Water numeric nutrient criteria: what, why and 4Rs
State water quality agencies have the major responsibility of ensuring the protection of water resources from nutrient (N and/or P) pollution.
For decades, lake, stream, and river water quality nutrient levels have been periodically monitored by either state agencies or federal agencies.
Historically, most states have relied on narrative nutrient criteria that stipulate that designated uses of water resources shall not be impaired by nutrients.
Since about 2000, the U.S. EPA has emphasized the need for more robust numeric nutrient criteria by states, based on sampling of reference waters and human-impacted waters. The EPA has advocated use of overlapping frequency distributions of nutrient levels from least-impacted reference streams/rivers and monitored streams/rivers, in selecting numeric nutrient criteria.
Last year, in its 2013 release of the 2008-2009 National Rivers and Streams Assessment Report on water quality (http://water.epa.gov/type/watersheds/monitoring/aquaticsurvey_index.cfm), the EPA used a more narrowed statistical approach than in the past, in interpreting stream/river quality conditions. Overall, the U.S. EPA found that 55% of the monitored rivers and streams were in poor biological condition. Compared to the 2004 EPA wadeable stream assessment, 7% fewer stream miles were in good biological condition and 19% fewer stream miles were in good condition for P.
The good news was that 9% more stream miles were in good condition for N, 17% more stream miles were in good condition for in-stream fish habitat, and 12% more stream miles were in good condition, as measured by riparian disturbance.
Currently, seven states have statewide or partial N numeric criteria for rivers/streams; while 11 have state-wide or partial P numeric criteria for rivers/streams. More states are making progress (http://cfpub.epa.gov/wqsits/nnc-development/), and the number of states with statewide or partial numeric criteria may double by 2016. According to a 2013 paper published in the Journal of Environmental Quality (https://www.agronomy.org/publications/jeq/pdfs/42/4/1002?search-result=1), among the several states with site-specific nutrient criteria for streams and rivers, only Wisconsin and Florida have statewide criteria which are well-supported by peer-reviewed and technical papers documenting the process of criteria development. A number of states have developed new nutrient loss reduction strategies. Several state strategies are designed to primarily encourage agriculture, and other nonpoint or diffuse sources of nutrient loss from land to water resources, to become more aggressive with their best management practice implementation and mitigation actions.
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