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EPA released its Final Aquatic Life Ambient Water Quality Criteria for Ammonia  on August 22. The revised criteria are significantly more stringent than the existing criteria, last updated in 1999, but how the criteria are implemented will ultimately dictate their impact on the clean water community. NACWA members are assessing the impacts on their utilities and are evaluating their options regarding site-specific recalculation of the criteria.

This Advocacy Alert builds on an analysis developed by NACWA member the Los Angeles County Sanitation Districts and provides additional insight into the final criteria, including some of the implementation flexibility that is available. NACWA also discusses some potential next steps now that the criteria are final.

Background and Overview of the Revised Criteria

NACWA has been following EPA’s efforts to revise the ammonia criteria since 2004 and has raised numerous objections on the data and methodologies EPA was employing. Many of NACWA’s concerns had not been addressed when EPA released its draft revised criteria in 2009. At that time, EPA proposed including two criteria values – one for waters where mussels are present and the other for waters where mussels are absent. While NACWA supported this bifurcated approach, it again raised a number of technical and policy concerns in its comments on the draft criteria.

Following the release of the 2009 draft revisions, EPA spent several years reviewing a number of new studies and the comments it received on the draft criteria. NACWA understands that concerns were raised by stakeholders, including the U.S. Fish and Wildlife Service, that EPA’s bifurcated approach was not sufficiently protective; that a single, nationwide number would be more protective; and that new research suggested that gill-breathing snails, like mussels, were similarly sensitive to ammonia. In finalizing the criteria, EPA included only one acute and one chronic value for ammonia (or rather an equation that establishes the criteria values as a function of temperature and pH), which will apply to all waters regardless of what aquatic species are present. However, EPA does highlight that the development of site-specific criteria using a recalculation procedure to better account for the organisms that reside at a particular site, is an available option.

While by no means perfect, the final criteria generally address NACWA’s most pressing technical concerns. The significant differences between the existing ammonia criteria (1999) and the revised criteria (2013) are:

1. EPA now includes toxicity data for highly-sensitive unionid freshwater mussels. These are the primary driver for the final 2013 criteria values.
2. Sensitive, but questionable, Hyalella azteca toxicity data that were included in the 1999 criteria are excluded from the 2013 criteria.
3. The bifurcated approach with different “mussels present/mussels absent” criteria as proposed in 2009 was abandoned. EPA does, however, reaffirm that a recalculation procedure can be used to “correct” the objectives in areas where unionid mussels are absent. EPA even includes the “recalculated” formulas for when unionid mussels are absent (see Appendix N of the final 2013 Ammonia Criteria document).

These changes result in the 2013 criteria being about one half of the 1999 criteria, in other words significantly more stringent. However, where mussels, in particular unionid mussels, are found to be absent and a recalculation is done, the resulting criteria are about two times higher (less stringent) than the 1999 criteria. This is due to the fact that EPA removed the Hyalella azteca data from the criteria database. The following tables provide a summary of the criteria values at a range of pH values and temperatures:

This Excel spreadsheet is provided so members can evaluate the potential impact the 2013 revised criteria may have on acute and chronic ammonia water quality objectives for any pH and temperature. Entering pH (column A) and temperature (column B or C) will result in the calculation of the 1999 and 2013 acute and chronic water quality objectives.¹

What are Unionid Mussels?

As explained above, whether unionid mussels are ‘present or absent’ could have a dramatic effect on how the 2013 ammonia criteria are implemented. The extent to which the criteria can be recalculated to reflect the absence of these mussels – factoring in the complexity of the issue; lack of data or historical information; regulator willingness; etc. – will dictate how implementation proceeds and the overall impact of these revisions on the clean water community.

Unionid mussels are a very diverse group of about 300 species of freshwater mussels that exist across North America, nearly three quarters of which are endangered or presumed extinct. Decades-long population declines have made these mollusks one of North America’s most critically endangered group of organisms. Unionid mussels have a very unique and complex life history that includes a larval life stage (glochidia) that must parasitically attach and embed into the gill of a fish to continue development. In addition, specific mussel species require a specific fish species (or limited number of potential species) as a host fish to continue their lifecycle with some mussel species exhibiting very creative and decorative “appendages” that mimic the host species or their prey in order to lure a suitable species within reach.

While regions east of the Rockies contain, by far, the most species, even the western U.S., including arid regions such as Arizona, were once known to have supported or currently are supporting several species of unionid mussels. In the 1800s and into the early 1900s, unionid mussels in the central U.S. supported a large and profitable button industry. Today, shells from some unionid mussels are highly valued in the cultured pearl industry where they are made into beads and inserted into marine oysters to stimulate the formation of high quality pearls. The primary reasons for the dramatic decline of unionid mussels most often cited are hydrologic alterations (dams and water diversions), habitat degradation (channelization, dredging, excess sedimentation), loss of native fish species, and introduction of exotic species. However, in the last ten years or so, ecologists and toxicologists have discovered that these mussels are also extremely sensitive to various water quality parameters including ammonia, copper, and pesticides. Although certainly not the primary cause for their widespread decline, at their currently endangered levels, anthropogenic introductions of these pollutants could limit unionid mussel recovery or worse, result in additional extinctions.

