Dating groundwater with tritium

Added: Luisalberto Brumit - Date: 15.09.2021 08:17 - Views: 36051 - Clicks: 5562

The assessment of nuclear objects sites in Lithuania, including groundwater characterization, took place in the last few years. Tritium activity in groundwater is a very useful tool for determining how groundwater systems function. Natural and artificial tritium was measured in 8 wells in different layers from 1.

The were compared with other regions of Lithuania also. The evaluated tritium activities varied from 1. The data show, that groundwater at the nuclear power objects sites is not contaminated with artificial tritium. In this work, the vertical tritium transfer from soil water to the groundwater well at nuclear objects site was estimated. The data show that the main factor for vertical tritium transfer to the well depends on the depth of wells.

Lithuania is planning to construct a new nuclear power plant NPP nearby the closed one. A near surface repository for low and intermediate-level short-lived radioactive waste will be built on the Stabatiskes site in the vicinity of Ignalina NPP during decommissioning works. The disposal capacity can also be used for the waste stored in the temporary repositories of the Ignalina NPP.

Engineering barriers are used in the repository for radioactive waste; however, in long-term evolution scenario radionuclides can spread into the environment, extend in the biosphere, and cause define the external exposure of the environment due to the natural and premature prescheduled degradation of the engineering barriers of the repository [ 1 ].

Groundwater dating is a very useful tool for determining how groundwater systems function [ 2 , 3 ]. The radionuclides transport in water systems depends on many factors: the physicochemical condition, pH, salinity, hydrological, biological, and so on. The tritium in groundwater can be treated as an indicator of radionuclide release from nuclear energy objects.

Tritium is a radioactive isotope of the element hydrogen. Tritium occurs naturally in the upper atmosphere when fast neutrons interact with nitrogen. It is also produced during nuclear weapons explosions and commercially in nuclear reactors producing electricity. Anthropogenic tritium sources include fallout from nuclear weapons testing, nuclear reactors, future fusion reactors, fuel reprocessing plants, heavy water production facilities and commercial production for medical diagnostics, radiopharmaceuticals, luminous paints, illumination, self-luminous aircraft, airport runway lights, luminous dials, gauges and wrist watches, and others [ 4 ].

Commercial uses of tritium for only a small fraction of the tritium used worldwide. Tritium is a pure beta-emitter with a half-life of The most common form of tritium is in water, since both radioactive tritium and nonradioactive hydrogen react with oxygen in the same way to form water.

Tritium replaces one of the stable hydrogen in the water molecule, H 2 O, creating tritiated water, which is colorless and odorless. Tritium is a radioactive isotope of hydrogen which occurs in precipitation. In groundwater studies tritium measurements give information on the time of recharge to the system, with the tritium content of precipitation being used to estimate the input of tritium to the groundwater system [ 6 ]. The turnover of tritium is very fast, except where it is fixed in glacier ice or groundwater, and when it is incorporated into organically bound tritium in organisms [ 11 ].

The tritium origin and hydrological cycle is presented in Figure 2. The transfer of tritium as HTO tritiated water from soil surface to the groundwater is essentially a process of vertical downward infiltration. This is a simplification of the process because there is also horizontal flow in the aquifer, if a gradient exists, as well as upward flow where the aquifer intersects a surface watercourse, that is, a discharge zone Figure 2. Both longitudinal and lateral dispersion occurs along the horizontal flow path, so the plume arising from the source area becomes wider, but centre-line concentrations are reduced along the flow path.

Tritium concentrations are also reduced by decay along the flow path. Horizontal flow is the most important for a receptor i. In other situations, the transfer from soil water to groundwater may be conservatively represented as a vertical downward infiltration. In the case of tritium, the is zero, since water does not orb to soil, and the term drops from the equation. The effective porosity represents an average for the entire pathway between the ground surface and the intake zone of the well. The soil moisture content can be considered an effective porosity in the unsaturated zone above the water table.

The porosity of the most unconsolidated porous media below the water table is in the range of 0. Background levels of tritium in groundwater can be ificant, depending on depth, because tritium inputs from atmospheric deposition have been higher in the past. Tritium levels in global air and precipitation began to increase around as a result of nuclear weapons testing and peaked in during the Nuclear Test Ban treaty between the United States and the Soviet Union. They have been decreasing since that that time. Depth profiles of tritium in groundwater usually show a peak at a depth level corresponding to The reduced concentration in groundwater in this zone was attributed mainly to dilution by discharge of deeper uncontaminated groundwater into the top layer of the aquifer.

The residence time in the top layer was estimated at 3 years, which is insufficient for appreciable decay of tritium. The difference in concentration is partly due to decay and partly due to longitudinal dispersion during vertical migration. The rate of vertical migration was estimated at 0. The average infiltration rate over the — period was estimated at 0.

The nuclear objects proposed to be constructed in Visaginas and both potential sites are located next to the former, closed Ignalina nuclear power plant. This paper deals with the assessment of the tritium activity in groundwater at the nuclear object sites.

