Subject: Spreadsheets for Water Quality based NDPES Permit Calculations


http://olympus.dis.wa.gov/www/access/ecology/pwspread.html


   
   
       SPREADSHEETS FOR WATER QUALITY-BASED NPDES PERMIT CALCULATIONS
                                       
   Updated August 4, 1995 by Greg Pelletier
   
   The spreadsheets identified below were developed by the Washington
   State Department of Ecology's Environmental Investigations and
   Laboratory Services Program to aid NPDES permit writers. These
   spreadsheets are referenced in Ecology's Permit Writer's Manual
   (Department of Ecology Publication Number 92-109). All of the
   spreadsheets are in separate Lotus 1-2-3 WK1 files and also have been
   combined and reformatted into an Excel 5 workbook.
   
     * DOSAG2.WK1: This spreadsheet calculates critical sag of dissolved
       oxygen downstream from a point source using the Streeter-Phelps
       equation.
       
       A detailed description and user instructions are available. See
       below for FTP access to the description and user instructions.
       
       DOSAG2.WK1 also is available to download via FTP.
       
     * IDOD2.WK1: This spreadsheet calculates concentrations of dissolved
       oxygen at a mixing zone boundary accounting for dilution of
       dissolved oxygen and initial dissolved oxygen demand.
       
       A detailed description and user instructions are available. See
       below for FTP access to the description and user instructions.
       
       IDOD2.WK1 also is available to download via FTP.
       
     * NH3FRESH.WK1: This spreadsheet calculates freshwater un-ionized
       and total ammonia criteria from temperature and pH from the
       formulas in the EPA Gold Book (EPA 440/5-86-001) and state water
       quality standards (Chapter 173-201A Washington Administrative
       Code).
       
       A detailed description and user instructions are available. See
       below for FTP access to the description and user instructions.
       
       NH3FRESH.WK1 also is available to download via FTP.
       
     * NH3FRES2.WK1: This spreadsheet calculates freshwater un-ionized
       and total ammonia criteria from temperature and pH from the
       formulas modified by EPA which are proposed to be adopted in the
       1995 revision to the state water quality standards. The modified
       formulas in this spreadsheet are recommended in the Permit
       Writer's Manual.
       
       A detailed description and user instructions are available. See
       below for FTP access to the description and user instructions.
       
       NH3FRES2.WK1 also is available to download via FTP.
       
     * NH3SALT.WK1: This spreadsheet calculates saltwater total ammonia
       criteria from temperature, pH, and salinity to meet the un-ionized
       ammonia criteria.
       
       A detailed description and user instructions are available. See
       below for FTP access to the description and user instructions.
       
       NH3SALT.WK1 also is available to download via FTP.
       
     * PHMIX2.WK1: This spreadsheet calculates the pH of a mixture of two
       sources from temperature, pH, and alkalinity.
       
       A detailed description and user instructions are available. See
       below for FTP access to the description and user instructions.
       
       PHMIX2.WK1 also is available to download via FTP.
       
     * RIVPLUM4.WK1: This is a simple dilution model for rivers.
       
       A detailed description and user instructions are available. See
       below for FTP access to the description and user instructions.
       
       RIVPLUM4.WK1 also is available to download via FTP.
       
     * WQBP2.WK1: This spreadsheet calculates water quality based permit
       limits to meet acute and chronic aquatic life criteria for
       specific chemicals.
       
       A detailed description and user instructions are available. See
       below for FTP access to the description and user instructions.
       
       WQBP2.WK1 also is available to download via FTP.
       
     * PWSPREAD.XLS: This Excel 5 file contains all of the above
       spreadsheets.
       
       PWSPREAD.XLS is available to download via FTP.
       
   
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Detailed Descriptions and User Instructions

   The complete set of detailed descriptions and user instructions for
   all of the above files also is available to download via FTP.
   
