How global geologic processes lace your glass of water with arsenic

More than 100 million people around the world today are exposed to dangerously high levels of naturally-occurring groundwater arsenic in sedimentary aquifers. However, the ultimate source of this stubborn bulk-arsenic in these aquifer sediments is unknown. Now, in a new study, researchers claim to have established the link between tectonics in the Andes and Himalayan mountain chains and local-scale groundwater pollution of arsenic influencing human health.

Taking a stab at answering why arsenic occurs in groundwater at some places on Earth but not others, researchers in India, Sweden, the United States, France and Australia found that major swathes of groundwater arsenic polluted areas across the world are mostly located in basins that run parallel to mountain ranges along convergent (tectonic) plate boundaries.

“Our study shows that only aquifers in sedimentary basins adjacent to mountain chains along present or ancient convergent tectonic margins are arsenic-enriched. The primary source of this arsenic may be traced to volcanic activities in these mountain chains,” study author Abhijit Mukherjee of IIT Kharagpur, India, told Mongabay-India.

For Asia, this basin adjacent to the Himalayas is the Indo-Gangetic plain, spanning India, Nepal, Pakistan, Bhutan, and Bangladesh. This plain is one of the most densely populated regions in the world.

Mukherjee pointed out that such global scale pattern (of arsenic presence in groundwater) can’t be a sheer coincidence and must be related to some processes inherent to the planet’s evolution, which we are yet to understand. This thread of reasoning led the researchers to propose the hypothesis of connectivity between global tectonic upheavals and locally reported groundwater pollution of arsenic.

“In order to get ground proof of this hypothesis, we have tried to integrate our two-decades of study-obtained data from Ganges basin (India) and Andes basin (Argentina and Bolivia), and have pretty much found a remarkable pattern of continuity,” Mukherjee said.

The key observation is that a particular sequence of geologic processes appears to control the cycling of arsenic, according to co-author Alan E. Fryar, professor, Department of Earth and Environmental Sciences, University of Kentucky.

Breaking down the sequence Fryar elaborated: “Where tectonic plates collide and subduction occurs, volcanic fluids or magmas form. As those fluids or magmas rise toward the surface, they leach arsenic from the continental crust. Volcanic ash and other rocks exposed at the surface above the subduction zone can be concentrated in arsenic.”

As mountain chains rise and those rocks are weathered, arsenic-bearing sediments are transported downstream along river valleys. Those sediments accumulate in basins where certain chemical reactions cause arsenic to be leached into groundwater in downstream aquifers.

Although tectonic activities take place on a continental scale and span millions of years, the groundwater arsenic problem is typically reported locally (districts of West Bengal, Bihar etc.) and over shorter spans of time (such as a decade). 

“But according to our hypothesis, the two things are intertwined, where these enormous geological processes actually influence drinking water in your groundwater well,” said Mukherjee.

“So, the global scale geological processes and setup do influence the groundwater quality at a local scale and the health of people consuming it,” he said.

The new contribution of the study to the existing scientific literature is a synthesis of data from multiple geologic settings around the globe where arsenic occurs and, by contrast, where it does not.

“This work helps by providing a conceptual model that may identify less-studied regions where arsenic may exist in groundwater,” added Fryar.

Co-author and groundwater chemist Prosun Bhattacharya of KTH Royal Institute of Technology, Sweden emphasised that more than 70 countries suffer from the scourge of arsenic. “All these occurrences globally are a function of geologic functions. By identifying the less-studied regions, we can monitor and check and inform about the arsenic level in these areas that have gone unnoticed,” Bhattacharya said.

Following the hypothesis, Mukherjee said the researchers experimented and predicted the presence of groundwater arsenic in certain places (such as in northeast India and parts of Jammu and Kashmir), where, previously, it’s occurrence is not very well known.

“We are studying these areas (such as the Brahmaputra basin in Assam, Indus basin in Jammu and Kashmir) for last four to five years, and again, we found the connectivity between the mountain-derived arsenic sources and the pollution of groundwater arsenic along groundwater flow paths from these sources,” Mukherjee said.

Sustainable arsenic mitigation

But finding safe water is another story in India where roughly 40 million people in India reside within the risk zone of arsenic contamination in states such as West Bengal, Bihar, Uttar Pradesh, Jharkhand, Assam, Manipur, Punjab, Haryana, Chhatisgarh and Karnataka. 

