EPA Nutrients
EPA’s use of empirical methods (as opposed to cause-effect or dose-response) for developing TMDL
endpoints for phosphorus in five Pennsylvania watersheds was called into question in several appeals of
the TMDLs before the EHB. EPA used these methods (e.g. conditional probabilities) and the subsequent
identification of nutrient thresholds as a line of evidence in a weight of evidence approach detailed in a
document called Development of Nutrient Endpoints for the Northern Piedmont Ecoregion of
Pennsylvania: TMDL Application in November 2007. Pennsylvania does not currently have numeric
criteria for nutrients; therefore, these TMDL endpoints were derived by EPA’s technical contractor to
meet federal consent decree ordered deadlines. The scope of these stringent, empirically-derived
endpoints was limited to the five previously mentioned TMDL watersheds in PA. Implementation of
these numbers in the permitting process was to be deferred until 2014 or until Pennsylvania develops
numeric nutrient criteria, as EPA has mandated for all states.
Subsequently, EPA summarized this approach in a document entitled Empirical Approaches for Nutrient
Criteria Derivation. This document suggested EPA’s support of this methodology, not only for
development of TMDL endpoints in waterbodies adversely affected by excess nutrients, but also for the
development of statewide nutrient criteria nationally. Upon request, EPA convened a Scientific Advisory
Board (SAB) panel to review this document and issue recommendations regarding the validity of the
methods for use in the development of statewide nutrient criteria; the SAB issued their findings in an
April 2010 report. The report basically stated that the use of conditional probability analyses, though a
valid method for use in a weight of evidence approach, should not be used in isolation to develop nutrient
criteria. The rationale provided by the SAB for that recommendation was that these empirical analyses do
not demonstrate and quantify the causal link between excess nutrients and an adversely affected aquatic
macroinvertebrate community. The aquatic macroinvertebrate community forms the cornerstone of the
assessments that DEP uses to determine the ability of a waterbody to support its aquatic life use. To our
knowledge, EPA has not backed away from their support of these methods. Instead, they made rather
non-substantive changes to the document in question (e.g. changing the title to Using Stressor-response
Relationships to Derive Numeric Nutrient Criteria) and re-issued it in November 2010.
In response to appeals of EPA’s TMDLs brought before the EHB, DEP requested and was granted a
jurisdictional hearing. As of December 2010, witness testimony has concluded and a decision has not yet
been rendered. If the cases are remanded to federal court, as DEP argued they should, then EPA will have
to decide whether to defend the empirical methods applied in the derivation of site-specific TMDL
endpoints or withdraw the TMDLs.
PA Nutrient Criteria Development
Since 2005, Pennsylvania DEP has been collaborating with Penn State and Stroud Water Research Center
on the types of cause-effect based research studies that the EPA Scientific Advisory Board identified as
critical to the development of scientifically defensible nutrient criteria. The Penn State and Stroud Water
Research Center studies detail the causal relationships between nutrient levels, algal growth and aquatic
macroinvertebrate response. These cause-effect studies will help provide scientific support for any
decisions involving nutrient standards as PA continues in the data collection and analysis phase of the
nutrient criteria development process. The most recent Nutrient Criteria Development Plan (NCDP)
submitted to EPA calls for the adoption of nutrient criteria into state water quality standards by 2015.
This NCDP is not something for which EPA issues an official approval, rather it represents a mutual
agreement and may be revised annually.