River-groundwater hot spot for arsenic release

Naturally occurring arsenic contamination has been traced by a team of Australian and Swiss scientists using 50-year-old clues from the nuclear age.

Researchers from Flinders University, CSIRO and the University of Western Australia, with the Swiss Federal Institute of Aquatic Science and Technology have used computer modelling to integrate years of data into computer simulations mimicking interactions between groundwater flow, solute transport and geochemical reactions.

The models helped scientists analyse field observations to unravel which chemical and physical processes play a role, and to predict the behaviour of arsenic within aquifers. The findings help indicate where and when pollution might occur in the future.

The results of the study have now been published in the latest issue of Nature Geoscience.

The research team selected a highly arsenic-polluted site near Hanoi in Vietnam to develop and test the computer model.

The team measured tiny concentrations of tritium that had entered the groundwater system from the atmosphere after nuclear bomb testing, and its decay product helium, to reconstruct the speed and direction of groundwater flows over the last five decades.

Once the model simulations matched the measured concentrations, the model simulated how arsenic was mobilised and transported in the aquifer.

Over the years Hanoi has had to markedly increase groundwater extraction to satisfy steadily increasing water demand; this showed to be the main trigger for arsenic pollution in the aquifer.

The computer modelling allowed the researchers to pinpoint the source of arsenic as the Red River muds regularly deposited at the river’s slow-flowing zones.

The organic matter contained in the muds fuelled a biogeochemical reaction that led to the release of arsenic which then travelled into the aquifer underlying the Van Phuc village, a process that continues to this day.

Employing their developed computer model in predictive mode, the researchers were able to illustrate the interplay of four key factors on the evolution and longevity of arsenic release at surface water/groundwater interfaces, namely: the abundance of reactive organic matter; the abundance of iron oxides; the magnitude of groundwater flow and the river mud deposition rate.

Original publication: Ilka Wallis, Henning Prommer, Michael Berg, Adam Siade, Jing Sun, and Rolf Kipfer (2020). The river-groundwater interface as a hotspot for arsenic release. Nature Geoscience. https://doi.org/10.1038/s41561-020-0557-6

Source: Flinders University

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