About the Nutrient Model

Introduction

EPA developed a model in the Loading Tool to better identify facilities operating in industries likely to discharge nutrients and estimate the amount of nutrients discharged from these facilities. EPA focused on nutrient discharges due to the significant environmental impacts on the Nation's water resources (e.g., Gulf of Mexico and Chesapeake Bay hypoxic zones).

EPA created the "Nutrient Model (Hypoxia Task Force Search)" to provide access to aggregated nitrogen and aggregated phosphorus loads (including modeled loads) for facilities. Users can search by location/watershed, industry, and pollutant concentration ranges, and permit limit exceedances.

For nutrient data in ICIS-NPDES, the Loading Tool uses the following procedures to aggregate loads for nitrogen compounds (e.g., nitrate, nitrite, TKN) to estimate total nitrogen, and to aggregate loads for phosphorus compounds (e.g., phosphate) to estimate total phosphorus. 

  • For Total Nitrogen (as N):
    • If Total Nitrogen is reported, then use that load for total nitrogen;
    • If Total Nitrogen is not reported and TKN is reported, then use the TKN load combined with any nitrate or nitrite loads (converted to pounds of N) to estimate total nitrogen; and
    • If Total Nitrogen and TKN are not reported and Organic Nitrogen is reported, then use the Organic N load combined with any nitrate, nitrite, or ammonia loads (converted to pounds of N) to estimate total nitrogen.
       
  • For Total Phosphorus (as P):
    • If Total Phosphorus is reported, then use that load for total phosphorus; and
    • If Total Phosphorus is not reported, then use the Phosphate load (converted to pounds of P) estimate total phosphorus.

For this model, the Loading Tool does not aggregate loads for facilities that only report measurements of nitrite, nitrate, or ammonia.

The Loading Tool then calculates flow-weighted average concentrations for the aggregated nitrogen and aggregated phosphorus discharges. The Nutrient Model uses these aggregated nutrient concentrations to generate summary statistics, calculate typical pollutant concentrations (TPCs), and estimate nutrient discharges for facilities that are likely discharge nutrients but do not have nutrient data in ICIS-NPDES. The Nutrient Model methodology is described in detail in the steps below.


Nutrient Model Methodology

1. Infill Missing Standard Industrial Classification (SIC) Code Data

The Nutrient Model uses the facility SIC code to identify facilities in industries that are likely to discharge nutrients. The Loading Tool uses SIC code data from EPA's Integrated Compliance Information System - National Pollutant Discharge Elimination System (ICIS-NPDES) and Federal Registry Service (FRS) to identify the industrial sector of the discharger. The Loading Tool uses FRS SIC code data if there are no SIC code data in ICIS-NPDES. Although EPA policy requires authorized NPDES permit programs to enter SIC code data into ICIS-NPDES for all facilities (majors and minors), SIC code data are not available for 35% of permits in ICIS-NPDES. EPA used the following procedure to improve the completeness and accuracy of the SIC code data in the Loading Tool database.

  • Apply SIC Code Corrections from U.S. Geological Survey (USGS): EPA used the error correction data supplied to EPA by USGS. Maupin and Ivahnenko (2011) estimated nutrient pollutant discharges from point sources in six major U.S. watersheds (Major River Basins 1 through 5, and 7).1
  • Crosswalk North American Industry Classification System (NAICS) codes to SIC Codes: If a facility does not have an SIC Code but has a North American Industry Classification System (NAICS) code in ICIS-NPDES or FRS, EPA assigned an SIC Code based on the NAICS description. EPA used FRS NAICS code data if there were no NAICS code data in ICIS-NPDES. Note that EPA policy does not require authorized NPDES permit programs to enter NAICS code data into ICIS-NPDES.
  • Assign SIC Codes to Publicly-Owned Treatment Works (POTWs): EPA assigned an SIC Code of 4952 to all facilities identified as POTWs.

2. Calculate Summary Statistics for Loading Calculations

The Nutrient Model generates summary statistics using aggregated nitrogen and aggregated phosphorus concentrations calculated from the most recent complete calendar year of DMR data in ICIS-NPDES (e.g., throughout 2018, the model will use 2017 DMR data).

The following statistics are generated for nitrogen and phosphorus for each SIC code2, facility flow class (i.e., <1 MGD, 1 - 5 MGD, >5 MGD), and season3:

  • Concentration (mg/L)
    • Average
    • Median
    • Minimum
    • Maximum
  • Median wastewater discharge flow (MGD)
  • Number of nutrient measurement observations
  • Number of wastewater discharge flow measurement observations
  • Number of unique facilities with nutrient measurement observations
  • Number of unique facilities with wastewater discharge flow measurement observations
  • Total number of unique facilities in 4-digit SIC code group

EPA decided to group concentration data by SIC code, facility flow class, and season based on the approach used by USGS to estimate annual nutrient loads in McMahon et al. (2007)4.

