Missouri Water Seminar, September 10-11, 2015, Columbia, MO

1. Using Mixing Zones Higher in the
Watershed Under New Stream
Classification System
Allison Pearson
Barr Engineering Co.

2. Applicable Definitions
 Mixing zone - An area of dilution of effluent in the receiving
water beyond which chronic toxicity criteria must be met (10
CSR 20-7.031(1)(Q))
 Low-flow conditions – the minimum amount of stream flow
occurring immediately upstream of a wastewater discharge and
available, in whole or in part, for attenuation of wastewater
pollutants (10 CSR 20-7.031(1)(O))
 7Q10, 30Q10, 1Q10…

3. How is a mixing zone applied?


4.  If you have upstream flow
data…Calculate your low-flow
– DFLOW, Log-Pearson Type III
Distribution
– Biologically-based
 If you don’t have upstream flow
data…
Methods for determining mixing zones

5. How it works…
 Computed Statistics at Streamgages, and Methods for Estimating
Low-Flow Frequency Statistics and Development of Regional
Regression Equations for Estimating Low-Flow Frequency
Statistics at Ungaged Locations in Missouri
Number of
Gages on
Stream
Calculation Method
2+ Power Curve Equations
1 Drainage-Area Ratio Method
0 Regional Regression-Equation

6. Regional Regression Equations
 Streamflow-variability Index
 Drainage Area
 Longest Flow Length

7. Streamflow-variability Index
 Region 1
– Impacted by clays and shales
– Smallest Low Flows
 Region 2
– Influence by underground reservoirs
– Losing streams possible
– Largest low flows
 Region 3
– Man-made changes for agriculture
– Influenced by alluvial groundwater

8. Drainage Area and Longest Flow Length

9. Final Calculation

10. Conclusions
 Hydrologically-based vs.
Biologically-based
 No guarantees you’ll get the flow
you hope for
 Mixing zones are approved at the
discretion of MDNR
 Less time consuming than
previous options
 If you can demonstrate
7Q10>0.1cfs, mixing zone can be
used for waste load allocation
calculations
 May offer some relief from
restrictive effluent limitations
Limitations Benefits

11. apearson@barr.com
or
(573) 638-5007
Questions?