A radiocarbon (14C) activity analysis in the tree rings around Ignalina nuclear power plant (INPP) has been carried out with the aim to test the hypothesis to use 14C tree-ring analysis data as a tool for the reconstruction of gaseous releases from NPP to the environment. The INPP has been in decommissioning state since the end of 2009. Tree-ring samples for 14C analysis were collected 7 yr after final power unit shutdown from the INPP vicinity. The samples from 5 sampling locations were collected, prepared and measured using the Single Stage Accelerator Mass Spectrometer (SSAMS). Data analysis represents observable Ignalina NPP influence by 14C increase up to 15 pMC (percent modern carbon) in tree rings. Good correlations of the 14C concentrations and wind direction were obtained. The main purpose of this article was to match 14C measurement data along with the atmospheric dispersion modeling of emissions in order to retrospectively characterize the emission source.

The vertical distribution of radiocarbon ( 14 C) was examined in the bottom sediment core, taken from Lake Drūkšiai, which has served as a cooling pond since 1983 for the 26 years of the Ignalina Nuclear Power Plant (INPP) operation using two RBMK-1500 reactors (Russian acronym for”Channelized Large Power Reactor”). 14 C specific activity was measured in alkali-soluble and -insoluble fractions of the sediment layers. Complementary measurements of the 210 Pb and 137 Cs activity of the samples provided the possibility to evaluate the date of every layer formation, covering the 1947–2013 period. In addition, 14 C distribution was examined in the scales of pelagic fish caught between 1980 and 2012. Our measurements reveal that, during the period 1947–1999, the radiocarbon specific activity in both fractions exhibits a parallel course with a difference of 5 ± 1 pMC (percent of modern carbon) being higher in alkali-soluble fraction, although 14 C specific activity in both fractions increased by 11.4–13.6 pMC during the first 15 years of plant operation. However, during the 2000–2009 period, other than previously seen, a dissolved inorganic carbon (DIC) → aquatic primary producers → sediments 14 C incorporation pattern occurred, as the radiocarbon specific activity difference between alkali-soluble and -insoluble fractions reached 94, 25, and 20 pMC in 2000, 2006, and 2008, respectively. Measurements in different sediment fractions allowed us to identify the unexpected organic nature of 14 C contained in liquid effluences from the INPP in 2000–2009. The discrepancy between 14 C specific activity in fish scales samples and DIC after 2000 also confirmed the possibility of organic 14 C contamination. Possible reasons for this phenomenon might be industrial processes introduced at the INPP, such as the start of operation of the cementation facility for spent ion exchange resins, decontamination procedures, and various maintenance activities of reactor aging systems and equipment.

In this study, we examined how land use and urbanization changes in adjacent areas affected biological productivity and carbon cycling in a lake ecosystem over 100 years and how these changes are reflected in carbon isotope variations. We performed radiocarbon (14C) activity and stable carbon isotope ratio analysis in two organic fractions: humin and humic acids of lake sediment. Additionally, we performed pigment and diatom analysis and determined the carbonate and organic matter (OM) content in sediments. Over the last century, the estimated 14C reservoir age in both sediment organic fractions varied from 1136 ± 112 yr to 5733 ± 122 yr. The increase in the reservoir age by 1175 ± 111 yr was related with higher inputs of pre-aged organic carbon and 14C depleted hard water due to the opening of the channel connecting two lakes. Nuclear weapons tests caused an increase in the reservoir age of up to 5421 ± 135 yr and 5733 ± 122 yr in humin and humic acids, respectively. 13C values in the humic acid fraction showed a tendency to decrease, depending on the content of autochthonous versus allochthonous OM in sediments, while changes in the sources of OM had a minor impact on the stable carbon isotope composition in the humin fraction.

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<p>The difference of radiocarbon (<sup>14</sup>C) concentration between terrestrial and aquatic samples is called the freshwater reservoir effect (FRE). The FRE is a potential issue for archaeologists dating fish bones, shells, human bones, or food crusts on pottery from sites near rivers or lakes. Studies on the FRE showed its variability in space and time, significant variations within one river or lake, different aquatic plants, and animals, or even single fish species of the lake [1, 2 and the references therein]. Therefore, dating the artifacts, it is very important to understand the nature of the FRE by studying processes that determine the redistribution of carbon isotopes in water ecosystems. It is important to obtain new knowledge on temporal variation of the FRE of a water system as due to climate change and anthropogenic activities it could be completely different at ancient times since such periods as Mesolithic, Neolithic and Early Bronze Age when aquatic resources were an important contribution to human nutrition are relatively poorly studied. The objective of the research was to examine how known anthropogenic factors affected carbon cycling in the lake systems, including how these changes are reflected in carbon isotope variations as well as the FRA of lake sediments and different species of fish.</p><p>Two completely different lake systems of eastern Lithuania were studied. Lake Tapeliai belongs to the huge drainage system and is permanently affected by hydrological changes. When Lake Drūkšiai served as a cooling pond for the Ignalina Nuclear Power Plant, its average temperature increased by 3-4 °C. Results revealed that over the last century the estimated radiocarbon freshwater reservoir age (FRA) in sediments of Lake Tapeliai varied from 1136±112 y to 5733±122 y. These changes were caused by old organic carbon import to the lake from a neighboring peat bog. The FRA in samples of different fish species differed by up to 500 y, whereas the variations in the FRA measured in samples of the same species reached up to 300 y. Radiocarbon activity measurements in the samples of fish caught in Lake Drūkšiai during the operation of the nuclear power plant were performed. During 1984-1999 years measurements showed that <sup>14</sup>C activity in fish slightly exceeded (up to 5 pMC) atmospheric activity. However, during 2000-2009 it exceeded by 40 pMC. Unfortunately, no information about increased activity levels of aquatic effluents or different chemical agents used could be found in INPP reports. Data of the fish scales <sup>14</sup>C activity measurements are in good agreement with the data of the humic acid fraction of lake bottom sediments.</p><p>This data clearly indicates that there was an event in the year 2000 when substances from NPP with elevated <sup>14</sup>C content were introduced into the lake, although not exceeding the permissible levels.</p><p> </p><p>This research was funded by a grant (No. S-MIP-19-16) from the Research Council of Lithuania</p><p> </p><p>References</p><p>[1] Heritage Science (2013) 1(1), 1–622.</p><p>[2] Quaternary Science Reviews (2012) 48: 67–79.</p>