Neshaminy Creek Watershed Nutrient Total Maximum Daily Loads (TMDL); Rationale for Withdrawal
[38 Pa.B. 1640]
[Saturday, April 5, 2008]
The Department of Environmental Protection (Department) developed, and the United States Environmental Protection Agency (EPA) approved, Total Maximum Daily Loads (TMDL) for nutrients in the Neshaminy Creek Watershed on December 9, 2003. TMDLs were completed for total phosphorus (TP) to address the problems identified on Pennsylvania's 1996-2002 Clean Water Act section 303(d) lists of impaired waters (impaired waters list). The water quality impairments identified on the lists include nutrients and excessive algal growth. The listed waters included the Neshaminy Creek mainstem, West Branch Neshaminy Creek, North Branch Neshaminy Creek, Pine Run, Cooks Run, Core Creek, Little Neshaminy Creek, Mill Creek, Park Creek and Little Neshaminy Creek. Allocations were established for 21 point source discharges requiring permit limits of 1.0 mg/l TP at existing flows or 0.8 mg/l TP at expanded flows.
The Department does not have numeric criteria for nutrients. Therefore, the Department developed an interpretation of the narrative standard contained in 25 Pa. Code § 93.6(a). This narrative standard requires that point and nonpoint source contributions to Waters of the Commonwealth not contain substances in amounts sufficient to be inimical or harmful to the water's protected uses. While the TMDLs represented the best interpretation of the narrative criteria available at the time, more recent intensive studies of Pennsylvania waters and an exhaustive scientific literature conducted by EPA as part of the Skippack Creek TMDL Withdrawal Rationale indicate that the allowable TP is much lower than the levels required by the TMDLs.
The Department is withdrawing the existing Neshaminy Creek nutrient TMDLs that were established in 2003. EPA will establish the nutrient TMDLs for Neshaminy Creek.
Based on subsequent reviews of existing data and EPA's scientific literature search, conducted as part of the Skippack Creek TMDL withdrawal process, the Department believes that the TP endpoint selected for the Neshaminy Creek TMDL is not sufficient to restore water quality to the point where waters are supporting all water uses. The basis for this determination is presented as follows.
Following establishment of the nutrient TMDLs for the Neshaminy Creek watershed, EPA developed nutrient TMDLs for an adjacent watershed, Skippack Creek, using an endpoint derived using a periphyton nutrient relationship to control excessive algal growth. Originally, the Neshaminy Creek TMDLs were to be revised with an endpoint derived for the Skippack Creek TMDL. The periphyton/nutrient relationship for Skippack Creek was based on a study by Dr. Hunter Carrick, Associate Professor of Aquatic Ecology at Pennsylvania State University. This study determined that the Skippack watershed was overloaded with nutrients and the growth of periphyton biomass was no longer limited by either nitrogen or phosphorus. The reader is directed to the April 8, 2005, Skippack Creek watershed TMDL report for Dr. Carrick's complete report. Based upon previous work, algal chlorophyll 'a' levels in streams that range from 100--150 mg/m2 are considered to be excessive or at nuisance levels (Welch, et al., 1989). EPA selected 100 mg/m2 as the appropriately protective goal for chlorophyll 'a' levels in the Skippack Creek watershed. Using regression equations (Dodds et al., 2002 and Cattaneo, et al. 1987), Dr. Carrick calculated the TP concentration in surface water that would yield the chlorophyll 'a' target of 100 mg/m2. The TP concentrations based on these equations ranged from184 ug/L to 233 ug/L. EPA selected the higher concentration as the TMDL endpoint. While the response-based endpoint is still considered the most scientifically defensible means of setting TMDLs, the method used to derive the endpoints for the nutrient TMDLs must be refined for various reasons discussed as follows.
