Numeric Nutrient Criteria

Wyoming’s surface water quality standards currently only include narrative criteria to protect designated uses from nutrient pollution. Numeric nutrient criteria, concentrations of nitrogen and/or phosphorus and response variables such as chlorophyll a, are expected to more effectively protect Wyoming’s surface waters because they can be used to more consistently: identify waters that are meeting water quality criteria and need to be protected; identify waters that are not meeting water quality criteria and need to be restored; establish restoration targets for waters that are not meeting water quality criteria; and develop effluent limits for point sources.

Wyoming’s Plan to Develop Numeric Nutrient Criteria 

In 2008, WDEQ and Tetratech developed the Wyoming Nutrient Criteria Development Plan. The plan summarized existing data, identified a general framework for developing and adopting criteria, prioritized lakes and reservoirs, and established timelines to achieve milestones. During development of the 2017 Wyoming Nutrient Strategy, the Wyoming Nutrient Work Group provided additional input on how to prioritize waters for numeric nutrient criteria development; WDEQ incorporated this and other changes into the 2018 Wyoming Nutrient Criteria Development Plan.

Current Status of Numeric Criteria Development – Wyoming Basin Lakes and Reservoirs

WDEQ is currently focused on developing numeric criteria for small to mid-sized lakes and reservoirs in south-central Wyoming (i.e., Wyoming Basin). The criteria are intended to protect aquatic life and have been derived primarily using a stressor-response approach corroborated with other lines of evidence. WDEQ has drafted a technical support document (TSD) describing the analyses, results, and recommendations for numeric criteria. The TSD recently underwent external peer review. WDEQ is in the process of evaluating the reviews, and will be updating the analyses, recommendations, and document as necessary. WDEQ plans to share the TSD with the Wyoming Nutrient Work Group. Following review by the Wyoming Nutrient Work Group, WDEQ will begin the process of formally adopting the criteria into Wyoming’s surface water quality standards. 

Other Lakes and Reservoirs Prioritized for Numeric Criteria Development in the Short-Term

WDEQ anticipates developing numeric nutrient criteria for the following lakes and reservoirs in the short-term: Bighorn Basin lakes and reservoirs, Boysen Reservoir, Southeast Wyoming lakes and reservoirs, and Seminoe Reservoir. Additional details are included in the 2018 Wyoming Nutrient Criteria Development Plan

Data Compilation and Collection Efforts

As an initial step in developing numeric nutrient criteria, WDEQ and Tetratech compiled the Wyoming Nutrient Database in 2008 to store nutrient and associated variable data. Since that time, WDEQ has updated the database with additional lake and reservoir data, identified data gaps, developed sampling and analysis plans to fill data gaps, and collected additional data on lakes and reservoirs an annual basis since 2013. Sampling and analysis plans describing WDEQ’s data collection efforts on lakes and reservoirs are included in the “Resources” at the bottom of this page. In addition, as part of the WDEQ/USGS Cooperative Monitoring Network, quarterly or monthly nutrient data collections were initiated in 2015 at USGS gages on major tributaries to and outlets from a handful of large reservoirs. 

WDEQ’s Monitoring Program has also collected nutrient related data at both new and previously monitored stream and river sites. This has included over 200 probabilistic sites as part of the Bighorn/Yellowstone, Northeast, Green, and Platte superbasin surveys conducted in 2010, 2011, 2015 and 2016, respectively. Additional monitoring as part of targeted studies for designated use support, effectiveness of best management practice, TMDL development, reference site re-visits, and new reference site identification has also occurred annually on multiple streams and rivers since 2007. Details regarding this monitoring can be found in the Monitoring Program’s Surface Water Monitoring Strategy and Annual Work Plans.


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