2012 NGWA Ground Water Summit: Innovate and Integrate

Using d15N and d18O measurments to identify sources of nitrate in karstic springs in the Paris basin (France)

Monday, May 7, 2012
Fatima-Zahra Jihane El Gaouzi, PhD, Université Pierre et Marie Curie Laboratoire Bioemco - CC 120, Tour 56-66;
Mathieu Sebilo, Ph.D., Pierre & Marie Curie University ;
Pierre Ribstein, Pierre & Marie Curie University ;
Valérie Plagnes, Pierre & Marie Curie University ;
• Manon Zakeossian, EAU DE PARIS company;

In order to supply Paris on drinking water, many springs, located in karst Senonian chalk and Beauce limestone, are exploited by Eau De Paris Company. This study is focused on five groups of springs located up to 150 km south of Paris. They are placed in two neighbouring watersheds: Villemer, Villeron are situated in the Lunain watershed (agricultural activity) and Bourron, La Joie and Chaintréauville are in the Loing watershed (a big part is covered by forests).

They have distinct geochemical characteristics. Nitrate concentrations show seasonal variations and vary between 27 mg/L as minimum value registered at Bourron, and 60 mg/L as maximum value in Chaintréauville and La Joie springs. High turbidity sometimes with bacterial contamination is observed in Villemer.

The objective of the study is to understand the origin of the water in the springs using isotopic characterization of nitrate content in different waters, as: chalk and limestone groundwater, surface water, agriculture-derived water and sewage. The combined use of δ15N and δ18O of nitrate isotopic results showed that the springs are supplied by a mixture of different waters. The contributions in each site were quantified by using the Bayesian model SIAR.

In Villemer spring, nitrate originates at quasi-equal proportions from chalk, agriculture-derived water (30%), sewage (12%) and loosing Lunain river water (21%). This could results from a diffuse contribution of waters via a complex karstic network. Villeron site is essentially supplied by chalk (31- 45%) groundwater strongly impacted by agricultural practices (41- 50%). In the Loing sites, the chalk and limestone groundwater and agriculture-derived water similarly (around 30%) are responsible for the intake of nitrate in Chaintréauville and la Joie springs. In Bourron site, nitrate and therefore water come from the chalk and limestone aquifers at about 30%, respectively, and from the Loing river at 29-39%.