Groundwater

I know, I know, It’s been a while since my last posting, but I really have an excuse – I’ve switched graduate programs and am fixing to be a environmental engineering graduate student come January 13th.  I’m looking forward to a change in pace, a different project, and the math!!  I’ve always been attracted to applied science (i.e. how can science help everyday people, such as biomedical engineering, waste water treatment design, etc.) and I think this will be a better fit for me.

Well enough about me, I wanted to talk about ground water today.  Now most people that I talk to have heard of the term groundwater, they associate it with a well and water that’s clean (with a bit of a funny aftertaste is some cases).  I remember once that co-worker of mine thought that groundwater is just underground rivers flowing in caves.  There are caves that have underground rivers flowing, but this is by far the least common form of ground water.

So let’s take a scientific look at what groundwater is.

What do we know?  First, we know that when it rains, not all water evaporates or flows over the land into rivers, some does, but not all of it.  Second, we know that springs are found in peculiar spots.  Third, we know that we can get water if we drill a well.

So where does the water go if it doesn’t all evaporate or flow into rivers?  – If you take a look at dirt, you notice that it’s composed of small grains of clay, silt and sand. If you pour water on the ground, it wicks away into the ground like water into a sponge.  The spaces in between the clay, silt and sand particles create voids, and these voids go all the way down into the ground until they hit bedrock.  These voids are called porosity, or the volume that can be occupied by either air or water.  I know that it doesn’t look like it, but a soil’s porosity can be a lot.  For example, a gravel soil may have up 25% porosity, or 25% of the total volume is empty, a sandy soil can be 35% porosity, and a clay soil can be up to 70% porosity!!  Now you may notice that if you try to squeeze a handful of wet sand, water come out, but when you squeeze a handful of clay mud, it just remains sticky.  This brings me to another term – permeability.  permeability is the connectedness of each pore to each other.  Think of a Styrofoam cup, if you cut it open and look closely at it, you’ll notice that it’s composed of lots of small air pockets.  A Styrofoam cup may have 95% porosity, but because those pores aren’t connected, they have zero permeability.  That’s why your hot coffee doesn’t seep through the cup and burn your hand.

So what’s a spring?  Well, as water percolates through the soil, it’ll eventually reach an area of the soil where it’s saturated, like a saturated sponge.  Here it forms a boundary between saturated and unsaturated soil – this is called a water table, and everything below is called an aquifer.  The water table moves up and down with the seasons and from pumping the aquifer.  The water table generally follows the landscape topography, so when we get a steep hill, and the water table hits the surface, we get a spring.

So what are pumping from a well?  A well is just a straw that we drill into the ground to access the groundwater.   You can imagine that a well is made up of a long tube that has a lot of tiny slits cut into it where we find good water called a screen.  It’s from this screened area that we pump water.  In developing countries, they usually dig the well by hand and line it with stones, then use well bags to draw the water up.  So why do some wells dry up?  If the water table drops below the screened area, then all we’re pumping is air in the pore space.  This can happen due to drought or more commonly due to over pumping the aquifer.

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