Perchlorate is an environmental contaminant of growing concern due to its potential human health effects and widespread occurrence in surface water and groundwater. It is considered potentially toxic even at low concentrations (> 4 ppb) as an endocrine disruptor, especially for fetuses and infants. Analyzes have highlighted presence of perchlorate in drinking water of Champagne-Ardenne (NE France) with two suspected sources: a military source related to the WWI (World War I) and an agricultural one related to past use of Chilean nitrates.
In order to define the origin and fate of perchlorate in groundwater, a study area of 500 km2 has been selected east of Reims, where drinking water catchments of the chalk aquifer are concerned with perchlorate contamination. Chalk groundwater flow and geochemistry are monitored for 2 years from June 2017 to June 2019 at 35 sampling points including boreholes, springs and rivers (mainly form chalk aquifer drainage). Water samples are collected monthly to determine the major and trace elements contents, stable water isotopes, perchlorate and organic pyrotechnic molecules. Perchlorate were detected at almost all sampling points (32 of 35) with a maximum value of 63 ppb and an average value of 11 ppb. High levels of perchlorate (> 4 ppb) were detected mainly downstream of the Monronvilliers military camp, where quantities of ammunitions were used, stored and destroyed during and after the WWI. Statistical analysis shows no significant correlation between perchlorate and other major ions. The isotopic signature of perchlorate was analyzed and showed a synthetic origin, proving for the first time the military source of contamination in this area. In addition, groundwater dating using CFCs and SF6 indicated an average residence time of < 30 years, implying that perchlorate contamination is related to sources that may still subsist in the subsoil after the WWI (e.g. unexploded ammunitions) rather than military activities during the conflict. Perchlorate concentrations are relatively stable with time for most points except for decreases observed in September 2017 and 2018. Stable isotopic analysis of water and monitored groundwater table showed that the chalk aquifer is recharged by precipitation mainly during the winter season. Therefore, the decrease of perchlorate levels could possibly be explained by the very few recharge and low groundwater table in September, as less perchlorate is dissolved and transferred into the aquifer. This research provides insights on a combined use of hydraulic, geochemical and isotopic approaches to study the origin and transfer of contamination in chalk groundwater. The chalk aquifer properties and the long-term influence of the WWI on groundwater quality in NE France are better clarified, with the aim to provide appropriate recommendations in terms of water management.