A major breakthrough in natural decontamination methods for groundwater

Pollution of rivers and groundwater by agricultural runoff, and nitrates in particular, has severe environmental consequences. In 2017, the French scientific community came together to solve the problem of eutrophication, a process caused by this type of pollution affecting many aquatic systems around the world. Since then, research has focused on improving our understanding of how groundwater tables operate when faced with this type of pollution. Following a significant development in the area, we interviewed Gilles Pinay, biogeochemist and research director at Irstea Lyon.

At the behest of the Ministries of Agriculture and the Environment, you led a collective expert report on eutrophication.1 Could you spell out the causes and effects of this phenomenon and describe the current situation?

Gilles Pinay:The damage caused by eutrophication, such as loss of biodiversity or blooms of occasionally toxic algae, is the result of over a century of nitrogen and phosphorus discharge into the environment. The culprits are industrial waste, household waste inadequately treated by wastewater treatment plants, and more significantly, agriculture, which has long invested in nitrogen fertilizers (nitrates) to guarantee high yields.
Measures have been taken to curb the process (restricting slurry spreading, reducing inputs, etc.). Unexpectedly, however, the situation has not improved very much over the past two decades. This observation can be partly explained by the role of groundwater in water flow and the nitrogen it transports. Recent studies have in fact shown that large amounts of nitrates remain in groundwater for years, or even decades. These historical nitrate deposits are gradually re-released into rivers, which limits the long-term impact of any measures implemented.

You just participated in a study looking at the role of groundwater and aquifers, the geological formations in which groundwater is found, in the water denitrification process.2 Published in January 2019 in PNAS,3 this study uncovered a significant result. Could you explain?

Gilles Pinay: We know that the nitrates contained in water can be broken down by specific bacteria that live in aquifers.4 In the absence of free oxygen, they can decompose nitrate molecules to use the oxygen released by this chemical reaction. Our aim was to gain a better understanding of these denitrification properties and evaluate them. To achieve this, we developed an original method that combined hydrogeological modeling (simulating the flow and residence time of water in the aquifer) with the use of biogeochemical tracers (molecular nitrogen, oxygen, nitrates, CFCs and sulfates that flow through the groundwater in their dissolved form). Using this approach, we recreated the flow of water through aquifers and analyzed the distribution of denitrification zones.

What specific advantages does this new method provide?

Water flow diagram
Diagram representing the flow of water through aquifers and denitrification zones (in red). @Irstea

Gilles Pinay: Using regular samples taken from wells and the analysis of tracking indicators, it was possible to locate denitrification zones, areas low in oxygen where the bacteria were required to use oxygen from nitrates to breathe, within the aquifers. In particular, we were able to determine whether these were distributed evenly, closer to the surface or deeper within the aquifer. Where we saw that the denitrification zones were on the surface, we surmised that nitrates would be filtered quickly as the water's flow path to these zones is short. This type of aquifer would therefore be relatively well protected from pollution and efficient at decontaminating water before it returns to the rivers. Conversely, if the denitrification zones were mainly located deep within the aquifer, the water's flow path would be longer. Moreover, not all the water would go through the zones. This meant that some of the water leaves the aquifer without being treated.
This information is significant since it can be used to predict the pollution risk of groundwater as well as to determine whether nitrates can be eliminated from a polluted aquifer and how long this will take. This data is crucial in developing facilities: by identifying the highest risk groundwater aquifers, it is possible, for example, to choose those that are best adapted to drinking water abstraction.

Following on from this significant development, what are you working on next?

Gilles Pinay: As we have just seen, groundwater plays a key role in water quality, particularly during low-water periods when rivers are at their lowest and are fed by the groundwater. At Irstea Lyon, we will continue our work in order to gain a better understanding of both surface and subterranean water flows through river basins with significant agricultural activity in order to better grasp the deterioration and improvement processes involved in water quality in these regions. A methodological development project currently submitted to the ANR and a project on quantitative and qualitative resource management at the head of agricultural river basins that we are currently submitting to Europe (H2020) should help us achieve this.

For more information

1- Produced by CNRS, Inra, Ifremer and Irstea, the expert report involved around 40 experts from various fields and was based on an analysis of over 4,000 scientific publications. It aimed to provide an overview of eutrophication, designed to help public authorities in their decision-making process.
2- Nitrate decomposition.
3- Kolbe T., de Dreuzy J.R., Abbott B.W., Aquilina L., Babey T., Green C.T., Fleckenstein J.H,Labasque T., Laverman A.M., Marçais J., Peiffer S., Thomas Z., Pinay G. Stratification of reactivity determines nitrate removal in groundwater. PNAS, Jan 2019. 201816892;10.1073/pnas.1816892116.
4- Geological structures that are often heterogeneous, formed of fractured rocks, stones or sand, within which groundwater circulates.