As mentioned previously, the concentration for TP selected as the endpoint for the control of excessive algal growth was, in part, based on an equation developed by A. Cattaneo in 1987 using data from lakes. Also, the data that served as the basis for this equation was collected in Canada. EPA and the Deprtment now believe that this equation is not appropriate for use in the determination of TP levels in free-flowing waters such as the Skippack and Neshaminy Creeks. Use of only the Dodds equation would result in a TP concentration of 184 ug/L in order to meet the TMDL selected in-stream goal for chlorophyll 'a' biomass of 100mg/m2. However, subsequent to EPA's establishment of the nutrient TMDLs for the Skippack Creek watershed, Dodds et al. (Dodds, et al, 2006) published an erratum that corrected the original 2002 equation. The erratum states that, ''This correction is related to two issues. First, a small number of data points from the literature data set had incorrect values of TP entered by the first author. Also, upon review of the entire literature data set, several chlorophyll values were found that did not match the criteria required for inclusion in the data set. Correcting these values resulted in modest changes . . . '' Application of the revised equation that relates total nitrogen (TN) and TP to chlorophyll 'a' values shows a much steeper slope to the curve. See Figure 1 as follows.
Due to the significant changes in one of the primary regression equations used to derive the TP endpoint and the belief that the target may not have been sufficient to meet water quality standards, EPA has withdrawn the Skippack Creek TMDLs. EPA will propose new Skippack and Neshaminy Creeks TMDLs. EPA has committed to re-evaluate the nutrient endpoint issue for these TMDLs.
The Department is withdrawing nutrient TMDLs for the Neshaminy Creek Watershed established on December 9, 2003. The Department's basis for this withdrawal is that the existing TMDLs are not sufficient to attain and maintain existing water quality standards and uses. The Department believes that a response-based endpoint derived for the protection of the waters from excessive algal growth is the most scientifically and ecologically desirable TMDL target. Thus, EPA's current effort to define nutrient endpoints and re-establish TMDLs for the Skippack Creek watershed represents an opportunity to establish TMDL targets that will be effective in restoring water quality in Neshaminy Creek.
By letter, dated January 31, 2008, EPA notified the Department that it concurred with this withdrawal. In addition, EPA confirmed that it will develop and establish revised nutrient TMDLs for the Neshaminy Creek Watershed.
References--Nutrients, Chlorophyll and Other Factors Impacting Algal Growth
Cattaneo, A. Periphyton in Lakes of Different Trophy. Canadian Journal of Fisheries and Aquatic Science, Vol. 44, 296-303, 1987.
Carrick, Hunter J., Using Periphyton to Estimate TMDL Endpoints and Assess Impairment in an Urban-Suburban Stream (Skippack Creek, Pennsylvania), School of Forestry, The Pennsylvania State University, March 30, 2005
Carrick, Hunter J., and Casey M. Godwin, TMDL Endpoint Estimates for a Urban-suburban Stream Based Upon In-stream Periphyton Assemblages (Neshaminy Creek, Pennsylvania), School Of Forestry, The Pennsylvania State University, December 15, 2005
Dodds, W. K., Val H. Smith and Kirk Lohman, Nitrogen and Phosphorus relationships to Benthic Algal Biomass in Temperate Streams, Canadian Journal of Fisheries and Aquatic Science, Vol 59, 865-874, 2002
Dodds, W. K., Val H. Smith and Kirk Lohman, Erratum: Nitrogen and Phosphorus relationships to Benthic Algal Biomass in Temperate Streams, Canadian Journal of Fisheries and Aquatic Science, NRC Research Press Web site, April 21, 2006
Dodds, W. K., Eutrophication and Trophic State in Rivers and Streams, Limnology and Oceanography, 51(1, part2), 671-680, 2006
U. S. Environmental Protection Agency, Guidance for Water Quality-based Decisions: The TMDL Process, EPA-440-4-91-001, 1991
U. S. Environmental Protection Agency, Nutrient Criteria Technical Guidance Manual, EPA-822-B-00-002, July 2000
Welch, Eugene B., Richard R. Horner and Clayton R. Patmont, Prediction of Nusiance Biomass: A Management Approach, Water Research 23:4, 401-405, April 1989
KATHLEEN A. MCGINTY,
[Pa.B. Doc. No. 08-634. Filed for public inspection April 4, 2008, 9:00 a.m.]
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