The summary statistics are recalculated during each weekly data refresh and provided in HTF Nutrient Model information.

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3. Calculate Typical Pollutant Concentrations (TPCs)

For each SIC code, the Nutrient Model calculates an overall median Total Nitrogen concentration (mg/L) and median Total Phosphorus concentration (mg/L) using the most recent complete calendar year of DMR data.

The TPCs are recalculated during each weekly data refresh and provided in HTF Nutrient Model information. The TPCs specifically for POTWs (facilities with SIC code 4952) were established based on averages of reported DMR data and are not refreshed weekly; see Rationale for Re-evaluation of POTW TPCs for the background and methodology for the TPC values for SIC code 4952.

The Nutrient Model flags SIC codes as "Likely to Discharge Nitrogen" if:

  • The median Total Nitrogen concentration is 1.0 mg/L; and
  • More than 10% of the total number of unique facilities in the SIC group had Total Nitrogen measurement observations.

The Nutrient Model flags SIC codes as "Likely to Discharge Phosphorus" if:

  • The median Total Phosphorus concentration is 0.2 mg/L; and
  • More than 10% of the total number of unique facilities in the SIC group had Total Phosphorus measurement observations.

4. Calculate Nutrient Discharges

The Nutrient Model uses the following methodology to calculate 'Total Nitrogen' and 'Total Phosphorus' loadings, based on the type of facility and availability of DMR data:

  • Facilities with flow and nutrient DMR data: The Loading Tool calculates  aggregate nutrient discharges using nutrient DMR data in ICIS-NPDES as outlined above. The Nutrient Model does not estimate any additional nutrient discharges for these facilities.
     
  • Facilities with flow and no nutrient DMR data: The Nutrient Model identifies facilities that are likely to discharge nitrogen and/or phosphorus based on their SIC code and statistics in the TPC table (see Step 3 above). If a facility is classified in an SIC code that the Nutrient Model identified as likely to discharge nitrogen and/or phosphorus, then the Nutrient Model calculates loads for the facility using the facility's reported effluent flow data (from DMR data in ICIS-NPDES) and the median total nitrogen and/or total phosphorus concentrations from the Summary Statistics table (see Step 2 above). This calculation step will match the facility flows with corresponding total nitrogen and total phosphorus concentration for the matching SIC code, flow class, and season. If the facility's flow data do not match exactly to a flow class and season in the Summary Statistics table, then the Nutrient Model uses the overall median concentrations for the facility's SIC code (from Step 3).
     
  • POTWs without DMR data: The Nutrient Model estimates nutrient loads for all POTWs. If a POTW does not have DMR data in ICS-NPDES (i.e., no flow or nutrient measurements), then the Nutrient Model estimates a flow for the POTW using service population data from the 2008 Clean Watersheds Needs Survey (CWNS). The Nutrient Model calculates the total POTW flow as the total flow from residents and non-residents:
    • Residential Flow (MGD) = Resident Population x 280 (L/day) x (1 gal/3.785 L) x (1 MGal/1,000,000 gal)
    • Non-Residential Flow (MGD) = Non-Resident Population x 55 (L/day) x (1 gal/3.785 L) x (1 MGal/1,000,00 gal)5

    The Nutrient Model calculates the POTW total nitrogen and/or total phosphorus loads using the estimated wastewater flow and the median nitrogen and/or phosphorus concentrations for SIC code 4952.

References

  1. Maupin, Molly A. and Tamara Ivahnenko. 2011. Nutrient Loadings to Streams of the Continental United States from Municipal and Industrial Effluent. Journal of the American Water Resources Association (JAWRA) 47 (5): 950-964. DOI: 10.1111/j.1752-1688.2011.00576.x
  2. Four-digit code that describes the primary activity of the facility. The two-digit code defines a major business sector; the four-digit code contains two additional numbers that denote a facility's specialty within the major sector.
  3. Seasons are Winter (December, January, February), Spring (March, April, May), Summer (June, July, August), and Fall (September, October, November).
  4. McMahon, Gerard, Tervelt, Larinda, and Donehoo, William. 2007. Methods for estimating annual wastewater nutrient loads in the southeastern United States: U.S. Geological Survey Open-File Report 2007-1040. 81 p.
  5. Metcalf and Eddy, Inc. 2003. Wastewater Engineering: Treatment and Reuse. McGraw-Hill, Inc. 4th edition.

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