Arsenic in Nepal's Terai Groundwater: District-by-District Status

Arsenic is a naturally occurring, tasteless and odourless element that leaches from Himalayan sediments into the shallow aquifers of Nepal's Terai (southern plains). Because it cannot be seen, smelled or tasted, contaminated tube-well water looks perfectly clean, which is why systematic laboratory testing is the only reliable way to know whether a well is safe. Long-term consumption is the danger: arsenic accumulates in the body over years, so the harm from drinking mildly contaminated water is slow and cumulative rather than immediate.

The stakes in the Terai are large. Roughly 47% of Nepal's population lives in the Terai, and an estimated 90% of Terai households rely on groundwater as their main drinking-water source, drawn from more than 200,000 shallow tube wells installed across the region. Tube wells were promoted from the 1980s and 1990s as a clean alternative to bacteria-laden surface water, and they did cut diarrhoeal disease sharply. The unintended consequence was mass exposure to naturally occurring arsenic in the aquifer, a pattern Nepal shares with neighbouring West Bengal (India) and Bangladesh.

Two numbers frame every discussion of safety. The WHO provisional guideline for arsenic in drinking water is 10 micrograms per litre (10 ppb), while Nepal's Interim National Drinking Water Quality Standard sets a higher permissible limit of 50 micrograms per litre (50 ppb). One microgram per litre equals one part per billion, so the two thresholds are often written simply as 10 ppb and 50 ppb. Water between 10 and 50 ppb is legal in Nepal but exceeds the international guideline, so it is best treated as a caution zone rather than fully safe.

Between roughly 2005 and 2008 (about 2062-2065 BS), the National Sanitation Steering Committee and the National Arsenic Steering Committee, working with the Department of Water Supply and Sewerage (DWSS), UNICEF, the Environment and Public Health Organization (ENPHO), WaterAid Nepal and other partners, carried out a nationwide 'blanket' testing campaign. Blanket testing means attempting to test every tube well in the affected districts rather than sampling a few, and it produced the most complete picture Nepal has of where arsenic occurs.

The campaign analysed on the order of 737,000 groundwater samples across the Terai and inner-Terai districts. According to the summary reported by the National Arsenic Steering Committee, about 92.6% of tested wells were below 10 ppb, about 5.6% fell in the 10-50 ppb band, and about 1.8% exceeded Nepal's 50 ppb national limit. In round terms, roughly one well in fourteen (about 7%) exceeds the stricter WHO guideline, while roughly one in fifty-five exceeds the national standard.

Those percentages sound modest, but they translate into hundreds of thousands of people. Government and research estimates put around 0.5 million Terai residents at risk from water above 50 ppb, and a 2011 government estimate suggested that some 2.7 million people, nearly 10% of Nepal's population at the time, were drinking water above the WHO 10 ppb guideline. Contamination is also highly uneven: it is concentrated in specific districts and even specific villages, so a national average badly understates the risk faced by people living in a hotspot.

Arsenic contamination clusters along the central and western Terai. The most intensively studied and most affected districts are Nawalparasi, Bara, Parsa, Rautahat, Rupandehi and Kapilvastu. Across these districts, the share of tested tube wells exceeding safe limits has been reported to range from roughly 2% in Rupandehi at the low end to more than 25% in Nawalparasi at the high end, illustrating how sharply risk varies over relatively short distances.

Nawalparasi (now split into Nawalparasi East / Nawalpur and Nawalparasi West / Parasi) is consistently identified as Nepal's arsenic epicentre. Depending on the survey and the threshold used, roughly a quarter of Nawalparasi wells fail the WHO 10 ppb guideline and on the order of 15% exceed Nepal's 50 ppb national limit. The eastern-central Terai districts of Bara, Parsa and Rautahat form a second cluster of concern, while parts of Rupandehi, Kapilvastu, Banke, Bardiya, Kailali and Kanchanpur show lower but still locally significant contamination.

Because these figures come from different studies conducted in different years with different well counts, they should be read as indicative of relative risk rather than as a single fixed ranking. The consistent, durable finding across all sources is the ordering: Nawalparasi is worst, the central Terai (Bara-Parsa-Rautahat) is a secondary hotspot, and the far-western and eastern Terai are generally less affected but not risk-free. The only way to know a specific well's status is to test that well.

