Analyze an environmental problem doing calculations

Pumping an aquifer

Units 4 and 5
10 points
About 24 minutes

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

You work through an environmental scenario using numbers: set up and solve calculations (showing your work), then explain the science and propose solutions. Several of the 10 points come from the calculations, usually one for the setup and one for the answer. On the exam: Question 3 of 3 in the 70-minute free-response section (Section II is 40% of the exam score).

The question

Farms in a dry region irrigate their crops with groundwater from an unconfined aquifer. The aquifer lies under 2,000 km² of land. Its saturated layer of sand and gravel is 30 m thick on average, and water fills 20% of the layer's volume (its porosity is 0.20). The farms pump out 3.5 × 10⁸ m³ of water per year. Rain and snowmelt that soak into the ground recharge the aquifer with 0.5 × 10⁸ m³ per year. (1 km² = 1,000,000 m²)

Suggested time: 24 minutes

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Part (a)

2 points

Calculate the volume of water, in m³, now stored in the aquifer. Show your work.

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Part (b)

2 points

Calculate the number of years until the aquifer would be completely drained if pumping and recharge stay at their current rates. Show your work.

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Part (c)

2 points

Water leaving the aquifer lowers the water table. Calculate the average drop in the water table, in meters per year. Show your work.

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Part (d)

1 point

Explain why rain recharges an aquifer faster where the surface soil is sandy than where it is mostly clay.

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Part (e)

1 point

Describe one consequence for the region if the water table keeps falling.

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Part (f)

1 point

Use the data given to explain why the farms' current use of the aquifer is not sustainable.

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Part (g)

1 point

Propose one realistic way the farms could reduce the amount of groundwater they pump.

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