Atmosphere, water pathways and geochemical cycles
Track matter between reservoirs and explain how permeability, circulation and runoff affect observations.
What you will tackle
- Use atmospheric composition and circulation models
- Follow water-cycle processes
- Distinguish porosity and permeability
- Connect rivers, lakes and oceans through transport
- Read carbon-cycle reservoirs and transfers
Air and water move
Earth's lower atmosphere is mainly nitrogen and oxygen, with smaller amounts of other gases including variable water vapour. Pressure generally decreases with altitude; unequal heating and rotation contribute to circulation. Evaporation transfers liquid water to vapour, condensation forms liquid droplets, and precipitation returns water to the surface. Infiltration enters the ground; runoff flows over it. A reservoir's amount changes when total inputs differ from total outputs.
Below and across the surface
Porosity is the fraction of pore space; permeability describes how readily connected pathways transmit water. A porous material can have low permeability when pores are poorly connected. An aquifer stores/transmits groundwater; flow follows hydraulic conditions rather than always matching the surface river direction. Rivers transport water and sediment into lakes/oceans; flow speed, gradients and particle size affect transport and deposition.
Matter cycles
A geochemical cycle follows matter through reservoirs and transfers. In a simple carbon model, photosynthesis moves carbon into organic material, respiration/decomposition return carbon to the environment, and combustion transfers stored carbon into atmospheric products. Matter conservation does not require every reservoir to remain constant: an imbalance can enlarge one reservoir while reducing another.
Build your notes, then practise
Work on paper before opening an explanation. Guided tasks provide a starting point; independent tasks remove the scaffolding; application tasks ask you to justify a method or interpretation in context.
Guided practice
Guided 1
Sequence evaporation, condensation and precipitation in a water-cycle pathway. A lake receives 12 units/day and loses 9, with other inputs/outputs ignored: find daily change. A sample has many isolated pores; must it transmit groundwater readily?
Show a starting hint
Account for every stated input and output before claiming balance.
Compare with the explained answer
- Water can evaporate, condense into droplets and return as precipitation, with many additional pathways possible.
- Net lake change is 12 − 9 = +3 units/day under the stated simplification.
- High pore volume does not guarantee connected flow; low permeability remains possible.
Answer / response criteria: Pathway as stated; +3 units/day; no, pore connectivity matters.
Independent practice
Independent 1
Compared with a vegetated permeable surface, an impermeable paved surface receives the same rainfall. Predict likely changes in infiltration and runoff. Explain why river sediment may deposit as flow slows entering a lake. In a carbon diagram, distinguish photosynthesis from respiration transfer direction.
Compare with the explained answer
- Under otherwise comparable conditions, paving tends to reduce infiltration and increase surface runoff.
- Slower water can carry less of some particle sizes, promoting deposition; particle properties also matter.
- Photosynthesis moves carbon into organic products; respiration returns carbon to environmental products such as carbon dioxide.
Answer / response criteria: Less infiltration, more runoff; reduced carrying capacity; opposite transfers in the simplified carbon model.
Application practice
Application 1
An atmospheric carbon reservoir gains 8 units/year from listed sources and loses 6 to listed sinks. Predict its change. Does that violate matter conservation? A groundwater tracer reaches a well after rain, but no surface channel connects them: explain a possible route and identify evidence needed to establish it.
Compare with the explained answer
- The listed net gain is +2 units/year; other reservoirs supply the transferred matter, so total matter need not increase.
- Rainwater can infiltrate and move through connected subsurface pores toward the well under suitable hydraulic conditions.
- Tracer timing, subsurface permeability and hydraulic-gradient measurements can test that pathway; arrival alone does not identify every intermediate route.
Answer / response criteria: +2 units/year; no conservation violation; a testable subsurface-flow explanation with additional evidence.
What can you explain now?
An atmospheric carbon reservoir gains 8 units/year from listed sources and loses 6 to listed sinks. Predict its change. Does that violate matter conservation? A groundwater tracer reaches a well after rain, but no surface channel connects them: explain a possible route and identify evidence needed to establish it.
Compare with the explanation
+2 units/year; no conservation violation; a testable subsurface-flow explanation with additional evidence.
Explain the reasoning before comparing. This review records your self-assessment rather than an official score or mastery decision.
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Source and review status
Coverage follows the named requirements and examples in the 2026–2027 ACT test guide and subject descriptions. That guide explicitly does not list every possible topic. The broader coded College and Career Readiness Standards are also mapped to conceptual teaching and practice; this does not calibrate a learner to a score band. The separate scientific-background context list and reading-complexity rubric have their own stated limits.
Original teaching and explained self-check practice are available. Specialist review and difficulty calibration remain pending. Essays require self-check or teacher review and are not automatically marked or awarded official scores.
Official source specification / guide