By Wendy Langhans
According to this U.S.Geological Service website, “there is a hundred times more water in the ground than is in all the world’s rivers and lakes.” Until this week, I didn’t know that. My understanding of water comes from what I’ve seen above ground – rain, rivers, marshes, estuaries and the ocean. But what goes on underground -well – that’s much more hidden and mysterious.
Luray Caverns in Virginia.
“A hydrogeologist is a person who studies the ways that groundwater (hydro) moves through the soil and rock of the earth (geology).” I could have used one last week, when I attended a meeting of the Wittaker-Bermite Citizen’s Advisory Group.
View north from Wittaker-Bermite. Photo courtesy G. Bona.
At that meeting I found myself listening intently, trying to make sense out of what was being reported about ground water contamination at the Wittaker-Bermite site and the on-going clean-up efforts. Two questions kept nagging at me – how does water move underground? Is it the same or different than water moving above ground?
I thought about what I already knew about hydrology (the study of surface water): water falls from the sky, soaks into the ground, flows downhill, eventually reaches the ocean and evaporates back into the sky. You can see the basic concepts of the Water Cycle in this chart from the U.S. Geological Survey.
But that didn’t help me understand what goes one below the surface of the ground? How does water move below ground? How fast does it move? What direction does it take? How is it similar to water flowing above ground? How is it different? Because I was curious, I decided to do a bit of research and share what I learned with you folks.
This week, I want to begin by introducing some basic concepts. Next week, I want to to talk about the factors that determine groundwater speed. And the following week, about the factors that determine groundwater direction. So, using the model from the popular “XX for Dummies” series and examples from my kitchen, here are the “Hydrogeology for Dummies” Parts of Tens:
1) Infiltration. Rainwater soaks into and through the soil, just as, in a coffeepot, water soaks into and through the coffee grounds. This is know as infiltration.
Rainwater at Quigley Canyon.
2) Unsaturated zone. Soil consists of solid particles and spaces between the particles, just like a loaf of bread consists of crumbs and air holes. The shallow area just below the soil’s surface contains both air and water. This is known as the unsaturated zone. Or technically, the vadose zone (“vadose” means shallow in Latin).
3) Saturated zone. As you go deeper, the holes in the soil become full of water; this is known as the saturated zone. Or technically, the phreatic zone (“phrear” means well or spring in Greek). Visualize a slice of bread soaking in milk, before you make French toast.
4) Water table. The boundary between the unsaturated (vadose) and saturated (phreatic) zone, which is “rarely flat, and it can change its shape and position”. Don’t picture this as a kitchen table with a flat, level surface. Instead, think of a tablecloth that scrunches and drapes.
5) Ground water. Water that is found in the saturated (phreatic) zone. Visualize a kitchen sponge.
6) Aquifer. Located within the saturated zone, you will find two different types of geologic formations, those that are permeable to water and those that are impermeable. To visualize this, think of a oatmeal sitting in a ceramic bowl. The moist oatmeal is the Aquifer. By the way, aquifers are not, as some people think, an underground river or lake. They are a solid, although porous, geologic formation “that will yield a usable quantity of water to a well or spring.”
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7) Confining bed. Technically known as an aquitard, this is “a geologic unit which is relatively impermeable and does not yield usable quantities of water” These impermeable layers are often made of clay or rock and can “restrict the movement of ground water into and out of adjacent aquifers”. Going back to the oatmeal example, the confining bed (aquitard) is the ceramic bowl.
8) Confined aquifer. An aquifer can either be confined and unconfined. A confined aquifer is like an oreo cookie, with the moist middle of sitting between two layers of cookie.
9) Unconfined aquifer. An unconfined aquifer has no confining bed above it and is usually open to water infiltration from the soil’s surface. To visualize this, think of a slice of bread with peanut butter on it – before you add that dollop of jam.
10) Plume. If you’re a bird-watcher, a plume is a large and showy feather, like a peacock feather. But if you are a hydrogeologist, a plume is contaminated groundwater, from a specific source, that spreads throughout the aquifer. To picture this, think about what happens when you add milk or cream to your cup of coffee.
That’s all I have for this week. Next week, let’s look at the factors that influence the speed of groundwater.
Upcoming Outdoor Events:
Saturday, August 20th, 8:00 – 10:00 AM, Early Morning Bird Hike at Towsley Canyon. We think of summer as being a cushy time for animals. But for our local bird residents, it’s a challenging time, with soaring heat, little water, and vegetation displaying all the signs of stress. Join us to see who the tough ones are. Beginning birders are welcome. Binoculars optional. Easy walk, 2 hours. Meet at Towsley Canyon’s front parking lot. For directions and trail maps, click here.
Trail Maintenance Schedule. Come join our volunteers as they help maintain our trails. Contact Steve at firstname.lastname@example.org for time and place.
Wednesday mornings, August 3, 10, 17, 24, & 31.
Saturday mornings, August 13 & 27.
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