By Wendy Langhans
Imagine you are commuting along the 405 in the San Fernando Valley. How fast are you going? At 5 AM, you’re probably moving at about 65-70 mph. And at 5 PM, maybe 5-10 mph? But if you were were a drop of groundwater in that valley, traveling underground through the aquifer towards the LA River, you might be travelling 5 feet per year. And once you reached the narrows of the LA river, you’d pick up speed – maybe 1,300 feet per year. But even at that rate, it still makes the 5 PM commute on the 405 look like the Indianapolis 500.
There’s no doubt about it, groundwater moves s…l…o…w…l…y. But still, there’s quite a difference between 5 and 1,300 feet per year. What accounts for this difference?
There are two factors that determine the speed of groundwater through an unconfined aquifer: (1) the hydraulic gradient (slope) of the water table and (2) the rock composition of the aquifer.
(1) Hydraulic gradient (slope) of the water table. It’s gravity at work. Just like riding a bicycle, the steeper the slope, the faster the bike (or the water) moves. To visualize this, compare these two above-ground pictures – one of Tejon Creek and the other of Sandrock Creek. Which creek has faster moving water? Which creek has the steeper slope?
Similar forces are at work underground. If you recall last week’s story, an aquifer rests on top of a confining layer of impervious rock (an aquitard). The slope of the underlying confining layer is the minimum slope of the water table. To get an idea of how steep the slope can be, take a look at this rock outcropping near Castaic.
There are two other factors that can influence the slope of the water table: a change in the rate of recharge – water entering the aquifer and or a change in the rate of discharge – water leaving the aquifer, either through natural means or through pumping. Since an aquifer can extend for many miles, the water infiltrating the aquifer through the recharge zone can increase the slope of the water table. So can water being discharged or pumped out. Take a look at this illustration to see what I mean.
(2) Rock composition of the aquifer. “The rate of groundwater flow is controlled by two properties of the rock: porosity and permeability.” Porosity is the percentage of open space in the rock. In dense igneous rocks like granite, open space is created by the cracks and fractures within the rock. In sedimentary rocks like sandstone or limestone, open spaces occur in the spaces between the grains of the rock, as well as the open spaces created by fractures and cracks within the rock.
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Permeability is the ease at which fluid moves through a rock This depends on the conectivity of the open spaces between the rocks. The more connectivity, the more permeable the rock and the faster the water can move. Gravel is the most permeable; clay the least.
To visualize water moving through the aquifer, think of trying to drive to your favorite shopping center. You need parking spaces and sidewalks (open spaces) between the stores, but you also need roads (cracks and fractures). And you need connectivity between these parking spaces, sidewalks and roads.
Hydrogeologists have developed a mathematical model that describes groundwater velocity: Darcy’s Law. Here’s a non-mathematical description of that law: groundwater velocity = (hydraulic conductivity x hydraulic gradient) / porosity.
(And for you Jane Austin fans out there, there is no relation between the French hydraulic engineer Mr. Henry Darcy and the fictional character Mr. Fizwilliam Darcy in “Pride and Prejudice”.)
For an easy-to-read guide to groundwater hydrology, check out this publication from the University of California. Next week, we’ll talk about the factors that determine groundwater direction.
Upcoming Outdoor Events:
Saturday, September 10th, 5:00 – 7:00 PM, Changing Seasons Evening Stroll at Towsley Canyon. With summer on the waning edge, this is a great time to explore the park and see what seasonal changes occur as we move into fall. You’d be surprised how the flora and fauna cope with our summers and prepare for their upcoming rainy season. 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.
You can listen to stories like this every Friday morning at 7:10 a.m. on “The SCV Outdoor Report”, brought to you by your hometown radio station KHTS (AM1220) and by the Mountains Recreation and Conservation Authority.
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