Arsenic in Bangladesh; sharing wells

Many of the wells used for drinking water in Bangladesh and other South Asian countries are contaminated with natural arsenic, affecting an estimated 100 million people. Arsenic is a cumulative poison, and exposure increases the risk of cancer and other diseases.

Is my well safe?

One of the challenges of reducing arsenic exposure is that there’s no easy way to tell if your well is safe. Kits for measuring arsenic levels exist (and the evidence is that aresenic levels are stable over time in any given well), but we and other groups are just beginning to make these kits widely available locally.

Suppose your neighbor’s well is low in arsenic. Does this mean that you can relax? Not necessarily. Below is a map of arsenic levels in all the wells in a small area (see the scale of the axes) in Araihazar upazila in Bangladesh:

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Blue and green dots are the safest wells, yellow and orange exceed the Bangladesh standard of 50 micrograms per liter, and red and black indicate the highest levels of arsenic.

Bad news: dangerous wells are near safe wells

As you can see, even if your neighbor has a blue or green well, you’re not necessarily safe. (The wells are located where people live. The empty areas between the wells are mostly cropland.) Safe and dangerous wells are intermingled.

Good news: safe wells are near dangerous wells

There is an upside, though: if you currently use a dangerous well, you are probably close to a safe well. The following histogram shows the distribution of distances to the nearest safe well, for the people in the map above who currently (actually, as of 2 years ago) have wells that are yellow, orange, red, or black:

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Switching and sharing

So if you are told where that safe well is, maybe you can ask your neighbor who owns that well to share. In fact, a study by Alex Pfaff, Lex van Geen, and others has found that people really do switch wells when they are told that their well is unsafe. We’re currently working on a cell-phone-based communication system to allow people in Bangladesh to get some of this information locally.

General implications for decision analysis

This is an interesting example for decision analysis because decisions must be made locally, and the effectiveness of various decision strategies can be estimated using direct manipulation of data, bypassing formal statistical analysis.

Other details

Things are really more complicated than this because the depth of the well is an important predictor, with different depths being “safe zones” in different areas, and people are busy drilling new wells as well as using and measuring existing ones. Some more details are at our papers in Risk Analysis and Environmental Science & Technology.