Under Our Feet
Groundwater and Watersheds
Learning Objectives
Explain how groundwater forms and moves through underground layers
Describe how watersheds collect and channel surface water
Analyze human impacts on water quality in local systems
Under Our Feet
~5 minutesUnder Our Feet
When it rains, where does the water go? Some runs off into streams and rivers. But much of it soaks into the ground, beginning an underground journey that can last from days to thousands of years.
Groundwater supplies drinking water to over half of Americans, including most rural communities. Understanding how water moves underground—and how easily it can be contaminated—is essential for protecting this hidden resource.
At the surface, all land drains somewhere. Every location on Earth is part of a watershed—an area where all water flows to a common outlet. What happens in your watershed affects everyone downstream.
Essential Question: How do human activities affect groundwater and surface water quality?
Groundwater: Water Underground
~15 minutesGroundwater: Water Underground
How Water Enters the Ground
When precipitation falls on land, it can: 1. Evaporate back into the atmosphere 2. Run off the surface into streams 3. Infiltrate (soak into) the soil
The water that infiltrates continues moving downward through soil and rock until it reaches a layer it cannot pass through. This process is called percolation.
Key Properties of Soil and Rock
Porosity = The amount of empty space (pores) in soil or rock
• Sand: ~30-40% pore space (high porosity)
• Clay: ~40-50% pore space (high porosity, but...)
• Solid rock: <1% pore space (low porosity)
Permeability = How easily water can flow through the material
• Sand: High permeability (large, connected pores)
• Gravel: Very high permeability (very large pores)
• Clay: LOW permeability (tiny pores, water can't flow easily)
• Solid rock: Very low permeability (unless cracked)
Key insight: Clay has HIGH porosity (lots of space) but LOW permeability (water can't move through it easily). The pores are too small and not well connected!
The Water Table and Aquifers
Zone of Aeration (Unsaturated Zone)
• Area below the surface but above groundwater
• Pores contain both air AND water
• Water drains through this zone
Zone of Saturation (Saturated Zone)
• All pores are completely filled with water
• This IS the groundwater
• Begins at the water table
Water Table
• The upper boundary of the saturated zone
• Not flat! Follows the terrain (higher under hills, lower in valleys)
• Can rise (wet seasons) or fall (drought, overuse)
• Where water table meets surface = springs, lakes, wetlands
Aquifer
• An underground layer that holds and transmits usable groundwater
• Can be rock, sand, gravel, or other permeable material
• Confined aquifers: sandwiched between impermeable layers (under pressure)
• Unconfined aquifers: water table forms upper boundary (easier to pollute)
The Ogallala Aquifer underlies 8 states in the Great Plains and is one of the world's largest aquifers.
- Supplies 30% of US irrigation water
• Took thousands of years to fill
• Being depleted faster than it recharges
• Some areas have dropped 150+ feet since 1950
• At current rates, much could be unusable within 50 years
This is "fossil water"—essentially non-renewable on human timescales.
Watersheds: Surface Water Systems
~15 minutesWatersheds: Surface Water Systems
What is a Watershed?
A watershed (also called a drainage basin or catchment) is an area of land where all water drains to a common outlet—a stream, river, lake, or ocean.
Key Concept: Watersheds are defined by topography (the shape of the land). Ridges and hills form the boundaries between watersheds, called drainage divides.
Think of a watershed like a bathtub: every drop of water that falls in the tub eventually reaches the drain. The rim of the tub is the drainage divide.
Watershed Hierarchy
Watersheds fit inside each other like nesting dolls:
• Small streams drain small watersheds
• Small streams join to form larger streams (larger watersheds)
• Rivers drain entire regions
• Major rivers drain to the ocean
Example: A raindrop falling in the North Carolina mountains might flow: 1. Into a small creek (small watershed) 2. That creek joins a larger stream 3. That stream joins the Yadkin River 4. The Yadkin becomes the Pee Dee River 5. The Pee Dee flows to the Atlantic Ocean
You are ALWAYS in a watershed. Every point on land drains somewhere!
