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7, Water Pullution (WATER QUALITY IN LAKES AND RESERVOIRS (Lakes and…
7, Water Pullution
WATER QUALITY IN LAKES AND RESERVOIRS
Lakes and reservoirs “age” with time
Natural process known as eutrophication
Young lake: Low nutrient content, little plant life
Oligotrophic
Clear water
Little sediment
Excess oxygen
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Lakes and reservoirs “age” with time
Natural process known as eutrophication
THOUSANDS OF YEARS
Old lake: High nutrient content, excess plant life
Euthrophic
Murky water
Sediment build up
Depleted oxygen
Lake becomes shallower
Marsh or bog
Accelerated eutrophication: Cultural eutrophication
Decades
Municipal waste water, industrial wastes, agricultural run off
Stimulate algae growth and diminish water quality
Anaerobic conditions at the bottom of the dam
Toxicity of water increases due to H2S and heavy metal leaching
Eutrophication only one of the risks to lakes/reservoirs
Think inlet and outlets vs fixed volume
Acid rain
CONTROLLING FACTORS OF EUTROPHICATION
Nutrient addition to lake/reservoir
Simplified to nitrogen and
phosphorus addition
Determine the LIMITING nutrient
Slow the algae growth
Blue-green Cyanophyta: Lakes
Obtain N directly from atmosphere
Fallout N from combustion sources
Rough guideline freshwater:
N concentration (mg/l) 10 times
that of phosphorus: P limited
N concentration (mg/l) less that
10:1 of P: N limited
Rough guideline marine:
N/P < 5: N limited
THERMAL STRATIFICATION
Density of water “max’s” out at 4°C
Summer: Layer of warm water
Winter: Ice forming with more dense “warmer” water blow
THERMAL STRATIFICATION
CONTROLLING FACTORS OF EUTROPHICATION
Eutrophication = algae growth
Sunlight
Nutrient concentration
Oligotrophic lake: Photosynthesis at deep depths of lake
Eutrophic lake: Photosynthesis at shallow depths of lake
Sunlight not used as controlling factor
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POINT SOURCE PLUG FLOW MODEL
Two key processes:
Removal of oxygen by microorganisms during biodegradation
DEOXYGENATION
This is the rate at which microorganisms are removing oxygen from the
water during biodegradation.
Replenishment of oxygen through aeration at the interface between the
river surface and atmosphere
REAERATION
This is the rate at which oxygen is being replenished
Assumption
Continuous discharge of waste at given location
Uniform mix of water and waste at any given cross section
No dispersion of waste in
direction of flow
OXYGEN SAG CURVE: Streeter-Phelps
Critical point downstream of a discharge point where DO drop rapidly
DO <<
River conditions at their worst
Organic matter diminished so much that the rate of reaeration is greater than the deoxygenation rate
Effect on temperature also NB to curve
Wastes decompose quicker due to metabolic rate increase
Saturated value of DO drops so reaeration slows
Other factors as well such as ?
Sludge
Tributaries
Load on river
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