Metals Calculators

This website has been developed to assist NPDES permit holders with assessing the economic impact of proposed dissolved metal standards based on estimated permit limits and implementation guidelines. The purpose of the calculators is to help municipal and industrial permittees estimate NPDES Permit Limitations based on the proposed Dissolved Metal Standards for freshwater streams. These calculations are based on current NPDES implementation policies and are subject to change as the regulations proceed through the Administrative Procedures Act process. Complete information on all the proposed Surface Water Quality Standards can be viewed at: /about/divisions/water-resources/water-resources-rules/dwr-classifications-standards-rulemaking/swtrirev. Only the Surface Water standards applicable to dissolved metals are addressed in the calculator spreadsheets.

These calculators are not intended for saltwater, lake, or stormwater discharges. Permittees with stormwater discharges should consult the Stormwater Permitting Unit.

Calculator 1

This calculator estimates Total Recoverable Metal Permit Limitations for dissolved metals using a hardness of 25 mg/L, expressed as CaCO3 or Ca+Mg. This is proposed to be the lower boundary of the applicable hardness and therefore should provide conservative permit limit estimates.


  • NPDES permitted flow (MGD)
  • Receiving stream's summer 7Q10 (cfs)


Calculator 2

This calculator estimates Total Recoverable Metal Permit Limitations for dissolved metals using a combined hardness based on the permittee's 8-digit HUC hardness and a facility's effluent hardness. A default hardness of 25 mg/L is used if the mass balance of the combined hardness data is less than 25 mg/L.


  • NPDES permitted flow (MGD)
  • Receiving stream's summer 7Q10 (cfs)
  • HUC Hardness (mg/L)
  • Facility effluent hardness (mg/L)

Go to Facility Information on NPDES Permittees to access data on a facility's NPDES Permitted Flow, Receiving Stream's 7Q10 and 8-Digit HUC. Facility information should be verified with the Permittee's current NPDES permit. If your facility is not listed, look on your permit or fact sheet to get your facility's NPDES Permitted Flow and 7Q10 information.

Retrieve median HUC hardness data: 8-Digit HUC Hardness


For NPDES permit limit calculations and limits affecting Pretreatment Programs, contact Michael Montebello, NPDES Municipal Permitting Unit, at 919-707-3624.

How do the Calculators Determine the Permit Limitations?

Follow the steps listed below:
  1. Some metals standards are calculated using the hardness of the ambient water. Go to Dissolved Metals and Hardness Determination to review the freshwater standard equations and how to determine hardness for use with these equations.
  2. US EPA Translators are calculated using EPA Default Partition Coefficients for streams. Go to US EPA Translator to review coefficient equations and how to determine Translators.
  3. Calculate Total Metals to be allocated to the Permittee by using the following equation:
     Dissolved Metals Translator=Total Metals
  4. NPDES Permit Limitations are based on stream dilution or the Instream Wastewater Concentration (IWC). Use the following equations to determine your facility’s IWC. Facility information on Permitted Flow and 7Q10s for individual NPDES permitted facilities can be found online: Click Here
    • Fill in:
    • Permitted Flow (MGD) = _________
    • Receiving stream 7Q10 (MGD) = _________
    • Receiving stream 1Q10(MGD) = 0.843 {(7Q10)0.993}
    • IWC chronic = Permitted Flow (MGD)
       [Permitted Flow (MGD) + 7Q10(MGD)]
    • IWC acute =Permitted Flow (MGD)
       [Permitted Flow (MGD) + 1Q10(MGD)]
  5. Estimate the facility's Total Recoverable Metal Permit Limits by dividing the Total Metal by the corresponding IWC. For zero low-flow stream's the summer 7Q10 equals zero and the IWC = 1.00
    • Total Metal (chronic) ÷ IWC (chronic) = Permit Limit (chronic)
    • Total Metal (acute) IWC acute = Permit Limit (acute)
  6. Reasonable Potential Analyses (RPA) will be performed on Permittees' sampling data to determine whether the facility will get a limit and/or additional monitoring. However, to assess costs use the following guidelines:
    • Copper, Silver & Zinc limitations are based on surface water quality standards and currently are limited in NPDES permits. Monitoring is required if the facility's effluent concentrations equal or exceed the estimated Permit Limits for these parameters.
    • For all other metals (Cadmium, Lead, Nickel, Chromium, Arsenic & Beryllium) if effluent sampling data equals or exceeds the estimated NPDES Permit Limits, assume limits and monitoring will be put in the facility's next permit renewal.
    • For Cadmium, Lead, Nickel, Chromium, Arsenic & Beryllium if all the effluent sampling data for the last five years shows the pollutant at concentrations less than the Practical Quantitation Limit (PQL), it is not likely a limit or monitoring will be put in the permit. However, if the estimated NPDES Permit Limit is less than the Practical Quantitation Limit and the pollutant is believed to be present, to assess compliance with the new standards and for future permit limit development, quarterly monitoring for the pollutant will be required.
    • For facilities that have demonstrated Total Chromium concentrations less than the Practical Quantitation Limit or have tested and demonstrated the absence of Chromium VI in their wastewaters, simply assume Total Chromium = Chromium III for Permit Limit and compliance purposes.
    • Pretreatment Facilities: To determine minimum Pass-Thru Loading, use the estimated chronic NPDES permit limit for each metal. Use the chronic Total Metal Concentration as the Stream Standard in the Pass-Thru Loading spreadsheet, if a NPDES permit limit is not expected for that metal. Click here to go to the Pretreatment website and view the section on Water Quality Standards.