Water Quality in
Lakes & Streams
Dr. Philip Bedient
Introduction
 Water
quality management is the
science that predicts how much waste
is too much for a body of water
 Assimilated- amount of waste that
can be tolerated by a body of water
 Determined by knowing the type of
pollutants discharged and their effect
on water quality
Water Quality
Management
 Water
quality is affected by natural factors:
 Historical uses in the watershed
 Geometry of the watershed area
 Climate of the region
 Good water quality protects drinking water
as well as wildlife

Point Sources of
Pollutants
Point sources include domestic sewage and
industrial wastes
 Point sources - collected by a network of pipes
or channels and conveyed to a single point of
discharge in receiving water
 Municipal sewage - domestic sewage and
industrial wastes that are discharged into
sanitary sewers - hopefully treated
 Point source pollution can be controlled by
waste minimization and proper wastewater
treatment
Nonpoint Sources

Urban and agricultural
runoff that are
characterized by
overland discharge
 This type of pollution
occurs during
rainstorms and spring
snowmelt
 Pollution can be
reduced by changing
land use practices
Combined Sewer Flow
 Nonpoint
pollution from urban storm water
collects in combined sewers
 Combined sewers- carry both storm
water and municipal sewage - older cities
Combined Sewer Overflow

Eliminating this involves:
 Construction of separate storm and sanitary
sewers
 Creation of storm water retention basins
 Expanded treatment facilities to treat the
storm water
 Combined sewers are not prohibited by the U.S.
because removal would disrupt streets, utilities,
and commercial activities
Oxygen- Demanding
MATERIAL


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Dissolved Oxygen (DO)- fish and other higher forms of
aquatic life that must have oxygen to live
Oxygen- Demanding Material- anything that can be
oxidized in the receiving water resulting in the
consumption of dissolved molecular oxygen - BOD, COD
Almost all naturally occurring organic matter contributes
to the depletion of DO
Nutrients

Nitrogen and phosphorus are considered
pollutants when too much present in high conc.
 High levels of nutrients cause disturbances in
the food web
 Organisms grow rapidly at the expense of others
 Major sources of nutrients (N, P):
 Phosphorus-based detergent
 Fertilizer and agricultural runoff
 Food-processing wastes
 Animal and human waste
Pathogenic Organisms

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Include bacteria, viruses, and protozoa from diseased
persons or animals
Water is made unsafe for drinking, swimming, and
fishing
Antibiotic-resistant bacteria are the most dangerous
Bacteria are found in both urban and rural environments
with no observable pattern
Pathogenic Organisms
Serious Outbreaks of these cause great suffering





E. Coli - indicator of fecal coliform bacteria
Salmonella (typhoid fever)
Shigella (dysentery)
Cryptosporidium - protozoa
Giardia- protozoa
Suspended Solids

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


Suspended solids- organic and inorganic
particles that are carried by wastewater into a
receiving water
A slower flow causes particles to settle and form
sediment
Colloidal particles- do not settle, cause an
increase in the turbidity of surface water
Organic suspended solids- exert oxygen
demand
Inorganic suspended solids- result from soil
erosion
Suspended Solids
 With
an increase in the amount of
sediment comes:




Increase of turbidity
Decrease of light penetration
Increase in amount of bacteria
Increase in solids settled on the bottom which
causes animal habitats to be destroyed
Salts
 Total
dissolved solids - TDS
 Water collects salt as it passes over soil
during irrigation practice
 Too much salt can cause crop damage
and soil poisoning
 Arid regions - west and south Texas
Toxic metals and toxic
organic compounds





Agricultural runoff contains pesticides and
herbicides
Urban runoff contains zinc - from tires
Too many toxic metals and toxic organic
substances can leave a body of water useless
 James River in Virginia
 Passaic River in New Jersey
Toxic compounds can also make fish and
shellfish unsafe to eat - As, Hg, Pb, and PCBs
The new concern is pharmaceutical chemicals in
water and wastewater
Endocrine-Disrupting
Chemicals (EDCs)




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These include
 Polychlorinated biphenyls
 Pesticides
 Phthalates
No suitable method exists to
characterize EDC’s
Can mimic estrogens, androgens, or thyroid
hormones
Interfere with regular animal reproduction
Affects synthesis of hormones in the body
Arsenic

A naturally occurring element - As2O3 of real concern.
 Caused by minerals dissolving naturally from weathered
rocks and soils - iron oxides and sulfides
 Causes many health effects such as:
 Arsenic poisoning - interfere with ATP cycle
 Circulatory disorders
 Gastrointestinal upsets
 Diabetes
 Skin lesions & possible skin cancers
 Created a huge problem in Bangladesh wells in 1992
Arsenic - October, 2001
 EPA lowered
the MCL from 50 to 10 ug/L
 Mostly a problem in western U.S. and the
Midwest - naturally occurring
 Lifetime excess risk translates to 30/10,000
 Compares to other carcinogens - 1/105 to
1/106
 Major concern in water supplies now
Heat Impacts

An increase in the Temp of
water can cause:
 Increase in DO which
leads to a deterioration
in water quality
 Large fish kills
 Blocked migration of
fish
 Altered genetic makeup
in fish
Taste and Odor
Problems
 An

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
increase in MTBE concentration in water
Releases from USTs and watercraft engines
Has impacted many lakes nationwide
Created serious taste and odor problems
City of Dallas shut down main water supply
intakes due to largest pipeline spill in the U.S. in
2000
City of Santa Monica closed main wells - 1999
Many private wells impacted by MTBE
Water quality
management in rivers

