Now geoscientists have discovered that the answer could lie all around them, according to a paper published today in the first issue of the Geological Society of London's new journal, Geochemistry, Exploration, Environment, Analysis.

Zimapán, 200km north of Mexico City, has been a mining district since the 16th Century. Lead, silver and zinc have been extracted from mineralized ores related to Tertiary-age igneous intrusions.

These natural sources contaminate some water supplies, while others may be polluted by modest quantities of rainwater leaching through mine tailings. Some tailing leachates have a pH of 2, and As concentrations of almost 16 g/litre.

About half of the water supply samples tested by Mexico's National Water Commission have As concentrations above current WHO guidelines (0.01mg/litre).

In 17% of samples, levels exceeded 0.05mg/liter, (the Mexican drinking water standard, and the former WHO guideline). The wells used for municipal water supply are heavily contaminated, and even after dilution from unpolluted water sources, still have concentrations up to about 0.4mg/litre, with likely local health consequences.

Over 40% of valley residents are not connected to this municipal supply and rely on local springs and norias (bucket-wheel wells) for their drinking water. Unfortunately many of these are also polluted with As.

In a region where 72% of the population earned less than US$3.00/day in 1994, commercial point-of-use purification systems lie well beyond their reach.

In 1994, the Lois Ongley (Androscoggin Valley Environmental Center, Lewiston, USA) and Aurora Armienta (Instituto di Geofisica, Universidad Nacional Autonoma de Mexico) the paper's lead authors, established a collaborative international research experience project for undergraduate students, funded by the US National Science Foundation.

The team created experimentally contaminated water (ECW) by shaking pure water with samples of mine tailings.

This had a concentration of 0.6mgAs/liter. The ECW was then reacted with samples of various local rocks. The experiments demonstrate clearly that As is reduced below detectable levels in any sample of ECW that has been mixed with rocks of the local Soyatal Formation.

(Other experiments tested the effects of different grain size of rock sample, reaction time, and degree of agitation.)

The Soyatal Formation, which crops out at surface throughout the area of contamination, is a calcareous shale containing up to 15% clay minerals (kaolinite and illite). Both of these are known to adsorb As. The finding is supported by the fact that water sources emerging through the Soyatal Formation are uniformly low in As.

Where commercial purification mechanisms (which use ion exchange to resins, green sand filtration and reverse osmosis) are too expensive, this low-tech mechanism would be well within the pockets of local residents, requiring no more sophisticated equipment than a bucket.

The researchers believe that one or two kilograms of crushed rock, added to about 20 liters of contaminated water, stirred frequently over 24 hours, will effectively remove As to below acceptable levels. However, they warn that further research will be needed, including further quantification of the effect and field testing, before residents should try it for themselves at home.

(Editor's Note: For full text, go to this URL and type "Ongley" in the search engine to obtain a PDF version.)

[Contact: Dr. Lois Ongley]

02-Apr-2001