1. Paonia/Collbran Low Flow Presentation Water Quality Work Group Meeting June 9, 2004

2. 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.

3. 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.

6. 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?

7. 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.

8. 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).

11. No. 2 – CDPHE Excursion Procedures Allowable excursion calculations –Annual vs. Monthly Basis

13. 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

14. 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.

15. 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

16. 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.