On her wedding night, Gita Paul felt doomed. Her parents had arranged her marriage to a man she had never seen who lived in Kolsur, an impoverished village kilometers from her own home in this landscape of rice paddies and cattle paddocks and clusters of homes, near the eastern Indian city of Kolkata. Arranged marriages to strangers are common in the region. But when Gita laid eyes on her husband, she was horrified to find him covered in open lesions and scabs. Then she met his family. An elder brother had lost a foot to rot, a sister was sickly and another brother had died in his 30s. Many people in the village were ill. “I'd never seen anything like it,” Gita says, years later, during an interview, sitting on the rough steps of her family's tiny brick-and-mortar home. “I thought it was a contagious disease.”

By the time scabs appeared on Gita's skin, she had heard that the illness was not airborne—it was in the water. Scientists had come by with simple testing kits and with the bad news that the cool, clear water from the village wells was making people ill. It was poisoned with arsenic. Gita decided that she and her husband had to move. They spent all the money they had to relocate to a neighboring farming village. But people there were also dying, and villagers said the wells there were tainted, too.

The scientists and the villagers were right. In many of the region's villages, people were unwittingly poisoning themselves by drinking the water and cooking meals and washing their dishes in it. At least 140 million people in Asia are drinking arsenic-contaminated water. It pours out of countless tube wells, hand-cranked pumps attached to plastic or metal pipes sunk into the earth. More than 18 million of these small wells have been dug across India alone, often by hand, over the past three decades, according to government census figures. They were sunk in an attempt to bypass surface waters teeming with illness-causing bacteria or infused with industrial runoff. But death also lay underground.

The naturally occurring arsenic kills human cells, leading first to skin scarring and then, as it slowly builds up in the body, to brain damage, heart disease and cancer. Arsenic-laced groundwater has been found in at least 30 countries, from Argentina to China, Cambodia and Vietnam, as well as parts of Canada and the U.S.

Now the ever expanding use of groundwater wells—people need water to drink, and farmers need it to grow crops to feed a large population—has been making things worse. Pumping out this water has changed the courses of underground streams, so previously clean water now flows through arsenic-laden sediments, and wells that used to be pure in villages once healthy suddenly pump out catastrophes.

Scientists have recently been trying something new: mapping the underground landscape in an attempt to pinpoint safer places to sink the wells. But so far the subterranean flow changes and rates of chemical reactions have been outpacing the predictive ability of the maps. “It's a pathetic situation. It's just desperate,” says Dipankar Chakraborti, an environmental analytical chemist who has devoted 28 years to studying the problem at Jadavpur University in Kolkata, where he formerly headed the School of Environmental Studies. The university is now working to set up a research institution in his name—the DC Research Foundation—to further arsenic studies. “We're changing things so quickly underground, we can barely keep up.”

The well problem

Wealthier areas affected, such as the southwestern U.S., have the money and means to filter their water. But many of the worst-hit populations are also the world's poorest. In South Asia—considered one of the highest-risk zones—groundwater steeped in arsenic runs across a densely populated swath of land covering parts of India, Nepal and Bangladesh. Although the World Health Organization says arsenic concentrations are dangerous above 10 micrograms per liter of water, India's legal maximum remains 50 micrograms per liter—and many wells test far above even India's lenient standard.

The troubles in India date to the 1960s, when the country began drinking groundwater as an alternative to being poisoned by bacteria-infested surface water, often stagnant and unprotected from sewage or agricultural runoff. In 1969 India launched a $125-million program, aided by international groups such as UNICEF, to drill holes into the earth and sink more than a million simple wells. More programs followed. There seemed little other choice. India had almost no infrastructure for storing, distributing or filtering water—a situation that remains today, except in the largest cities.

The tube wells were hailed as an inexpensive and lifesaving solution. Of India's 1.25 billion people, about 80 percent of the rural population and 50 percent of the urban population use groundwater for drinking, cooking, and irrigating crops and garden plots. Groundwater solved another acute problem: famine, which had threatened parts of the nation through the 1980s. Today India uses a staggering 91 percent of its irrigation water to grow rice, wheat and sugarcane.

But the agricultural boon came with consequences. Most tube wells were sunk to depths of 50 to 200 meters, stopping when they reached the first layer of bacteria-free water. Unfortunately, this depth corresponded precisely with where most of the region's arsenic is found, something not known at the time. Drill a bit farther, and the water is usually potable. But drilling deeper wells takes more time and money, requiring sturdier materials that many impoverished villagers cannot afford.

