Developing Nutrient Criteria for Delta Waters: Expectations and Challenges

Author(s): Caviness, K.

The Clean Water Act (CWA) requires each State to establish and maintain water quality standards (WQS) to meet the two objectives expressed in Section 101(a), which are as follows: (1) restore and maintain the chemical, physical, and biological integrity of the Nation's waters and (2) wherever attainable, achieve a level of water quality that provides for the protection and propagation of fish, shellfish, and wildlife and recreation in and on the water. Consequently, WQS serve as the foundation for a wide range of water quality management programs under the CWA. WQS serve multiple purposes that include defining the water quality goals for a specific waterbody and providing the regulatory basis for establishing water quality‐based effluent limits (WQBELs) beyond the technology‐based levels of treatment required by CWA Sections 301(b) and 306. WQS also serve as a target for CWA restoration activities such as total maximum daily loads (TMDLs).

The CWA also states that WQS must include the following three elements including (1) designated use(s) for each water body, (2) water quality criteria necessary to protect these designated uses, and (3) antidegradation requirements. Since WQS establish the environmental baselines used for measuring the success of CWA programs, protection of the designated uses (aquatic life, recreation, sources of drinking water, etc.) depends on States developing and adopting well‐crafted WQS.

In Mississippi, like many other states, excessive amounts of nitrogen and phosphorus are a major cause of surface water impairments. Since 2001, the Mississippi Department of Environmental Quality (MDEQ) has been working diligently to develop appropriate and protective numeric nutrient criteria (NNC) for Mississippi’s waters. Nitrogen and phosphorus are natural parts of aquatic ecosystems and they are essential to supporting the growth of algae and aquatic plants, which provide food and habitat for fish, shellfish, and smaller organisms that live in aquatic ecosystems. However, when too much nitrogen and phosphorus enter the environment, streams, rivers, lakes, estuaries, and coastal waters may be impacted.

Developing NNC is extremely complex at every step of the process from selecting the appropriate endpoint that demonstrates support of the designated use to ultimately determining the concentration of nitrogen and phosphorus that will achieve that desired endpoint. Additional complexity is added by the numerous other confounding factors that occur within aquatic environments and their biological communities that are also affected by habitat, sunlight, flow, and numerous other variables that are different for every water body. Establishing NNC becomes even more complicated in the MS Delta region of the State due to the extent of historical stream and channel alteration as well as other physical and chemical characteristics that are unique to this region of the state.

Designated uses are a critical component of WQS because the use of the water body is used to define the appropriate water quality goals to protect that use. Mississippi currently has a very simple water body classification structure. One aspect of the current classification structure that has been criticized is how the State classifies waters within the MS Delta. Even though there are numerous water body types present in the MS Delta, from large rivers to bayous, every water body currently has the same designated use and thus the same expectations. MDEQ is examining the current water body classification structure and investigating further refinement of this structure. Further refinement of water use classifications will allow MDEQ to provide a more accurate distinction between water bodies around the state and allow for more appropriate criteria (or goals), including the development of nutrient criteria, to be established for those various water bodies.

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