   
     _________________________________________________________________
   
Spreadsheet DOSAG2.WK1

   Revised October 19, 1993
   
   This spreadsheet replaces a previous version called DOSAG.WK1. This
   spreadsheet calculates the critical dissolved oxygen sag and
   concentration downstream from a point source load of BOD in a river
   using the Streeter-Phelps equations. The method used is documented in
   EPA/600/6-85/002a (Water Quality Assessment: A Screening Procedure for
   Toxic and Conventional Pollutants in Surface and Ground Water). This
   spreadsheet is recommended for use as a screening tool to determine
   the potential for dissolved oxygen standards to be violated. DOSAG.WK1
   may be overly simplistic for deriving limits for effluent BOD. If
   DOSAG.WK1 suggests the dissolved oxygen sag is close to or below the
   water quality standard, then a more sophisticated model such as QUAL2E
   or WASP5 should be used to derive appropriate effluent limits. Those
   water quality models are designed to more accurately simulate water
   movements, mass transport, and water column processes.
   
  USER INSTRUCTIONS FOR THE INPUT SECTION
  
   
   
   Step 1: Enter the permittees effluent characteristics, including
   permitted discharge and maximum (e.g, weekly) 5-day BOD (referred to
   as CBOD5 for "carbonaceous" 5-day BOD). Carbonaceous 5-day BOD is less
   than the total 5-day BOD if nitrification occurs during the test. The
   minimum national standards for carbonaceous 5-day BOD in effluent
   after secondary treatment are a monthly average of 25 mg/L and weekly
   average of 40 mg/L (40 CFR Part 133). Guidance for determining if
   carbonaceous 5-day BOD should be substituted for total 5-day BOD is
   contained in Ecology's Permit Writer's Manual (section V-3.6).
   
   Nitrogenous BOD (NBOD) should also be estimated if it is significant
   (e.g. if nitrification is not significant during secondary
   treatment). NBOD can be estimated as:
   
   NBOD = 4.57 * (Ammonia N + Organic N)
   
   where concentrations of NBOD, ammonia N and organic N are expressed in
   mg/L. Effluent temperature and dissolved oxygen for the analysis are
   also entered at this step.
   
   The spreadsheet may be used to estimate the maximum permissible
   effluent CBOD5 and NBOD that will meet the water quality standards for
   dissolved oxygen. A trial and error solution is necessary for this
   purpose. Trial values of effluent CBOD5 and NBOD may be entered until
   the dissolved oxygen at the critical sag meets the water quality
   standard.
   
   Step 2: Enter receiving water characteristics. These will generally be
   conditions at the 7Q10 discharge. Upstream CBOD5, NBOD, dissolved
   oxygen and temperature at the design river flow (e.g., 7Q10) should be
   entered. The local channel elevation and channel slope (e.g., from
   USGS topographic maps) downstream from the discharge should also be
   entered. Downstream average channel depth and velocity at the design
   flow should be entered also.
   
   If no receiving water data are available, it would be desirable to
   collect data. Channel cross-sections of depth and velocity can be
   measured during the critical season. If measurements are not taken
   near critical conditions, then Manning's equation may be used to
   estimate velocity and depths from the measurements. Several
   cross-sections proceeding downstream from the discharge may be needed
   to characterize the river to the point of critical sag if velocities
   and depths are not uniform. Dye studies to measure travel time may be
   useful if velocities are variable. If significant tributaries,
   groundwater inflows, or other pollutant loads occur before the
   predicted critical sag point, then a more sophisticated model should
   be used (e.g. QUAL2E).
   
   Measurements of water quality (e.g. dissolved oxygen, ammonia, BOD) in
   the receiving water from upstream and at intervals downstream to the
   critical sag point are also desirable for model calibration. If the
   model is applied without sufficient data to demonstrate calibration,
   then the model should mainly be used to screen for potential violation
   of standards. If effluent BOD is required to be more restrictive than
   current technology-based limits, then calibration data are probably
   needed. Separate calibration and verification data sets taken on
   different dates may be needed in many cases where the accuracy of the
   model is in question.
   