How Watersheds Function
Natural Watershed Processes:
| Process | What Happens | Effect |
|---|---|---|
| Infiltration | Water soaks into soil | Recharges groundwater; reduces runoff |
| Evapotranspiration | Plants release water vapor | Returns water to atmosphere |
| Natural filtering | Soil and plants filter water | Improves water quality |
| Flood storage | Wetlands absorb excess water | Reduces flooding downstream |
| Erosion/Deposition | Streams move sediment | Shapes landscape over time |
• Absorb rainfall, reducing flooding
• Filter pollutants naturally
• Maintain stream flows during dry periods
• Support diverse ecosystems
Human Impacts on Water Quality
~10 minutesHuman Impacts on Water Quality
How Human Activities Change Watersheds
Impervious Surfaces (Roads, Parking Lots, Buildings)
• Water cannot infiltrate concrete and asphalt
• More runoff, less groundwater recharge
• Runoff is faster, increasing flooding
• Pollutants wash directly into streams
Agriculture
• Fertilizers cause excess nutrients (algal blooms)
• Pesticides contaminate water
• Soil erosion increases sediment in streams
• Irrigation depletes groundwater
Urban Development
• Removes vegetation that filters water
• Increases impervious surfaces
• Creates "urban heat islands" that warm water
• Stormwater carries oil, chemicals, trash
Types of Water Pollution
Point Source Pollution:
• Comes from a specific, identifiable location
• Examples: factory discharge pipe, sewage treatment plant
• Easier to monitor and regulate
• Often has permits and treatment requirements
Non-Point Source Pollution:
• Comes from many diffuse sources across the landscape
• Examples: fertilizer runoff, oil from parking lots, pet waste
• Much harder to control
• Major cause of water quality problems today
Remember: groundwater has residence times of 100-10,000+ years.
Once polluted, an aquifer may take generations to clean naturally. Contamination sources include:
• Leaking underground storage tanks (gas stations)
• Agricultural chemicals
• Industrial waste
• Septic system failures
• Road salt
• Improper chemical disposal
Prevention is the only practical solution!
North Carolina's Watershed Challenges
The Neuse River Basin:
• Drains much of central North Carolina
• Suffered severe algal blooms in the 1990s
• "Nutrient Sensitive Waters" designation
• Strict regulations on nitrogen and phosphorus
Hog Farm Waste:
• NC has ~9 million hogs (more than people!)
• Waste lagoons can leak or overflow during hurricanes
• 2018 Hurricane Florence caused major spills
• Ongoing challenge for eastern NC water quality
Emerging Contaminants:
• PFAS ("forever chemicals") in Cape Fear River
• Microplastics in waterways
• Pharmaceuticals in wastewater
• Many not yet regulated
Summary
~5 minutesSummary
Key Concepts Review
Groundwater Basics:
• Water infiltrates soil and percolates downward
• Porosity: Amount of empty space in material
• Permeability: How easily water flows through
• Water table: Upper boundary of saturated zone
• Aquifer: Underground layer that stores/transmits groundwater
Watersheds:
• All land drains to a common outlet
• Bounded by drainage divides (ridges)
• Watersheds nest inside larger watersheds
• Everything upstream affects everything downstream
Human Impacts:
• Impervious surfaces increase runoff, reduce infiltration
• Point source pollution: Specific, identifiable sources
• Non-point source pollution: Diffuse, harder to control
• Groundwater contamination is nearly permanent
Key Principle: We all live in a watershed, and our actions affect water quality for everyone downstream—and for future generations.
Check Your Understanding
What is the difference between porosity and permeability?
Clay has high porosity (lots of pore space) but low permeability. Why?
The water table is:
What defines the boundaries of a watershed?
In a watershed, what happens upstream affects:
Which type of pollution is HARDER to control and regulate?
Why is groundwater contamination considered more serious than surface water contamination?
How do impervious surfaces (like parking lots) affect watersheds?
An aquifer is:
Match each term with its correct definition:
Match each material with its characteristics:
Match each type of pollution with its characteristics:
Which of the following are functions of a HEALTHY watershed? (Select ALL that apply)
Select all that apply.
Put these steps of groundwater formation in the correct order: