Point Source

Quaquaversal musings on the geosciences and public information.

[Image of how groundwater occurs.]

Hydrologically Incorrect Cake (and why I care)

I was very excited when @rockbandit shared this great “geology” cake photo. As a baker, I had to inspect the photo closely to see how they did it. This was when I noticed the problem:

The groundwater is shown as a giant underground pool.

Now, I don’t expect my baked goods to be geologically correct. I’m also a huge sci-fi fan and happily suspend disbelief in the face of all manner of geological fallacies.

The Groundwater Myth

The problem is that most people think these misrepresentations of groundwater are accurate:

It is common for people to believe the myth that all groundwater exists in underground pools and rivers.

This makes for exciting movies, like Journey to the Center of the Earth (original or remake), where people understand that there are no dinosaurs in the middle of the earth but think the underground rivers and lakes could be real.

This misunderstanding about how groundwater occurs has real repercussions:

  • It affects how people think about water rights
  • It affects how people think about water use and availability
  • It supports the use of unscientific practices like water dowsing

How and where does groundwater really occur?

The reality of groundwater is much less exciting and dramatic: most groundwater occurs in the spaces or cracks in the soil, sand, or rocks in the ground below us.

This is a good groundwater diagram:

[Image of how groundwater occurs.]

Groundwater diagram, (c) Natural Resources Canada.

The bottom half of this image shows how groundwater can fill the spaces (pores) between sand, gravel, and other materials, and how it can flow through small cracks (fractures) in the bedrock.

The best basic groundwater video I’ve seen comes out of the King County (Washington) Groundwater Protection Program. It’s a great combination of cute animation, catchy tunes, and accurate science, not to mention a great example of effective science communication. Just try to watch the whole thing without singing along or tapping your toes!

Just to be clear, though: I’d still eat the cake.

Update 1/29/2011: Dan points out below that sometimes groundwater does occur in large open spaces or voids in the subsurface. This is true, but the issue is that many people mistakenly think this is true for all groundwater, which is not correct. Check out Dan’s good comment and visit the USGS karst aquifer web site for more information and photos.

Learn more about groundwater:


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2010 My Year in Geo-Travel Adventures


AW#30: Hydrogeologically Correct Cake


  1. Good eye! I generally don’t pay that much attention to such details, but then again I’m not a hydrologist. Now I don’t have any more excuses!

  2. It’s a geologically incorrect cake even farther down-section. Horizontal layers –> angular unconformity (?!) –> horizontal layers. No!

  3. Dan Doctor

    Though I’d rather nibble on that cake than quibble on it, I will offer an alternative viewpoint, and one that is often lost on my esteemed colleagues in the groundwater community: water can (and does) exist in large open voids as lakes, pools, and streams underground, in areas we collectively refer to as karst.
    Caves are the manifestation of formerly filled phreatic voids, formed by the dissolution of the rocks that contain them. The water that formed these voids is, in fact, groundwater; however, in karst areas the groundwater very often is “young”, and began its life as surface water in a stream before it encountered a sinkhole and disappeared underground. So, I would submit that the savvy geologist who baked this cake had an understanding of karst, and that the uppermost dark and light brown layers are carbonate rocks, limestones or dolomites. The stream encountered a sink point, and flowed down through the carbonate before encountering a contact with an insoluble layer of sandstone or shale (the creamy-white layer), where it happily continued its way horizontally through an underground cave. Try going caving in an active stream passage some time–you’ll never see groundwater the same way again. 🙂

    • cian

      Dan – Thanks for the comment! (Sorry for the delay, it got trapped in my spam.) You raise good points. I am familiar with karst and agree with most of your comments in that regard. I usually try to be more careful in my language so as not to say groundwater “never” occurs in open subsurface voids, and I’ll revise my post to be more clearly worded. The problem, though, is that people tend to think *all* groundwater occurs in karst-like formations, and that is not accurate. But your point is well-taken: we need to dispel the myth while not discounting the instances where the description is accurate.

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