   Step 3: Enter the reaeration rate (base e) at 20 degrees C in cell
   D27. Suggested values using empirical equations referenced in
   EPA/600/6-85/002a are given below cell D27 for guidance in selecting
   an appropriate value. If the calculated values are used, select the
   most appropriate equation based on applicable depth and velocity
   (e.g., if depth is Step 4: Enter the BOD decay rate (base e) at 20
   degrees C in cell D36. A calculated value based on the Wright and
   McDonnell equation referenced in EPA/600/6-85/002a is provided and may
   be entered in cell D36 at Step 4 if desired.
   
  USER INSTRUCTIONS FOR THE OUTPUT SECTION
  
   
   
   The user does not need to change or enter any values or formulas in
   the Output Section. The travel time and distance to critical sag,
   deficit at critical sag, and dissolved oxygen concentration at
   critical sag are displayed in the Output Section.
   
   
   
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Spreadsheet IDOD2.WK1

   Revised October 19, 1993
   
   This spreadsheet replaces a previous version called IDOD.WK1. This
   spreadsheet calculates the dissolved oxygen concentration at a mixing
   zone boundary from dilution of dissolved oxygen in the effluent and
   ambient background and immediate dissolved oxygen demand of the
   effluent. The method used is presented in EPA/600/6-85-002b (Water
   Quality Assessment: A Screening Procedure for Toxic and Conventional
   Pollutants in Surface and Ground Water - Part II Revised 1985) and
   EPA/430/9-82-011 (Revised Section 301(h) Technical Support Document).
   
  USER INSTRUCTIONS FOR THE INPUT SECTION
  
   
   
   Step 1: Specify the dilution factor for effluent at the chronic mixing
   zone boundary. This value should represent dilution at critical
   conditions if the spreadsheet is being used for developing NPDES
   permit limits. The dilution factor used should represent the
   reciprocal of the volume fraction of effluent present at the mixing
   zone boundary (see Permit Writer's Manual section VI-2.1).
   
   Step 2: Enter the background dissolved oxygen concentration in the
   receiving water. The 10th percentile during the critical season is
   recommended as a reasonable worst case. If no data are available it
   would be desirable to collect data describing background dissolved
   oxygen concentrations during the critical season (e.g. upstream from
   the discharge to a river).
   
   Step 3: Enter the effluent dissolved oxygen concentration. The 10th
   percentile during the critical season is recommended as a reasonable
   worst case.
   
   Step 4: Enter the immediate dissolved oxygen demand (IDOD) of the
   effluent if known. The IDOD represents the oxygen demand of reduced
   substances which are rapidly oxidized (e.g. sulfides to sulfates). If
   the effluent contains measurable dissolved oxygen, then the IDOD may
   be negligible. If IDOD is to be determined experimentally, the
   procedures in Standard Methods 1979 edition could be followed.
   However, the method was omitted from Standard Methods in the 1985
   edition because of concerns about the accuracy of the test.
   
  USER INSTRUCTIONS FOR THE OUTPUT SECTION
  
   
   
   The user should not enter or change the value or formula in the output
   section. The dissolved oxygen at the mixing zone boundary is presented
   in the output section.
   
   
   
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Spreadsheet NH3FRESH.WK1 and NH3FRES2.WK1

   Revised October 19, 1993 (NH3FRESH.WK1) and December 12, 1994
   (NH3FRES2.WK1)
   
   These spreadsheets replace a previous version called AMMONIA.WK1.
   NH3FRESH.WK1 calculates the freshwater acute and chronic criteria for
   un-ionized and total ammonia for specified temperature and pH using
   the procedure described in the EPA Gold Book (EPA 440/5-86-001) and
   listed in WAC 173-201A. NH3FRES2.WK1 contains the formulas modified by
   EPA that are proposed to be adopted in the 1995 revision of the state
   water quality standards. The spreadsheets also calculate the amount of
   un-ionized ammonia present in a sample if total ammonia, temperature,
   and pH are known.
   
