1. Freshwater and Water Quality Notes

2. Freshwater
Since 70% of Earth’s freshwater is frozen in ice, we split the remaining usable freshwater into two categories: groundwater and surface water.
Surface water is found in lakes, streams, and rivers, while groundwater is found underground in aquifers.
Both forms are replenished by the water cycle through precipitation, however, runoff water will feed lakes, rivers, and their tributaries (streams that flow into rivers) while water that infiltrates and drips through the ground will feed the aquifers there.

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As the runoff flows over land it can pick up pollutants and other materials that negatively affect its quality.
Groundwater on the other hand, is filtered as it drips through the many layers of rock that lead to the aquifer.

4. Water Quality
Because both freshwater and saltwater resources are so vital to living things on Earth, it is important that we maintain their quality by keeping them clean.
In general, water quality refers to the cleanliness of water and is determine by looking at a variety of physical and chemical factors.
For example, the health of a river system is best judged by nitrate levels and turbidity while a lake used for freshwater, is assessed by temperature and pH.

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In the United States, the Environmental Protection Agency (EPA) is responsible for regulating the quality of natural resources such as air and water by setting limits on the 90 most common pollutants and contaminants since excessive amounts of tem are considered dangerous to our health.
Physical aspects of water quality include its temperature, turbidity (how cloudy the water is), and rate of water movement.
Even though living things can survive in a range of temperatures, aquatic animals generally need to be in cooler water because at warmer temperatures, the water contains less dissolved oxygen and other nutrients which are needed to support them.

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Because the increased temperature can kill off or force the migration of a species away from the area, the entire ecosystem can easily become unstable or collapse.
Water with higher turbidity is cloudier because there are more solids and sediments suspended in it.
Since many of these solids come from pollutants, lower turbidity is better for most living things.
This cloudiness can also prevent sunlight from getting into the water reducing the amount of photosynthesis present.

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Scientists also look at various chemical aspects when assessing water quality which must remain in balance.
For example, aquatic plants need phosphates and nitrates in order to survive and remember that dissolved oxygen is important for animals to survive and should ideally remain high.
Dissolved oxygen comes from aquatic plants releasing it during photosynthesis.
So, if phosphate and nitrate levels get too low and plant life decreases, so will dissolved oxygen and animal life.
Similarly, if phosphate and nitrate levels get too high, events called algae blooms can occur.

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Algae blooms are rapid growths of algae, and when they die, are quickly decomposed by bacteria in the water which uses up all of the dissolved oxygen making it unlivable.
Algae blooms can also release toxic chemicals into the water depending on which types of algae are present.
Scientists also monitor the pH of water.
The pH scale measures how acidic or basic a substance is.
This scale goes from 0 to 14, with pure water having what is called a neutral pH of 7, making it neither acidic (lower than 7) nor basic (above 7).
This can be bad for both people and other living things.
If drinking water is high in acidity or alkalinity (another word for how basic a substance is), it can dissolve the metals such as lead in pipes making it toxic and while different living things can tolerate a range of different pH levels, if the pH of a water sample is above 9 or below 4, no fish can survive.

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The pH level of water can also alter the effects of different chemicals such as ammonia or mercury making it more dangerous.
In general, the farther a pH rating is from 7, the worse it is.
Sometimes scientists can chemically test to find exact concentrations (amounts present I a certain amount of water) of pollutants.
When they cannot, indicator species (also known as bioindicators), are used.
These are species with a known tolerance for pollutants whose presence in the water helps to indicate its cleanliness.
For example, if a large number of different bioindicators including some that are sensitive to pollution are found in a stream, the level of pollution present is very low because if it were higher, the sensitive organisms would have been killed.

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Similarly, in a heavily polluted stream or pond, the few indicator species found will be organisms with a very high tolerance for pollutants, such as leeches or worms.
People negatively change the quality of water all the time.
This called pollution.
There are two main forms of pollution that people introduce to the water: thermal and chemical.
Thermal pollution happens when people pull water from a river or lake, heat it through use in power generation or other industries, and return it to its source.
As a result, the overall temperature of the water is raised having negative effects o n the wildlife in it.

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Chemical pollution occurs when waste material, rather than heat energy, is added to the water.
Chemical pollution can be either point or non-point depending on where it cam from.
Point sources pollutants, such as chemical wastes from factories and other industrial facilities, can be easily traced back to where they cam from.
Non-point pollution, however, is less specific and it source cannot be easily identified.
Examples of non-point sources of pollution include run-off from farms, wastes from sewer and septic systems, and animal wastes.
Non-point pollution is a major source of the phosphates and nitrates being added to our water supplies.

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Given the constantly changing quality of our water supplies, stewardship and treatment are very important.
Stewardship refers to the ways in which we manage a resource such as reducing usage and consumption and recycling.
Through proper stewardship, availability of clean freshwater can increase but used water still needs to be treated before it is returned to the lakes and rivers it came from.
First, the water is collected in large basins.
In the second step, chemicals like lime and alum are used to get particles in the water to clump together.
This process is called coagulation.

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The water is shaken to get these pieces to stick to together into larger clumps called flocs.
The third step, sedimentation, allows the flocs to settle to the bottom and the fourth step, filtration, pumps the water through a filter leaving the flocs behind.
In the final step, disinfection, things like chlorine, or UV rays are used to kill off harmful bacteria.
This make the water potable, or safe to drink.