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The U.S. Environmental Protection Agency (U.S. EPA) has developed guidance on regional nutrient criteria for lakes and reservoirs, rivers and streams, and estuaries. This WERF project (Project 99-WSM-3) has developed methods, procedures, and guidance for setting site-specific nutrient criteria for these types of water bodies. Site-specific means that the criteria would be applicable only to a water body or a segment of the water body. States and tribes will be required to adopt regional nutrient criteria and standards. If states and tribes do not develop their own criteria by the end of 2004, U.S. EPA may promulgate criteria for them. If states, tribes, or other entities believe that U.S. EPA regional nutrient criteria are not appropriate for a particular water body because they are under- or over-protective, they may develop their own site-specific criteria. To be acceptable, site-specific criteria must be scientifically defensible and must be approved by U.S. EPA and state regulatory agencies.

These methods are intended to complement U.S. EPA's guidance for developing regional nutrient criteria. When the methods for developing site-specific nutrient criteria are applied, they should be used together with U.S. EPA's methods. In the application of these methods, in most cases, regional nutrient criteria are assumed to have already been developed or proposed. Most often, the implementation of regional nutrient criteria generally will be the motivation for developing site-specific nutrient criteria.

A tiered approach is proposed to specify SSNC. This approach would be implemented if the initial evaluation identifies a need for such criteria. The overall approach offers a sequence of increasingly detailed, site-specific methods for deriving nutrient criteria. The approach also involves an increasing demand for site-specific information. While the approach was designed as a sequence of increasingly detailed and data intensive procedures, the process may be initiated at any of the three tiers. The main objective of each tier is to specify site-specific criteria. The tiers differ in the underlying data and analytical methods used to develop the criteria.

Tier 1 uses available data, if adequate, or requires new data collection, if extant data are inadequate, and simple data analyses to derive site-specific criteria. Tier 1 site-specific nutrient criteria are selected from published nutrient thresholds or algal limits, or more commonly, from distributions of TN and TP concentrations that are associated with acceptable Chl a values. The 25th percentile of all stream values, or some other agreed-upon percentile, may be selected as the site-specific nutrient criteria.

After selection and adoption of site-specific criteria, nutrient management plans are designed and implemented to evaluate the performance of the SSNC with regard to achieving water quality objectives. If the near-term performance evaluation of the SSNC is consistent with achieving and maintaining the water quality objectives, the Tier 1 assessment concludes with the development of a longer term monitoring plan and continued evaluation of the SSNC performance.

A Tier 2 analysis is implemented when the Tier 1 criteria are determined to be either underprotective or over-protective of designated uses, or the published criteria and distributions used in Tier 1 to select criteria are unacceptable. The development of Tier 2 site-specific nutrient criteria is based on empirical models of nutrient-biological response relationships developed from data on the water body being evaluated (Figure 3). Tier 2 uses both existing data and new site-specific data.

Successful completion of the Tier 2 analysis will result in SSNC that achieve and maintain the desired water quality objectives. Under these circumstances, Tier 2 concludes with the design and operation of a long-term monitoring program. The performance evaluation may demonstrate that the Tier 2 SSNC are inadequate to meet and maintain the water quality objectives. If the performance is not acceptable to the stakeholders, a decision to proceed to Tier 3 may result. The Tier 3 assessment process parallels Tier 2 with two important modifications. One, Tier 3 emphasizes the collection of site-specific data necessary to support the derivation of SSNC. Two, the Tier 3 analyses focus on a process-level evaluation of the site-specific relationships between nutrients and the values of the selected response variables. Correspondingly, Tier 3 offers the possibility of producing SSNC based on state-of-the-art scientific understanding backed up by in-depth analysis of site-specific data and limited only by the time and resources committed to the assessment. Tier 3, as a result, is also the most costly of the three-tiered approach.
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Document Type: Research Article

Publication date: 2002-01-01

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