                             Technical Memorandum
   Health and Ecological Criteria Division/Ecological Risk Assessment Branch
                               November 3, 2020
                                       
Derivation of Acute and Chronic Nationally-Representative Criteria Values for Aluminum and Copper in Support of Multi-Sector General Permits for Industrial Stormwater Discharges
                                       
                                       
Background
The US EPA Office of Wastewater Management/Water Permit Division is generating nationally-representative aluminum and copper aquatic life values for use in Multi-Sector General Permits (MSGP) for industrial stormwater discharges. EPA is proposing in the 2020 MSGP to update the benchmarks for aluminum and copper to align with current acute and chronic aquatic life criteria for these chemicals, which vary based on specific water quality parameters (pH, hardness and dissolved organic carbon (DOC) for aluminum and temperature, pH, DOC, Ca, Mg, Na, K, SO4, Cl and alkalinity for copper).
In support of this effort, EPA Office of Water calculated water qualtiy criteria values for aluminum and copper based on water quality parameters measured in representative surface water samples collected from monitoring locations across the conterminous United States (CONUS). Acute and chronic criteria were calculated based on the input water chemistry parameters determined from these monitoring data. The 5[th], 10[th], 15[th], 20[th] and 25th percentile values of the acute and chronic water quality criteria for both metals are provided.
Data Summary and Analysis
Water quality data reported in the USGS National Water Information System (NWIS) database and collected from surface waters within the conterminous US between 1984 and 2018 were evaluated for use in this analysis. The following water chemistry parameters (representing one day averages) were evaluated: calcium (Ca), magnesium (Mg), sodium (Na), sulfate (SO4), chloride (Cl), potassium (K), alkalinity, temperature, hardness, pH and DOC. EPA also included the following supporting information: sampling station ID number, sample date, sample season, state, EPA region, stream order, location name, latitude, longitude and ecoregion. 
After initial compilation, data were evaluated for usability based of the following:
 Only data collected from locations that were sampled for a minimum of 10 days during three or more meteorological seasons, and with a minimum of two sampled days per season were included in the analysis, to ensure the adequate characterization of each location included in the analysis;
 Samples with incomplete or missing water chemistry parameters were excluded from the analysis; 
 Water quality data were inspected to ensure that minimum and maximum values fell within expected ranges; datapoints falling outside these bounds were excluded; and,
 It was verified that all sample location data (within the conterminous US; state, EPA region and Ecoregion III) agreed with the latitude/longitude data at each station. ESRI ArcGIS Desktop 10.8.1 GIS software was used to QA the sample location data.
The final database included a total of 686 NWIS sample stations and 38,603 records (Attachment A). These USGS NWIS stations were mapped using Geographic Information System (ArcGIS Desktop 10.8.1) and shown in Figure 1.
For this analysis EPA employed the Aluminum Criteria Calculator R Code V2.0 and the Copper Biotic Ligand Model (BLM) V2.2.1 for criteria derivation. For information on the 2018 aluminum criteria and the 2007 copper criteria, please refer to publications "Final Aquatic Life Ambient Water Quality Criteria for Aluminum  -  2018 (EPA-822-R-18-001)" and "Aquatic Life Ambient Freshwater Quality Criteria - Copper 2007 Revision (EPA-822-R-07-001)", respectively. 
The input files of water chemistry parameters were processed from the QA'd water chemistry dataset in spreadsheets. For all water quality records, the parameters serving as inputs to the Aluminum Criteria Calculator (pH, hardness and DOC) were compared to the bounded values of each parameter, and values outside of the model bounds were identified and flagged in the Aluminum Criteria Calculator output tabulations (Attachment A). In those instances where water chemistry data inputs fell outside of model bounds, caps will be placed on the inputs and reflected in the derived criteria (refer to the 2018 aluminum criteria document for details). The parameters serving as inputs to the copper BLM (temperature, pH, DOC, Ca, Mg, Na, K, SO4, Cl and alkalinity) were compared to the "Input Check Range" provided in the BLM Help section and parameter values outside of the "check range" were identified and flagged in the BLM output tabulations (Attachment A). Values outside the model bounds for copper were used to derive criteria values. The batch-processed Aluminum Criteria Calculator and the Copper Biotic Ligand Model outputs were checked on a random basis against model outputs for the water quality realizations.
The 5[th], 10[th], 15[th], 20[th] and 25th percentiles of the acute and chronic water quality criteria for both metals, based on modeled outputs for all 38,603 realizations in the QA'd water quality dataset, were calculated using the Percentiles function in Interactive Data Language (IDL, 2020). The Percentiles function calculates nonparametric (i.e., rank) percentiles, following the recommendations of Dierickx (2008) and Hyndman and Fan (1996). Percentiles calculated using the built-in MS Excel PERCENTILE function were also tabulated for comparison. For this large dataset, the results are virtually indistinguishable from the percentile values calculated using the more robust IDL function. 

Results
The criteria derived for aluminum and copper are presented in Attachment A  (`National Data and Models Output'). The calculated percentiles for acute criteria (criterion maximum concentration; CMC) and chronic criteria (criterion continuous concentration; CCC) for aluminum and copper are presented in Table 1.
Table 1. Calculated 5[th], 10[th], 15[th], 20[th] and 25[th] percentiles acute and chronic criteria for aluminum and copper (n=38,603).
       
                                  Percentile
                             Aluminum  CMC (ug/L)
                             Aluminum CCC (ug/L)
                              Copper CMC (ug/L)
                              Copper CCC (ug/L)
                                      5%
                                      680
                                      340
                                     2.87
                                     1.78
                                      10%
                                     1100
                                      460
                                     5.19
                                     3.22
                                      15%
                                     1400
                                      550
                                     7.18
                                     4.46
                                      20%
                                     1700
                                      620
                                     8.90
                                     5.53
                                      25%
                                     1900
                                      700
                                     10.66
                                     6.62
      	
Figure 1. Map showing the locations of all NWIS stations included in this analysis (n=686).

References
Dierickx, T. 2008. Computing Percentiles  -  Are Your Values Correct? (http://www.data-for-all.com/documents/computing-percentiles.pdf)
Hyndman and Fan, 1996. Sample Quantiles in Statistical Packages. American Statistician. 50(4): 361-365.
Interactive Data Language (IDL). Harris Geospatial Solutions, Inc. 2020.
US EPA (2007). Aquatic Life Ambient Freshwater Quality Criteria - Copper 2007 Revision. Office of Water Office of Science and Technology Washington, D.C. February 2007. EPA-822-R-07-001. https://www.epa.gov/sites/production/files/2019-02/documents/al-freshwater-copper-2007-revision.pdf
US EPA (2018). Final Aquatic Life Ambient Water Quality Criteria for Aluminum  -  2018. Office of Water Office of Science and Technology Health and Ecological Criteria Division Washington, D.C. December 2018. EPA-822-R-18-001. https://www.epa.gov/sites/production/files/2018-12/documents/aluminum-final-national-recommended-awqc.pdf


 

