---
title: "Water Underground"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 17
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/17-water-underground
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 17 — Water Underground

On a hot summer day, the fields are dry and the streams are low. Yet at the foot of a [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), water keeps rising out of the ground, cold and clear, as it did all winter, and fills a pool. It has not rained for weeks. Where does this water come from, and how does it get out of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock)?

![A spring rises at the foot of a rocky bluff and fills a pool: Big Spring, Missouri (United States). (Photo: Johnston9494, CC BY-SA 4.0, via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-water-underground/img-895f678f14a2.jpg)

*A [spring](#def-g6-water-underground-spring) rises at the foot of a rocky bluff and fills a pool: Big Spring, Missouri (United States). (Photo: Johnston9494, CC BY-SA 4.0, via Wikimedia Commons.)*

**You already know.**

Almost all the water on the Earth is [salt water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh); [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) is a small part, kept mostly as ice and under the ground ([Chapter 3](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#ch-g2-water-on-earth)). [Rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) such as sandstone and limestone are made of grains or shells stuck together, while clay and shale are made of very fine particles ([Chapter 15](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#ch-g6-rocks-and-minerals)). In the physics book, you have followed water around the water cycle: it evaporates from the [seas](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), condenses into [clouds](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) and falls back as [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) or [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation).

## 17.1 Rain soaks in

When [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) falls on the land, the water can take three paths. Part of it evaporates back into the air. Part of it flows over the surface, down the slopes, into streams and [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water): this is *run-off*. The rest soaks into the ground.

**Definition 17.1 (Infiltration).**

*Infiltration* is the soaking of [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) water, or melted [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation), into the ground, through the soil and down into the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

How much water soaks in depends on the ground. On a steep slope, on bare or hard ground, on a road or a car park, or when the soil is already soaked, most of the water runs off. On a gentle slope, under grass and trees, on sandy soil, more of it has time to soak in.

![The three paths of rain water. In the meadow much of it soaks in; on the road most of it runs off.](https://one-course.com/images/onecourse/chapters/earth-1/g6-water-underground/fig-0adbef859e45.svg)

*The three paths of [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) water. In the meadow much of it soaks in; on the road most of it runs off.*

## 17.2 Porous and permeable rocks

Look at a handful of dry [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) through a hand lens: between the grains there are tiny empty spaces. Pour water on it and the water fills these spaces. Many [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) are like that: they are full of small holes between their grains, or of thin cracks.

**Definition 17.2 (Porosity).**

The *pores* of a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) are the empty spaces between its grains and in its cracks. A [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) with pores is *porous*. Its *porosity* is the share of its volume made of pores, written as a percentage.

**Definition 17.3 (Permeability).**

A [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is *permeable* when water can flow through it, because its [pores](#def-g6-water-underground-porosity) are large enough and connected to one another. It is *impermeable* when water cannot flow through it. *Permeability* is how easily water flows through a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

The two ideas are not the same. Clay is very [porous](#def-g6-water-underground-porosity): half of its volume is made of [pores](#def-g6-water-underground-porosity). But its [pores](#def-g6-water-underground-porosity) are so tiny that the water stays stuck in them: clay is *[impermeable](#def-g6-water-underground-permeability)*. [Sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) has fewer [pores](#def-g6-water-underground-porosity) than clay, but they are wide and connected: [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) is *[permeable](#def-g6-water-underground-permeability)*. Granite without cracks has almost no [pores](#def-g6-water-underground-porosity) at all.

![The pores (in blue) between the grains of a sand and between the flakes of a clay, much enlarged. Water finds a way through the sand; in the clay, it stays stuck.](https://one-course.com/images/onecourse/chapters/earth-1/g6-water-underground/fig-fca9eb14ce49.svg)

*The [pores](#def-g6-water-underground-porosity) (in blue) between the grains of a [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and between the flakes of a clay, much enlarged. Water finds a way through the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand); in the clay, it stays stuck.*

The table gives the [porosity](#def-g6-water-underground-porosity) of a few materials, and the share of water that drains out of them by itself when they are full of water. The rest stays stuck to the grains.

