Groundwater

Here's something wild. There's way more fresh liquid water hiding underground than in all the rivers and lakes on Earth combined, about 30 times more. It's called groundwater, and if you live in eastern Idaho, there's a good chance it's what comes out of your faucet.

When rain or snowmelt soaks into the ground (infiltration), it trickles down through tiny spaces between grains of soil and rock.

Two properties decide how well this works:

• Porosity: how much open space a rock or soil has. A sponge has high porosity.
• Permeability: how easily water can flow through those spaces. Gravel is very permeable. Clay is not; its spaces are too tiny and not connected.

A rock can have lots of space but still not let water move. Clay is like a parking garage with no exits.

Dig straight down and you'll pass through:

• Zone of aeration: the upper layer, where spaces hold both air and some water.
• Water table: the top of the soaked zone. Its depth changes with the seasons, rising in wet times and dropping in droughts.
• Zone of saturation: below the water table, where every space is filled with water.

An aquifer is a layer of permeable rock or sediment that holds and moves groundwater. Think of it less like an underground lake and more like a giant soaked sponge made of rock.

A layer that blocks water, like clay or solid granite, is called an aquitard (or confining layer).

The Eastern Snake River Plain Aquifer is one of the most productive aquifers in the world. It stretches under about 10,800 square miles of southern Idaho. Water moves through cracks and gaps in layers of basalt lava. It supplies drinking water for hundreds of thousands of people and irrigates a huge share of Idaho's farms. Some of the water takes decades or even centuries to travel through it.

At Thousand Springs near Hagerman, that aquifer water bursts right out of the canyon walls of the Snake River in spectacular waterfalls.

• A well is a hole drilled below the water table so water can be pumped up.
• A spring is where groundwater naturally flows out at the surface.
• An artesian well taps an aquifer under pressure, so water rises on its own, sometimes even gushing out without a pump.

If people pump water out faster than nature refills it, the water table drops. Wells can run dry, and in some places the ground actually sinks. Refilling an aquifer is called recharge.

Groundwater is slightly acidic, so over thousands of years it can dissolve limestone. That creates:

• Caves and caverns: underground hollows, sometimes huge.
• Stalactites and stalagmites: icicle-shaped mineral formations. Stalactites hang tight to the ceiling. Stalagmites might reach the ceiling someday.
• Sinkholes: when the roof of a cave collapses, the ground above suddenly drops. Florida is famous for these.

When groundwater sinks near hot rock or magma, it heats up and rises again as a hot spring. Idaho has more soakable hot springs than almost any other state.

A geyser is a hot spring that erupts. Water deep in a narrow tube gets super-heated, flashes to steam, and blasts out. Old Faithful in Yellowstone, just next door, erupts about every 90 minutes on average.

Pollution from fertilizer, pesticides, leaky septic tanks, and spilled chemicals can soak down into aquifers. Cleaning an aquifer is extremely hard, so preventing pollution in the first place is key.