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.
Understanding Groundwater & Aquifers
Groundwater begins as precipitation that soaks into soil during rain or snowmelt. Some water is taken up by plants or returns to the air. The rest may move downward because of gravity.
This process works best where land is not covered by pavement and where soil has openings. A forest, field, or wetland can allow more infiltration than a parking lot.
Recharge is often slow. In dry regions, a large storm may produce only a small amount of recharge because much of the water evaporates or runs across the surface.
The size of spaces underground is not the only thing that matters. Clay can contain many tiny pores, so it may have high porosity. Yet those pores are so small and poorly connected that water moves through clay very slowly.
Gravel may have less total pore space than clay, but its larger connected spaces let water travel more easily. This difference explains why geologists test both porosity and permeability when they search for useful water supplies. Layers of clay or solid unfractured rock can act as barriers that slow groundwater movement and separate underground water bodies.
Some aquifers are unconfined, meaning their upper boundary is the water table. Their water level can rise after wet seasons and fall during drought. Other aquifers are confined beneath a low permeability layer.
Water in a confined aquifer may be under pressure because recharge occurred at a higher elevation. A well drilled into such an aquifer can make water rise inside the pipe without a pump. This is called an artesian condition.
It does not mean the water will always reach the ground surface. The pressure must be high enough for that to happen.
Groundwater moves much more slowly than water in a river. It may travel only a short distance in a year through some materials. This slow movement matters when pollution enters the ground.
Fertilizer, leaking fuel tanks, septic systems, road salt, and landfill chemicals can be carried downward with infiltrating water. Once contamination reaches an aquifer, cleanup can take many years and cost a great deal.
Wells near pollution sources need careful monitoring. Students should pay attention to the direction of groundwater flow, since a pollutant usually spreads downhill in terms of hydraulic head rather than simply following the visible shape of the land.
Pumping changes the natural balance of an aquifer. A well removes water faster than nearby recharge can replace it, causing water levels to drop around the well. Neighboring wells may then need to be drilled deeper or pumped harder.
Near coasts, heavy pumping can pull saltwater inland into freshwater aquifers. Lower groundwater levels can reduce flow to springs, streams, and wetlands because these places often depend on groundwater discharge.
When reading diagrams, identify the recharge area, the saturated zone, confining layers, wells, and discharge locations. Then trace the likely path of water from its entry at the surface to where it eventually leaves the ground.