| **material** | **[porosity](#def-g6-water-underground-porosity) (%)** | **water that drains out (%)** |
| --- | --- | --- |
| soil | 55 | 40 |
| clay | 50 | 2 |
| [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) | 25 | 22 |
| [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) | 20 | 19 |
| limestone | 20 | 18 |
| sandstone | 11 | 6 |
| granite | 0.1 | 0.09 |

**Method 17.4 (How much water does a rock hold?).**

1. Find the volume of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) (for a block: length $\times$ width $\times$ height).
2. Multiply by the [porosity](#def-g6-water-underground-porosity) and divide by 100: this is the volume of the [pores](#def-g6-water-underground-porosity) , so the water the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) holds when it is full.
3. For the water that drains out, use the second percentage in the same way. (1 cubic metre is 1000 litres.)

**Example 17.5 (A cubic metre of sand).**

One cubic metre of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), full of water, holds $1000 \times 25 \div 100 =
250$ litres of water in its [pores](#def-g6-water-underground-porosity). Of these, $1000 \times 22 \div 100 = 220$ litres drain out by themselves; 30 litres stay stuck to the grains. One cubic metre of clay holds twice as much water, 500 litres, but only 20 litres drain out.

**In the lab — Sand or clay? (in class, with the teacher).**

Two flowerpots with a hole at the bottom stand over two glasses. One is filled with dry [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), the other with dry clay soil pressed down. The teacher pours the same glass of water into each, at the same time. Through the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), water soon drips into the glass below. On the clay, the water stays on top as a puddle and only a few drops come through, slowly: the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) is [permeable](#def-g6-water-underground-permeability), the clay is not.

## 17.3 Groundwater and the water table

Dig a hole in the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) of a beach, a few steps from the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean): after a while, water appears at the bottom of the hole. You have reached the place where all the [pores](#def-g6-water-underground-porosity) of the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) are full of water.

**Definition 17.6 (Groundwater).**

*Groundwater* is the water under the ground that fills the [pores](#def-g6-water-underground-porosity) and cracks of the soil and of the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

**Definition 17.7 (Water table).**

Below a certain depth, all the [pores](#def-g6-water-underground-porosity) of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) are full of water. The *water table* is the top of this water-filled zone. Above it, the [pores](#def-g6-water-underground-porosity) hold air and a little water; below it, they are full of water.

The [water table](#def-g6-water-underground-water-table) is not flat. It follows the shape of the land, but more gently: higher under the [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), lower under the [valleys](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). It also moves up and down with the seasons. After a rainy season, the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) that has soaked in raises it; after a long dry season, the water slowly flows away to [springs](#def-g6-water-underground-spring) and [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), and the [water table](#def-g6-water-underground-water-table) goes down.

[Groundwater](#def-g6-water-underground-groundwater) is a great store of [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh). Of all the [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) of the Earth, about 69 % is ice and permanent [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation), and about 30 % is [groundwater](#def-g6-water-underground-groundwater); the [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) of all the [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) makes only about a quarter of one percent. [Groundwater](#def-g6-water-underground-groundwater) is more than a hundred times as abundant as the [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) of the [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

![Where the Earth’s fresh water is. The bars for the lakes and for everything else (rivers, the air, the soil, swamps, living things) are too short to see.](https://one-course.com/images/onecourse/chapters/earth-1/g6-water-underground/fig-20bf23773da5.svg)

*Where the Earth’s [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) is. The bars for the [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and for everything else ([rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), the air, the soil, swamps, living things) are too short to see.*

## 17.4 Springs and wells

**Definition 17.8 (Spring and well).**

A *spring* is a place where [groundwater](#def-g6-water-underground-groundwater) comes out of the ground by itself, where the [water table](#def-g6-water-underground-water-table) meets the surface. A *well* is a hole dug or drilled down below the [water table](#def-g6-water-underground-water-table), so that [groundwater](#def-g6-water-underground-groundwater) fills its bottom and can be lifted out.

The section below crosses a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) of the Varden Hills, the region of our exercises. The [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) are made of chalk, a white, soft, [porous](#def-g6-water-underground-porosity) and [permeable](#def-g6-water-underground-permeability) limestone. Under the chalk lies a layer of shale, which is [impermeable](#def-g6-water-underground-permeability). [Rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) soaks into the chalk, fills its [pores](#def-g6-water-underground-porosity) and stops on the shale.

![A north–south section through the Varden Hills, from the river up the northern hillside (heights exaggerated about 2 times). The water held in the chalk stops on the shale and comes out as a spring where the base of the chalk meets the slope. Well A always reaches the water; well B reaches it only after the rainy season.](https://one-course.com/images/onecourse/chapters/earth-1/g6-water-underground/fig-8a18e63bb436.svg)

*A north–south section through the Varden Hills, from the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) up the northern hillside (heights exaggerated about 2 times). The water held in the chalk stops on the shale and comes out as a [spring](#def-g6-water-underground-spring) where the base of the chalk meets the slope. [Well](#def-g6-water-underground-spring) A always reaches the water; [well](#def-g6-water-underground-spring) B reaches it only after the rainy season.*

The [spring](#def-g6-water-underground-spring) flows where the base of the chalk meets the hillside: the water cannot go down through the shale, so it flows along the top of the shale and comes out on the slope. As long as there is water in the chalk above the shale, the [spring](#def-g6-water-underground-spring) keeps running, even in a dry summer: that is the answer to the question asked at the start of the chapter.

Look at the two [wells](#def-g6-water-underground-spring). [Well](#def-g6-water-underground-spring) A is 100 m deep; its bottom is below the [water table](#def-g6-water-underground-water-table) in every season, so it always gives water. [Well](#def-g6-water-underground-spring) B is even deeper, 105 m, but it was dug higher up the [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform): its bottom is below the [water table](#def-g6-water-underground-water-table) after the rainy season and above it after the dry season. In summer, [well](#def-g6-water-underground-spring) B runs dry. What matters is not how deep a [well](#def-g6-water-underground-spring) is, but how far its bottom goes below the lowest [water table](#def-g6-water-underground-water-table).

![Children fill a bucket at the hand pump of a village well. (AI illustration.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-water-underground/img-0d98eba60f5b.jpg)

*Children fill a bucket at the hand pump of a village [well](#def-g6-water-underground-spring). (AI illustration.)*

**Remark 17.9 (A well can run dry).**

Pumping water out of a [well](#def-g6-water-underground-spring) lowers the [water table](#def-g6-water-underground-water-table) around it. If water is pumped out faster than the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) refills the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), the [water table](#def-g6-water-underground-water-table) goes down and down: the [well](#def-g6-water-underground-spring) gives less and less water and can run dry. Two neighbours pumping from the same [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) share the same water: one pump can dry out the other’s [well](#def-g6-water-underground-spring).

## 17.5 Aquifers

**Definition 17.10 (Aquifer).**

An *aquifer* is a layer of [porous](#def-g6-water-underground-porosity) and [permeable](#def-g6-water-underground-permeability) [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), full of [groundwater](#def-g6-water-underground-groundwater), that lets this water flow out to [springs](#def-g6-water-underground-spring) and [wells](#def-g6-water-underground-spring). The [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) that soaks in refills it.

The chalk of the Varden Hills is an [aquifer](#def-g6-water-underground-aquifer): it is [porous](#def-g6-water-underground-porosity), it is [permeable](#def-g6-water-underground-permeability), and the farms of the [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) draw their water from it. The shale under it is not an [aquifer](#def-g6-water-underground-aquifer): water cannot flow through it. Deeper still, under the shale, lie a limestone and a sandstone, which can hold water too; to reach it, a [well](#def-g6-water-underground-spring) has to be drilled right through the shale.

[Groundwater](#def-g6-water-underground-groundwater) is part of the water cycle. [Rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) soaks into an [aquifer](#def-g6-water-underground-aquifer); [groundwater](#def-g6-water-underground-groundwater) flows slowly through the [pores](#def-g6-water-underground-porosity); it comes out at [springs](#def-g6-water-underground-spring) and into [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), which carry it to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean); there it evaporates, and falls again as [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation). Taking water from an [aquifer](#def-g6-water-underground-aquifer) is like taking water from a tank that the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) fills slowly: it lasts only if we do not take more than the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) brings back.

