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N.C. Department of Environment and Natural Resources

NC Department of Environment and Natural Resources
Water Quality - Option 2

Water Quality

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Calculator 2

Estimate NPDES Permit Limitations based on the proposed
Dissolved Water Quality Standards & Stream/Effluent Hardness

NOT intended for saltwater, lake or stormwater discharges

v. 5/27/2010

This calculator will assist Municipal and Industrial permittees' estimate NPDES permit limitations based on the proposed dissolved Metal Standards for freshwater streams. It was designed to provide NPDES permitting information to enable permittees to assess the economic impact of the proposed metal standards based on estimated permit limits and implementation guidelines. 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 Standards can be viewed at: http://portal.ncdenr.org/web/wq/ps/csu/swtrirev

Only the Surface Water Standards applicable to dissolved metals are addressed in this calculator.

This calculator estimates Total Recoverable Metal Permit Limits for dissolved metals using a combined hardness based on the permittee's 8-digit HUC hardness and the 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.

Individual facility's NPDES Permitted flow, 7Q10's, and 8-Digit HUC's can be found here: Facility Information

  • Step 1: Enter the facility's Receiving Stream's summer 7Q10 flow (cfs), in the first box in Table A below - click Enter.
  • Step 2: Enter the facility's NPDES Permitted Flow limit (MGD), in the fourth box in Table A below - click Enter.
  • Step 3: Enter the hardness for the 8-digit HUC, in which the facility is located, in the second to last box in Table A below - click Enter. To determine the 10th percentile Hardness in your 8-digit HUC, click here.
  • Step 4: Enter the facility's effluent hardness (10th percentile of 12 samples taken monthly, if available) in the last box in Table A below - click Enter. A Combined Hardness using a mass balance equation will be determined, as shown in the center of Table A, and used to recalculate the Dissolved Metals. If the combined Hardness is <25 a default value of 25 mg/L will be used. If no effluent hardness is available it is best to use Calculator 1 to determine estimated permit limitations.
  • Step 5: Review the estimated NPDES Permit Limits listed in red. For HQW streams the limits are reduced to one-half the value as shown in the columns to the right of the spreadsheet.
  • Step 6: Reasonable Potential Analyses 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 NPDES Guidelines for Permit Limits:
    • Copper, Silver & Zinc limitations are based on action level standards and currently are not limited in the NPDES permit unless the Permittee is failing its Whole Effluent Toxicity tests. 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 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.
    • In accordance with Federal Regs, permit limitations must be written as Total Metals per 40 CFR 122.45(c).


Table A

Receiving
Stream
summer
7Q10
(CFS)
Receiving
Stream
summer
7Q10
(MGD)
Receiving
Stream
1Q10
[MGD]
NPDES
Flow Limit
[MGD]
Total
Suspended
Solids
[mg/l]
Combined
Hardness
[mg/l]
Instream
Wastewater
Concentration
(Chronic)
Instream
Wastewater
Concentration
(Acute)
8-digit HUC
Hardness
Concentration
(10th %)
Effluent
Hardness
(10th %)
           

Table B

  Dissolved
Metals
US EPA Translators -
using Default Partition Coefficients (streams)
Total Metals
[Dissolved Metal ÷ Translator]
Class C, C-NSW
C-Swamp, B.
and WS streams
  HQW streams
Estimated NPDES
Permit Limits (e)
[Total Metal ÷ IWC]
Estimated NPDES
Permit Limits (e)
[Total Metal ÷ IWC]
Chronic Acute Chronic Acute Chronic Acute Chronic Acute
PARAMETER * [µg/l] [µg/l] [µg/l] [µg/l] [µg/l] [µg/l] [µg/l] [µg/l]
Cadmium (a)                    
   Cd - Trout streams (a)                  
Chromium III (a)                  
Chromium VI N/A N/A N/A N/A N/A N/A N/A N/A N/A
Chromium, Total N/A N/A N/A N/A N/A N/A N/A N/A N/A
Copper (a,b)                  
Lead (a)                  
Nickel (a,c)                  
   Ni - WS streams                  
Silver (a,b)                  
Zinc (a,b)                  
Dissolved Metals that are not hardness dependent - proposed standards the same or greater than existing standards
Beryllium                  
Arsenic (d)                  

Pretreatment POTWs should print a copy of the calculator and proceed to Pretreatment Program's Home Page and view the section on Water Quality Standards.


  1. Denotes metals for which Aquatic Life Criteria are expressed as a function of hardness.
  2. Copper, Silver and Zinc are action level standards and currently are not limited in the NPDES permit unless the Permittee is failing its Whole Effluent Toxicity tests.
  3. The Human Health standard for Nickel in Water Supply Streams is 25 ug/L which is more stringent than the aquatic life criteria.
  4. The Human Health standard for Arsenic is 10 ug/L which is more stringent than the aquatic life criteria. The Permit limit for Arsenic would be determined by taking the Human Health standard and dividing it by the IWC based on the mean annual flow (QA) of the receiving stream and the permitted plant flow.
  5. NPDES Permit Limits are rounded up one significant digit from what is shown in the spreadsheet.

Questions?

  • On NPDES Permit Limit calculations, contact , NPDES Complex Permitting Unit: 919-807-6389
  • On how the NPDES Limits effect Pretreatment Programs, contact , PERCS Unit: 919-807-6311
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