New study reveals underestimated pathway for arsenic pollution in water

A team of scientists has uncovered a hidden risk to freshwater quality that could affect millions worldwide: the shift of lake and river sediment from trapping toxic arsenic to releasing it, driven by the widespread loss of aquatic plants. Their research, recently published in Energy Environment Nexus, shows that the decline of submerged macrophytes, vital underwater plants, can fundamentally change how arsenic moves through aquatic environments, posing an unanticipated threat to water safety.

From Pollution Shield to Pollution Source

For decades, submerged macrophytes like Vallisneria natans have played a quiet protective role in many freshwater ecosystems. Their roots actively release oxygen into the sediment, which encourages beneficial chemical reactions and microbial activity. This causes arsenic, a notorious toxin linked to cancer and other health issues, to become trapped in iron plaques that form around the roots. As a result, sediment under healthy aquatic plants acts as a sink, keeping arsenic from leaching into the water.

However, the new research has found that when these plants die, a phenomenon now accelerating worldwide due to water pollution and climate change, the situation reverses. Without living roots to provide oxygen, sediments switch from aerobic (oxygen-rich) to anaerobic (oxygen-poor) conditions. This environmental shift dissolves the iron plaques and sets arsenic free, transforming the sediment into a source of contamination that releases toxic compounds back into the water above.

“Our experiments revealed a dramatic transition,” said Dr. Cai Li, lead author of the study. “During plant growth, arsenic is effectively immobilized. But after plant death, as the roots decompose, arsenic mobility can increase several-fold, creating localized hotspots of contamination.”

High-Resolution Microscale Insights

The research team utilized advanced microscale sampling and high-throughput genetic sequencing to closely monitor arsenic, iron, and microbial activity in the sediment, both before and after plant decay. They observed that after plant death, the abundance of iron-reducing bacteria surged by over 80 percent, contributing to a nearly 90 percent loss of arsenic once bound within the root’s protective iron plaque. Genetic analysis indicated a sharp drop in bacterial genes responsible for turning the most toxic forms of arsenic into safer ones, leading to greater risk of arsenic remaining available and mobile in the environment.

These findings suggest that the decline of submerged aquatic vegetation could tip formerly stable sediments into new pollution hotspots, potentially elevating arsenic levels in water and increasing exposure risks for people, wildlife, and crops.

Implications for Lake Management and Restoration

According to the authors, submerged macrophyte coverage in global lakes has plummeted by more than 30 percent in just the past two decades. As these crucial plants vanish, their protective ecosystem functions are lost, and so is the sediment’s capacity to keep arsenic locked away.

“Most water restoration projects rightly focus on removing pollutants, but our results show that losing aquatic vegetation can trigger unexpected water quality threats,” explained co-author Dr. Shiming Ding. “Restoration strategies must address both the removal of contamination and the long-term stability of plant communities in lake and river sediments.”

The researchers recommend adding materials with oxygen-releasing or iron-stabilizing properties to sediment surfaces in lakes undergoing plant loss, to slow or prevent the release of arsenic and other hazardous elements. Their work provides a new perspective on how aquatic plant growth and decay affect pollution risks, highlighting the importance of ongoing monitoring and habitat protection.

Warning Sign for Aqueous Arsenic Pollution

The study’s authors hope their findings will help agencies, managers, and the public recognize the “double-edged sword” of aquatic plant beds: as sinks under healthy conditions, but as sources of pollution after plant loss or death. Protecting and restoring submerged macrophytes should now be seen as a critical part of any long-term water quality management plan, particularly in regions with known arsenic contamination.

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Journal reference: Li C, Ma X, Jiang X, Gong Y, Wang X, et al. 2025. Unveiling an overlooked pathway of water arsenic contamination: microscale evidence of enhanced arsenic mobility from the rhizosphere to detritusphere of macrophytes. Energy & Environment Nexus 1: e008     https://www.maxapress.com/article/doi/10.48130/een-0025-0003   

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About Energy & Environment Nexus:
Energy & Environment Nexus is an open-access journal publishing high-quality research on the interplay between energy systems and environmental sustainability, including renewable energy, carbon mitigation, and green technologies.

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