## 17.6 Keeping groundwater clean

Water that soaks into the ground is filtered on its way: leaves, soil particles and small creatures are held back by the soil and the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). That is why water from a [spring](#def-g6-water-underground-spring) or a [well](#def-g6-water-underground-spring) usually looks crystal clear. But substances *dissolved* in the water are not held back: they travel down with it to the [water table](#def-g6-water-underground-water-table).

[Groundwater](#def-g6-water-underground-groundwater) can be spoilt by what people put on the ground: fertilisers and pesticides spread on fields and gardens, salt spread on icy roads, fuel leaking from a tank, waste dumped in the open, the overflow of badly kept waste-water tanks of houses. Once dissolved in an [aquifer](#def-g6-water-underground-aquifer), these substances are hard to remove, and the water may no longer be safe to drink without treatment. So the water of a [well](#def-g6-water-underground-spring) has to be tested before it is drunk, and fertilisers, fuel and waste are best kept far from [wells](#def-g6-water-underground-spring) and [springs](#def-g6-water-underground-spring).

Dissolved substances travel with the [groundwater](#def-g6-water-underground-groundwater). Below the [water table](#def-g6-water-underground-water-table), the water creeps slowly downhill: from where the [water table](#def-g6-water-underground-water-table) is high, under the [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), to where it is low, at the [springs](#def-g6-water-underground-spring) and the [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). A spill that soaks in therefore does not stay where it fell. It sinks to the [water table](#def-g6-water-underground-water-table), then drifts downhill with the [groundwater](#def-g6-water-underground-groundwater), towards whatever lies below it on the slope.

![A spill on a hillside (a sketch). The dissolved fuel sinks to the water table, then drifts downhill with the groundwater (blue arrows): it reaches well B, below it on the slope, and in the end the stream. Well A, uphill of the tank, stays clean.](https://one-course.com/images/onecourse/chapters/earth-1/g6-water-underground/fig-3db6dc318663.svg)

*A spill on a hillside (a sketch). The dissolved fuel sinks to the [water table](#def-g6-water-underground-water-table), then drifts downhill with the [groundwater](#def-g6-water-underground-groundwater) (blue arrows): it reaches [well](#def-g6-water-underground-spring) B, below it on the slope, and in the end the stream. [Well](#def-g6-water-underground-spring) A, uphill of the tank, stays clean.*

**Method 17.11 (Is a well at risk?).**

1. Find where the [water table](#def-g6-water-underground-water-table) is high and where it is low: it is high under the [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and low near the [springs](#def-g6-water-underground-spring) and the [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) .
2. The [groundwater](#def-g6-water-underground-groundwater) flows from high to low: draw its direction with an arrow.
3. A [well](#def-g6-water-underground-spring) or a [spring](#def-g6-water-underground-spring) that lies downhill of a source of pollution, on the path of that arrow, is at risk; a [well](#def-g6-water-underground-spring) uphill of it is not.

**Example 17.12 (A new fuel tank for the farm).**

A farmer on a chalk hillside drinks the water of her own [well](#def-g6-water-underground-spring), halfway up the slope, and wants to put a new fuel tank in one of two fields: one uphill of the [well](#def-g6-water-underground-spring), one downhill of it, near the stream. The [groundwater](#def-g6-water-underground-groundwater) flows downhill. A leak from the field uphill would drift down towards the [well](#def-g6-water-underground-spring); a leak from the field downhill would drift away from it, towards the stream. The field downhill of the [well](#def-g6-water-underground-spring) is the better choice for the [well](#def-g6-water-underground-spring) — and a tank on a tray that holds any leak is better still, since the stream matters too.

**In the field — Spotting springs.**

On a walk with an adult, look along the foot of a slope. A [spring](#def-g6-water-underground-spring) often shows itself as a patch of wet ground, a line of greener grass or of reeds in a dry field, a small stream that starts from nowhere, or a stone basin or a trough built to catch the water. On a detailed walking [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map), [springs](#def-g6-water-underground-spring) are marked with a small symbol. Observe from the path: never drink the water of a [spring](#def-g6-water-underground-spring) that has not been tested.

