Вестник МГТУ. 2020, Т. 23, № 1.

Gamma-active radionuclides in the sediments of Lake Ukonlampi (Fig. 1) were measured at the Radiochemistry Department of Moscow State University using an ORTEC GEM-C5060P4-B gamma-spectrometer and ultra- pure germanium (HPGe) semiconductor detector with a beryllium window (relative efficiency 20 %). The spectra were processed in the software suite SpectralineGP. Statistical processing of the data was done in Microsoft Excel 2007. The results of the study were visualized in Inkscape 0.48.4. Results and discussion This study of the collected core samples of recent sediments from small lakes in Karelia revealed an upward trend in the concentrations of chemical elements, primarily HMs, from the deeper layers of sediment to the more recent ones. This trend is most pronounced for urban lakes, mainly in the city of Petrozavodsk, which is associated both with the powerful impact of local pollution sources and with the long-range transport of pollutants (Слуковский, 2018; Medvedev et al., 2019). For example, in the sediment core sample from Lake Lamba located near a thermal power station in Petrozavodsk, abnormally high concentrations of V, Ni, Cr, and other metals were found in layers 0 to 20 cm deep, which is associated with the practice of burning fuel oil (Слуковский и др., 201 7). In addition to HMs, recent sediment layers in urban lakes demonstrate an increased concentration of lithophilic elements, e. g. lanthanides (Слуковский, 2019). One of the explanations for this phenomenon may be the deposition of fine urban dust, composed of particles of silicate minerals, which are the primary concentrators of lanthanides. The dust may also originate from construction sites in the city, as well as with the practice of winter sanding of roads with a clay-sand mixture, which is easily carried by the wind in summer. In non-urbanized areas, which can conventionally be described as reference areas, long-range transport is the main factor in the accumulation of HMs in lake sediments (Vinogradova et al., 2017). According to numerous studies, Pb, Cd, Hg, Tl, Sb, and Bi can be classified as the main agents of long-range transport (Sarkar et al., 2015; Mcconnell et al., 2008; Bartnicki, 1994). Typically, these elements are closely correlated with each other, which further supports the validity of the hypothesis about the influence of long-range transport. Most of the other elements, especially lithophilic elements, do not show pronounced abnormally high concentrations in the upper sediment layers compared to the deeper layers (Слуковский, 2019; Slukovskii et al., 2019). However, the study of sediment core samples from some regions of southern Karelia revealed that such elements as Zn, Cu, Sn can also exhibit the behavior typical of long-range transport agents. Considering that there are no local sources of these metals near the respective lakes, it is logical to hypothesize that these elements could also enter water bodies through atmospheric transport. In particular, this situation is observed in the lakes of the southwestern part of Karelia, which can be explained by the effects of industrial emissions from the Leningrad Region and St. Petersburg on the geochemistry of the recent sediments in Karelia (Slukovskii et al., 2019). To establish the background level of chemical elements in the collected sediment core samples from the studied small lakes in the south of Karelia, two factors were taken into account: sedimentation rate in small lakes of neighboring regions (as no such data is available for Karelia) and direct analysis of the vertical distribution of marker elements in the studied sediment core samples. Pb was chosen as a marker element, because it is the most reliable indicator of historical changes in the environment 150-300 years ago (Sarkar et al., 2015; Karlsson et al., 2006; Escobar et al., 2013; Hosono et al., 2016; Dauvalter et al., 2010). The average sedimentation rate in the small lakes of Northern Fennoscandia and Murmansk Region, which geology is similar to that of Karelia, is known to vary from 0.3 to 1.25 mm per year (Даувальтер, 2012; Rognerud et al., 2000; Hakanson, 1984). Taking into account that the southern part of Karelia has a greater share of forested areas compared to Northern Fennoscandia and Murmansk Region and that the sedimentation rate can be higher in Karelia due to the high flow rate of organic matter into the lakes, a sedimentation rate of 1.25 mm per year was adopted. Thus, sediments 30 cm thick could form over the course of approximately 240 years. However, the most pronounced changes in the accumulation of HMs in the northern environments around the world started to happen later than indicated, namely 150-170 years ago (Keinonen, 1992; Dauvalter et al., 2011; Моисеенко и др., 2000). Therefore, in the average lake, sediment layers deeper than 20 cm can be regarded as reference layers that formed in pre-industrial time. Examination of the sediment core samples from the lakes that were investigated as part of this study largely supported these assumptions. Fig. 2 shows that in all five lakes in the reference regions of southern Karelia, a noticeable increase in Pb concentrations begins at a depth of 15-25 cm. Other elements behave similarly, namely Sb, Cd, Tl, Bi, and other markers of the effects of global anthropogenic emissions on the geochemistry of the recent sediments in small lakes (Michinobu et al., 2013). This is also seen in Fig. 3, which shows that the increase in the concentrations of Pb, Cd, and Tl in the recent sediment in Lake Ukonlampi dates from the early 20th century, which is associated with the industrial development in Europe. The most noticeable increase in the concentrations of chemical elements was observed in the post-World War II period, which is associated with the active growth of the industry in the USSR. The average recent sedimentation rate in Lake Ukonlampi, according to a dating study, is 1.25 mm, which coincides