Study and evaluation of the chemical composition of bottom sediments of the Volga-Akhtunisky flood

. The chemical composition of the bottom sediments of the Volga-Akhtuba floodplain was assessed. Chlorides (36 mg/kg) were found to be one of the predominant anions, while calcium (27 mg/kg) and sodium (27 mg/kg) were the predominant cations. In terms of pH, the bottom sediments were weakly alkaline throughout the studies. The agrochemical composition of bottom sediments of the studied water body corresponded to lime sapropel due to the content of organic matter in them (4.21%), as well as common forms of nitrogen (0.58%), phosphorus (0.27%) and potassium (1.1%). To assess the chemical and toxicological parameters in samples of bottom sediments, the quantitative content of metals and pesticides was determined. It was found that their content does not exceed the maximum permissible concentrations. An analysis of the IR spectrum data made it possible to establish that the IR spectrum contains absorption bands characteristic of bentonite clays, which have good fertilizing and water-retaining properties.


Introduction
It is known that due to catastrophic changes in ecosystems, the global consequences of intensive land use, leading to soil degradation, it is required to improve agricultural production.The solution of the global environmental problem of land degradation is possible through the use of non-traditional soil ameliorants [1][2][3][4], but due to the lack of scientific and industrial infrastructure in the field of processing, recycling and disposal of waste, technologies for the secondary use of resources are practically not used.Bottom sediments are sediments that cover the bottom of a water body.They have a diverse chemical, mineral and granulometric composition.The composition of bottom sediments may reflect the totality of biological, chemical, and physical processes occurring in a water body [5].Large volumes of bottom sediments create a serious environmental problem for the environment.For their storage for a long time, specially equipped territories are required, but this requires significant material costs.At the same time, silting of reservoirs with bottom sediments is one of the important problems in the management of water resources, mainly due to a decrease in the capacity of the accumulation basin.Bottom sediment (sapropel) can be used as a fertilizer or ameliorant in the cultivation of various crops in order to increase productivity, as well as to recultivate unproductive lands and increase soil fertility.Together with organic waste, river silt can form organomineral composts.Such organomineral mixtures play a huge role in improving both the physical and chemical properties of the soil [6].For example, according to some data [7], the use of bottom sediments of a pond for the recultivation of technogenically transformed soils contributes to a significant increase in the content of organic matter.There is evidence of the possible use of bottom sediment as an additional source of mineral nutrition for fertilizers, as well as for land reclamation after technogenic intervention [8].The purpose of this study is to study and evaluate the chemical composition of bottom sediments sampled in the Volga-Akhtuba floodplain (erik Dudachenok).

Materials and Methods
The object of research is located on the territory of the Kuibyshev and Sukhodolsk rural settlements of the Sredneakhtubinsky district of the Volgograd region (erik Dudachenek).Bottom sediments were taken when clearing the erik.Sampling was carried out according to RD 52.24.609-2013.The content of water-soluble cations and anions in bottom sediments was measured by capillary electrophoresis according to the methods PND F 16.1:2:2.3:2.2.69-10 and PND F 16.1:2:2.2:2.3.74.The agrochemical composition of bottom sediments was determined by the following methods: acidity pH of the salt extract was determined according to GOST 26483-85, organic matter according to GOST 26213-91, total nitrogen according to PND F 16.1:2:2.3.82-2013,total phosphorus according to PND F 16.2:2.3.73-2012,general potassium according to GOST 26718-85.The content of heavy metals in samples of bottom sediments was determined by atomic absorption spectrometry according to the method PND F 16.1:2.2:2.3:3.36-2002 and PND F 16.1:2.2:3.17-98.Determination of the mass fraction of pesticides in samples of bottom sediments was carried out by gas chromatography using an electron capture type detector using separating phases of different polarity by comparing the height (area) of the peak of the analyzed and calibration solutions (RD 52.18.180-2011).

