Contamination of groundwater with naturally occurring (geogenic) arsenic (As) is a serious problem extending throughout the world. Many mechanisms responsible for As mobilization from sediments to groundwater have been proposed, some more general, others rather specific for certain locations. Our study adds a new twist to this discussion and offers an explanation for the observed As release that seems to be relevant for many aquifers where methane (CH₄) concentration is high.

Groundwater at our study site in Vietnam (about 15 km southeast from Hanoi) showed a remarkable abundance of CH₄. This turned out to be true also for neighboring villages, where in some cases the concentration of this potent greenhouse gas was so high that apparently some wells could even be set on fire. These observations were the motivation for our study. Microbes need an electron donor such as carbon (C) to catalyze reductive dissolution of As-bearing Fe(III) minerals that then leads to As mobilization. It has been shown that organic carbon infiltrating from the earth's surface into the subsurface as well as organic carbon already present in the subsurface can fuel this microbial process. However, how about CH₄, the simplest hydrocarbon, consisting of one fully reduced carbon atom? A compound that is very abundant at our field site and, obvious to us after careful literature research, also at other As-contaminated field sites. Could microbes gain enough energy using the CH₄ dissolved in the water to reduce Fe(III) minerals and release the As that was bound to them? Our first experiment that lasted 100 days unfortunately did not support our hypothesis. After intense discussions in our research team and after other studies on microbial CH₄ oxidation coupled to Fe(III) reduction were published showing that microorganisms mediating this process are rather slow, we had decided to repeat this experiment. This time, our experiment lasted much longer, 225 days (nearly a third of my Ph.D.—it was all or nothing!). To our big excitement, the second round indeed supported our hypothesis and demonstrated that CH₄ can be a driver for microbial As mobilization. Therefore, now, we are adding another mechanism that can lead to As groundwater contamination.

Many of us, particularly those living in countries where efficient water treatment facilities ensure access to safe drinking water, may not be concerned. Yet, in many places around the world, particularly in the rural areas of South and Southeast Asia, access to safe water is a privilege rather than a basic human right. In consequence, the health of up to 220 million people might be compromised. 

Water is the major component of the human body, and regular supplies of about 1.5 L of water per day is required for it to function properly. It is therefore not surprising that water is deemed as the most essential natural resource. However, access to this precious resource is often taken for granted. In consequence, water reservoirs are currently under huge pressure. Besides contamination with As, freshwater systems are threatened by human activities related to agriculture, industry, or mining. Future generations will have to face yet another danger—climate change. Water stress will continue to increase as a drastic decline in rainfall and increasing salinization of surface and near-surface water is expected. Following the WHO, by 2025 half of the world’s population will be living in water-stressed areas. As a result of decreasing access to safe and clean water, many people think that the 21^st century’s conflicts will be fought over water. Therefore, protection and sustainable use of this vital resource should be a priority for all of us.

For more information see: Glodowska, M., Stopelli, E., Schneider, M., Rathi, B., Straub, D., Lightfoot, A., Kipfer, R., Berg, M., Jetten, M., Kleindienst, S., Kappler, A. & AdvectAs team members. Arsenic mobilization by anaerobic iron-dependent methane oxidation. Commun Earth Environ 1, 42 (2020). https://doi.org/10.1038/s43247-020-00037-y