Existing Use Determinations


​                           A stream in a forest    Description automatically generated with low confidence

What are Existing Uses (EUs) and Why are they Protected?

In addition to defining designated uses as part of water quality standards, the implementing regulations of the CWA, found in Code of Federal Regulations (CFR) Title 40 § 131.3(e), also establish the concept of an “existing use”, one of the foundational principles for antidegradation policy. Here, 40 CFR § 131.3(e) defines existing uses as “…those uses actually attained in the water body on or after November 28, 1975, whether or not they are included in the water quality standards”. Federal regulations additionally discuss existing uses in 40 CFR § 131.12(a), part of the Code of Federal Regulations that establishes the basis for EPA and state’s Antidegradation Policy. Here the regulation asserts that “at a minimum…(1) Existing instream water uses and the level of water quality necessary to protect the existing uses shall be maintained and protected.” Existing uses therefore represent the highest level of use and water quality (necessary to support that use) that has been achieved since 1975. In this manner, existing uses function as the baseline or floor of water quality that must be maintained regardless of the codified designated use.

Maryland’s Code of Regulations (COMAR) also establishes protections for existing uses. Specifically, Maryland’s Antidegradation Policy regulation (COMAR regulation echoes the federal regulations by saying “A. Waters of this State shall be protected and maintained for existing uses and the basic uses of water contact recreation, fishing, protection of aquatic life and wildlife, and agricultural and industrial water supply as identified in the Use Class designations​.” and “B. Consistent with the Federal Act, existing uses and the level of water quality necessary to protect existing uses for any water body shall be maintained.” ​

Further, Maryland’s regulations provide Antidegradation Policy Implementation Procedures for the Tier I level of protection in COMAR which state that “A. All waters of the State shall receive Tier I protection which requires the protection and maintenance of existing uses and designated uses.” and “B. Protections. Waters that have demonstrated an existing use that is not protected by the water quality criteria specified by the current designated use for this water body shall be protected so as to maintain the existing use and the water quality necessary to protect the existing use.”

How are Existing Uses Identified and What is the MDE's Policy on EU's?


MDE formed a Cold Water Advisory Committee composed of stakeholders and subject matter experts to assist the Department in developing MDE’s policy and procedures for identifying cold water existing uses and outlining steps designed to reduce regulatory uncertainty.  This policy and procedures were formalized as the “Cold Water Existing Use Determinations: Policy and Procedures” document that was incorporated by reference in COMAR

Completed Existing Determinations

Recent data have shown that the current use classification of some Maryland surface waters was not protective of their existing use and specifically that of a cold water existing use. The existing uses of these streams are formally recognized by the department and, as a result, some surface waters were reclassified to use class III (or III-P) where data demonstrated that the class III designated use and criteria were being attained. The following existing use determination documents provide justification for and describe the scale of existing use determinations and use classification changes where applicable. These determinations received public review and comment with the 2019 Triennial Review​ in March-April of 2022 and were made final in October 2022. MDE received EPA approval for the 2019 Triennial Review on June 28, 2023. These existing uses are also shown on 
MDE’s Designated Use Map.

Pending Existing Use Determinations

These are existing use determination documents which have not yet undergone public review.

  • ​In production!​

​Contact Information 

For more information, please contact Melinda Cutler at​ melinda.cutler@maryland.gov.  ​

Last Updated 08/2023

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