Results and discussion
Bottom sediments of water bodies are characterized by the ability to accumulate various chemical elements that pollute components that can subsequently enter water bodies as a result of anthropogenic impact.Analysis of the chemical composition of samples of bottom sediments revealed that the content of water-soluble forms of cations and anions was as Studies show that chlorides mg/kg) are one of the predominant anions, while calcium (27 mg/kg) and sodium (27 mg/kg) are the predominant cations.In terms of pH, the bottom sediments were weakly alkaline throughout the studies.Table 2 presents the results of the analysis of the agrochemical composition of samples of bottom sediments taken during the clearing of the erik.The agrochemical composition as a whole satisfies the requirements of GOST 54000-2010, however, the content of organic matter (4.21%) was somewhat less than the standard (at least 5%).The content of total nitrogen was 0.58%, total phosphorus -0.27%, total potassium -1.1%, which fully complies with the standard for lime sapropel.It is known that the main factor determining the ecological well-being in the regions is the content of heavy metals in various natural environments, including bottom sediments.Note that the maximum permissible concentrations of heavy metals in bottom sediments have not been developed.The comparison was carried out with the standards specified for the use of bottom sediments as sapropel.Analysis of samples of bottom sediments (Table 3) showed that according to all normalized indicators, except for zinc, the bottom sediment corresponds to the first class of suitability.The zinc content was 145.0 mg/kg, which significantly exceeded the norm, however, for the second suitability class, the zinc content is allowed at the level of 300-600 mg/kg.
Pesticides are persistent organic pollutants, toxic substances that are widely used in many areas of human activity.To date, data on the study of the content of pesticides in water bodies, as well as in the system "soil -water -bottom sediments" are practically absent.In the ecological aspect, the movement of pesticides along trophic chains poses a serious threat, first of all, this applies to those compounds that actively accumulate during migration in each of the links (DDT, HCCH, etc.).Given the above, the issues of pollution of water bodies, and especially bottom sediments of water bodies, with pesticides are relevant, since, accumulating in bottom sediments for decades, they can migrate along trophic chains.Data on the content of the residual amount of pesticides in samples of bottom sediments are presented in Table 4.The study showed that the following pesticides were found in bottom sediments: alpha-HCCH, gamma-HCCH, P,P'-DDE, P,P'-DDT.It was found that the content of haloorganic pesticides in the samples is below the determination ranges presented in the methodology.There are currently no maximum permissible concentrations of pesticides in bottom sediments.The obtained data were compared with the maximum permissible concentrations of pesticides in the soil, approved in the guidelines for the examination of soils of agricultural lands and crop products for the content of heavy metals, residual amounts of pesticides and radionuclides (MU Ministry of Agriculture of Russia dated 15.12.1995).A comparative analysis showed that the actual content of pesticides did not exceed the MPC specified in the standard.
Also samples of bottom sediments were studied by IR spectroscopy.IR spectroscopy is a method for the analysis of substances and materials based on the selective absorption of infrared radiation by a substance when this radiation passes through it.This method was used to study and identify the qualitative composition of the bottom sediment.The IR spectrum contains absorption bands characteristic of bentonite clays.The corresponding IR spectrum is shown in Figure 2. The studied sediment contains, in addition to other minerals, bentonite clays.Natural minerals, including bentonites, have the ability to swell and retain water, contain valuable macro-and microelements, and when added to fertilizers, they can be given controlled properties.

Conclusions
When clearing water bodies from bottom soils and silt, there is a problem with their disposal.A comprehensive study of bottom sediments of the Volga-Akhtuba floodplain was carried out, the agrochemical composition of bottom sediments, the content of water-soluble forms of cations and anions, as well as the content of heavy metals and especially hazardous pesticides were studied.When evaluating the data on the agrochemical composition of bottom sediments, it was found that they have some fertilizing properties and can be classified as calcareous sapropel.The samples of bottom sediments were characterized by the presence of chlorides, sulfates, as well as calcium and sodium.To assess the chemical and toxicological parameters in bottom soil samples, heavy metals and the residual amount of pesticides were determined.It was found that their content does not exceed the maximum permissible concentrations.In the IR spectrum, there are absorption bands characteristic of bentonite clays, which have good fertilizing and water-retaining properties.Given the data obtained, it can be assumed that with proper processing and cleaning of bottom sediments and additional research, it is possible that they can be used in agriculture, urban or forestry as a fertilizer.

Table 1 .
).The content of water-soluble cations and anions in bottom sediments

Table 2 .
Agrochemical composition of bottom sediments

Table 3 .
Chemical and toxicological parameters of the bottom sediment