The introduction of reactive nitrogen to the waters of the world is causing serious ecological damage. All the more important, then, are natural purification processes in the subsurface, which can eliminate at least a part of this pollution.
An international research team led by the University of Rennes (France), in which hydrologists from the University of Bayreuth and from the Helmholz Centre for Environmental Research – UFZ also participated, come to the conclusion that the capacity of many deep aquifers to remove nitrate is likely higher than is often assumed. In the renowned scientific journal PNAS, they present a robust method for assessing this self-purifying potential.
Reactive nitrogen enters the soil through agricultural fertilisers as well as from the atmosphere. There, a part of it is taken up by plants, but the rest, especially in the form of nitrate, leaches into deeper soil layers and finally reaches the groundwater. “The processes which take place in deeper soil layers, however, can hardly be comprehended using our usual measuring equipment. Hence it is usually difficult to determine how much nitrogen is transported down to the groundwater, and to the rivers it feeds”, says Dr. Tamara Kolbe, a scientist at Sweden’s University for Agricultural Science in Uppsala, and lead author of the study.
With the help of a new method, an international team of scientists has now managed to evaluate the purification potential of the subsurface.
The basis of the research work is data on the quality and age of groundwater from more than 50 groundwater wells in France and the USA. To the surprise of the scientists, in 80 percent of the wells there were signs of a significant breakdown of nitrate underground. Kolbe and her co-authors trace this to the existence of energized minerals in the subsurface. This connection can be explained by the ability of certain microorganisms to transpose nitrate instead of oxygen through respiration, i.e. breathing. In doing so, nitrate is transformed into harmless nitrogen gas, which constitutes the greatest part of our atmosphere. For this transformation process to take place, the microbes require a source of energy. As a rule, they find this in the organic carbon in soils (e.g. from plant remnants). Still, only a small part of this organic carbon reaches deeper aquifers. Typically, however, groundwater saturated rock is rich in energetic minerals like iron and sulphur compounds. Some microorganisms can utilize these rock minerals to remove nitrate, even after the organic carbon has long since been used up.
“This is very good news, for many reasons, including the fact that drinking water is commonly sourced at great depth from such aquifers”, explains Prof. Stefan Pfeiffer from the University of Bayreuth, and co-author of the study. From this finding, however, it does not follow that nitrogen fertilizers can be heedlessly spread on agricultural land in unlimited quantities. “The availability of mineral energy sources for the microbial breakdown of nitrate in the subsurface is finite, and the protective potential of the subsurface is therefore limited”, warns co-author Prof. Jan Fleckenstein from the Helmholz Centre for Environmental Research – UFZ.
The results of the study could also help explain why nitrate from groundwater can still enter flowing waters when its introduction into soils has long since been reduced or even stopped. In fact, polluted water can remain in circulation in the subsurface for long periods without meeting with the right conditions for nitrate breakdown. Hence, there can be a considerable time delay between more environmentally friendly land management and healthier ecosystems. “The methods we developed as part of our study allow us to better estimate the recovery periods for contaminated aquifers. This knowledge could also help those responsible in environmental politics from creating unrealistic expectations”, Tamara Kolbe adds.
Tamara Kolbe, Jean-Raynald de Dreuzy, Benjamin W. Abbott, Luc Aquilina, Tristan Babey, Christopher T. Green, Jan H. Fleckenstein, Thierry Labasque, Anniet M. Laverman, Jean Marçais, Stefan Peiffer, Zahra Thomas, and Gilles Pinay: Stratification of reactivity determines nitrate removal in groundwater. https://doi.org/10.1073/pnas.1816892116