  USER INSTRUCTIONS FOR THE INPUT SECTION
  
   
   
   Step 1: Specify the temperature (design condition at the mixing zone
   boundary) for which un-ionized ammonia criteria or concentrations are
   to be estimated. If the spreadsheet is being used to calculate
   criteria for a NPDES permit limit, the 90th percentile temperature
   during the critical season is recommended for a reasonable worst-case
   condition. If no data are available it may be desirable to collect
   data during the critical season to describe temperature at the mixing
   zone boundary.
   
   Step 2: Specify the pH (design condition at the mixing zone boundary)
   for which un-ionized ammonia criteria or concentrations are to be
   estimated. If the spreadsheet is being used to calculate criteria for
   a NPDES permit limit, the 90th percentile pH during the critical
   season is recommended for a reasonable worst-case condition. If no
   data are available it may be desirable to collect data during the
   critical season to describe pH at the mixing zone boundary.
   
   Step 3: Specify the sample total ammonia concentration if known.
   Entering a value here only affects Output Step 2 (calculation of
   un-ionized ammonia present in a sample). No input is required at this
   step if the spreadsheet is being used only to calculate criteria from
   temperature and pH (i.e. values entered at this step do not affect
   criteria calculations).
   
   Step 4: Specify "Acute TCAP" according to the Gold Book (enter 20 if
   salmonids are present; 25 if salmonids are absent).
   
   Step 5: Specify "Chronic TCAP" according to the Gold Book (enter 15 if
   salmonids are present; 20 if salmonids are absent).
   
  USER INSTRUCTIONS FOR THE OUTPUT SECTION
  
   
   
   The user should not enter or change any values or formulas in the
   Output Section. The spreadsheet calculates the amount of un-ionized
   ammonia present in a sample at Output Step 2 if the sample total
   ammonia was specified at Input Step 3. Output Step 3 provides the
   acute and chronic criteria for un-ionized ammonia expressed in ug/L as
   NH3-N. Output Step 4 provides the acute and chronic criteria for total
   ammonia expressed in ug/L as NH3-N.
   
   
   
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Spreadsheet NH3SALT.WK1

   Revised October 19, 1993
   
   This spreadsheet replaces a previous version called HAMPSON.WK1. This
   spreadsheet calculates water quality criteria for ammonia in saltwater
   using the method specified in EPA 440/5-88-004 (Ambient Water Quality
   Criteria for Ammonia (Saltwater)-1989).
   
  USER INSTRUCTIONS FOR THE INPUT SECTION
  
   
   
   Step 1: Specify the temperature (design condition at the mixing zone
   boundary) for which un-ionized ammonia criteria are to be estimated.
   If the spreadsheet is being used to calculate criteria for a NPDES
   permit limit, the 90th percentile temperature during the critical
   season is recommended for a reasonable worst-case condition. If no
   data are available it may be desirable to collect data during the
   critical season to describe temperature at the mixing zone boundary.
   
   Step 2: Specify the pH (design condition at the mixing zone boundary)
   for which un-ionized ammonia criteria to be estimated. If the
   spreadsheet is being used to calculate criteria for a NPDES permit
   limit, the 90th percentile pH during the critical season is
   recommended for a reasonable worst-case condition. If no data are
   available it may be desirable to collect data during the critical
   season to describe pH at the mixing zone boundary.
   
   Step 3: Specify the salinity (design condition at the mixing zone
   boundary) for which un-ionized ammonia criteria are to be estimated.
   If the spreadsheet is being used to calculate criteria for a NPDES
   permit limit, the 10th percentile salinity during the critical season
   is recommended for a reasonable worst-case condition. If no data are
   available it may be desirable to collect data during the critical
   season to describe salinity at the mixing zone boundary.
   
