Paonia/Collbran Low Flow Presentation

Water Quality Work Group Meeting

June 9, 2004

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 CDPHE’s modified DFLOW model.

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.

DFLOW Calculated 30E3 Flow, Minimum Single Day Recorded Flow, and Harmonic Mean of Lowest Monthly Flow on Record for May -- Paonia

16

121

228

0

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200

250

Flo

w (

cfs)

DFLOW 30E3Min.Single Day on RecordMin. Month on Record

North Fork Gunnison RiverGage Flow Data Month of May2000 - 2004

Day 2000 2001 2002 2003 2004

1 1290 1990 285 955 514 Harmonic mean of all points 2 1390 2240 259 866 473 635.743 1630 1990 201 903 5614 1940 1510 197 997 801 CDPHE 30E3 Flow5 2320 1330 233 823 1040 16 cfs6 2560 1270 306 710 14507 2370 1270 345 709 1719 Minimum Single Day Flow8 2700 1030 343 668 1850 121 cfs9 2120 819 259 624 187010 1580 889 224 619 2110 Days Flow under CDPHE Flow11 1750 890 218 530 2070 0 days12 1590 1180 229 632 158013 1130 1380 177 931 134014 900 1800 161 1130 97315 710 1820 188 1820 77916 605 1890 273 2180 74517 782 1960 279 2870 86018 570 1820 356 3690 96819 459 2260 396 3990 137020 393 2260 386 3770 180021 394 2110 422 3760 161022 520 1570 328 3530 140923 1080 1480 247 3439 98924 1920 1600 188 3780 90425 2030 1690 138 3540 85626 1470 1530 121 3090 80327 1050 1080 131 3340 80928 1130 1430 167 3950 83429 1550 1310 219 3820 93830 1720 1070 266 3830 73631 1510 1070 358 3110 525

Harmonic 1027.7 1412.5 228.8 1305.8 953.9

Year

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?

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 1980’s 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 EPA’s original methodology.

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).

Impact From 29-day Forward AveragingDaily Streamflow April 2 thru June 29, 2002 vs. CDPHE 30E3 Low Flow for May -- Paonia

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April 1 April 11 April 21 May 1 May 11 May 21 May 31 June 10 June 20 June 30

Date

Flo

w (

cfs)

Daily Flow

30E3 Flow

7.4 cfs16 cfs

Month of Interest

80 cfs 67 cfs

(15 Days Outside Period)

Impact From 29-day Backward AveragingDaily Streamflow October 3 thru December 29, 2001 vs. CDPHE 30E3 Low Flow for

November -- Paonia

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120

Oct 1 Oct 11 Oct 21 Oct 31 Nov 10 Nov 20 Nov 30 Dec 10 Dec 20 Dec 30

Date

Flo

w (

cfs) Daily Flow

30E3 Flow

22 cfs

9.6 cfs

Month of Interest

45 cfs47 cfs

(15-days Outside Period)

No. 2 – CDPHE Excursion Procedures

• Allowable excursion calculations– Annual vs. Monthly Basis

CDPHE 30E3 Low Flow vs. Harmonic Mean for Lowest Flow Month in Period of Record -- Paonia

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Flo

w (

cfs) CDPHE 30E3

Minimum Month on Record

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

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.

• 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

• 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.