## 17.7 Exercises

**Exercise 17.1 ★.**

Sort these materials into “[permeable](#def-g6-water-underground-permeability)” and “[impermeable](#def-g6-water-underground-permeability)”: [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), clay, [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), granite without cracks, shale, chalk.

**Solution of Exercise 17.1.**

[Permeable](#def-g6-water-underground-permeability): [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), chalk. [Impermeable](#def-g6-water-underground-permeability): clay, granite without cracks, shale.

**Exercise 17.2 ★.**

Match each word with its meaning: [spring](#def-g6-water-underground-spring); [well](#def-g6-water-underground-spring); [water table](#def-g6-water-underground-water-table); [aquifer](#def-g6-water-underground-aquifer). (a) The top of the zone where all the [pores](#def-g6-water-underground-porosity) are full of water. (b) A [porous](#def-g6-water-underground-porosity) and [permeable](#def-g6-water-underground-permeability) layer that holds [groundwater](#def-g6-water-underground-groundwater). (c) A place where [groundwater](#def-g6-water-underground-groundwater) comes out by itself. (d) A hole that goes down below the [water table](#def-g6-water-underground-water-table).

**Solution of Exercise 17.2.**

(a) [water table](#def-g6-water-underground-water-table); (b) [aquifer](#def-g6-water-underground-aquifer); (c) [spring](#def-g6-water-underground-spring); (d) [well](#def-g6-water-underground-spring).

**Exercise 17.3 ★.**

(a) Name the three paths that [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) water can take when it falls on the land. (b) After a [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm), where does more water run off: on a car park or on a meadow? Why?

**Solution of Exercise 17.3.**

(a) It evaporates back into the air; it runs off over the surface into streams; it soaks into the ground ([infiltration](#def-g6-water-underground-infiltration)). (b) On the car park: its hard surface lets almost no water soak in, so the water runs off. In the meadow, the soil and the grass let much of it soak in.

**Exercise 17.4 ★.**

True or false? Correct the false sentences. (a) A [porous](#def-g6-water-underground-porosity) [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is always [permeable](#def-g6-water-underground-permeability). (b) Below the [water table](#def-g6-water-underground-water-table), the [pores](#def-g6-water-underground-porosity) are full of water. (c) The [water table](#def-g6-water-underground-water-table) is flat, like the surface of a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). (d) There is more [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) under the ground than in all the [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

**Solution of Exercise 17.4.**

(a) False: clay is very [porous](#def-g6-water-underground-porosity) but [impermeable](#def-g6-water-underground-permeability), because its [pores](#def-g6-water-underground-porosity) are tiny. (b) True. (c) False: it follows the shape of the land, more gently, and it goes up and down with the seasons. (d) True: about 30 % of all [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) is [groundwater](#def-g6-water-underground-groundwater), against about a quarter of one percent in the [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

**Exercise 17.5 ★★.**

A bucket holds 10 litres of dry [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). The [porosity](#def-g6-water-underground-porosity) of the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) is 25 %. How much water can you pour into the bucket before it overflows? How much water would drain out through a hole in the bottom (22 %)?

**Solution of Exercise 17.5.**

$10 \times 25 \div 100 = 2.5$: you can pour 2.5 litres before it overflows. $10 \times 22 \div 100 = 2.2$: 2.2 litres would drain out through the hole.

**Exercise 17.6 ★★.**

A block of limestone measures 2 m by 1 m by 1 m. (a) What is its volume, in cubic metres and in litres? (b) Its [porosity](#def-g6-water-underground-porosity) is 20 %: how many litres of water does it hold when it is full? (c) Of this water, 18 % of the volume drains out. How many litres drain out, and how many stay stuck?

**Solution of Exercise 17.6.**

(a) $2 \times 1 \times 1 = 2$ cubic metres, that is 2000 litres. (b) $2000 \times 20 \div 100 = 400$ litres. (c) $2000 \times 18 \div 100 =
360$ litres drain out; $400 - 360 = 40$ litres stay stuck.

