Lake Pollution Flashcards
Streams vs Lakes
Streams: the water is moving,
● There is a flushing action for incoming pollutants
● The organic pollutants affect the O2.
Lakes: the water is not moving much
● Pollutants can remain for many years
● Seasonal temperatures affect the water quality
● Water quality is affected by phosphorous and nitrogen (plant
nutrients)
● Excessive growth of algae (algae blooms).
The algae die and they accumulate at the bottom.
Bacteria and protozoa decompose them by using the DO.
At the bottom there is DO depletion
Lake with algae blooms
Lake with algae blooms:
● Algae forms slimy mats that float on the lake surface
● No good for recreation
● Raise the cost of treatment if the lake water is used for water supply
● Additional chemicals are required to control tastes and odors
Eutrophication and Cultural Eutrophication
Eutrophication: is the natural process of nutrient enrichment and gradual filling in of a lake. It is an inevitable and continual aging of the lake.
Cultural Eutrophication: is the acceleration and hastening of the natural aging process because of human activity in the drainage basin of a lake
Eutrophication is caused by:
Phosphorous and nitrogen.
A 0.02 mg/L inorganic phosphorous causes algae
blooms.
Inorganic nitrogen can be 10 times more.
Natural Life Cycle of a Lake
Natural Life cycle of a lake:
1. Oligotrophic (Deep,clear, nutrient, poor, little
aquatic life)
2. Mesotrophic (Nutrients and sediment beigin to
accumulate, increasing populations appear)
3. Eutrophic (Nutrient rich, shallow, warmer, other
aquatic organisms: algal blooms occur
4. Senescent (lake becomes marsh): Overgrown with
emerging rooted plant life
Controlling cultural eutrophication
- Control wastewater treatment effluents by advanced treatment of
sewage to remove much phosphorus and nitrogen.
This is an expensive method. - Control nutrients input from dispersed sources (surface runoff,
agriculture)
● More efficient use of fertilizers
● Control of soil erosion
● Surface water diversion - Divert the wastewater effluents around the lake into some other
body of water, such as a stream, which is less sensitive to the
nutrients. - Control algae growth by copper sulfate.
Copper sulfate kills the algae, but its dose must be carefully
controlled to prevent fish kills
Thermal stratification (Sketch)
Thermal Stratification: is a layering of the water in the lake, as well as a mixing or seasonal overturn of the water. This is caused by seasonal temperature differences.
In temperate climates: cycle stratification and overturn occurs twice a year
In warm climates: the cycle occurs once.
Summer Stratification Layers
Epilimnion: top layer with warm water, complete mixing by wind, energy from sun, algae growth.
Hypolimnion: bottom of the lake, cold dense water, cool dark, poor water quality, BOD due to decaying benthic sediments, depleted DO, anaerobic conditions.
Thermocline: thin layer, rapidly decreasing temperature from top to bottom.
It separates the epilimnion and hypolimnion. It prevents mixing
between top and bottom.
The epilimnion and water near the bottom of hypolimnion are of the poorest quality. The best quality water is just below the thermocline.
Fall, Winter, Spring
Fall Overturn: The epilimnion becomes cool, denser, and begin to sink towards the bottom.
Lake becomes completely mixed.
The poor quality water of the bottom is mixed with all volume of the
lake.
Winter Stagnation: Ice covers the lake surface.
The best-quality water is at the depth, below the ice cover.
Spring Overturn: Surface water warms to 4C (water is densest) it sinks towards the bottom.
The lake becomes completely mixed.
Control Stratification
How to control the stratification problem?
1. Compressed air is diffused into hypolimnion through
perforated pipes placed at the bottom to reoxygenate the
water.
2. Mechanical mixing
3. Pump the cold bottom water up to the surface
