The context of soil reception
- As part of the administrative authorizations for the reception of excavated soil on its sites in Villeneuve-sous-Dammartin (77) and Annet-sur-Marne (77), between June 2020 and April 2021, ECT received soil that complied with the requirements of the decrees but was potentially pyritiferous. This soil came from the digging of the Grand Paris Express metro lines.
- Since 2021, ECT has been in contact with the Seine-et-Marne Prefecture, the Ministry of Environment and the Geological and Mining Institute (BRGM). These exchanges have validated the consideration of the risk and the analysis methodology deployed by ECT.
- Two prefectural decrees of July 2022 have taken up and set out the procedures, controls and analyses conducted by ECT. They provide for two annual drilling campaigns.
What is the pollution risk?
Detection process
It is difficult to detect the presence of pyrite; if it is not seen directly in these sedimentary formations, most often, there is only indirect evidence of its presence due to oxidation. This ore may become contaminant only after oxidation, when exposed to water and air for a long time.
Chemical principles and measurements of oxidation
Pyrite (FeS2) can oxidize in the presence of air and water, which can generate an acidification of the medium. This oxidation is characterized by an increase in the rate of sulfide and an acidification of the medium.
The analyses must measure:
- The sulphur content
- The acidogenic potential = AP
- The neutralizing potential = NP
The NP/AP ratio is then studied to evaluate the risk of acidification of the environment. If this ratio is greater than 2, there is no risk for the receiving environment.
What are the results of the numerous analyses conducted on the sites?
The studies conducted so far show:
- Very high NP/AP values demonstrating the strong neutralizing power of the analyzed environment: at least 4 times the value that characterizes the absence of risk
- The total absence of acidification of the medium. Consequently, the absence of oxidation.
How was this soil managed by ECT?
- The soil received was used immediately and recovered quickly. The exchange phase with the air was therefore very short, about a few days. ECT receives large quantities of soil on these sites from traditional building sites in the Ile-de-France region, mostly composed of marly limestone, which is abundant in the region. The limestone (basic) neutralizes the risk of acidification.
- The techniques of implementation of the excavated soils on the ECT sites are very protective and neutralizing as regards the risk of pyrite oxidation.
Considering the risk of pollution of water catchments downstream of the Annet-sur-Marne and Villeneuve-sous-Dammartin sites: what is the situation?
An essential environmental and health issue.
The assessment and consideration of risk are systematically examined during the site authorization procedure. In the case of potential pyritiferous soils, the specific framework for taking the risk into account is as follows:
- The 2 sites are outside the perimeter of protection of water catchments.
The analyses of the received soils established on each of the sites show the absence of environmental acidification: there is thus no risk of a transfer towards the drinking water catchments. - For each of the sites, water analyses are conducted within the framework of the exploitation authorizations.
The storage of the soil in high racks and their rapid recovery guarantees:- an absence of peripheral runoff during the reception phase
- no contact between rainwater and potentially pyritiferous soil once the cell is covered
- The soil inputs occurred between June 2020 and April 2021. Throughout this period, all analyses at the two sites complied with the regulatory thresholds.
Specific elements of the Villeneuve-sous-Dammartin site
- The ECT site is located upstream of the Mitry-Mory drinking water catchment area.
- The site lies on a layer of clay, a naturally impermeable geological element, which effectively protects the deep-water table that supplies the drinking water catchment.
- 5 piezometers, arranged in the direction of the water table flow, allows us to monitor its analysis. The Biberonne river, which runs to the east of the site, is also subject to regulatory measurements. All the analyses carried out are in conformity with the regulatory thresholds.
Specific elements of the Annet-sur-Marne site
- The Annet-sur-Annet site is outside the protection zone of the Beuvronne river, a tributary of the Marne where the catchment area is located downstream. Note that the flow of the Beuvronne is very low (4.2 l/s) compared to that of the Marne (1000 m³/s).
- As previously mentioned, the methodology used for the storage and rapid recovery of the soil prevents the runoff of stormwater that has been in contact with the soil from reaching the Beuvronne.
- The prefectural decree of August 2021 reinforced the analytical measures that were put in place on the site at the time of the authorization to operate with three measurements upstream and downstream of the catchment point on the Marne.
Going further : FAQ
Can we say that there is pyrite in the 167,000 tons of soil received? Are there any abnormal concentrations at these sites?
The soils come from deep subsoils and are potentially pyritiferous. However, no analysis has confirmed the presence of pyrite in these soils. As a reminder, it is the oxidation that can be dangerous (and not its mere presence). The acidification of the environment is the consequence and the indicator. However, the analyses show basic pH levels and strong neutralization on the sites.
In July 2022, did the Prefecture « discover » the presence of pyritiferous earth on the ECT sites?
The prefectural orders of July 2022 formalize the exchanges that started a year earlier with ECT and BRGM. These orders confirmed the analysis methodology used.
Are we significantly above the standards set by BRGM at the Villeneuve-sous-Dammartin and Annet-sur-Marne sites?
The standards mentioned by the press refer to the sulfur content and not the pyrite content. It should be noted that the BRGM has not issued any standard for excavated soil from the Grand Paris Express. It has specified thresholds, if it goes beyond them, an analysis methodology should be put in place.
Is there a risk of gaseous emissions, sulphurous gases?
No! That would be scientifically untrue. The sulfurous gas, H2S, comes from the decomposition of organic matter. The oxidation of such an ore does not produce sulphurous gas.
