Evolution of Uranium Isotopic Compositions of the Groundwater and Rock in a Sandy-Clayey Aquifer

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 234 U/ 238 U activity ratio were determined in the samples. Estimations were made of the 14 C and 234 U-238 U 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 Almost all U on the slopes has been replaced by a newly formed hydrogenic U with a higher 234 U/ 238 U activity ratio.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 234 U/ 238 U 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.

Study of the Mobilization of Uranium Isotopes in a Sandstone Aquifer in Combination with Groundwater Data

“…The results obtained partially confirm previously expressed ideas about the evolution of U isotopic compositions of the Vendian rock near the study area [20]. 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.…”

“…Further, daughter nuclides pass most actively into water from surface coatings [15], increasing the uranium isotopes activity ratio in water. In addition, the similarity of the activity ratios of 234 U: 238 U in groundwater and in the most easily leached fractions of water-bearing rocks was noted in the experiments of Lowson et al [16], Payne et al [17], and Dabous et al [18], with the fractions being defined as sorbed elements, carbonate minerals and amorphous iron minerals [19] which could indicate the opposite process which is the transition of nonequilibrium nuclides from water to rock with an increase in the U isotopes activity ratio in the rock [20]. Under such conditions, it is difficult to estimate the rate of chemical weathering and to date natural processes by using the currently developed methods [21,22].…”

“…At the same time, the lower average value of 234 U: 238 U activity ratio 0.92 [20] was established in the samples of green siltstones that were collected in the paleovalley, which was screened by the layer of sea clays. It is explainable by the fact that these deposits have reached a steady state of the were under reducing conditions for over 1 Ma.…”

Modeling Changes in the Composition of River Water with Discharged Wastewater: A Case Study in NW Russia

“…As shown in [60][61][62], Vendian siltstones and sandstones do not contain native uranium minerals. The uranium was in a dispersed state and was redistributed by groundwater along its streamlines from the recharge zones of the watersheds to the discharge zones in the river valleys.…”

Isotopes in Hydrology and Hydrogeology

“…Malov [19] analyzes the uranium redistribution in the sandstones and siltstones of aquifers of the Northern Dvina basin, a paleo-valley in Northwest Russia that is associated with the formation of increased concentrations of uranium in both rocks and groundwater. Malov highlights that, 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.…”