PaoniaCollbran Low Flow Presentation

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Paonia/Collbran Low Flow Presentation

• Water Quality Work Group Meeting
• June 9, 2004

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Introduction

• Paonia and Collbran
• Both are small towns located on the Western
  Slope.
• Paonia is located approx. 70 miles SE of Grand
  Junction on the North Fork of the Gunnison River
  and has a population of approx. 1500 people.
• Collbran is located approx. 45 miles SE of Grand
  Junction on Plateau Creek and has a population of
  approx. 500 people.
• Each Town operates a lagoon system to treat its
  municipal wastewater.
• Both Towns discharge to streams that experience
  rapid rise and fall in the hydrograph as a result
  of seasonal flows.
• Both Towns have installed USGS gauging stations
  near their outfalls to accurately monitor
  in-stream flows. Paonia has approx. 4 years of
  data. Collbran is in its first year of data
  collection.
• Effluent limitations for both Towns are based on
  low flows calculated using CDPHEs modified DFLOW
  model.

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Statement of Problem

• CDPHE DFLOW calculations underestimate monthly
  low flows in highly fluctuating runoff driven
  streams.
• For smaller communities with lagoon systems these
  underestimated flows will reduce flexibility in
  operations and could require unnecessary
  improvements to meet more stringent effluent
  limits.
• By Example
• The DFLOW estimated chronic 30E3 low flow for
  May in the North Fork of the Gunnison River near
  Paonia is 16 cfs. From 2000 to 2004 (2002 being
  an extreme drought year) data collected at the
  Paonia WWTP gauging station show the 30 day
  harmonic mean for the lowest May on record is 228
  cfs, over 14 times the estimated 30 day average
  calculated by the model. Further, the single
  lowest recorded daily flow in May (i.e. the most
  extreme acute event) was 121 cfs, nearly 7 times
  the calculated 30 day average chronic low flow.

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Issue

• Should The Basic Standards and Methodologies for
  Surface Water regulations be revised to more
  specifically describe the procedures to be used
  in establishing low flow?

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Overview of DFLOW

• Prior to DFLOW, hydrologic statistical analysis
  (Log Pearson Type III regression) was used to
  determine the low flow (7Q10)
• EPA developed DFLOW in mid 1980s to establish a
  method for calculating biologically based design
  flows.
• Empirical biologically based flow method intended
  to examine frequency to biological exposure
• Calculates annual design flows for a 4 day
  average, 1-in- 3 year recurrence chronic
  concentration (CCC) and 1 day 3 year recurrence
  acute concentration (CMC).
• The 4 day average is a rolling (forward) harmonic
  mean used to develop annual design flow from
  April 1 thru March 27 does not roll into
  succeeding year. Harmonic mean is always less
  then arithmetic mean.
• EPA allows averaging period to be lengthened to
  30 days where low variability of effluent
  pollutant concentration and resultant
  concentrations in receiving water.
• Annual excursions from the design flows are
  allowed based on a 1-in-3 year recurrence
  interval.
• CWQCC adopted DFLOW approach in 1988. Basic
  Standards provide that 30 day averaging period
  for 3 year recurrence frequency for chronic
  standards (30E3) and 1 day 3 year for acute
  standards (1E3).
• CSU in conjunction with Low Flow Task Force
  Committee, evaluated low flow calculation
  methods, including DFLOW. (C. Paulson and T.
  Sanders, 1987. Evaluation of Design Flow
  Criteria for Effluent Discharge Permits in
  Colorado - 1987).
• CDPHE developed a hybrid DFLOW Model from EPA
  DFLOW Model to calculate 30E3 and 1E3 on monthly
  basis.
• Monthly 30E3 flow based on harmonic mean
  averaging includes 29 days prior and 29 days
  after specific month of interest or period of
  interest.
• Excursions apparently calculated on annual basis
  based on EPAs original methodology.

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Issues Regarding CDPHE DFLOW Model

• No. 1 - Averaging Period too Broad
• Use of 29 days outside period of interest
  results in low flows from a preceding or
  succeeding month being attributed to the month of
  interest. Such an approach does not reflect
  actual stream flow conditions.
• CSU report recommended that at least half of days
  used in each harmonic mean calculation be from
  period of interest (for example, for a 30 day
  averaging period at least 15 days should fall
  within the month of interest).
• Current methodology can result in 1 day acute
  flow (1E3) and 4 day chronic flow (4E3) being
  higher than 30 day average flow (30E3).

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No. 2 CDPHE Excursion Procedures

• Allowable excursion calculations
• Annual vs. Monthly Basis

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Possible Remedies

• Modify monthly harmonic mean calculations to
  include at least half of the days in the month of
  interest (Overlapping Procedure)
• Other Remedies/Considerations
• Allow use of 4E3, rather than 30E3, during months
  with highly varying flows
• Allow discharger to use real time gage data as
  variance to effluent limits if higher than DFLOW
  calculations
• Develop/define monthly excursion method
• Legal/political remedies as part of discharge
  permit renewals

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Low Flow Regulations

• Regulation 31 Basic Standards and Methodologies
• Section 31.9 FLOW CONSIDERATIONS
• (1) Low Flow Exceptions
• Water quality standards shall apply at all times
  provide, that in developing effluent limitations
  or other requirements fro discharge permits, the
  Division shall normally define critical flow
  conditions using the following low-flow values
  the empirically based 30-day average low flow
  with an average 1-in-3 recurrence interval (30E3)
  for chronic (30-day) standards or the empirically
  based 1-day low flow with an average 1-in-3-year
  recurrence interval (1E3) for acute (1-day)
  standards, or the equivalent statistically-based
  flow. For certain substances, such as ammonia,
  the low flow exception may be based on periodic
  or seasonal flows. The length of the periods
  will be determined on a case-by-case basis by the
  Division.

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• Section 31.14 Integration into Discharge Permits
• (8)The flow associated with the duration and
  frequency of exceedence criteria as defined in
  section 31.7, 31.9 and 31.16 shall be utilized in
  determining permit limitations

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• Regulation No. 61 Colorado Discharge permit
  System Regulations
• Section 61.8 Definition of Effluent Limitation
• (b) Water Quality Standards-Based Effluent
  Limitation.
• (viii) For discharges which contain ammonia or
  metals (see table II and III, Basic Standards
  Regulation) in sufficient quantities to
  potentially cause exceedance of the assigned
  water quality standard, the Division shall assign
  limitations which protect both the acute and
  chronic water quality standards. Such
  limitations shall be derived utilizing the stream
  low flow as defined in regulation No. 31, section
  31.9(1) of the Basic Standards.