Legacy phosphorus

From the 1950s, the Seine, like all rivers in France, suffered the consequences of the growth of cities and agriculture intensification. Massive quantities of chemical fertilizer and wastewater containing nitrogen and phosphorus, were released into the river network. Today, and since the 2000’s, wastewater treatment plants remove phosphorus from wastewater before it reaches the rivers. As a result, the quantities of phosphorus released into the environment have drastically decreased. However, the problem has not been completely resolved.

In the framework of the PIREN SEINE programme, Guillaume Morin and his team (IMPMC -CNRS/UPMC/IRD) are interested in the phosphorus stored in riverbeds, and in trying to discover if it will remain trapped there forever, or cause further disturbances.

This video follows one of the X-Ray Fluorescence Spectroscopy experiments carried out by Guillaume Morin at the LUCIA beamline of Synchrotron SOLEIL, as part of this topic.

Legacy phosphorus

Audio Transcription

VOICE-OVER
From the 1950s, the Seine, like all rivers in France, suffered the consequences of the growth of cities and agriculture intensification. Massive quantities of chemical fertilizer and wastewater containing nitrogen and phosphorus, were released into the river network. 

Gilles Billen - biogeochemistry researcher and research director of the National Center for Scientific Research (CNRS)
The main source of phosphorus in rivers, was not so much caused by agriculture but by sewage wastewater that instead of being spread in gardens or fields was concentrated in a sewage system to then be discharged into rivers.
This massive contamination causes disorders in aquatic environments, expressed as algal blooms: known as eutrophication. 

VOICE-OVER
Today, and since the 2000’s, wastewater treatment plants remove phosphorus from wastewater before it reaches the rivers. As a result, the quantities of phosphorus released into the environment have drastically decreased.
However, the problem has not been completely resolved. 

Live (G.B) : It's odd, phosphorus, because we should be rid of it! Yet, it's still here, as much in agricultural soil as well as in sediments. That's the problem. It has accumulated. Now the issue is how this legacy is little by little being put back into circulation. 
This is when we need you ! 

VOICE-OVER
Guillaume Morin and his team are interested in the phosphorus stored in riverbeds, and in trying to discover if it will remain trapped there forever, or cause further disturbances. To find out, the researchers have focused their attention on sediment. 

Guillaume Morin
To sample sediment for our study, we set up downstream of the Paris region, in the second curve of the Seine, just upstream of the large wastewater treatment plant in Achères, which collects and treats all wastewater from the Paris region. Sediment is taken from the riverbank with a manual core sample removal tool. It consists of a steel core tube with a corer of approximately fifty cm which as soon as it is taken out of the water is immediately placed in anoxic conditions (i.e. without oxygen), to preserve the chemical forms of phosphorus. 

VOICE-OVER
The research to trace phosphorus in the sediment is conducted at synchrotron SOLEIL. There, this complex medium, composed of minerals and organic matter produced by living beings will be explored and mapped. Great care is taken to not expose the samples to air, due to the risk of oxidizing certain constituents and losing the information researched. 

Guillaume Morin
In there we research the minerals and possibly the main molecules that carry organic phosphorus. And then how it changes after the deposit, we know that major biological reactions occur during the deposit. One of the questions is whether, during these reactions, the phosphorus mainly remains trapped in the sediment or whether, on the contrary, a part returns to the water column. 

VOICE-OVER
Sediments are host to a multitude of bacteria, which by breathing, literally transform the environment. Phosphorus, which until then had been fixed by the organic matter or imprisoned by the minerals, can be released and reach the river. To glean this information disseminated in the sediment,
"X-ray fluorescence" is of great help to the researchers. 

Guillaume Morin
What interests us, for example, might be the phosphorus that adheres to the surface of very small minerals - iron oxides - which when deposited will undergo chemical reduction phenomena, related to bacterial activity: iron respiration by bacteria,which will release the Fer2 in solution and then eventually release phosphorus. 

Live (G.M) : We can't really see any iron there and there we can... Is there calcium with it?

Guillaume Morin
The interest of the "LUCIA" beamline is that we have a beam of a few micrometres, so we can scan the sample with this beam to find information at each point, each pixel, on the chemical composition of the sample and return to certain points of interest where phosphorus is found. 

VOICE-OVER
The results of these experiments will enable better understanding of the phosphorus cycle and to anticipate its impact on the lives of rivers and their inhabitants. An indispensable tool for better management of algal bloom risk. 

Gilles Billen
Harmful algal blooms can still happen, less than before, but there is still a risk. Consequently, we can only truly be free of the problems of eutrophication when we have truly mastered the release of these legacy forms in agricultural soil as well as in sediments, streams and lakes.