Water Under All: Hydrogeology explained by Elle, Pea & Co

You may recall our helpful little diagram that showed the level of water underground; we named it the water table.

This is the level of water that is measured from a piezometer (remember those fancy pipes we put in the ground?). But not all measurements of water levels underground refer to the water table. It depends on what aquifer we are measuring. 

Aquifers come in two types: confined and unconfined. The confined aquifers have a seal or lid on them that doesn't let the water escape vertically through it. This confines the water to that aquifer. so some clever chap decided to classify it as confined. Not very inventive at all, we could have come up with a much more creative and entertaining name. But its too late for all that now, so we will go ahead with this naming for consistency. 

Now get those brain cogs cranking because if a confined aquifer has a seal and is confined, what do you reckon those unconfined aquifers are all about?

yup! the aquifer doesnt have a seal! So its not under any pressure of boundaries and can move freely like a hippy child through a daisy field :)

And as a treat for getting this far, we will reveal the scientific name of the seal that confines our aquifers. This one is a doozy!

Aquitard... thats right. Aquitard. 

Please take this gift and spread it around for all to share in its comical name! It makes such a good insult:

For example,

“Get out of my way, stop being such an Aquitard”

Until next time

-Elle, Pea & Co

Even spell check is having trouble with this word

What the heck is a piezometer?

We know that last time we focused on Henry Darcy...

... and we promised to have a look at the equation that he so cleverly came up with. However, we realised that the more we looked into explaining his law, the more complicated it would be without covering off some other basics.

Basics first, then juicy maths later :)

Ok, so back to piezometers - first things first, It is not pronounced “pie-zometer”. Well, at least not from where we were taught about it. Unfortunately, google is not backing us up on this one, probably because its hungry.

So, we pronounce it more like “pee-zometer” and its short hand name of “pee-zo” (spelt piezo, of course.)

Good old Wikipedia has a lovely confusing and convoluted way of explaining what it is: 

“ A piezometer is either a device used to measure liquid pressure in a system by measuring the height to which a column of the liquid rises against gravity, or a device which measures the pressure (more precisely, the piezometric head) of groundwater at a specific point.“

But here at Elle, Pea & Co we like to think of it as a piece of piping inserted into the ground like a straw.  Along the straw will be specifically placed holes, or screen, to allow water to travel into the pipe. the depth of where the water sits is known as the head height. The entire hole is plugged and sealed to ensure that the only water travelling in is from the aquifer we are interested in.

Let’s take a look at a little picture (they’re worth 1000 words after all, and it's beginning to hurt to type.)

And from the top:

Like an iceberg, all the stuff is happpening under the ground. 

Next time we will look at what the piezometer head height really means.

- Elle, Pea & Co

What’s his deal?

Darcy, the smart and charismatic Frenchman, derived the equation now known as: 

Darcy’s Law

This equation was derived to describe the flow of liquid through a porous medium. His experiment was measuring the flow of water through a pipe packed with sand. 

Kind of like the movement of water through an aquifer, wouldn’t you say? 

Now, a warning here, we are about to look at (*gasp*) Maths! Dont be scared, we are right alongside you. Fear not! One of the versions of the equation that Darcy put together is this: 

So, what does that all mean? 

Lets break it down next post

-Elle, Pea & Co

5 posts - well, I think we can do better than that!

- Elle, Pea & Co

So what’s one of the cool things about water? It’s fluid. And fluid things move, even underground.

When we look at an aquifer and its water table, we know that the level of the water table follows the topography. But what we can add to this is that the water in the aquifer is also moving like a slow, slow, slow and sometimes slower river.

Which way do they move? Like all fluids, gravity plays a part. you can see in the example below that the water is moving from its highest point to its lowest - feeding into a river.

How fast does water move underground Elle, Pea &Co?

What a great question, imaginary person! Well, the answer has got to do with this handsome chap…

No wait… Not that Mr Darcy, this Mr Darcy

Stay tuned for more!

- Elle, Pea & Co

Remember as a kid you would be a smart-ass with your drink and put a straw into your cup, cover the top of the straw with your finger and then lift it up out of the glass to wonder at the fact that there is still liquid inside the straw? You remember, right? And you would hover it over your mouth and a wave of absolute power would come over you while you decide when you will let go and let the drink fall into your parched throat? Or maybe flick or drop it over an unsuspecting sibling?

Well, the principals of that sensational experience are similar to what the water table is. Kind of, it’s more like the opposite effect, really. But both have got to do with pressure.

Unlike the straw, where you have created a pressure vacuum to hold the liquid, the water table is under no pressure, or at atmospheric pressure. So there is air in the pore spaces above it and no air (just water) in the pore spaces below it. That line dividing the two is the water table. Its kind of like looking at the top layer of water in a glass. A glass that is also full of dirt.

When it rains and water trickles down through the ground to the water table, the level will increase. The water table will also follow the topography (peaks and dips of the land). Seasons and tides (if close to the ocean) can also affect the height of the water table.

- Elle, Pea & Co

We will be using the schematic above from time to time, so lets get acquainted! This is a great depiction of groundwater in a basic environment. See that solid blue line at the top of the water filled pores?

That’s the Water Table

not to be confused with a table of water (or coffee in the gif below)

Ok, we’re dizzy now looking at that gif, and now we want coffee.

Check back soon for more!

- Elle, Pea & Co

Ok, so what are the basics of hydrogeology and how do we keep it interesting?

Gif’s are probably the answer along with lots of pictures and keeping it short.

You wouldn’t believe it, but there are lakes under the ground - crazy, right?

“But there are rocks and dirt under the ground, where is the room for the water?”

Well, its got to do with Pore Spaces.

When we zoom in on the dirt below us, and zoom in some more, and a little more, we can see the individual grains of sand, clay, silt, what have you. In this example below, we are looking at three different levels of pore space: none, some and lots.

Each of these levels describes how porous the grains are. That is, how little or how much cavity is available between the grains for water to be stored. Non-porous = nada, porous = plenty of space!

You want a fancy word to swing around the office? Well then try this one out

Permeability - this is the ability of a porous material to allow fluids to pass through it.

Such uses included:

“My, this sponge must be quite permeable, considering all the water its holding”

and:

“thank goodness my coffee mug is impermeable, or I would be wearing it all over my pants!”

[Please note: do not use these examples around your workplace if you want to be taken seriously… we hold no responsibility for loss of employment due to bad humour]

Check back in next time for some more basics

- Elle, Pea & Co