My Science Box

Teacher Background
This activity, like the Soil Analysis activity are fabulous springboards for future independent investigations and real world connections. Water quality is one of the first things analyzed by environmentalists, government agencies, aquarium owners, gardeners, and business men and women when asked about the health of an ecosystem. It is a question you can ask about the water that comes out of your tap and that goes down the drain. This lesson gives students experience with 3 very simple water quality measures although there are many many more tests available to try (salinity, phosphates, chlorine, nitrates, fecal coliform, and others).

The first test is temperature which simply measures how hot or cold something is. Water temperature significantly affects the type of life that can survive. High temperatures reduce the amount of dissolved oxygen. The rate of photosynthesis gradually increases as water temperature rises, although beyond 32 degrees Celcius, the rate of photosynthesis falls again. Similarly the metabolic rate of plankton, insects, and other water life varies according to the temperature of the water. Human impacts that can affect water temperature include thermal pollution from industry, deforestation resulting in fewer trees to shade water, and soil erosion adds sediments to the water increasing the absorption of solar heat.

The second test measures pH. pH is a measure of the hydrogen ion concentration in liquids and other substances. Acids such as vinegar and lemon juice have high concentrations of hydrogen ions and register below 7 on the pH scale. Bases such as soap and milk have low concentrations of hydrogen ions and register above 7 on the pH scale. Pure deionized water has a neutral value of 7. Pure rainwater has a pH closer to 5.6 although natural freshwater sources such as creeks, ponds, and lakes fall between 6.5 and 8.5. Some water sources such as bogs (with a pH as low as 4.2) have naturally low pH. Human impacts such as acid rain, pollution, and chemical spills can affect pH values.

The final test, dissolved oxygen, measures the presence of oxygen in the water, an essential ingredient for animal life. Dissolved oxygen is measured in parts per million or ppm. Although some organisms can survive at very low dissolved oxygen levels, 7-14 ppm is generally considered healthy for most fish and other aquatic life. Levels of 3-5 ppm are considered stressful and 0 is termed anoxic. Oxygen is introduced into water from the atmosphere (running water and wind increases the amount of oxygen) and from aquatic plants. A reduction in dissolved oxygen often is the result of increased temperature, changes in stream flow such as damming a river, the build up of organic wastes from pesticides and fertilizers, and eutrophication (if too much algae and plant life grows, as the plants die the bacteria population explodes, virtually eliminating all dissolved oxygen from the area).

Student Prerequisites
none