**Exercise 17.7 ★★.**

Use the section through the Varden Hills. (a) At what height is the [spring](#def-g6-water-underground-spring)? the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water)? (b) Is the [spring](#def-g6-water-underground-spring) above or below the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water)? Where does the water of the [spring](#def-g6-water-underground-spring) go? (c) Why is there no [spring](#def-g6-water-underground-spring) in the middle of the chalk hillside?

**Solution of Exercise 17.7.**

(a) The [spring](#def-g6-water-underground-spring) is at about 203 m, the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) at about 132 m. (b) The [spring](#def-g6-water-underground-spring) is about 70 m above the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water): its water runs down the slope into the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). (c) Inside the chalk hillside the [water table](#def-g6-water-underground-water-table) is below the ground; it meets the surface only where the base of the chalk, resting on the [impermeable](#def-g6-water-underground-permeability) shale, comes out on the slope.

**Exercise 17.8 ★★.**

At [well](#def-g6-water-underground-spring) A, the ground is at 401 m; the [water table](#def-g6-water-underground-water-table) is at 336 m after the rainy season and at 314 m after the dry season. (a) How deep is the water below the ground in each season? (b) By how many metres does the [water table](#def-g6-water-underground-water-table) go down between the two seasons?

**Solution of Exercise 17.8.**

(a) After the rainy season: $401 - 336 = 65$ m; after the dry season: $401 - 314 = 87$ m. (b) $336 - 314 = 22$ m.

**Exercise 17.9 ★★.**

A farmer wants a new [well](#def-g6-water-underground-spring) at the top of the [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), at the right end of the section. Must the [well](#def-g6-water-underground-spring) be deeper or shallower than [well](#def-g6-water-underground-spring) A? Explain with the [water table](#def-g6-water-underground-water-table).

**Solution of Exercise 17.9.**

Deeper. The [water table](#def-g6-water-underground-water-table) follows the shape of the land, but more gently: at the top of the [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) it is higher than under [well](#def-g6-water-underground-spring) A, but the ground rises much more, so the water lies farther below the surface (on the section, more than 110 m down).

**Exercise 17.10 ★★.**

A cubic metre of clay holds 500 litres of water but gives only 20 litres. Explain why a [well](#def-g6-water-underground-spring) dug into clay gives almost no water, although the clay is full of it.

**Solution of Exercise 17.10.**

The [pores](#def-g6-water-underground-porosity) of clay are tiny: the water stays stuck to the flakes and cannot flow through the clay towards the [well](#def-g6-water-underground-spring). Clay is [porous](#def-g6-water-underground-porosity) but [impermeable](#def-g6-water-underground-permeability), so only a little water (20 litres out of 500) drains out.

**Exercise 17.11 ★★★.**

Of all the [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) of the Earth, 30.1 % is [groundwater](#def-g6-water-underground-groundwater) and 0.26 % is in [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). (a) How many times more [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) is there under the ground than in the [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water)? (b) Of all [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh), about 69 % is ice and [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation). What share is left for everything else ([groundwater](#def-g6-water-underground-groundwater), [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), the air, the soil…)? (c) Why is [groundwater](#def-g6-water-underground-groundwater) so important for drinking water?

**Solution of Exercise 17.11.**

(a) $30.1 \div 0.26 \approx 116$: more than a hundred times more. (b) $100 - 69 = 31$: about 31 % is left, and [groundwater](#def-g6-water-underground-groundwater) alone is about 30 of them. (c) It is by far the largest store of liquid [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh); the ice is frozen and mostly far away, in the polar regions and on [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). And [groundwater](#def-g6-water-underground-groundwater) is found almost everywhere under our feet.

**Exercise 17.12 ★★★.**

Two farms stand side by side on the same chalk. One summer, the first farm installs a powerful pump and fills a large pond; a few weeks later, the [well](#def-g6-water-underground-spring) of the second farm runs dry, although nobody touched it. Explain what happened, and what the two farmers could do.

