Part 2: Why is there Poop in My Margarita? my subterranean mexican blues.

My vicarious experience with Montezuma’s Revenge over the Christmas holiday raised some questions. Where, I wondered, does Mexico get its freshwater supply? And how does fecal matter make its way into this water? And what does Mexico do to filter or process its water to make it fit for human consumption?

The answer to the first question lies in the area’s geology. The Yucatan peninsula is made out of a thick layer (hundreds of meters) of limestone. Limestone is fairly ubiquitous in tropical environments because it forms in warm shallow seas from the compacted shells of sea creatures. Later, during a period of uplift, the bottom of the sea is pushed up and becomes the ground beneath our feet, or bedrock. When limestone gets broken down into sand during the process of weathering, the result are those white sandy beaches that we are used to associating with warm island vacation spots. (The characteristic turquoise blue of the water is also due to the limestone sand).

In addition to gorgeous beaches, you’re also likely to find caves wherever you find limestone. This is because the rock is composed mostly of the mineral calcium carbonate, which dissolves easily in acidic water. Thanks to the combination of high rainfall and high atmospheric carbon dioxide, much of the Yucatan’s limestone bedrock has dissolved, resulting in an extensive network of underground caves, cracks, and fissures, and a landscape dominated by classic karst topography*.

Rainwater infiltrates quickly through the highly permeable limestone until it reaches the water table—the level below which all of the empty spaces within the rock are filled with water (a large body of rock with water in all of its empty space is referred to as an aquifer). The entire population of the Yucatan peninsula gets its fresh water from natural springs or wells that tap this supply of groundwater.

In much of the Yucatan peninsula—and especially near the ocean—the water table happens to be fairly close to the ground surface. On the one hand, this makes it easy to dig a well. On the other hand, the high water table means that pollutants from the surface seep quickly and easily into the groundwater supply—these pollutants include toxic industrial, medical, human, and agricultural waste. In addition to the pollutants’ “natural” flow into the aquifers, many communities on the Yucatan actively pump liquid waste underground as a method of disposal. Solid wastes are frequently burned and then added to unlined landfills, where contaminants are leached by rainwater and carried underground.

Furthermore, because the network of cracks and caves is so extensive throughout the Yucatan, once underground pollutants can spread far from their source, contaminating vast sections of the groundwater supply.

Considering these facts, it isn’t surprising that there’s poop in the water.

In the more industrial parts of the Yucatan, such as Merida—the peninsula’s largest city—there are chemicals whose toxicity far exceed that of a few pesky strains of e. coli. One recent study of the groundwater in Merida found high levels of cadmium, arsenic, lead, iron, and chromium, none of which are beneficial to human health.

The Riviera Maya—the area south of Cancun including Playa del Carmen, Tulum, and a string of resorts—has undergone rapid development over the past ten years, bringing both huge numbers of tourists and a large industry-support population to the area. It seems wildly irresponsible to engage in rapid development of an area such as this based on its natural beauty without taking into account the environmental and—ultimately—economic impact of that development. I believe this is a matter of putting the cart before the horse, of leaping before we look, and I believe we should know better than that in this twenty-first century of ours.

We find ourselves always moving forward into an uncertain future. Often, the nature of progress requires us to rush forward in some new direction before we have the capacity to understand the consequences of our actions. The problem, as I see it, arises when we do have the capability to explore the impact of some human venture or activity, but we choose not to. Certainly, this is the case with the state of freshwater in the Yucatan peninsula, as well as many other places in the developing and developed world.

It is well within the means of science to explore the current hydrogeological state of the Yucatan peninsula. Our findings could tell us how much water is there, and to what extent it is contaminated. More importantly, we could apply this knowledge to the sustainable development of the area. How much will it cost? Can we afford not to do it?


*A karst landscape is made out of limestone and has a characteristic dry, lumpy look to it. The lack of moisture is due not to a lack of rainfall, but to the high permeability of limestone rock—when it rains, the water infiltrates quickly downward until it reaches the water table. Underground cave networks give way and fall in on themselves, giving rise to an uneven, “lumpy” ground surface. Check out Ireland’s burren for some good-looking karst landscape.

Pictures:

Limestone Beach

Aquifer

Riviera Maya

References:

Sustainable Management of Groundwater in Mexico:
Hydrogeology of a contaminated sole-source karst aquifer,
Mérida, Yucatán, Mexico

The Oldest Stuff on Earth

The Cosmic Pathway (coolest name for an exhibit ever) at the American Museum of Natural History spirals down through thirteen billion years of universal happenings and drops its visitors off at the entrance to the Gottesman Hall of Planet Earth. The Hall of Planet Earth appeals enormously to my inner geology geek, with displays ranging from chunks of black smokers (hydrothermal vents on the ocean floor where sulfide deposits build up and where life, it is theorized, may have begun on Earth) to lava samples from around the world to parts of an ice core from Greenland that’s been crucial to our understanding of climate change. Thrilled as I am to get to see these things, I am always a little disappointed by the specimen that I think should be the coolest part of the hall: a zircon crystal, found in Australia in the 1990’s, that is one of the oldest known pieces of Earth, at 4.3 billion years.

Zircon is an impossibly resilient mineral that scientists love because it contains uranium, which is unstable and breaks down to lead over a known time period—thus, we can use radiometric dating to find out how old the zircon crystal is. In fact, the radioactive material inside zircon crystals sometimes destroys the mineral itself over millions of years by emitting radiation, so that the mineral's structure is destroyed and the crystal is discolored (picture: right). If you catch sight of one of these so-called ‘metamicts’ in the field, it looks just like a tiny radioactive bomb went off—a brownish dot surrounded by little fractures in the surrounding rock.

In the Hall of Planet Earth, however, the zircon crystal just looks like a little lump of rock, which for some reason disappoints me. I suppose it seems that a remnant of Earliest Earth should be more obviously imbued with some quality of specialness—some sort of iridescent, diamond-in-the-rough sparkle (perhaps the Hall of Planet Earth, while appealing to my inner geology geek, fails to appeal to my inner Lord of the Rings geek).

At any rate, the zircon is special because it is one of the oldest materials on Earth, having formed just a couple hundred million years after the planet itself (Earth’s estimated age is about 4.6 billion years). While the 4.3 billion year-old zircon was certainly a fun find for science, the mineral had out-survived the rest of the rock with which it formed—in other words, while the other minerals in the rock melted and/or deformed and became new rock again, the zircon mineral remained solid, eventually re-incorporating as part of a newer, younger rock.

What scientists have been searching for, then, is rock that is as old as the zircon crystals inside—rock that formed just as the Earth was settling out into layers and cooling to a temperature that allows rock to solidify (the geologic word for this earliest eon of Earth is the Hadean: Earth then was a fiery, violent mess replete with extraterrestrial bombardments and volcanic activity). Because Earth’s crust is so often created and destroyed in the generative and destructive processes of tectonics and erosion, really old rock is tough to find-- the oldest rock in New York City is about 1.3 billion years, and a good deal of the land below us is much younger.

Scientists working in northern Quebec on the Hudson Bay believe they may have found some of this very old rock—a little exposure of Earth’s proto-crust. The dated material in this rock (it’s an area composed mostly of a rock named faux-amphibole, whose chief mineral component is Cummingtonite….I remember the day in mineralogy class when a fairly humorless professor lectured on this very mineral-- suffice it to say that we geology students, ever fond of questionable-sounding jargon, had a tough time keeping our faces straight) is about 4.28 billion years old. If the rock itself is as old as some of the minerals inside, scientists have an opportunity to take a close look at some of the first crust that ever formed on Earth. In doing so, they can begin to answer questions about how crust formed back then and to compare Earth’s early geologic processes with the processes occurring today.

If the rock does turn out to be proto-crust from 4.28 billion years ago, I hope the Hall of Planet Earth gets a chunk of it-- even if it’s not much to look at.