The nuclear objects in Lithuania are concentrated in the eastern part. A thin 0. Groundwater perched accumulates in it only seasonally. This layer as a whole is drained by local boggy depressions. The aquifer has little or no connection with the boggy local depressions; it is drained to a larger extent by Lake Druksiai.

Hydraulic conductivity of low permeable layers ranges from 3. The systematic investigations of groundwater system have been begun before Ignalina NPP construction in The activity of tritium in the groundwater was measured in many observation wells constructed in the region of Ignalina NPP. Water samples were taken from different layers from 1. Thus, the water samples of all observations points were measured several times. Tritium activity was measured with the method of liquid scintillation beta-spectrometry. The most systematic observations were fulfilled in the wells and of Ignalina NPP site in —, in Stabatiske site one observation well since Visaginas NPP site 5 observation wells was investigated The tritium measurements in groundwater have been performed in four different regions of Lithuania also; two of them belong to the eastern part of Lithuania: Zarasai, Ignalina regions.

In — tritium activity concentration was observed in groundwater of karst region, Birzai North part of Lithuania. The tritium activity concentration in Nida region western part of Lithuania was observed in — The magnitude of the measured tritium value on the nuclear object territories could have been influenced by global sources, local releases from the ly operated Ignalina NPP, precipitation input, soil moisture transfer rate, groundwater residence time, and so forth. Although these parameters affect distribution of tritium in groundwater, their impact on the tritium at all studied sites is rather uniform due to a relatively small distance between these sites.

The tritium activity concentration in groundwater wells of closed Ignalina NPP varied from 1. The maximum of tritium activity concentration in water was observed in — and they are coming to background level. According to Mazeika , the annual average tritium activity concentration in precipitation of Ignalina NPP region was ed— The tritium activity concentration was detected 1. The similar tritium activities were determined in other Lithuania cities also. The tritium activity concentration in site of planned near-surface radioactive waste repository Stabatiske site was measured in the water samples collected in more than 20 observation wells.

In the present work are given the of tritium activity investigations in the well 6 of Stabatiske site because of the largest of measurements The tritium activity concentration varies from 0. The last tritium activity concentrations measurements in Visaginas NPP site indicate the downtrend of tritium activity concentration in the groundwater.

The tritium activity concentration in groundwater wells varies from 2. The measurements were done from 3 to 39 times and the measurements errors of Ignalina and wells are presented in Figure 3. The error of measurements from activity concentration was estimated and consists of 10 percent.

The measurements errors are shown Figure 4 of the two these wells where most of measurements have been done. In the other regions of Lithuania the tritium activity concentration was measured in the different periods. The tritium activity in the northern Karajimiskis and in the north-eastern Sakiai, Budiniai Lithuania varies from 1. The tritium activity concentration in the wells of above mentioned regions of Lithuania is about background level 1. In this work the vertical tritium transfer from soil water to the groundwater well at nuclear objects site was estimated also 1.

The estimated parameters of soils were used. The effective porosity is an average value for the entire pathway between the ground surface and the intake zone of the well, which includes the zones above and below the water table.

The effective porosity in this area varies from 0. This assumption implies that only top part of well supplies water to the well [ 12 ]. The infiltration rate was calculated couple of times according to tritium and volumetric moisture data [ 19 ]. The parameter shows vertical tritium transfer to the well and describes what part of tritium could reach well water. By taking advantage of earlier identified parameters in investigated sites the calculations showed that the vertical tritium transfer can vary from 0.

The vertical tritium transfer depends on the soil thickness and effective porosity and groundwater in the region of nuclear sites could be enriched by tritium activity concentration equal of atmospheric tritium. The in Figure 6 show that tritium vertical downward transport depends on effective porosity, and the tritium activity would not reach the groundwater.

Tritium values of nuclear object sites in Lithuania were inificantly higher in comparison with the tritium values in other towns of Lithuania Figure 5. of investigations clearly demonstrate that no local radiometric anomaly exists at the sites.

The obtained of the tritium activity concentration at the nuclear power object sites show that the groundwater is not contaminated with artificial radionuclides. In the western part of Lithuania tritium activity is less and varies from 1. The data analysis demonstrates that no local radiometric anomaly existed at the NPP sites. Variability of vertical tritium transfer versus effective porosity and soil depth was also estimated. The Figure 6 show that tritium vertical downward transport is more dependent on the soil depth than the effective porosity.

Vertical tritium transfer is higher near surface. The authors declare that they have no conflict of interests regarding the publication of this paper. This is an open access article distributed under the Creative Commons Attribution , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article of the Year Award: Outstanding research contributions of , as selected by our Chief Editors.

Read the winning articles. Academic Editor: Joonhong Ahn. Received 28 Aug Revised 23 Jan Accepted 25 Jan Published 12 Mar Abstract The assessment of nuclear objects sites in Lithuania, including groundwater characterization, took place in the last few years. Introduction Lithuania is planning to construct a new nuclear power plant NPP nearby the closed one. Figure 1. Figure 2.

Dating groundwater with tritium

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