Are Unionid Mussels Present or Expected To Be Present?

Concurrent with the release of the final 2013 criteria, EPA also released a Technical Support Document  for conducting and reviewing freshwater mussel occurrence survey data. Page 5 of the document contains specific language regarding what is meant by presence and absence:

The Recalculation Procedure is dependent on the species that occur at the site. As stated in Revised Deletion Process for the Site-Specific Recalculation Procedure for Aquatic Life Criteria (USEPA 2013b), the equivalent terms “resident” and “occur at the site” include life stages and species that meet one of the following elements:

- Are usually present at the site.
- Are present at the site only seasonally due to migration.
- Are present at the site intermittently because they periodically return to or
extend their ranges into the site.
- Were present at the site in the past, are not currently present at the site due to
degraded conditions, but are expected to return to the site when conditions
improve, or
- Are present in nearby bodies of water, are not currently present at the site due
to degraded conditions, but are expected to be present at the site when
conditions improve.

The terms “resident” or “occur at the site” do not include life stages and species that meet one of the following elements:

- Were once present at the site but cannot exist at the site now due to permanent (physical) alterations of the habitat or other conditions that are not likely to change within reasonable planning horizons.
- Are still-water life stages or species that are found at a flowing-water site solely and exclusively because they are washed through the site by stream flow from a still-water site.”

Therefore, even if freshwater mussels can be found within close proximity to an outfall, if the habitat conditions, including various hydrologic alterations or extirpation of native fish species needed for glochidia development, prevent the area from being suitable for freshwater mussel colonization, a case could be made that the less restrictive “mussels absent” criteria may be appropriate.

So What Can Clean Water Utilities Do?

The primary opportunity for flexibility available to clean water utilities in determining what criteria apply will be in establishing that unionid mussels are not present and that a recalculation of the criteria for that particular receiving water is appropriate. Documenting that habitat and/or hydrologic conditions are the limiting factor preventing the restoration of unionid mussels in receiving waters may represent the best alternative for relief. While the EPA’s Technical Support Document lacks sufficient detail, it does provide a basic framework for getting started.

Also, utilities can consider what pH and temperature should be used to calculate objectives – single maximum result, monthly average, receiving water or effluent conditions, etc. This has a tremendous effect on the ultimate criteria objectives/values and various regulating entities have used a wide range of differing approaches. Other considerations include how to demonstrate protection of downstream beneficial uses, assuming mussels are absent below an outfall but present several miles further down. An important element will be demonstrating how quickly ammonia is assimilated to the point that downstream populations are not impacted.

In at least one state, utilities are working together to propose an alternative process to state regulators where the new criteria would not be automatically applied without first ensuring that unionid mussels are present. Several options are being considered, but one approach would create a clearer path to recalculation, giving dischargers the option to conduct the required studies and other work necessary to make the showing of presence/absence and propose new site-specific criteria.

Possible Next Steps

Although the 2013 criteria include some “new” data regarding snail toxicity that were not previously available for review and comment, the unionid mussels remain the primary driver for the extremely low criteria values (excluding only the unionid mussel data and leaving in everything else results in objectives far less restrictive than the 1999 criteria). While NACWA has not ruled out a challenge to the final criteria, an administrative challenge focused on the new information that was previously not available for comment would likely have little impact on the criteria.

The only way to effectively mount a viable technical challenge to the criteria is to focus on the unionid mussel data and studies. In general, however, through comments by NACWA and others, EPA has addressed or at least partially addressed our most significant concerns regarding the environmental relevance of long exposures to unique life stages of unionid mussels as well as the use of “questionable” growth endpoints. A significantly large amount of peer-reviewed and published ammonia sensitivity data have been generated using the unionid mussels over the past ten years - by a very diverse and respected group of noted experts. A technical challenge focused on these data would be difficult.

While NACWA continues to evaluate all of its options, the best opportunity for utilities to minimize the impact of these criteria at this point is to explore using the flexibility EPA has expressly provided for in its final recommendations and accompanying guidance documents regarding the recalculation procedure.

If you have any questions or would like to discuss this issue further, please contact Chris Hornback at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

¹The formulas contained in the spreadsheet were taken directly from the 1999 and 2013 EPA criteria documents and were crossed checked with the tables included in the documents. However, even though it has been cross checked, it is recommended that these calculations be used for informational purposes until they have been verified by your own staff.