**Solution of Exercise 17.12.**

Both [wells](#def-g6-water-underground-spring) draw on the same chalk [aquifer](#def-g6-water-underground-aquifer). The powerful pump took water out faster than the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) refills the chalk, so the [water table](#def-g6-water-underground-water-table) went down all around, below the bottom of the second [well](#def-g6-water-underground-spring), which ran dry. The farmers could agree to pump less, especially in summer, share one [well](#def-g6-water-underground-spring), or deepen the second [well](#def-g6-water-underground-spring); above all they should not take more water than the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) brings back.

## 17.8 Problem: The farm well of the Varden Hills

**Problem 17.1.**

Weekend problem — the rocks under a farm of the Varden Hills, the water stored in its chalk, and how many days it would last without rain

A farm of the Varden Hills stands on the chalk, next to [well](#def-g6-water-underground-spring) A of the section. The farmer wants to know how long the [well](#def-g6-water-underground-spring) could keep the farm going in a long [drought](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-drought), if no [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) at all came to refill the chalk.

**Part I — The rocks under the farm.**

1. Name the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) on which the farm stands, and the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) under it. Which one is [permeable](#def-g6-water-underground-permeability) ?
2. Explain why a [spring](#def-g6-water-underground-spring) flows at the foot of the slope below the farm.
3. Use the section: the ground at the farm is at 401 m and the [water table](#def-g6-water-underground-water-table) is at 336 m after the rainy season. How far down is the water?
4. After the dry season, the [water table](#def-g6-water-underground-water-table) is at 314 m. [Well](#def-g6-water-underground-spring) A is 100 m deep. Does it still reach the water? By how many metres?

**Part II — The water in the chalk.** The farmer can drain the chalk around the [well](#def-g6-water-underground-spring) in a block 100 m long, 50 m wide and 2 m thick, below the [water table](#def-g6-water-underground-water-table). The chalk is a limestone: use a [porosity](#def-g6-water-underground-porosity) of 20 % and a share of water that drains out of 18 %.

5. What is the volume of the block, in cubic metres?
6. How many cubic metres of water fill its [pores](#def-g6-water-underground-porosity) ?
7. How many cubic metres of water drain out of it into the [well](#def-g6-water-underground-spring) ? How many stay stuck in the chalk?
8. Write the water that drains out in litres.

**Part III — How long it lasts.**

9. The farm (house, animals and vegetable garden) uses 18 cubic metres of water a day. How many litres is that?
10. How much water does the farm use in a week?
11. Why does the store of water in the chalk not run out in a normal year?
12. Without any [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) at all, for how many days would the water draining out of the block supply the farm?

**Solution of Problem 17.1.**

**1.** The farm stands on chalk; under it lies shale. The chalk is [permeable](#def-g6-water-underground-permeability), the shale is not.

**2.** [Rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) soaks into the chalk and stops on the shale. Where the base of the chalk comes out on the slope, the [water table](#def-g6-water-underground-water-table) meets the ground and the water comes out: that is the [spring](#def-g6-water-underground-spring).

**3.** $401 - 336 = 65$ m below the ground.

**4.** The bottom of the [well](#def-g6-water-underground-spring) is at $401 - 100 = 301$ m, below the [water table](#def-g6-water-underground-water-table) at 314 m: the [well](#def-g6-water-underground-spring) still reaches the water, by $314 - 301 =
13$ m.

**5.** $100 \times 50 \times 2 = 10\,000$ cubic metres.

**6.** $10\,000 \times 20 \div 100 = 2000$ cubic metres.

**7.** $10\,000 \times 18 \div 100 = 1800$ cubic metres drain out; $2000 - 1800 = 200$ cubic metres stay stuck.

**8.** $1800 \times 1000 = 1\,800\,000$ litres.

**9.** $18 \times 1000 = 18\,000$ litres a day.

**10.** $18 \times 7 = 126$ cubic metres a week.

**11.** The [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) of the wet season soaks into the chalk and refills it every year (as long as the farms do not pump more than the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) brings back).

**12.** $1800 \div 18 =$ **100 days**: a little more than three months without any [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation).