Saturday, January 24, 2009

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:
References:

Wednesday, January 21, 2009

La Navidad Mexicana, Part 1. Or, When You're Sliding Into First...


Every Christmas holiday, the members of my family uproot from their various corners of the continental U.S. and converge on an area of the Greater Northern Detroit suburbs known as Lake Orion, where our parents have lived for the past ten years. Michigan is, in general, a flat state, and Lake Orion is characterized by a distinctly two-dimensional element—an element amplified by the single, white, depthless cloud that covers the sky like a non-retractable dome for the majority of the wintertime. The only notable topography in the area is a giant landfill by the highway, across from a sign that reads “Lake Orion: Where Living is a Vacation.”

And then there’s the weather. In Michigan, in late December, there are always variables—it may or may not be snowing, it may or may not be windy—but it is always cold. Really, really cold.

Regardless of the fact that we always have an excellent time in Michigan over Christmas, the family decided this year to try for a spot where living might actually feel like a vacation. And so, instead of blustery Michigan, we met up in Akumal, Mexico for a week of sunshine, relaxation, and fun—and a little diarrhea.




Everyone in the family is pretty private about these kinds of things, so I don’t really know exactly who got it and exactly how bad it was, but I know it made, at least, a brief pass through the villa.

Which made me wonder: Why are people always getting sick when they go to Mexico? Why aren’t we supposed to drink the water? What’s in the water there that makes so many visitors to the area sick? And why aren’t the native Mexicans sick? And what is diarrhea, anyway?

In Mexico, Traveler’s Diarrhea goes by the less formal name “Montezuma’s Revenge.” The name comes from the Aztec king Moctezuma whose empire fell to the Spanish in the mid-16th century (historical details are foggy, but what’s clear is that Moctezuma welcomed the Spanish explorer Hernan Cortes into his capital city of Tenochtitlan and ultimately got totally screwed by him. Sound like a familiar story?). The idea is that old Moctezuma is taking revenge on subsequent interlopers by afflicting them with vigorous bouts of nausea, cramping, vomiting, and diarrhea.

There’s also a more scientific explanation.

Traveler’s Diarrhea affects about 40% of people who visit Mexico, and is prevalent in many other underdeveloped nations around the world. While there are several bugs (parasites, bacteria, and viruses) responsible for TD, the most common culprit is a strain of e. coli referred to as enterotoxogenic e. coli, or ETEC. While it shares a proclivity for causing diarrhea with its notorious relative O157:H7, ETEC is not a deadly strain of e. coli. The good news is that most strains of e. coli are completely harmless (many take up permanent residence in our colons); the bad news is that where there is e. coli, there is poop. Which means that-- even if you don’t like to think about it-- there’s a little bit of fecal matter in the water in Mexico. (We’ll get into why that is later).


In order to understand how ETEC causes diarrhea, let’s make like an e. coli-tinged ice cube and take a hurried trip through the human GI tract:

Food begins to break down in our mouths, with the physical work of chewing and the chemical work of enzyme-laden saliva. The saliva also makes the food wet and mushy, so that it slides with relative ease* down the esophagus and into the stomach, where it hangs out for several hours. Here, gastric acid breaks the food down into a gooey, milky substance called chyme. This acidic environment also serves to kill off a number of potentially harmful bacteria.

From the stomach, the chyme makes its way into the small intestine by way of the pyloric sphincter. The small intestine is a hotbed of digestive activity where food gets broken down by a rich cocktail of bile, pancreatic juice, and digestive enzymes. The folded walls of the small intestine are covered in millions of little projections (think of a trimmed Koosh ball) that increase the surface area of the organ and allow for increased absorption of nutrients (converted glucose, fats, and amino acids). Excess liquids are also absorbed through the walls of the small intestine.


After the small intestine, food makes its last stop at the large intestine, or colon. Thanks to trillions of bacteria, this is where poop really becomes poop. The bacteria creates gas, turns the waste brown, and makes it smell like—well, shit. The colon is also theoretically where fecal matter becomes solid, due to absorption of liquid through the walls of the large intestine…

But here we need to take a step back and remember that we’re talking about diarrhea rather than semi-solid, healthy, normal feces. If the enterotoxogenic e. coli in your margarita’s ice cubes survives the gastric juices of the stomach and makes it to the large intestines, it produces a toxin that prevents the absorption of liquids and instead stimulates the opposite process. Instead of sucking moisture out of your poop, the walls of your large intestines let water in.

You know the rest of the story.

*The motion of food through the entire digestive system is aided by the action of peristalsis—the contraction and relaxation of muscles in the organ’s walls. Think of squeezing a tube of toothpaste or, perhaps more accurately, squeezing raw sausage out of it’s intestinal casing. If you’ve ever felt the urge to go to the bathroom right when you start eating, it’s because this action in the esophagus triggers a wave of peristalsis throughout the whole GI tract—all the way to the colon.

Pictures:
Digestive System
Michigan Winter
Akumal
E. Coli
Chyme

References:
The CDC
The Way We Work, David Macaulay
Textbook of Bacteriology