Main goal is to control the discharge of
pollutants so that water quality is not degraded
above the natural background level
 Controlling waste involves:
1) Measuring pollutants levels (x,z, t)
2) Predicting their effect on the water quality
3) Determining background water quality that
would be present without human intervention
4) Evaluate the levels acceptable for intended
uses of the water
River Pollution Impacts
Waste Input
Receptor
Simple Mass Balance
Input rate - Output rate - decay rate =
Accumulation rate
Steady state conservative system
Stream
Qs, Cs
C = Qw Cw + Qs Cs
Qs + Qw
Waste Input Qw, Cw
Simple Mass Balance
Input rate - Output rate - decay rate =
Accumulation rate
Steady state conservative system
Qs = 10 m3/s
26.67 mg/L
Cs = 20 mg/L
C = 20 (10) + 40 (5)
(10 + 5)
Waste Input Qw = 5 m3/s
Cw = 40 mg/L
Transport
characteristics that
affect concentration
• Velocity
• Dilution (mixing)
v
• Dispersion
• Degradation (mass loss)
• Adsorption (to soils)
• Sedimentation (to bottom)
• Aquatic Life (attached)
Effect of Oxygen-demanding
wastes on rivers

Depletes the dissolved oxygen in water
 Threatens aquatic life that require DO
 Concentration of DO in a river is determined by
the rates of photosynthesis of aquatic plants and
the rate of oxygen consumed by organisms
Biochemical oxygen
demand

Biochemical oxygen demand (BOD)- oxidation
of an organic compound is carried out by
microorganisms using the organic matter as a
food source
 Biossay- to measure by biological means
 BOD is measured by finding the change in
dissolved oxygen concentration before and after
bacteria is added to consume organic matter
Biochemical oxygen
demand

Aerobic decomposition- when organisms use
oxygen to consume waste
 The rate at which oxygen is consumed is directly
proportional to the concentration of degradable
organic matter remaining at any time
 BOD is a first order reaction L = BOD
dL/dt = -kL
Lt = Lo e-kt
where Lo = ultimate BOD
BOD

Ultimate BOD- maximum
amount of oxygen
consumption possible
when waste has been
completely degraded
 Numerical value of the
rate constant k of BOD
depends on:
 Nature of waste and T
 Ability of organisms in
the system to use the
waste
Nature of the waste
 Materials
that are rapidly degraded have
large BOD constants
 Materials that degrade slowly are almost
undegradable in the BOD test
 BOD rate constant depends on the relative
proportions of the various components
 Easily degradable organics are more
completely removed than less readily
degradable organics during wastewater
treatment
Ability of Organisms to
use waste

Many organic compounds can be degraded by
only a small group of microorganisms
 The population of organisms that can most
efficiently use wastes predominates
 BOD test should always be conducted with
organisms that have been acclimated to the
waste
 This created a rate constant that can be
compared to that in the river
Temperature


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Oxygen use speeds up as the
temperature increases and slows
down as the temperature
decreases
Oxygen use is caused by the
metabolism of microorganisms
BOD rate constants depend on:
1) Temperature of receiving water
throughout the year
2) Comparing data from various
locations at different T values
Temperature Eqns

The BOD rate constant is adjusted to the
temperature of receiving water using this:
kT=k20()T-20
• T= temperature of interest (in °C)
• kT= BOD rate constant at the temperature of
interest(in days -1)
• k20= BOD rate constant determined at 20 °C
(in days -1)
• = temperature coefficient.
5 day Bod test
1) A special 300 mL BOD bottle is filled with a
sample of water that has been appropriately
diluted and inoculated with microorganisms
2) Blank samples containing only the dilution
water are also placed in BOD bottles and
sealed
3) The sealed BOD bottles containing diluted
samples and blanks are incubated in the dark
at 20°C for the desired number of days
4) After five days has elapsed, the samples and
blanks are removed from the incubator and the
dissolved oxygen concentration in each bottle
is measured.
Dissolved Oxygen DO

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If the discharge of oxygendemanding wastes is within the
self-purification capacity, the DO
is high
If the amount of waste
increases, it can result in
detrimental changes in plant and
animal life
Aquatic life cannot survive
without DO
Objective of water quality
management is to assess the
capability of a stream to absorb
waste
Do Sag Curve

DO concentration dips as oxygen-demanding
materials are oxidized and then rises as oxygen is
replenished from atmosphere and photosynthesis
 Major sources of oxygen:
 Reaeration from the atmosphere
 Photosynthesis of aquatic plants
 Factors of oxygen depletion:
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BOD of waste discharge
DO in waste discharge is less than that in the river
Nonpoint source pollution
Respiration of organisms and aquatic plants
Use of Ponds for Water Quality
Oxygen Deficit Equation
Define deficit D = DOs - DO in mg/L
L = ultimate BOD (mg/L)
V (dD/dx) = kd L - kr D
Where kd = deoxygenation rate constant (day-1)
kr = reaeration rate constant (day-1)
Since t = x / V, can write the above in time as
dD/dt = kd L - kr D (reaeration vs oxygen use)
Solution to this eqn gives the DO sag curve
Oxygen Deficit Equation
At t = 0, D = Da and L = La - Initial values
Solving the equation for Dt = deficit at any time t
Dt = kdLa e-kd t - e-kr t
+ Da e-kr t
Kr - kd
Critical DO
Dt = DOs - DO
DO
X