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Porous Rocks

They say you can't get blood from a stone, and that's true. But you can certainly get water through rock--at least, some kinds of rock--and that's a very good thing. If water couldn't flow through rocks we'd have no groundwater, that priceless and fragile natural resource relied on by billions of people.

Many rocks consist of tiny particles that slowly, slowly become compacted as they're buried. But spaces--the even tinier pores and channels between the particles--remain. Water can flow through these spaces and be held there. When that happens, the rock becomes part of the groundwater system, or aquifer, which is Latin for "water bearer."

Rock Solid?

The spaces between rock particles are only visible under high magnification; to the unaided eye the rock would look . . . rock solid. Rocks that make up good aquifers not only have pores, but pores that are interconnected. These connections allow the groundwater to flow through the rock.

Sandstone: Fine-grained rocks such as sandstone make good aquifers. They can hold water like a sponge, and with their tiny pores, they are good at filtering surface pollutants.

Dolomite: This type of rock can easily be dissolved by slightly acidic water. Underground caverns can develop in regions where dolomite or limestone is common--the water eats away the rock.

Granite: This rock consists of several different types of crystals that form in molten rock at high pressure and temperature. The crystals are tightly interlocked, so the granite isn't very porous.

When it Rains, it Drains

Recently, engineers have been developing types of concrete and asphalt that have pores, mimicking natural surfaces. Landscape architects are incorporating decorative bricks with drainage holes into their designs. Use of these surfaces in parking lots, patios, and driveways can help turn the problem of stormwater runoff into a benefit--aquifer recharge.

American Museum of Natural History

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