Transfer dynamics of chlorinated solvents in the chalk aquifer of northern France

The metropolis of Lille (more than 1 million inhabitants) produces 40% of its drinking water through well fields tapping the chalk aquifer to the agricultural, urban, and industrial region southwest of Lille. The groundwater quality is threatened by the presence of chlorinated solvents amongst others pollutants. In fact, many industries using chlorinated solvents are or were established on the well field’s territory as paint factories, industrial laundries and metallurgical plants. The chlorinated solvent concentrations in groundwater often exceed the regulation limit for drinking water usage in France ([perchloroethene] + [trichloroethene] < 10 µg.L^-1) and then limit its use for drinking water production. The understanding of the chlorinated solvents dynamics and space distribution in the aquifer is a major issue for the metropolis of Lille. In addition, the quantities of available water with good quality is currently decreasing due to repeated annual droughts in the recent years. Thus, the metropolis of Lille, the French Geological Survey and the University of Lille explored the transfer and degradation mechanisms of the chlorinated solvents in the well fields in the two research projects RESEAU (2016-2019) and COHMET (2017-2020).

18 wells and 9 piezometers were sampled during 3 years in order to evaluate the chlorinated solvents concentrations. In order to assess a possible migration of the compounds, three piezometers were sampled along the water column using passive samplers. In addition, a more detailed hydrochemical characterisation of groundwaters (chemical elements markers of the reducing conditions, physico-chemical parameters) was performed in the same three piezometers. Furthermore, the possible chlorinated solvent sources were identified with the databases BASIAS and BASOL, which list the past and current industrial plants, polluted soils and sites on the French territory. Finally, the chlorinated solvent degradation mechanisms were investigated with a compound-specific carbon isotope analysis.

The three-year concentration monitoring highlights complex dynamics of the chlorinated solvents in the aquifer. A wide variety of compounds is detected in the well fields (perchloroethene, trichloroethene, cis and trans-1,2-dichloroethene, 1,1-dichloroethene, 1,1,1-trichloroethane, 1,1-dichloroethane, 1,2-dichloroethane and vinyl chloride) with maximal concentrations ranging from 1.2 (vinyl chloride) to 155 µg.L^-1 (cis-1,2-dichloroethene). The highest concentrations are measured downstream three former industrial laundries in the south of the territory. The chlorinated solvent concentrations are stratified along the wells water columns and increased with depth. These concentration increases are consistent with water inlets along the wells originated from the fractured chalk. Despite the measure of favourable physico-chemical conditions, the δ¹³C ratios comparison do not indicate biodegradation of the chlorinated solvents, except in two wells. The concentrations changes are essentially due to the migration of compounds in depth. Then, the δ¹³C ratios indicate the presence of several major sources of chlorinated solvents.