Abstract
Uranium isotopes have been used as mechanistic or time scale tracers of natural processes. This paper describes the occurrence and redistribution of U in the Vendian aquifer of a paleo-valley in NW Russia. Forty-four rock samples were collected from nine boreholes with depths up to 160 m, and 25 groundwater samples were collected from 23 boreholes with depths up to 300 m. The U, Fe concentration, and 234U/238U activity ratio were determined in the samples. Estimations were made of the 14C and 234U-238U residence time of groundwater in the aquifer. It has been established that the processes of chemical weathering of Vendian deposits led to the formation of a strong oxidation zone, developed above 250 m.b.s.l. The inverse correlation between the concentrations of uranium and iron is a result of removal of U from paleo-valley slopes in oxidizing conditions, accumulation of U at the bottom of the paleo-valley in reducing conditions, and accumulation of Fe on the slopes and removal from the bottom of the paleo-valley. Almost all U on the slopes has been replaced by a newly formed hydrogenic U with a higher 234U/238U activity ratio. After, dissolution and desorption of hydrogenic U occurred from the slopes during periods with no glaciations and marine transgressions. Elevated concentrations of U are preserved in reduced lenses at the paleo-valley bottom. In these areas, the most dangerous aspect is the flow of groundwater from the underlying horizons, since during the operation of water supply wells it can lead to the creation of local zones of oxidizing conditions in the perforated screens zone and the transition of uranium into solution. For groundwater under oxidizing conditions, an increase in the concentration of uranium is characteristic of an increase in the residence time (age) of water in the aquifer. Also, the 234U/238U activity ratio increases with increasing radioactivity of groundwater. Therefore, the most rational approach is to use groundwater for drinking water supply from the slopes of the Northern Dvina basin.
Highlights
It is not uncommon for uranium isotopes to be used as mechanistic or time scale tracers of natural processes [1,2]
Of special interest is the study of the uranium redistribution in the sandstones and siltstones of aquifers of paleo-valleys, which are associated with the formation of increased concentrations of uranium in both rocks and in groundwater [13]
The aim of this paper is to carry out the paleoreconstruction of hydrodynamics of Northern Dvina basin (NDB) with an assessment of the direction of the evolution of the uranium-isotope composition of rocks and Thegroundwater aim of thisduring paper is to carry out the paleoreconstruction of hydrodynamics of NDB with the Pleistocene-Holocene transition
Summary
It is not uncommon for uranium isotopes to be used as mechanistic or time scale tracers of natural processes [1,2]. Uranium isotopes may be applied to determine the radiological suitability of groundwater for drinking purposes [3,4,5,6], applied in ore bodies exploration [7,8,9], or applied to assist in understanding historical climates [10,11,12]. A typical area with similar conditions is the Northern Dvina basin (NDB)—a paleo-valley in NW Russia (39◦ 300 –41◦ 570 E and 64◦ 060 –64◦ 480 N) (Figure 1). Groundwater has the best quality, but the rationale for the radiological safety of its use is required
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