There have been other obstacles. Widespread ignorance and institutional apathy stymie efforts to educate people about the risks. Seemingly simple solutions, such as harvesting rainwater or treating water on the spot, prove too complex for the illiterate and are easily misunderstood. Harvesting efforts break down as plastic tarps and pipes are poorly maintained. Filtering water though sand-filled buckets is often seen as an onerous and time-consuming chore. Treatment tablets passed out by scientists and activists are misused by villagers who cannot read instructions or understand the chemistry. More permanent solutions, such as large-scale filtration plants that could eliminate the guesswork for millions, have proved both costly and technologically cumbersome, suffering many of the pitfalls of poor oversight.

“The best solution, of course, is to avoid the contaminated water altogether,” says Michael Berg, who heads water contamination research at the Swiss Federal Institute of Aquatic Science and Technology, known as Eawag. “But compared with surface water contaminated with pathogens, the groundwater is seen as the lesser of two evils.”

Geology of a killer

Arsenic is a relatively common element. Tasteless, colorless and odorless, it was long a favorite tool for assassins. It is toxic to most life-forms, even at very low doses.

The plains below the Himalayas are some of the most arsenic-rich lands on earth. After the giant mountains were formed through tectonic collisions, arsenic-laden pyrite minerals in their slopes were exposed and eroded away by swift-flowing rivers, whose waters carried the sediments through India, Bangladesh, China, Pakistan and Nepal. As the dissolved arsenic churned through the water, it underwent chemical reactions to combine with oxygen and iron or other heavy metals, forming granules that fell to the riverbed and leaving striated layers of arsenic-infused soils at random depths. Over millennia the muddy deposits built up the ancient deltas of the Ganges-Meghna-Brahmaputra plain—now a densely populated area of some 500 million people covering almost 700,000 square kilometers of land.

By the natural order of things, most of that arsenic should have stayed underground. But the tube wells tapped into it, even in places where rivers no longer run. “You can't just look where the rivers are flowing now,” says Chakraborti, tracing out water paths with his fingers while sipping coffee from a laboratory beaker in his university office, where filing cabinets and visitors shelter under a green canopy of potted plants. “You have to consider how the rivers' paths have changed. At one point, this was all awash in water. That means there are a lot more possibilities for finding arsenic.”

Not all of the earth's arsenic is leached from soils into water; certain geologic conditions must be present. Scientists studying the problem have outlined two general scenarios that prompt arsenic's release, and this understanding has opened possibilities for predictive modeling on probable risk.

The first scenario—alkaline arsenic release—takes place in oxygen-rich soils, where water with an alkaline high pH is circulating, such as in arid regions of Argentina or the southwestern U.S. The water triggers a chemical reaction that breaks apart oxidized iron and other metals coating soil particles. That frees any arsenic that had bonded to those charged molecules, allowing it to dissolve and contaminate surrounding groundwater.

The second scenario—reductive arsenic release—takes place in soil low in oxygen but rich with organic carbons. These conditions are typical in deltas, floodplains and river basins, where surface soil is often new enough to still be infused with bacteria. These conditions correlate with some of the world's most populated lands, including northern India, Bangladesh and Southeast Asian countries such as Vietnam. In this case, the bacteria promote the chemical reactions using catalyzing enzymes to bust up iron oxides on which arsenic has bonded. So if one were to take a handful of soil from an area with arsenic-free groundwater—say, in North Carolina—and bury it in Bangladesh, it would release arsenic.

The process goes on as long as there is enough organic carbon to feed the bacteria, growing more scarce at greater depths. Fertilizers, which are used heavily in India, can prolong the process. It can be mitigated by salinity, particularly sulfides, which will also bond with arsenic to create precipitates. But that only holds for as long as the oxygen remains low. Any new oxygen introduced can be used by bacteria to break up the sulfides and, again, release any bonded arsenic. So if aquifers are depleted and recharged at a rapid pace, sending freshly oxygenated water trickling back into the ground, that circumstance can prompt a new wave of arsenic release. Recharging aquifers is also common in India, providing perfect conditions for the ongoing release of arsenic over long periods.

