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Aquatic Toxicology. Water Quality Criteria and TMDLs. Background. Previosly  origin/effect/toxicity of some pollutants Today  talk about how pollutant levels are controlled (mostly point source). What’s the difference? A Review. Criteria are recommendations

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Water quality criteria and tmdls

Aquatic Toxicology

Water Quality Criteria and TMDLs


  • Previosly  origin/effect/toxicity of some pollutants

  • Today  talk about how pollutant levels are controlled (mostly point source)

What s the difference a review
What’s the difference? A Review

  • Criteria are recommendations

    • Usually made by EPA, based on scientific data

    • Allowable levels of toxicants in water or effluent discharge

    • National level

  • Standards are laws/regulations which must be followed

    • May or may not be based on criteria

    • Regional/state level

      Note: National Pollutant Discharge Elimination System (NPDES) permitting enforces either criteria or standards at the local level

Types of criteria
Types of Criteria

  • Drinking Water Criteria

    • Protect human health

    • Fewest # of standards (25 vs 129 for acute/chronic) because many things harmful to aquatic organisms that are not toxic to human (i.e. copper  limit for drinking water based on taste)

  • Water Quality Criteria

    • Acute criteria

      • Short-term/accidents designed to protect

      • Maximum values aquatic organisms

    • Chronic

      • Long-term

      • Average values

        Note: WQC is in-stream concentration, not effluent concentration

Factors on which wqc are based
Factors on which WQC are based

  • Toxicity (1°)

  • Taste/odor

  • Aesthetic (Fe staining clothing)

  • General water chemistry (do not want to release large quantities of anything (even if not toxic)

Regulatory background for wqc
Regulatory background for WQC

  • Clean Water Act (CWA)

    • “No toxic compounds in toxic amounts are to be placed in water in the US”

    • Fosters solution to pollution is dilution

      • Although it is against the law to do, can “stack the deck” by creating conditions/use conditions which will bring [x] to WQC

        • Increase impervious surfaces upstream  increase stream flow

        • Cut trees (logging) upstream  also increase stream flow

Regulations for npdes
Regulations for NPDES

B. National Pollution Discharge Elimination System (NPDES permits)

  • implemented 1975

  • Initial goal  zero discharge within 10 years

  • Goal not met but amount of pollution has significantly decreased

  • Designed to control amount (load) of specific pollutants discharging into receiving water so as not to be toxic to 95% of exposed organisms.

  • Based on many minor, two major assumptions:

    • Toxicologists can provide “biological effects concentrations”  forms basis for WQC

    • By meeting WQC, discharger is protecting aquatic biota

Additional assumptions
Additional assumptions

  • Know [x]up (upstream concentration of regulated compound)

  • Low flow (minimum flow) – may have to guess based on drainage basin

  • Maximum discharge of water based on

  • Maximum [x] of discharge design

    (conc. of regulated cmpd) considerations

    Note: above used to determine waste load allocation (Total Maximum Daily Load or TMDL)  mass of compound allowed to be discharged in effluent to meet (not exceed) the in-stream WQC

Look at last three assumptions in more detail
Look at last three assumptions in more detail

  • Minimal low flow (September low flow)

    • Based on upstream low flow (if it has been measured)

    • 7Q10, Q7,10 weekly low flow which repeats over 10 years (usually occurs between August and October)  drought conditions

More assumptions for determining tmdl
More assumptions for determining TMDL

# 5 Maximum discharge of water from source

#6 Maximum concentration of discharge

Both of the above are based on the particular processing requirements of the discharger  may be controlled by discharger (manufacturing) or patially uncontrolled by discharger (sewage treatment plant may not know what is coming “down the pipe” or may get excessive storm drainage in sewer system)

Notation for mass balance at outfall
Notation for mass balance at outfall

  • Need to know to understand rest of lecture

  • Basically values downstream are based on upstream values + addition/changes from effluent discharge

Waste load allocation

Qu --------------------------------------------------- Q (= Qu + Qe)


Mixing zone

Qe, Se

Outfall (effluent)

Waste Load Allocation

QuSu + QeSe = Q(s)

Qu assumed to be the 7Q10

Qe “ “ “ “ maximum design flow

S “ “ “ “ water quality criterion (WQC from EPA)

Su is the concentration of toxicant upstream

Tracer study showing longitudinal plume in a river

Waste outfall from a chemical plant (pre-CWA)

 Direction of flow

All pictures from S. Socolofsky and K-A Chang, Texas A&M University

Joining of three rivers (decreasing sediment load from left to right)


  • Unknown 7Q10? (AR has many small dischargers on small strams without gaging station

    September Qest

    Qlow (in CFS) = a[(DA)b1]Pb2 Snb3

    Where DA = drainage area

    P = annual precipitation (inches)

    Sn = “ snowfall “


    a = 1.57 x 10-5 b1 = 1.06 (all are

    b2 = 2.90 b3 = -0.28 unitless)

    Note: above constants are used in eastern US

    above estimate by be off by as much as 100%!


  • WQC Unknown? (1985 only 65 known, now 129)

    May use Application Factor (AF) method to estimate s from LC50 data

    WQC = application factor x LC50 where AF = 0.1, 0.01, 0.001  higher toxicity  use lower AF

    i.e. Zn  find LC50 for resident species, multiply by 0.01 and use as WQC

More fun with units
More fun with units

Flow units = MGD or CFS or CMS (m3/sec)

Load units

  • Load = flow x concentration

  • Lbs/day = vol./day x mass/V = mass/day