Вестник МГТУ. 2020, Т. 23, № 1.

Introduction Studying the background concentrations of chemical elements, including those regarded as environmental pollutants, is an important component of environmental research (Даувальтер, 2012). In "classical" geology, averaged or reference concentrations of chemical elements are mainly used to normalize the concentrations of the respective elements in the studied matter, primarily in rocks and sediments (Интерпретация..., 2001). One example is the so-called clarke values, named after the American geochemist F. W. Clarke (1847-1931) and representing the average content of chemical elements in the Earth's crust, hydrosphere, the Earth, cosmic bodies, geochemical or cosmochemical systems, etc., relative to the total mass of the respective system. Normalization using clarke values reveals any abnormal concentrations of individual chemical elements, which is important in geological prospecting and commercial evaluation of mineral deposits (an applied task), as well as identifying the general patterns of migration and accumulation of chemical elements in geosystems (a theoretical task) (Юдович и др., 2011). Normalized concentrations of chemical elements are known as concentration factors or enrichment factors. When studying environments capable of accumulating the incoming chemical elements (water, soil, bottom sediment (or just sediments) of water bodies, living organisms), clarke values are replaced by background concentrations, which may vary between regions. Background normalization makes it possible to identify abnormal concentrations of chemical elements caused by anthropogenic impacts on the environment ( Даувальтер, 2012; Водяницкий, 2008). Based on background values, various indices and indicators can be calculated in the context of environmental and geochemical studies, which is aimed at an integrated assessment of the anthropogenic impact to characterize pollution levels and environmental damage caused. In this regard, background values are often compared with maximum permissible concentrations (MPC) in different standards or methodological guidelines. However, this comparison is not entirely correct, since standards are calculated taking into account the possible impact on living organisms, including humans, without taking into account regional geological and geochemical features, while background concentrations, on the contrary, emphasize these features without an explicit reference to any impact on the biotic components of the given environment. Considering that organisms, even those belonging to the same ecosystem, can respond very differently to anthropogenic impacts, as well as the fact that contemporary MPC tend to fail to take account of the regional climatic conditions and the adaptations that living organisms have developed, background concentrations seem to be a more reliable reference for standards governing the concentrations of chemical elements in soil, water, and sediments. Moreover, in the environmental interpretation of geochemical data, as opposed to the use of regulatory standards, it is advisable to take into consideration the behavior of living organisms themselves. Currently, the most studied is the problem of background concentrations of chemical elements in the waters and soils in different regions of Russia and the world. The available research on the regional geochemistry of the sediments of water bodies is fragmented. Moreover, there are still no national regulatory standards for sediments, which in the absence of background values significantly complicate the interpretation of the collected geochemical data on the sediment of water bodies (Даувальтер, 2012). Adopting standards based on either background or normalized values estimated for soils, based on the clarke values of the Earth's crust or sedimentary rocks of the world can invalidate the results of environmental and geochemical studies. Thus, at present, the study of background concentrations of chemical elements in sediments is one of the most relevant research problems in environmental geochemistry and geoecological research. This problem is especially pressing in the northern regions of Russia with their abundance of rivers and lakes, where the key challenge facing environment limnologists and geochemists is to assess the present condition of water bodies taking into account the growing anthropogenic pressure on the environment around the world. One such region is the Republic of Karelia, a region of Russia with nearly 60 thousand lakes, most of which are small lakes (Каталог., 2001). The aim of study is to estimate the background concentrations of chemical elements, including heavy metals (HMs), in the sediments of the small lakes in the southern part of the Republic of Karelia. This research problem has never been studied before in Russian Karelia. Materials and methods Sediment sampling in 15 small lakes in the southern part of the Republic of Karelia (Fig. 1) took place in 2016-2018, both in summer and in late winter - spring. The five studied lakes are urban water bodies located in the cities of Petrozavodsk (Lakes Lamba and Chetyrekhverstnoe), Medvezhegorsk (Lakes Plotichie and Kitaiskoe), and Suoyarvi (Lake Kaipinskoe) (Слуковский, 2018). The other lakes are located mainly in forested or swampy forested areas at a distance from larger centers of population. The samples were mostly collected using a Limnos sampler, allowing to extract stratified sediment core samples up to 60 cm long and separate these into 1 cm or thicker layers. Considering that sediments can differ in density and water content, the core samples ranged from 28 to 48 cm. In addition, in Lakes Lamba, Chetyrekhverstnoe, Gryaznoe, Rakhoilampi, and Dennoe, the sediment was drilled to the rock bed, silt, or clay, underlying the sediment. For this purpose, a manual drill (the so-called Russian drill) was used, which allows