MEMORANPUM 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

NOY 5 1997 OFFICE Of' 

WATER 

SUBJECT: Establishing Site Specific Aquatic Life Criteria 

Tudor T. Davies, Director ~~ 

I to Natural Background 

) 

FROM: 

Office of Science and Techn~1A 

TO: Water Management Division Directors, Regions 1-10 

State and Tribal Water Quality Management Program Directors 

In the course of reviewing State and Tribal water quality standards (WQS), EPA has 

identified several issues pertaining to the establishment of site specific numeric criteria on the 

basis of natural background conditions. EPA is issuing this policy to provide greater clarity and 

direction for States and Tribes who are considering establishing site specific criteria equal to 

natural background conditions, and for EPA Regional Offices reviewing State and Tribal water 

quality management programs. 

BacklW>und 

Site specific criteria are allowed by regulation and are subject to EPA review and 

approval. The Federal water quality standards regulation at 40 CFR 131.11 (b)( I) requires States 

and authorized Tribes to adopt numeric water quality criteria that are based on section 304(a) 

criteria, section 304(a) criteria modified to reflect site-specific conditions, or other scientifically 

defensible methods. Under 40 CFR 131.5(a)(2), EPA reviews State WQS to determine whether 

a State has adopted criteria to protect the designated uses. Existing guidance and practice are that 

EPA will approve site specific criteria developed on the basis of sound scientific rationales. 

Currently, EPA guidance has specified three procedures for States and Tribes to follow in 

deriving site specific criteria. ·These are the Recalculation Procedure, the Water-Effect Ratio 

Procedure and the Resident Species Procedure. These procedures can be found in the Water 

Quality Standards Handbook (EPA-823-B940005a, 1994). EPA also recognizes there may be 

naturally occurring concentrations of pollutants which may exceed the national criteria published 

under section 304(a) of the Clean Water Act. 
Policy 

This policy applies only to site specific numeric aquatic life criteria based on natural 

background. States and Tribes may establish site specific numeric aquatic life water quality 

criteria by setting the criteria value equal to natural background. Natura) background is defined 

as background concentration due only to non-anthropogenic sources, i.e., non-manmade sources. 

In setting criteria equal to natural background the State or Tribe should, at a minimum, include in 

their water quality standards: 

(1) a definition of natural background consistent with the above; 

(2) a provision that site specific criteria may be set equal to natural background; 

(3) a procedure for determining natural background, or alternatively, a reference in their 

water quality standards to another document describing the binding procedure that will be 

used. 

Discussion 

A State or Tribal procedure for detennining natural background will need to be specific 

enough to establish natural background concentration accurately and reproducibly. States and 

Tribes should also provide for public notice and comment on the definition, the provision, the 

procedure and the site specific numeric criteria derived from the procedure. The State or Tribe 

will need to document the resulting site specific numeric criteria in the State or Tribal water 

quality standards, including specifying the water body segment to which the site specific criteria 

apply. This can be accomplished through adopting the site specific criteria into the State or 

Tribal WQS, or, alternatively, by appending the site specific criteria to the WQS. In either case, 

the State or Tribe must comply with the public participation requirements of 40 CFR 131.20 and 

40 CFR Part 25, and State and Tribal citizens should be able to readily detennine the water 

quality criteria applicable to specific water bodies. 

For aquatic life uses, where the natural background concentration for a specific parameter 

is documented, by definition that concentration is sufficient to support the level of aquatic life 

expected to occur naturally at the site absent any interference by humans. The State or Tribe 

should consider refining the designated use for the water body to more precisely define the 

existing aquatic life use. 

This policy does not apply to human health uses. For human health uses, where the 

natural background concentration is documented, this new information should result in. at a 

minimum, a re-evaluation of the human health use designation. Where the new background 

information documents that the natural background concentration does not support a human 

health use previously believed attained, it may be prudent for the State or Tribe to change the 

human health use to one 'he natural baclcground concentration will support (e.g., from drinking 

water supply to drinking water supply only after treatment). 

2 
Conclusion 

This policy explains and clarifies the use of natural background conditions in establishing 

site specific criteria for protection of aquatic life uses. In addition to the three procedures listed 

above for deriving site specific criteria as discussed above. States and Tribes can address natural 

background conditions through refining the designated use to more accurately reflect the aquatic 

community present within the stream segment. EPA recognizes that there are other options 

available to StateslTribes to account for other ambient conditions (e.g .• concentrations due to 

non-naturaI. man-made conditions) which exceed the national criteria. One such option is for a 

State or Tribe to conduct a Use Attainability Analysis. consistent with the requirements of 40 

CFR 131.10, and adopt a use which is less than the 101(a) goal uses of the Clean Water Act, e.g., 

less than ufishablelswimmable", or modify a 101(a) goal use such that less stringent criteria are 

required. In any case, the existing uses of the water body segment must be maintained and 

protected. 

If you have any questions or concerns regarding this policy, please contact me or have 

your staff contact Elizabeth Southerland, Acting Director, Standards and Applied Science 

Division, at 202-260-3966. 

cc: Lepow, OGC 

Wayland, OWOW 

Cook,OWM 

Dougherty, OGWDW 

3