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Earth Science
Grade 7-11
Groundwater & Aquifers Cheat Sheet
A printable reference covering porosity, permeability, aquifers, water tables, recharge, discharge, and groundwater flow for grades 7-11.
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Groundwater is water stored in the spaces and cracks below Earth’s surface, and aquifers are underground layers that can hold and move this water. This cheat sheet helps students understand how groundwater forms, moves, and supplies wells, springs, rivers, and communities. It also connects Earth science ideas to real problems such as drought, pollution, overpumping, and safe drinking water.
Key Facts
- Porosity is the percent of open space in a rock or sediment, calculated as porosity = volume of pore space / total volume x 100%.
- Permeability is how easily water can flow through connected pores or cracks in a material.
- The water table is the top surface of the saturated zone, where all pore spaces are filled with water.
- An aquifer is a permeable underground layer that stores and transmits groundwater, such as sand, gravel, or fractured rock.
- Groundwater generally flows from higher hydraulic head to lower hydraulic head, often from recharge areas toward streams, lakes, wells, or springs.
- Hydraulic gradient is calculated as hydraulic gradient = change in hydraulic head / distance.
- Darcy’s law estimates groundwater flow using Q = K x A x hydraulic gradient, where Q is discharge, K is hydraulic conductivity, and A is cross-sectional area.
- Overpumping a well can lower the water table and form a cone of depression around the well.
Vocabulary
- Groundwater
- Water found underground in the spaces between soil particles, sediment grains, and cracks in rock.
- Aquifer
- An underground layer of permeable rock or sediment that can store and transmit usable amounts of groundwater.
- Water Table
- The upper boundary of the saturated zone where groundwater fills all available pore spaces.
- Recharge
- The process by which water from rain, snowmelt, rivers, or lakes soaks into the ground and adds to an aquifer.
- Porosity
- The percentage of a material’s total volume that is made of open spaces or pores.
- Permeability
- A measure of how well connected pores or cracks allow water to move through rock or sediment.
Common Mistakes to Avoid
- Confusing porosity with permeability is wrong because a material can have many pores but still block flow if the pores are not connected.
- Thinking all underground water is in rivers or lakes is wrong because most groundwater is stored in tiny pores and cracks within sediment and rock.
- Assuming groundwater always flows straight downward is wrong because it usually moves from higher hydraulic head to lower hydraulic head along sloping paths.
- Forgetting units in hydraulic gradient problems is wrong because gradient is change in head divided by distance and is usually written as a unitless ratio.
- Treating aquifers as unlimited water sources is wrong because pumping faster than recharge can lower the water table and dry up wells.
Practice Questions
- 1 A sediment sample has 30 mL of pore space and a total volume of 120 mL. What is its porosity as a percent?
- 2 The hydraulic head drops 6 m over a horizontal distance of 300 m. What is the hydraulic gradient?
- 3 Using Q = K x A x hydraulic gradient, find Q if K = 4 m/day, A = 20 m2, and hydraulic gradient = 0.05.
- 4 A town wants to build a landfill above a shallow aquifer made of sand and gravel. Explain why this location could threaten groundwater quality.