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Worksheet Grade 8 Science

Thornhollow Reservoir's Water Budget: Residence Time and Notation

A grade 8 worksheet where students use real residence-time data to compare how long water stays in the atmosphere versus two fictional aquifers.

Objectives

  1. Students will calculate and compare how long water spends in different parts of the water cycle using given residence-time data.
  2. Students will convert time measurements between days, hours, and years and express results in scientific notation.
  3. Students will explain, using energy from the sun and gravity, why water moves between the atmosphere and Thornhollow's aquifers.

Vocabulary

Residence time
Residence time is how long a drop of water typically stays in one part of the water cycle before moving on. Water passes through the air quickly but can stay underground for a very long time.
Aquifer
An aquifer is an underground layer of rock, sand, or gravel that holds water in the tiny spaces between its grains. Wells reach down into aquifers to bring water up to the surface.
Infiltration
Infiltration is when rain or melted snow soaks down into the soil instead of flowing away across the top of it. Water that infiltrates can feed plant roots or keep sinking to become groundwater.

Also: scientific notation

Teacher overview

The fictional town of Thornhollow draws its water from a reservoir fed by Thornhollow Ridge, where rain either runs off into the creek, soaks into a shallow aquifer, or sinks further into a deep aquifer. This worksheet uses real residence-time data, the length of time water typically spends in a given part of the water cycle, to compare how quickly the atmosphere recycles water against how slowly deep groundwater does. A common misconception is that all groundwater moves and returns to the surface at roughly the same pace; the calculations below show that a shallow aquifer and a deep aquifer can differ by many orders of magnitude, which is exactly what scientific notation is built to describe clearly.

Thornhollow Reservoir's Water Budget: Residence Time and Notation — Grade 8 Science
Name: ____________________________ Date: ______________

Student Worksheet

The fictional town of Thornhollow sits below a ridge where rain falls. Hydrologists there measured how long water spends in three places before it moves on: the atmosphere, a shallow aquifer, and a deep aquifer. Use the residence-time facts below for every question.

  • On average, water spends about 9 days in the atmosphere before it falls back to Earth as rain or snow.
  • Water entering Thornhollow's deep aquifer stays underground for about 1,000 years before it re-emerges. This fits the usual range for deep groundwater, which can stay underground for centuries or longer.
  • Water entering Thornhollow's shallow aquifer moves through much faster, in weeks to a few years. It travels through looser, more open soil close to the surface.
  1. In your own words, define "residence time." Use one of Thornhollow's three water pathways as your example.
  2. Convert the atmosphere's residence time of 9 days to hours. (Use 24 hours in one day.)
  3. Write your answer to item 2 in scientific notation.
  4. Convert the deep aquifer's residence time of 1,000 years to days. (Use 365 days in one year.)
  5. Write your answer to item 4 in scientific notation.
  6. Use the 9-day atmosphere figure and your answer to item 4. About how many times longer does water stay in the deep aquifer than in the atmosphere? Round to the nearest whole number.
  7. Explain why water moves upward out of the reservoir into the atmosphere. Then explain why water moves downward into Thornhollow's aquifers. Use the words "energy from the sun" and "gravity" in your answer.
  8. Thornhollow's shallow aquifer has a much shorter residence time than its deep aquifer. Based on the description above, give one reason the shallow aquifer moves water faster.
  9. Engineers are choosing a spot for a new well. They want the water supply to respond quickly to a rainy year. Should they draw from the shallow aquifer or the deep aquifer? Explain your choice using residence time.
  10. A student tells a classmate, "All groundwater takes about nine days to come back to the surface, just like rain." Explain what is wrong with that statement. Use your calculations from this worksheet.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: Residence time is how long, on average, water stays in one part of the water cycle before moving to the next. Water in Thornhollow's deep aquifer, for example, typically stays underground for about 1,000 years before it re-emerges.
  2. Answer: 216 hours.
    9 days × 24 hours/day = 216 hours.
  3. Answer: 2.16 × 10^2 hours.
    216 = 2.16 × 100 = 2.16 × 10^2.
  4. Answer: 365,000 days.
    1,000 years × 365 days/year = 365,000 days.
  5. Answer: 3.65 × 10^5 days.
    365,000 = 3.65 × 100,000 = 3.65 × 10^5.
  6. Answer: about 40,556 times longer.
    365,000 days ÷ 9 days ≈ 40,555.56, which rounds to about 40,556 times longer.
  7. Answer: Energy from the sun and gravity drive the two directions of movement. Energy from the sun heats the reservoir's surface, evaporating water into the atmosphere; gravity then pulls precipitation and infiltrating water back down into the ground, driving it into the shallow and deep aquifers.
  8. Answer: The shallow aquifer's looser, more connected soil and rock let water pass through quickly, unlike the denser material water must move through to reach the deep aquifer.
  9. Answer: The shallow aquifer. Its much shorter residence time means water entering it during a rainy year reaches the well and becomes available sooner than water entering the deep aquifer, which would take centuries to respond.
  10. Answer: The statement is incorrect. Only water in the atmosphere has a residence time of about 9 days; groundwater in Thornhollow's aquifers can take anywhere from weeks in the shallow aquifer to about 1,000 years in the deep aquifer to return to the surface, so groundwater does not move at the same pace as atmospheric water.

Differentiation

Support (below grade level)

Provide the two residence-time conversions already set up as multiplication expressions (9 × 24 = ___ and 1,000 × 365 = ___) so students only need to complete the multiplication, and give a printed scientific-notation reference card showing two worked examples before item 3.

Extension (above grade level)

Ask students to use only the worksheet's own numbers: if Thornhollow's shallow aquifer has a residence time of about 5 years, how many times longer is the deep aquifer's residence time than the shallow aquifer's? (1,000 ÷ 5 = 200 times longer.) Have them express the shallow aquifer's residence time in days using scientific notation as well (5 years × 365 days/year = 1,825 days = 1.825 × 10^3 days).

ELL supports

Pre-teach "residence time," "aquifer," and "infiltration" with a simple three-layer diagram (atmosphere, shallow aquifer, deep aquifer) labeled in English and, where possible, in students' home languages. Allow items 1 and 7 to be answered with labeled diagrams plus short phrases instead of full paragraphs.

Extensions

Have students contact a local water utility or search a state geological survey website, with teacher guidance, to find an approximate residence time for their own region's aquifer, then compare it to Thornhollow's shallow and deep aquifer figures using the same scientific-notation method practiced in this worksheet.