Common Water Quality Tests
And Why They Are Important
Temperature is one of the more important measurements to be considered when examining water quality.
Many biological, physical and chemical parameters are dependent on temperature.
It can dramatically affect the rates of chemical and biological reactions.
Some of the more common things which temperature can effect are the solubility of chemical compounds in water, the distribution and abundance of organisms, the rate of growth of biological organisms, water density, mixing of different water densities and current movements.
The amount of oxygen that can dissolve in water is dependent upon temperature.
As the temperature of a body of water increases, the amount of dissolved oxygen decreases.
Temperature, specific gravity, and salinity are also interrelated.
For a body of water at a given salinity, as the temperature of that water decreases, the specific gravity will increase and the water becomes more dense.
Conversely, as the same water warms up, the water will expand and the specific gravity will decrease.
This can be an important consideration in evaluating the mixing of waters of different densities.
Salinity is the concentration of salt dissolved in water.
It is normally expressed in parts per thousand or the grams of salt per 1000 grams of water sample (o/oo, ppt).
An important characteristic of seawater is the property of "Constant Composition".
This means that the ratio of the concentration of the major components is constant and is the same everywhere in the ocean.
The salinity of the water may increase or decrease due to the loss or gain of water from evaporation, rainfall, freezing, melting, or other processes.
During a rainstorm, freshwater will dilute the concentration of these components and the salinity will decrease.
If water evaporates, these components are left behind causing the salinity of the remaining water to increase.
Seawater has an average salinity of 35o/oo.
Probably the most important aspect of salinity with regards to water quality is its effect on aquatic organisms.
Salinity changes can affect the well being and distribution of biological populations.
Major Constituents (over 100 ppm)
| Ion | Percentage | Concentration
in g/1000 g |
|---|
| Chloride, Cl- | 55.04 | 19.35 |
| Sodium, Na+ | 30.61 | 10.76 |
| Sulfate, SO4-2 | 7.68 | 2.71 |
| Magnesium, Mg+2 | 3.69 | 1.29 |
| Calcium, Ca+2 | 1.16 | 0.41 |
| Potassium, K+ | 1.10 | 0.40 |
| Total | 99.28 | 34.92 |
Ocean Salinity
| Considering the average salinity of the world ocean is 35‰ by weight, a 1000 g sample of this water would contain 35 g of dissolved solids. Six ions account for 99.28% of the dissolved solids. |
Dissolved oxygen (DO) is one of the most important indicators of water quality.
It is essential for the survival of fish and other aquatic organisms.
Oxygen dissolves in surface water due to the aerating action of winds.
Oxygen is also introduced into the water as a by-product of aquatic plant photosynthesis.
When dissolved oxygen becomes too low, fish and other aquatic organisms cannot survive.
The DO test tells how much oxygen is dissolved in the water.
However, it does not tell you how much oxygen the water is capable of dissolving at the temperature at which it was measured.
When water dissolves all of the oxygen it is capable of holding at a given temperature it is said to be 100% saturated.
The colder the water is, the greater the amount of oxygen the water can hold.
As the water becomes warmer, less oxygen can dissolve in the water.
Salinity is also an important factor in determining the amount of oxygen a body of water can hold.
As the amount of dissolved salts in the water increases, the amount of oxygen the water can hold decreases.
Conversely, as the water becomes more fresh (lower salinity), more oxygen can dissolve into the water.
Oxygen levels may be reduced because the water becomes too warm or because there are too many bacteria or algae in the water.
After the algae complete their life cycle and die bacteria consume the dead algae.
During this decay process the bacteria also consume the oxygen dissolved in the water.
This consumption of oxygen by the bacteria can lead to decreased levels of dissolved oxygen and in some cases completely strip the water of all DO.
This process is termed eutrophication.
This decrease in dissolved oxygen can cause fish kills and death to other aquatic organisms.
When the dissolved oxygen drops below 3 mg/L is when fish kills can occur.
How acidic or alkaline (basic) a water sample is can be determined by a measurement of the water's pH.
When an acid dissolves in water, it releases positively charged hydrogen ions.
pH is defined as the negative logarithm of the concentration of these hydrogen ions in the water.
It is actually simple, the scale runs from 1 to 14.
Pure water is said to be neutral and has a pH of 7.0.
Because pH is a negative logarithm two things result.
First, a lower pH means more acid is dissolved in the sample.
Thus, a pH value less than 7 indicates the water sample is acidic.
A pH greater than 7 means the water has excess alkali (base) dissolved in it.
Second, pH changes as a power of 10.
A water sample with a pH of 3 is ten times more acidic than a water sample with a pH of 4.
A sample with a pH of 3 has 1000 times more acid than a sample with a pH of 6 (not just twice as much acid).
pH is an important water quality parameter.
The pH affects the solubility of minerals in water.
Human activities including chemical spills, agricultural runoff, storm water runoff and sewage effluent can all affect the pH of water.
The solubility of trace metals, some of which are toxic, are affected by changes in pH, generally becoming more soluble as the pH decreases.
The buffering capacity of water, its ability to resist changes in pH, is critical to aquatic life.
Aquatic organisms survival greatly diminishes as pH falls below 5 or increase above 9.
To meet Florida state standards the pH of the water must be between 6.5 and 8.5 .
Material that becomes mixed or suspended in water will cause the water to become more turbid and reduce the clarity of the water.
As the water clarity decreases, light will not be able to penetrate as far below the water's surface.
Many factors can contribute to decreasing water clarity.
During periods of rain, storm water runoff contains silt and sand washed from the streets, yards, and construction sites are carried into the water.
In shallow water winds, boats and people may stir up bottom sediments contributing to decreasing water clarity.
Decreases in water clarity can have a profound effect on penetration of sunlight below the surface of the water.
Plants need sunlight in order for photosynthesis to occur.
The plants which live on the bottom, typically seagrasses, are particularly affected by reduced penetration of sunlight.
If light levels become too low photosynthesis may stop altogether and the plants will die.
These plants produce oxygen for fish and other aquatic life, as well as providing food, shelter, nurseries, and habitat.
Less seagrass in turn means a lower population of aquatic organisms.
The Secchi disk provides a convenient method for measuring the penetration of light below the water surface, and thus limit of visibility in the water.
The Secchi disk is either all white or has alternating black and white quadrants with a premeasured line attached.
The disk is lowered into the water until it can no longer be seen by an observer at the surface.
This depth is know as the Secchi disk transparency.
As the amount of suspended matter in the water decreases, the deeper you will be able to still see the disk.
Shallow Secchi disk readings will occur during times when large amounts of suspended solids are present.
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Revised January 3, 2003