Nawalparasi has drawn the most scientific and media attention because contamination there is both widespread and severe. A 2008 government survey found that nearly 4,000 tube wells in the district exceeded the 50 ppb national standard, with an additional roughly 4,400 wells in the 10-50 ppb band; together these wells were relied upon by an estimated 140,000 people. Individual wells have tested extraordinarily high: water in Manari village was recorded at around 600 ppb, about sixty times the WHO guideline.

The southern belt of Nawalparasi West (Parasi), close to the Indian border, is the most heavily affected. Here the combination of shallow tube wells, sediment geochemistry that releases arsenic into groundwater, and near-total dependence on that groundwater has produced Nepal's clearest concentration of arsenic-related illness. Surveys in affected wards have found visible skin symptoms of arsenicosis in a substantial minority of long-term users.

Nawalparasi's experience shaped Nepal's national response, including the piloting of arsenic-removal filters, deep tube wells and community water supplies. It remains the reference district for understanding both the health toll of chronic arsenic exposure and the practical challenge of delivering safe water to dispersed rural households.

Chronic arsenic poisoning, known as arsenicosis, develops over years of drinking contaminated water. The earliest visible signs are skin changes: dark spots (melanosis) on the chest, back and limbs, and hardened, thickened patches (keratosis) on the palms and soles. These lesions are the classic markers health workers look for during field screening in the Terai, and their presence usually signals years of exposure.

Beyond the skin, long-term arsenic exposure raises the risk of cancers of the skin, bladder and lung, as well as cardiovascular disease, chronic lung disease, diabetes and adverse effects on child development and pregnancy. Because these outcomes take years to appear and have many other causes, the full health burden of Terai arsenic is difficult to count precisely and is likely underestimated. Studies in Nawalparasi have reported arsenicosis-related skin disease in the low single-digit percentages of exposed populations, with higher rates among the most heavily exposed households.

Two points matter for households. First, children and pregnant women are especially vulnerable, so reducing their exposure is a priority. Second, arsenicosis has no simple cure; management centres on stopping further exposure by switching to a safe water source, improving nutrition, and seeking medical follow-up for skin lesions or other symptoms. Prevention through safe water is far more effective than treatment after illness appears.

You cannot judge arsenic by looking at, smelling or tasting the water, and it is unrelated to whether the water is clear or the well is new. The only reliable answer comes from a laboratory or field test of that specific well. During the blanket survey, wells were tested and the result recorded for each spout so households would know their status; if your well was tested and marked, that mark reflects the reading at the time of testing.

If you do not know your well's status, arrange a test. District water-supply offices (DWSS), local government units, ENPHO and non-governmental water programmes in the Terai can test or advise on testing, and field kits can give a same-day indication. Because arsenic levels can change over time and vary between neighbouring wells, a well that tested safe years ago is worth re-checking, and you should never assume a neighbour's result applies to your own well.

As a practical rule while you arrange testing: if your well has ever tested above 10 ppb, treat the water as a caution and prioritise a tested-safe alternative for drinking and cooking. Water above 50 ppb should not be used for drinking or cooking at all. Arsenic-affected water is generally still acceptable for washing, bathing and laundry, since the main exposure route is ingestion rather than skin contact.

There are three broad strategies for getting safe water in an arsenic-affected area, and the best choice depends on local geology, cost and how many households share a source. The first is switching to a deeper aquifer: deep tube wells often draw water with much lower arsenic than shallow wells, but 'deep' is not automatically 'safe', and deep wells must still be tested before use because some deeper waters are also contaminated or carry other quality problems.

The second strategy is household or community arsenic-removal filters. Nepal's best-known technology is the Kanchan Arsenic Filter (KAF), developed from 2002 by an MIT-ENPHO team as a modified biosand filter that adds a bed of iron nails; rust released from the nails binds arsenic, which is then trapped in the sand. Field studies report arsenic removal in the range of about 85-95% when the filter is built and maintained correctly. Other options used in the region include SONO-type filters and other iron- or activated-alumina-based units. All filters need regular maintenance and periodic re-testing to confirm they are still working.

The third strategy is turning to alternate sources entirely: protected dug wells, rainwater harvesting, gravity-fed or piped surface-water supply, and community treatment plants that serve a whole village. Where feasible, a shared safe source is often more sustainable than thousands of individual household filters. In practice Nepal's response has combined all three - marking unsafe wells, distributing filters, drilling deeper or alternative wells, and building piped schemes - because no single measure fits every hamlet in the Terai.