The use of stable lead isotopes for the identification of the sources and transport processes leading to the release of radioactive contaminants downstream of former uranium mine sites
Résumé
Radioactive-enriched material produced during past uranium mining activities,such as tailings and mine
drainage water, potentially releases radioactive contaminants into the surrounding soils and sediments.
Activities of U and its decay products could be therefore significantly higher than those of the local
geochemical background. Moreover, weathering and erosion of the local bedrock could also lead to a
high level of uranium up to 4000 mg.kg-1
[1]. Thus, highlighting the potential sources and mechanisms
driving radioactive elements dissemination in the vicinity of former U mines is a key element for the
decision-making process, the effective management and remediation of contaminated sediments and
soils. This research aims to identify the sources of radioactive contaminants and their transport
mechanism by using stable Pb isotopes and radioactive disequilibrium of the 238U-decay series. The
study area is a wetland located downstream of U mine tailings, Rophin (France), considered a uranium
territory workshop (ZATU). The exploited uranium mineral is parsonsite Pb2(UO2)(PO4)2, rich in
common Pb [2].
High U concentration up to 3147 mg.kg-1
(34 mg.kg-1
in local bedrock) was observed in a soil core
extracted in the wetland. The results of stable Pb isotopic analysis indicate that U accumulation is linked
with a whitish clay soil layer below the organic topsoil layer. The 206Pb/207Pb ratio of this layer reveals
a radiogenic signature ~1.73 (local background ~1.20), which means it probably originates from
previous mining activities. The alignments of Pb isotope ratio data in three isotope diagrams highlight
the mining impact on the soils. Many studies have evaluated the percentage of mining contamination by
Pb binary isotope mixing models [3]. We remark here the high content of common Pb in parsonsite
could lead to an underestimation of mining impacts. So, the evaluation needs more cautious in this
context. Moreover, our results show the mobility of U in the soil core. However, more studies are needed
in order to better understand the processes governing the mobility of U and its decay products and to
shed new light on their codispersion or not with the radiogenic lead of the U-ore