As per government records, as many as 15,811 habitations in India are arsenic affected.

India recently concluded its elections and constituted a Jal Shakti Mantralaya, integrating the Ministry of Water Resources, River Development and Ganga Rejuvenation and Ministry of Drinking Water and Sanitation and aims to provide potable water to every rural household by 2024 under the newly announced Jal Jeevan Mission, as the country grapples with the worst water crises in decades.

Read more on India’s environmental issues not getting desired attention in the budget

Prosun Bhattacharya, who spearheads SASMIT (sustainable arsenic mitigation), a collaborative project between partners in Europe and south Asia, highlighted the significance of tapping into local well-drillers to mitigate the lethal impact of prolonged exposure to high levels of arsenic in drinking water.

The strategy targets and assesses safe aquifers in regions with high arsenic groundwater through comparison of geological characteristics and their relation to high arsenic concentration. It revolves around identification and targetting safe aquifers through scientific validation of the local drillers’ perception. Local drillers are the main driving force in tube well installation and if they can target safe aquifers, it will play a significant role in arsenic mitigation programmes countrywide, according to Bhattacharya.

In order to identify and exploit safe aquifers, researchers are able to map the colour and textural attributes and the geochemical characteristics of the targeted aquifer sediments by linking them to the groundwater pH, redox and a series of water quality parameters in accordance with the WHO drinking water guidelines. 

“Local drillers use this tool based on sediment colour concept to zero-in on safe aquifers so as to ensure drilling tube wells in arsenic-safe zones,” added Bhattacharya.

Discussing SASMIT’s application in neighbouring Bangladesh, Bhattacharya said despite significant progress in the understanding of the source and distribution of arsenic, and its mobilisation through sediment-water interactions, there has been limited success in mitigation attempts in the country.

“At present, the main problem is the huge gap between arsenic exposure and the pace of mitigation,” Bhattacharya said, adding their study and related research may help bridge the gap.

We need not invent the wheel for safe and adequate drinking water

Geochemistry and water quality researcher Avner Vengosh of the Nicholas School of the Environment, Duke University, USA, who was not associated with the research, opined there are complexities linked to the arsenic problem when one scales down to local scenarios.

“On a global scale, this is the place (as observed in the study) where you expect to find arsenic, but on the local scale, it is much more complicated than that because it also reflects the chemistry of the water. There are many places that do not fit into the model. For example, in the United States there are aquifers that are not part of the belt and still have groundwater with high arsenic. It is a good paper but it depends on what scale you are looking at,” Vengosh told Mongabay-India.

Vengosh added that the problem is much wider than arsenic hotspots.

“Safe water supplied by municipalities is not actually safe from the wide-range of contaminants that are present. Due to the lack of adequate monitoring, we are blind in seeing that it’s much wider than those hotspots of arsenic,” he said.

Vengosh emphasised that while there is heightened attention to droughts and water crisis in India, he saw the crisis arrive in the country a decade ago because of poor water quality.

He batted for a major shift in the water quality monitoring system in India. 

“I don’t think we need to invent the wheel. There are many already established monitoring protocols such as in the U.S. or European Union that can be followed. Typically common water quality monitoring in India is based on the major elements that are not precisely measured in most cases and this is not enough,” he said.

Better scientific understanding of contaminants will also help shape better policies.

“How you collect and analyse samples and the manpower…  is all severely lacking. Once you have a better understanding of the distribution of the contaminants and the mechanism of their occurrence and then you can have a better policy to deal with water quality,” Vengosh said.

Spelling out the mechanisms through which water quality can be improved and groundwater conserved, Vengosh underscored the application of drip irrigation, reuse of wastewater for agriculture and enhanced adoption of reverse osmosis desalination for removing contaminants.

“It is inevitable that the water crisis needs to be managed on a larger scale and I hope after the elections in India, when the dust has settled, people will have time to look at the real issues like water,” he added.

CITATION     

Mukherjee, A., Gupta, S., Coomar, P., Fryar, A. E., Guillot, S., Verma, S., … & Charlet, L. (2019). Plate tectonics influence on geogenic arsenic cycling: From primary sources to global groundwater enrichment. Science of The Total Environment, 683, 793-807.