# Under 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

## 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)

# Watersheds: 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 |

Healthy watersheds: - Absorb rainfall, reducing flooding - Filter pollutants naturally - Maintain stream flows during dry periods - Support diverse ecosystems

# Human 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

## 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

## 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.