Glimpse the future of water
Hiking the Sierran high country, following a stream to its source, we come to a small, wet mountain meadow, nestled snug to the base of a cliff. Springs issue from the cliff, watering the meadow. The stream itself seems to spring right out of the ground.
Whence that water – from where did it come?
Most of the world’s water resides in the oceans – calculations put it at more than 97 per cent.
The ancients knew this: Homer sang of the ocean as the ultimate source of all streams and seas, springs and wells.
But how does water get from the oceans to the mountains?
Raindrops and snowflakes are rather insubstantial things, so common sense led to some sort of underground passages – tunnels and such – that allowed ocean water to move through the continents.
In the 17th century, scientists totted up just how much rain and snow fell in the course of a year – enough, it turned out, to supply the world’s streams and seas, springs and wells, and a fair amount of evaporation, too.
Evaporating off the oceans, off the land and vegetation, water – as vapor – blows over the land, condenses, falls as rain or snow, and percolates into the ground, from which it can emerge to form springs and streams. Groundwater is water that fell from the sky.
If it doesn’t evaporate, or become bound up in plants or animals, water eventually makes it back to the ocean, completing the hydrologic cycle.
Needing water for factories and farms, perhaps even to drink, we sink wells and pump groundwater to the surface.
Gases – air, for instance – can be compressed. But liquids are incompressible. A mechanic pumps hydraulic fluid into a lift to raise a car over his head.
To let the car down, the mechanic drains the fluid back out.
Underground rivers are rare. Groundwater usually flows through fractures in the rock, or from one pore to the next. Water in those fractures and pores supports the rock around it. And just as a car comes down when the mechanic drains the fluid, sucking water from rock allows the rock above to subside.
Mexico City is built on a stack of ancient lakebeds, the remains of lakes that came and went as the climate grew wetter or drier. The porous lake sediments hold massive amounts of water.
Or used to.
In Mexico City, an unusual “monument” stands in the middle of town. It is a pipe. Maybe eight or so inches across, it rises, straight up, 20-plus feet above the ground.
The pipe rests on solid bedrock. The ground around it, however, rests on old lake sediments – sediments whose pores were once filled with water. But the water has been pumped out, and the rains that fall in the mountains and recharge the aquifer have not been able to keep up.
Unable to support the ground, the ancient lake beds have collapsed, taking the ground with them. The top of the bedrock-supported pipe rests at the same elevation it did years ago. The ground that once was level with the top of the pipe has fallen more than 20 feet.
As it sucks its aquifers dry, Mexico City sinks.
Sidewalks buckle, buildings tilt, pipes break. At the other end of the water system, wastewater that used to drain out of the city must now be pumped uphill.
Many regard Mexico City as a glimpse into the future of the world’s water situation. How to respond to this situation engenders fierce (though, to the public, nearly invisible) debate among governments, corporations, environmentalists and those who simply drink the stuff.
Alan Stahler trained as a biologist and is an amateur astronomer. He teaches private enrichment classes for students of all ages. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).
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