Mapping the danger

At the moment, most contaminated wells are found through a time-consuming, labor-intensive process of going village to village and testing each well with a chemical-reaction field kit. After the water is mixed with several reagents, a testing strip is inserted into a sealed container to absorb any arsenic released. The color of the strip after about 10 minutes gives a ballpark result: white indicates the water is clean; red shows it is tainted.

But the field kit offers only a blunt test that can detect contamination to a certain level. Beyond that or for more detail, the water needs to be tested in a lab.

Because the crisis is so widespread, inspectors rarely detect the problem in time, instead arriving at wells years after people have been drinking arsenic-laden water. So some scientists have begun to seek shortcuts, studying satellite images of land contours and mapping water flows to predict the types of sediments underground and to show where arsenic is most likely to be found. They say such methods can help governments save money and time by narrowing the number of tube wells that need testing, or they can raise red flags in areas previously thought safe.

In 2006 Berg and other scientists at Eawag began creating a global map of arsenic worldwide based on early predictive models built on parameters such as soil content, land slope and water flow. They published the first draft of their global risk-probability map in 2008 and plan a newer version incorporating the latest studies and more details soon.

These models “can make predictions where no testing exists,” says Berg, who has been leading the effort. For example, his team was able to predict that large areas of Sumatra, Indonesia, were in danger. “Then we went and tested, and our prediction was confirmed. That gave us a lot of confidence that this predictive modeling was not so bad.”

In 2013 the China Medical University teamed up with Eawag to build a China model after tests on some 445,000 tube wells in 2001–2005 revealed about 5 percent were contaminated above the Indian legal limit of 50 μg/L; many more exceeded the more conservative safety level set by the World Health Organization. With vast areas of the country still untested, the team wanted to help policy makers take action. “There is a barrier between science and society. Somehow we need to show policy makers that we can help solve real problems,” says Luis Rodríguez-Lado, a chemist now at the University of Santiago de Compostela in Spain and lead author on the paper, published August 2013 in Science. The China model, when compared with actual tube-well measurements, correlated 77 percent of the time. Such data, Rodríguez-Lado says, can help save lives, money and time by highlighting which tube wells need testing: “That's hugely satisfying for any scientist.”

There are limits, however. Because the models are based on surface conditions and recent knowledge about water flow, they are poor predictors of the contents of more ancient and unknown underground water bodies. “Our predictions are always related to what you see at the surface,” Berg says. “If they are related to older deposits, we cannot catch that.”

Building the models on accurate and up-to-date information is crucial for avoiding mistakes, Rodríguez-Lado says. He had gone into the China study assuming the model would follow alkaline conditions, with oxygen-rich soil and basic water, based on China's arid landscape and rainfall patterns. “Most of China was classified under oxidizing conditions for arsenic release,” he says. “But the information from China was poor,” and he quickly realized China's aquifers were anoxic, like those in India and Bangladesh. When he recalculated using these parameters, accuracy improved.

There are other limitations to predictive mapping, particularly in resolution. The China risk model set its grid sizes at 25-by-25 kilometers—too large to predict which villages will be hit. “The models can be useful, but they're not quite there,” says geochemist Alexander van Geen of Columbia University's Lamont-Doherty Earth Observatory. “Suppose a model predicts there is a 20 percent chance of arsenic in a certain area. Well, I'd still want to test my well, right?”

Failed solutions

Governments have attempted other ways to solve the water-supply problem, but they have failed to make a dent. A few years ago the West Bengal state government built a pipeline to carry Kolkata's arsenic-free municipal water east to rural villages. But the water runs only a few hours a day, if at all, and it does not reach every village. The black, plastic pipes are poorly maintained, and many lie broken, spilling water through jagged holes into muddy puddles by the side of the road.

Hundreds of arsenic-removal plants, each costing an average of about $1,500, have been installed across both West Bengal and neighboring Bangladesh. Chakraborti and others have shown that the simple, cylindrical filter mechanisms have been largely ineffective. One study found that only two of 13 plants from several manufacturers maintained arsenic levels below the Indian standard. None reliably met the WHO standard. By the time the study was published in 2005, that barely mattered: poor maintenance and oversight meant only three out of a total 18 plants were still working.

Digging deeper tube wells to bypass current layers of contamination is not only an expensive task, stretching village resources, but Chakraborti's research shows it is only a short-term fix. The low-lying aquifers, at 200 meters under the surface, are partly blocked off from higher, poisoned ones by a thick clay barrier. “Partly” is a key caveat. There are cracks and holes. So drawing from greater depths may buy time, but deadly waters can eventually trickle down and contaminate what is below.

