The study, published last week (20 August) in Proceedings of the National Academy of Sciences, found that the number of wells with arsenic contamination higher than ten micrograms per litre — the safe limit set by the WHO — is seven times higher in the deeper than shallower zones.
"The implication of these findings for the Mekong Delta region, and potentially other arsenic-prone aquifer systems like it, is that deep, untreated groundwater is not a safe long-term water source," the study says.
It adds that even deep wells that test as arsenic-free initially, may develop heavy contamination after around a decade, because of a pollution mechanism that has so far not been understood.
Arsenic occurs naturally in the Earth's crusts, particularly in areas of South and South-East Asia. Potential health effects include skin problems, cancer and cardiovascular disease.
Regionally, an estimated 100 million people are chronically exposed to arsenic poisoning through groundwater. High risk areas include the Irrawaddy Delta in Myanmar, parts of Indonesia's Sumatran coast, Bangladesh's Bengal Delta, and Vietnam's Mekong and Red River Deltas.
Vietnam's safe limit for arsenic is 50 micrograms per litre of water — five times the WHO limit.
Using groundwater data compiled by the government between 2002 and 2008, the study analysed an arsenic hotspot in Vietnam's portion of the Mekong Delta. In an area 50 kilometres from Ho Chi Minh City, where over 1,050 wells were contaminated, 84 per cent of them were in the zones deeper than 170 metres.
The findings appear to contradict recent surveys of arsenic hotspots in South Asia including from the WHO which reported that the percentage of arsenic-contaminated wells diminishes with depth.
"In general it has been assumed that deep groundwater would be safe to drink," says Steven Gorelick, a professor of hydrogeology at Stanford University, United States, and one of the study's authors. "This assumption is called into question [by the study]."
One reason for the contradiction may be that wells deeper than 200 metres have not been studied as extensively as shallower wells, the authors report.
They also say that the contamination may be due to the excessive pumping of groundwater, causing clay layers in the surrounding land to compact, squeezing out arsenic or arsenic-mobilising solutes, such as carbon, into the deep aquifers.
But this happens with around a ten-year lag, they say. "Deep wells that test clean upon installation, as do those bordering the [study's] focus area, may not remain arsenic-free over time as pumping promotes compaction and release of arsenic or arsenic-mobilizing solutes from deep clays."
To reduce the impacts of arsenic contamination from deep groundwater extraction, the study suggest that water managers should try to limit intensive extraction, and treat or blend extracted groundwater to meet health standards, among other strategies.
Because little is known about groundwater extraction and land subsidence in the Mekong Delta, the study is a "major contribution" to the field and a potential resource for Vietnamese policymakers, says Laura Erban, the study's lead author.
Le Anh Tuan, a research professor at Cần Thơ University, Vietnam, who was not involved in the research, tells SciDev.Net the study will be a good point of reference for scientists in the future.
He adds that Vietnam's government is aware of the risks of over-exploiting national groundwater resources, but does not have the budget, technology or alternative water sources to address the problem effectively.
Given these limitations, Tuan says, the typical approach in Vietnam has been to focus on reducing the number of new wells that are drilled and to expand efforts to analyse the scope of the damage.
Link to full study in Proceedings of the National Academy of Sciences
This article has been produced by SciDev.Net's South-East Asia & Pacific