waters, protected as drinking water sources, human exposure to nitrates is regulated through the Federal Safe Drinking Water Act, with the Maximum Contaminant Level set at 10 milligrams/liter (mg/L), and a nitrite standard set at 1 mg/L. However, there is little guidance for protection of United States waters from the impacts of nitrate toxicity to aquatic organisms. The importance of nitrate toxicity to aquatic organisms has been a concern to aquaculture management for many years. In the environment, nitrate toxicity has not been a subject of scrutiny compared to the more toxic ammonia and nitrite. This document will present the technical discussion of surface water exposures and resulting toxicity of nitrate to aquatic organisms, and will propose a draft water quality standard necessary for the protection of aquatic life.