Design an investigation

Warm water from a power plant

Units 2 and 8
10 points
About 23 minutes

You can use a calculator on this question, just like on exam day.

You read about a real-world environmental situation with a described figure or model, then pick out the parts of a scientific investigation (hypothesis, variables, controls), explain the science behind it, and propose solutions to the problems it raises. On the exam: Question 1 of 3 in the 70-minute free-response section (Section II is 40% of the exam score).

The question and its sources

A natural gas power plant pumps water from the Arden River to cool the steam that turns its turbines, then returns the warmed water to the river through an outflow pipe. Biologists noticed that a native minnow is rare near the outflow and want to know how water temperature affects it.

Figure 1: Map of the river near the power plant

The map shows the river flowing from north to south. The plant's intake pipe is upstream (north) of the plant, and its outflow pipe is just downstream.

Summer measurements are labeled at three points: upstream of the intake, 18 °C and 9.4 mg/L dissolved oxygen; at the outflow, 29 °C and 7.7 mg/L; and 2 km downstream of the outflow, 22 °C and 8.7 mg/L.

Source: Hypothetical map

Planned laboratory investigation

The biologists will collect 90 minnows of similar size from the river upstream of the intake and keep them at 18 °C for one week. They will then place 15 minnows in each of six identical aquariums held at 18, 21, 24, 27, 30 and 33 °C. Every aquarium will be aerated and the fish will be fed the same food. After 96 hours, the biologists will count the minnows still alive in each aquarium.

Source: Hypothetical study design

Suggested time: 23 minutes

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Part (a)

1 point

Identify the independent variable in this investigation.

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Part (b)

1 point

Identify the dependent variable in this investigation.

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Part (c)

1 point

Identify the control group in this investigation.

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Part (d)

1 point

Identify one variable, other than the amount of food, that the biologists should keep the same in all six aquariums.

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Part (e)

1 point

State a testable hypothesis for this investigation.

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Part (f)

1 point

Using Figure 1, explain why the dissolved oxygen concentration at the outflow is lower than upstream.

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Part (g)

1 point

Suppose most minnows survive the 96-hour test at 27 °C. Explain why minnows living at that temperature in the river might still decline over time.

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Part (h)

1 point

In winter, minnows sometimes crowd into the warm water near the outflow. Explain why many of these fish die when the plant shuts down suddenly for repairs.

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Part (i)

1 point

Propose one realistic change the power plant could make to reduce thermal pollution in the Arden River.

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Part (j)

1 point

Explain how the change you proposed in part (i) would help the minnow population near the outflow.

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