  USER INSTRUCTIONS FOR THE OUTPUT SECTION
  
   
   
   The user should not enter or change any values or formulas in the
   output section. The acute and chronic criteria are expressed three
   ways: 1) as unionized ammonia in mg/L as NH3 at Output Step 5; 2) as
   total ammonia in mg/L as NH3 at Output Step 6; and 3) as total ammonia
   in mg/L as NH3-N at Output Step 7. For derivation of total ammonia
   waste load allocations and comparisons with effluent total ammonia
   data, it is recommended that the criteria be expressed as total
   ammonia in mg/L as NH3-N for simplicity. [Note: the criteria in EPA
   440/5-88-004 Tables 2 and 3 are for total ammonia as mg/L as NH3,
   which should be multiplied by 0.822 to convert to mg/L as NH3-N.]
   
   
   
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Spreadsheet PHMIX2.WK1

   Revised October 19, 1993
   
   This spreadsheet replaces a previous version called PH-MIX.WK1. This
   spreadsheet calculates the pH of a mixture of two flows using the
   procedure in EPA's DESCON program (EPA, 1988. Technical Guidance on
   Supplementary Stream Design Conditions for Steady State Modeling. EPA
   Office of Water, Washington DC). The major form of alkalinity is
   assumed to be carbonate alkalinity. Also, alkalinity and total
   inorganic carbon are assumed to be conservative.
   
  USER INSTRUCTIONS FOR THE INPUT SECTION
  
   
   
   Step 1: Specify the dilution factor for effluent at the mixing zone
   boundary. This value should represent dilution at critical conditions
   if the spreadsheet is being used for developing NPDES permit limits.
   The dilution factor used should represent the reciprocal of the volume
   fraction of effluent present at the mixing zone boundary (see Permit
   Writer's Manual section VI-2.1).
   
   Step 2: Specify the upstream characteristics, including temperature,
   pH, and alkalinity. For development of NPDES permit limits for
   ammonia, the 90th percentiles during the critical season are
   recommended. If no data are available, it is desirable to collect data
   describing upstream temperature, pH, and alkalinity during the
   critical season.
   
   Step 3: Specify the effluent characteristics, including temperature,
   pH, and alkalinity. For NPDES permit limits, a reasonable worst case
   estimate of each may be estimated from DMR data (e.g. for ammonia
   limits use 90th percentile values from the DMR data during the
   critical season). If effluent data are not available then data should
   be collected during the critical season. In many cases, pH in ambient
   receiving water (at Step 2 above) may be assumed to represent the pH
   in the mixing zone.
   
  USER INSTRUCTIONS FOR THE OUTPUT SECTION
  
   
   
   The user does not need to enter or change any values or formulas in
   the Output Section. The spreadsheet calculates and displays the pH at
   the mixing zone boundary at Output Step 4. Some important factors that
   can influence pH are not included in this calculation. For example,
   photosynthesis in the receiving water may increase pH downstream from
   the mixing zone. In many cases where dilution is relatively large
   (e.g. greater than a dilution factor of 20) the pH in the mixing
   zone will be dominated by ambient conditions. This spreadsheet should
   be used mainly where effluent dilution is relatively low and effluent
   pH and alkalinity are much different than in the receiving water.
   
   
   
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Spreadsheet RIVPLUM4.WK1

   Revised May 19, 1995
   
   This spreadsheet calculates dilution at a specified point of interest
   downstream from a point discharge to a river. The procedure used is
   described in Fischer et al., 1979 (Mixing in Inland and Coastal
   Waters, Academic Press) and referenced in EPA/505/2-90-001 (TSD for
   WQ-based Toxics Control). The calculation for dilution factors
   incorporates the boundary effect of shorelines (Fischer et al.,
   equation 5.9) using the method of superposition.
   