This is already happening in India, where groundwater use is so intense that 60 percent of the country's aquifers will hit critical levels in 20 years unless the pumping is drastically curtailed, the World Bank says. In the Bengali village of Jaynagar, Chakraborti found once safe arsenic concentrations at eight tube wells had jumped drastically to perilous levels in just five years, from 1995 to 2000.

Arsenic also can move horizontally, from one dirty aquifer to a neighboring clean one, as well as up and down if the water pressure between the two reservoirs changes. This movement is currently imperiling Hanoi, which pulls its water from an arsenic-free aquifer that used to flow away from the city. That flow pushed water from a neighboring contaminated aquifer away. But as the Vietnamese metropolis has grown, it has drawn more and more water from the safe layer, and the change has reversed the flow. Water from the tainted aquifer, near the Red River, has begun to pour into the city's previously clean one. This is cause for worry, van Geen says, but he notes that so far the problem is developing slowly. His study found that arsenic is moving 16 to 20 times more slowly than the water itself, presumably still bound to other elements in the soil and only gradually being freed by underground chemical reactions.

In India, things have moved much faster, accelerating with the size of the country's population and the effort to feed its people. Hardly anyone is policing a 1986 law barring excessive use of groundwater. Even where fields are located next to lakes or rivers, farmers irrigate with groundwater. Even when no water is needed, landowners pump what they can to sell on the black market. And the arsenic is getting into the food chain. It is in the rice. It is in the cow milk and buffalo meat. Chakraborti has even found it in bottled sodas and vials of sterile water used by hospitals.

Struggling for safety

So although researchers agree on the problem and the cause, van Geen says that “what isn't clear is what to do about it.” Along with Chakraborti and others, he believes that while predictive modeling may be useful, it cannot replace the need for testing tube wells on the ground.

Van Geen advocates using inexpensive testing kits at the sites of the wells. They are not as accurate as lab tests but give immediate results at minimal cost. He has also uncovered a potential job market in testing. A study of 26 villages in the state of Bihar found that about two thirds of residents were willing to pay 20 rupees, or about 30 U.S. cents, for someone to test their wells.

“We can't handle all these private wells, so our angle is to promote a network of testers and give them a financial incentive for providing tests,” van Geen says. In Bangladesh, he and his colleagues managed to have enough wells tested and plotted with GPS location data to build a dynamic map of the country's safe and unsafe wells so villagers can easily find safe water.

Follow-up research also suggests that villagers who pay for testing are more likely to heed the results and switch to safer though less convenient wells, according to one of van Geen's research partners, hydrogeologist Chander Kumar Singh of TERI University in New Delhi. The two are also studying how socioeconomic factors such as income or caste identity might keep people from using safe wells shared by other castes or those with less money. “We haven't seen much concern from the government,” Singh says. “Maybe some of our work can help point the way.”

Chakraborti similarly has trained helpers to travel by bicycle or train to villages to collect well samples. He has organized international conferences and led teams of doctors, students and activists out to do health checks. He has also set up a fund to cover his research and free water tests for poor residents. And when Chakraborti's credentials fail to impress villagers, he sets aside his distaste for India's old hierarchies and plays up his upper-caste Brahman heritage: he dons the white loincloth and sacred threads worn by Brahman holy men and points families to currently safe wells. “I hate it, but I'll do it,” he says of the stunt. “All I have to do is get through to the mother. Then I know the family will be okay.”

In Gita's village, her frail husband, Srivas, struggles with headaches, constant pain and exhaustion. Calloused lesions cover his body, and his skin stings, especially in the sun. There is no known cure for arsenic toxicity. There are no drugs to reverse the chromosomal damage done. Chelation therapy, which involves injecting bonding agents into the blood, has been used historically in extreme cases of metal poisoning. But it is highly risky and prohibitively expensive in India. The best most can manage is to eat nutritious food and stop ingesting the poison. Still, Srivas counts himself among the lucky. He has a teenage son who helps to haul buckets of healthy water from a nearby hospital clinic, and Gita earns the family's income in her job as a maid.

“I have no complaint against anyone,” says a trembling Srivas, reflecting a fatalism that is so common among India's poor that some scientists worry it actually keeps villagers from searching for cleaner wells. “Even if I did want to complain, there is no one to listen.”