Geochemical Controls on the Uranium Cycle in a Lake Watershed

Understanding the uranium (U) cyclereservoirs
and processesat
the watershed scale is key to manage contaminated areas, to elucidate
ore formation processes, as well as to implement paleoenvironmental
research. Here, we investigated the different steps of the U cycle,
from sources to sinks, and the relative roles of redox processes and
organic matter in the control of U mobility in the naturally U-rich
small mountainous watershed of Lake Nègre (France). We interpret
the U repartition in U reservoirs through chemical, isotopic (δ238U and (234U/238U)), and speciation
analyses, in light of anterior studies of the site. We show that U(VI)
originates from the leaching of U-rich rock fractures and is transported
in dissolved forms. Wetlands and meadow soils then act as intermediary
sinks where U(VI) is complexed by organic matter (up to >5000 μg/g)
and subsequently partly reduced to U(IV). Dissolved U is also supplied
to the lake, in addition to particulate and colloidal U resulting
from soil physical erosion. After entering the lake, most U(VI)-bearing
organic particles settle in the sediments and U(VI) is reduced to
U(IV), resulting in high sedimentary U concentrations (up to >1000
μg/g), while a fraction of U is potentially desorbed from particles.
Remaining dissolved U is exported from the watershed through the lake
outlet stream. In this high-mountain lake catchment, the U cycle is
mainly controlled by organic matter complexation and particulate transport,
though U reduction in the lake sediments may help its long-term immobilization.