   This spreadsheet is based on the assumption that the discharge: 1) is
   a single point source, which is most appropriate for single port or
   short diffusers, or side-bank discharges; and 2) is completely and
   rapidly mixed vertically, which usually only occurs in shallow rivers.
   If the diffuser length occupies a substantial portion of the stream
   width, or the discharge is not vertically mixed over the entire water
   column within the acute mixing zone, an alternative model should be
   used such PLUMES or CORMIX. RIVPLUM4.WK1 is useful for estimating
   dilution in shallow rivers for side-bank discharges or single-port
   outfalls. This spreadsheet replaces a previous version called
   RIVPLUM3.WK1. RIVPLUM4.WK1 was modified to include calculation of the
   flux-average dilution factor.
   
  USER INSTRUCTIONS FOR THE INPUT SECTION
  
   
   
   Step 1: Enter the effluent design flow (see Permit Writer's Manual
   section VI-3.3.2).
   
   Step 2: Specify the receiving water characteristics, including average
   channel depth, velocity and width downstream from the discharge at the
   design flow (e.g, at 7Q10. NOTE: The product of depth*width*velocity
   should equal the receiving water discharge rate downstream from the
   discharge).
   
   Also enter either the channel slope downstream from the discharge
   (e.g., as measured from a USGS topographic map) or Manning's "n"
   coefficient for roughness. Finally, enter either 0 (if slope is
   entered above) or 1 (if Manning's "n" is entered above).
   
   The slope or Manning's "n" are used to estimate shear velocity and
   transverse mixing coefficients. Either method may be used, depending
   on which data are more readily available. It is not necessary to
   specify both slope and Manning's "n". If comparisons are made between
   the two methods then care should be taken to be sure that slope and
   Manning's "n" values are consistent with velocity, depth, and width
   data since all are related by Manning's equation.
   
   In general, it is not desirable to overestimate Manning's "n" because
   a lower value will generally be more protective since it will predict
   a lower transverse mixing coefficient. If the Manning option is used,
   the following values may be appropriate estimates for Manning's "n"
   (EPA/600/3-87-007 after Henderson, F.M., Open Channel Flow, Macmillan
   Co., New York, NY, 1966):
   
     * Artificial channel, earth, smooth, no weeds: 0.020
     * Artificial channel, earth, some stones and weeds: 0.025
     * Natural channel, clean and straight: 0.025 - 0.030
     * Natural channel, winding with pools an shoals: 0.033 - 0.040
     * Natural channel, very weedy, winding and overgrown: 0.075 - 0.150
       
   If no receiving water data are available, then data collection would
   be desirable. Measurements of channel cross-sections of width, depth,
   and velocity should be collected within the mixing zone at conditions
   near critical low flow (e.g. near 7Q10). If conditions are
   significantly different than 7Q10 during measurements, then data may
   need to be adjusted (e.g. using Manning's equation).
   
   Step 3: Enter the distance between the diffuser midpoint and the
   nearest shoreline of the river (e.g., for a side-bank discharge enter
   0).
   
   Step 4: Enter the location of the downstream point at which dilution
   factors will be estimated, including the distance downstream from the
   diffuser and the distance from the nearest shoreline. The "point of
   interest" is the location at which dilution factors will be estimated
   in the Output Section. The highest concentration of effluent
   downstream from the outfall will be the same distance from shore as
   the point of discharge. Therefore, the distance from shore for the
   point of interest should be the same as for the diffuser midpoint in
   Step 3 for a worst case. However, the dilution at any point downstream
   may be estimated using any combination of distances downstream and
   from shore for the "point of interest."
   
   Step 5: Enter the transverse mixing coefficient constant. A value of
   0.6 is recommended for most natural channels. Fischer reports that the
   transverse mixing coefficient can range from 0.1 to 0.2 for straight
   artificial channels. Curves and sidewall irregularities increase the
   coefficient such that in natural streams it is rarely less than 0.4.
   If the stream is slowly meandering and the sidewall irregularities are
   moderate, then the coefficient is usually in the range of 0.4 to 0.8.
   Therefore, a value of 0.6 is usually recommended in natural channels.
   Uncertainty in this constant is usually at least +/- 50 percent.
   
