A Stanford University study conducted in Cambodia has shed new light on the natural introduction of the poison arsenic into groundwater – an established problem in Cambodia that could be exacerbated by hydrological development, particularly dams, researchers say.

According to a report on their findings, published in the journal Nature Geoscience on Friday, the Stanford team carried out experiments on wetlands in Kandal province’s Kien Svay district in order to better understand how water is contaminated by the lethal toxin here and in other parts of South and Southeast Asia.

They discovered that in wetlands that went through the natural cycle of wet and dry seasons, arsenic was entirely absent or at very low levels. But in wetlands that were saturated all year round, with no dry period, the tiny microbes capable of producing the killer substance released much more arsenic into groundwater.

The findings could help predict how development might affect arsenic levels, which in South and Southeast Asia are frequently 20 to 100 times higher than World Health Organization-recommended limits, said professor Scott Fendorf Stanford’s School of Earth, Energy & Environmental Sciences.

“If you change the hydrology of a region by building dams or levies that change the course of the water, or if you change agricultural practices and introduce oxygen or nitrate into sediments where they didn’t exist before, that will alter the release of arsenic,” he said.

Cambodia’s first large hydropower dam, Kamchay in Kampot province, came online in 2011, and several more are planned for Mekong tributaries, which environmental scientists have suggested could lead to serious social and environmental impacts not fully appreciated by the government.

And arsenic in groundwater has already proven itself a problem in the past. UNICEF has estimated that in Cambodia alone 2.25 million people are at risk of arsenicosis (arsenic poisoning), and in 2013 it reported there were more than 500 cases of the incurable disease, which it said had led to numerous amputations and fatalities.

According to Stanford University, more than 100 million people have been poisoned by drinking arsenic-laced water in South and Southeast Asia.

A man pumps arsenic-contaminated water in Kandal province last year.Heng Chivoan

Arsenic historically entered Cambodia’s water sources after being washed down from the Himalayas, where it was bound to iron oxide compounds in rocks. These compounds were deposited in lowland basins and deltas in the form of sediments.

If there is a lack of oxygen, bacteria in these sediments start to break down arsenic and iron oxide particles to breathe, and in so doing, separate the arsenic from the iron oxide and transfer it to the groundwater.

Stanford researchers found that when wetlands experienced a dry season, the bacteria would consume all the available food – mostly digestible plant material – during this dry period. During these periods, the bacteria also enjoyed access to plenty of oxygen, removing the need to break down the arsenic-laced iron oxide.

Moreover, when the wet season returned, there would be no food left for the bacteria to eat, leaving nothing to fuel the separation of arsenic from iron oxide.

“The work suggests that, under normal conditions, microbes in seasonal wetlands don’t pose a significant threat for adding arsenic to groundwater,” Stanford University said in a press release.

“The team conducted a second type of experiment, in which they simulated the conversion of a small, remote seasonal wetland into a permanent one by digging out a seasonal wetland plot and keeping it permanently filled with water. As predicted, this resulted in the release of arsenic.”

Responding to the research, the team leader for non-communicable diseases and environmental health at the World Health Organisation in Cambodia, Steven Iddings, said current and planned developments along Cambodia’s main river risked making Cambodia’s arsenic problem worse.

“Damming of the Mekong and tributaries, combined with climate change, will change the hydrology of Cambodia in a way that could impact the release of arsenic into the ground water,” he said.

Arsenic release, he added, was just another “down-the-road impact” of damming and climate change.

Additional reporting by Alessandro Marazzi Sassoon