Water Quality Criteria and TMDLs

Aquatic Toxicology

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Background• Previosly origin/effect/toxicity of some

pollutants• Today talk about how pollutant levels

are controlled (mostly point source)

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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

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Types of CriteriaA. 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)

B. Water Quality Criteria1. Acute criteria

• Short-term/accidents designed to protect • Maximum values aquatic organisms

2. Chronic• Long-term• Average values

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

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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)

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Regulatory background for WQC

A. 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

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Regulations for NPDESB. 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:1. Toxicologists can provide “biological effects concentrations”

forms basis for WQC2. By meeting WQC, discharger is protecting aquatic biota

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Additional assumptions3. Know [x]up (upstream concentration of regulated

compound)4. Low flow (minimum flow) – may have to guess based

on drainage basin5. Maximum discharge of water based on 6. 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

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Look at last three assumptions in more detail

4. 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

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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)

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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

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Waste Load Allocation

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

Su

Qe, Se

Mixing zone

Outfall (effluent)

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

04/22/23 13Joining of three rivers (decreasing sediment load from left to right)

Tracer study showing longitudinal plume in a river

Waste outfall from a chemical plant (pre-CWA)

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

Direction of flow

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Exceptions1. Unknown 7Q10? (AR has many small dischargers on

small strams without gaging stationSeptember Qest

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

Where DA = drainage area P = annual precipitation (inches) Sn = “ snowfall “And

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%!

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Exceptions2. 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

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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