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Witchetty River Case Study
Transcript of Witchetty River Case Study
design by Dóri Sirály for Prezi
The Witchetty River rises in the mountains and flows into the ocean about 100km downstream. The city of Duncan is 20km upstream from the river mouth. Recently large numbers of dead perch were found in the river in the city area. We have been hired by the government to investigate the reason behind the death of the fish.
We decided to test three different factors that may have contributed to the death of the fish. These three factors include concentration of dissolved metals, pH levels and dissolved oxygen levels in the river water.
What's the Problem
It can be concluded that it was the rise in temperature of the water, presumably due to the large amount of industries and buildings in the articular area, that led to a decrease in the amount of dissolved oxygen in the water which led to the death of the fish.
What killed the fish?
Concentration of Metals
The only metal which increased in concentration over the time period of six months is mercury. Cadmium and lead were seen to decrease in concentration over the six months. It was also observed that arsenic stayed at the same concentration.
According to the limits of freshwater life, none of the metals that were found at point six had a high enough concentration to be considered a threat towards the fish species. The closest metal to reaching the concentration limit was cadmium and that was still 0.14 x 10^-2mg/l under the limit.
Affect towards the fish
For the water to be acidic, the pH reading would have to be below 7.0. This means that the water was only found to be acidic at point 1 (a reading of 6.8) and at point 8 (a reading of 6.6). The most acidic point in the river that a sample was taken from was point 8.
Why the Acidity of the water varies along the river?
The salt that is found in this river is a major contributor in keeping the pH of the water neutral. This is because the salt water is neither acidic nor basic. The amount of fresh water in the river is affected by rainfall patterns, structure, dams and plants. The amount of saltwater in the river changes with tidal flows and with mixing of freshwater and seawater by wind and currents. Human activity can also affect the salinity of the water. When water is taken from a river for human use (ie farming and irragation) there is less freshwater flowing downstream, and this causes the water to contain more salt in parts of the river compared to others. Storm water adds to the amout of freshwater found in the river, this can also affect the acidity of the water. This is found to be evident at point 1, the results support this theory.
Dissolved Oxygen Levels
As seen in the graph above, the temperature of the water (blue line) slowly increases the further along the river until it spikes up and then slowly begins to decrease again. The dissolved oxygen levels (red line) does the opposite of the temperature. When the temperature significantly increases, the dissolved oxygen levels drop, when it decreases, the oxygen levels begin to slowly rise again.
Temperature of the water
The temperature of the water was highest (25^c), 70km along the river at point 5 where the power station is located. A possible reason for this is the power plant using the water to prevent its devices from over heating, and then returning the same water to the river at a higher temperature.
Dissolved oxygen levels
The dissolved oxygen levels were lowest between the power plant and industries, where the City of Duncan is located and the dead fish were found. An explanation for this is in the increased water temperature found in this area. It has been scientifically proven that elevated temperature typically decreases the level of dissolved oxygen in the water which is sometimes called thermal pollution.
Although increases in water temperature can be known to directly kill fish, in this particular case it is extremely unlikely that this is what happened. It was a gradual change and the most common way increased water temperature directly kills entire fish species is through thermal shock where a sudden change in water temperature causes the animal to die. However, the increased water temperature does play a role in decreasing dissolved oxygen levels which can have a fatal effect on the fish.
Affect on fish
Affect on fish
The lack of dissolved oxygen levels in the water definitely could have caused the death of the fish in the Witchetty river. Fish rely on dissolved oxygen in the water they live in to be able to breath just like humans do on land. The lack of oxygen in the water means that the fish may not have been able to breath and could have been what caused their death
As seen in the past diagram, fish can only survive at a dissolved oxygen (D0)level of 3.0 - 5.0 mg/l for 12 to 24 hours. The yellow perch, the fish found dead in the Witchetty river, is known to have survived for 24 hours at a DO levels of 4.2 mg/l. At point six, The City of Duncan, where the dead fish were found, the dissolved oxygen levels were at its lowest, just 4.0 mg/l. These statistics along with background knowledge prove that it was the low dissolved oxygen levels, caused by an increase of temperature in the water that killed the fish.
A map of the Witchetty river.
One way to stop this same scenario from happening again in the future is to increase the amount of sustainable energy used along the Witchetty river. This will help the air to become cleaner which affects how healthy the river is.
Making sure the river is flowing at all times
Flowing water has a higher dissolved oxygen level then stagnant water. This means that by making sure the Witchetty river is constantly running, the chance of it's dissolved oxygen levels getting too low for the fish to survive will be a lot lower. Little things to help this include making sure there is no dams blocking river flow, regulating the amount of water taken from certain places that are at risk of restricting river flow and keeping a constant watch for non-flowing areas.
Was the Acidity responsible for the fish dying?
All of the fish were dying in the City of Duncan. According to the results in the table above the City of Duncan had a pH reading of 7.3. Anything below 7.0 is considered to be acid and anything above 7.0 is an alkaline. It was recorded at point 6 that the pH was alkaline therefore the acidity was not responsible for killing the fish.
To ensure that this does not happen again we believe that testing needs to become regular so that the levels of dissolved oxygen can be monitored.
It is necessary when keeping a healthy river that fertilizers are used only when they are needed. This is because they upset the nutrient balance in the river. It is also best to use them in spring or early autumn.
Growing local native plants around the river is very beneficial They need less water and fertiliser and attract native birds, lizards and insects