  USER INSTRUCTIONS FOR THE OUTPUT SECTION
  
   
   
   The user does not need to enter or change any values or formulas in
   the Output Section. The plume characteristics incorporating the
   shoreline effect are displayed at Step 5 of the Output Section,
   including the approximate distance downstream to complete mix,
   theoretical maximum available dilution at complete mix of effluent
   with the receiving water, flux-average dilution at the specified
   downstream distance, and the calculated dilution factor at the
   specified point of interest downstream from the discharge.
   
   The distance downstream to complete mixing is often overestimated
   because most natural channels contain sharp bends or changes that
   increase mixing beyond the processes included in the model. The model
   is most useful for predicting mixing where the channel is represented
   over a relatively short distance (e.g. to the mixing zone boundary).
   
   
   
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Spreadsheet WQBP2.WK1

   Revised October 19, 1993
   
   This spreadsheet replaces a previous version called WQBP-CON.WK1. This
   spreadsheet calculates water quality-based permit limits, including
   calculations of waste load allocations (WLAs) and permit limits
   incorporating effluent variability for specific chemical
   concentrations. The method used is documented in Box 5-2 of
   EPA/505/2-90-001 (Technical Support Document for Water Quality-based
   Toxics Control).
   
  USER INSTRUCTIONS FOR THE INPUT SECTION
  
   
   
   Step 1: Specify water quality standards/criteria that apply to the
   receiving water. These include the acute and chronic concentration
   values (e.g., mg/L or ug/L) appropriate for the specific chemical of
   interest. The acute and chronic water quality criteria that apply to
   the conditions (e.g., hardness, pH, temperature) at the point of
   compliance (e.g., end-of-pipe or within the mixing zone for acute and
   300 feet downstream from the discharge for chronic river) should be
   specified.
   
   Step 2: Specify the upstream or background concentration of the
   parameter of interest in the receiving water for the acute and chronic
   evaluations (e.g. at river flow of 7Q10). The 90th percentile
   concentrations during the critical season are recommended for a
   reasonable worst-case. If no data are available it would be desirable
   to collect data during the critical season to characterize upstream
   concentrations unless background can be considered negligible (e.g.
   background can usually be considered negligible for residual
   chlorine).
   
   Step 3: Enter the dilution factors that apply at the point of
   compliance with acute and chronic criteria (see Permit Writer's Manual
   section VI-2.1). Dilution factors should be defined as the reciprocal
   of the volumetric fraction of effluent present at the mixing zone
   boundary.
   
   If actual dilution factors have been reliably estimated from tracer
   studies or plume modeling, then those values should be entered in the
   spreadsheet. In rivers, the dilution factors for permit limit
   calculations should not exceed the dilution obtained from mixing the
   effluent design flow with 25% or 2.5% of the critical upstream river
   flow for chronic and acute criteria, respectively. If water quality
   criteria are required to be met at the end-of-pipe, then a dilution
   factor of 1 should be entered.
   
   Step 4: Enter the coefficient of variation for the effluent
   concentration of the parameter of interest (e.g., use 0.6 if less than
   10 effluent samples are available).
   
   Step 5: Specify the number of days for the chronic average (EPA
   recommends using 4 days).
   
   Step 6: Specify the number of samples per month that the permittee
   will be required to report to monitor compliance with the permit.
   
  USER INSTRUCTIONS FOR THE OUTPUT SECTION
  
   
   
   The user does not need to enter or change any values or formulas in
   the Output Section. The spreadsheet calculates permit limits
   incorporating effluent variability using the method described in the
   EPA TSD. Estimated daily maximum and monthly average permit limits are
   calculated and displayed in the Output Section at Step 4 in the same
   concentration units used for water quality criteria.
   
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