Geo-ecological aspects of the de-icing chemicals’ impact on the geological environment of urbanized areas.

02017


Introduction
Every year the number of cars on the roads is increasing, new roads are being built, the need for road surface maintenance and road safety is growing.The most common method of cleaning roads from snow and ice is the treatment of roads with de-icing chemicals, namely salt mixtures.The following salts are mainly used: sodium chloride, calcium chloride, magnesium chloride.These substances melt the ice, after which the chemical mixture penetrates the soil and groundwater.It should be borne in mind that these chemical agents not only melt the ice, but also further enter into chemical reactions with soil components, which can lead to negative consequences.Thus, soil fertility is significantly reduced under the influence of salts.De-icing chemicals, which include salts, have been used to melt ice on roads since the late 19th century.The de-icing chemical annually penetrates the soil and groundwater, causing techno-genic soil salinization.For such a long period of time the use of salt on the roads, it has not only penetrated into the upper layer of the soil, reducing its fertility, but also penetrated into the soil massif, complicating the current geo-ecological situation.
According to the results of soil phyto-testing contaminated with a de-icing chemical, a significant inhibition of plants caused by chloride poisoning was established -a delay in seed germination, a reduction in the size of shoots.Moreover, the inhibition of seedlings was observed already at a minimum concentration of sodium chloride of 20 g/m 2 , and at a maximum dose of 150 g/m 2 of all chloride agents, the suppression of the roots of seedlings reached 90-100%.Studies have shown that magnesium chloride-based agents have less effect on higher plants and soil microorganisms compared to calcium and sodium chlorides.[1,2] Due to incomplete purification of sodium chloride from impurities, the arsenic contained in the technical salt enters the soil.As a carcinogen, arsenic can have an extremely negative impact on soil microflora in the long term.[3] A direct dependence of soil pollution and the condition of roadside plants on traffic intensity has been established.Pollution of forest bio-geocenose can be traced from at least 300 m from the roadway to the maximum at the edges and in a strip up to 35 m from the roadbed.It should be noted that there are also a number of trees and shrubs that are resistant to pollution by de-icing chemicals.[4] Based on geochemical monitoring of the southern part of the Eastern Administrative District of Moscow with a dense road network, including the Moscow Ring Road, sh.Entuziastov, Svobodny avenue, st.Plekhanov, Metallurgov, a group of researchers at Lomonosov Moscow State University over a 21-year (1989-2010) period concluded that the mineralization of the soil solution and the dense residue in the district increased by an average of 1.5 times, compared with the regional background level -by 16-20 times.Since the beginning of the 2000s, solonetzic soils have been recorded on the major highways of Moscow, which did not exist in this area in the 1980s.The content of exchangeable sodium in the surface layer of soils reached 24-43% of the total cations.It should be noted that with a change in the compositions of de-icing chemicals used, the nature of soil salinization also changes.Thus, due to the increase in the prevalence of agents containing calcium chloride (CaCl 2 ), by 2010 the chloride-sodium salinization of soils was replaced by chloride-sodium-calcium.It makes sense to continue monitoring the techno-genic halogenation of urban soils and, based on it, taking into account the potential danger of long-term accumulation of salt agents in soils, to develop scientifically based application rates for de-icing chemicals containing salts [5][6].
To optimize the application rate of chemicals, you can also use modern automatic vehicle location systems (AVL) and road weather information systems.[7] In 2009, employees of the Department of Engineering and Ecological Geology of the Faculty of Geology of Moscow State University studied the effect of road samples with the remnants of a de-icing chemical on soil, the underground space of the city, skin, clothing, vehicles, humans and animals.The high mineralization of de-icing compounds and, as a result, their aggressiveness is also due to an increased pH (up to 8.2).Under the influence of de-icing chemicals, soil covers are alkalized, a reducing environment is formed in them, and a saline solution of residues of anti-icing reagents dissolves many organic and inorganic compounds.An increase in soil salinity leads to an increase in the level of electrical conductivity of soils, which in turn increases the intensity of migration of stray electric currents, the corrosive electrochemical aggressiveness of soils increases, corrosion of metals in the soil strata of the city is more intense, sections of pipelines are destroyed, water and wastewater leaks from underground utilities occur, which also leads to the development of negative engineering-geological processes, such as flooding, in-situ erosion, suffusion, and others.Groundwater pollution occurs, mass transfer processes are activated.The structure of the soil is changing, aggressiveness and filtration capacity are growing, which leads to increased flooding of building foundations.Under the influence of a de-icing agent, the composition and state of soil microorganisms, as well as the physical and mechanical properties of soils, decrease their bearing capacity, which causes the destruction of foundations, asphalt pavements and other structures.[8] V.A. Korolev in 2009 proposed to minimize the amount of de-icing chemicals used, to strengthen control over compliance with the technologies for their use and the consequences of their use, and to increase responsibility for their misuse.[9] In a study by V. A. Korolev and A. K. Gornyakov in 2018, the consequences of the use of anti-icing reagents were considered in detail.The ingress of residues of these reagents into soils leads to a technogenic change in their physical and mechanical properties.Especially critical is the impact of soil salinization on their bearing capacity, which decreases and may not comply with current standards.In addition, salinization of subsoils leads to the formation of large aggregates and an increase in pore sizes, which, in turn, increases the filtration capacity of soils.As a result, there is a danger of suffusion processes in soils.If suffusion is caused by water leakage from pipelines damaged by corrosion, then this process can progress to in-situ erosion, which can lead to the formation of dips on the surface and the occurrence of emergencies.He also notes that the assessment of the impact of DIC in the operation of the city's housing and communal services is not yet reflected in the regulatory technical documents on engineering and environmental surveys and environmental monitoring.Thus, the problem of the effect of de-icing chemicals on soils has its own serious consequences that require attention and research [10].
In 2003, Mikhail Zabelin in his work «Physico-chemical mechanisms of karst formation» noted that karst is a phenomenon of selective destruction of rocks, partially or completely consisting of soluble minerals, and described the features of carbonate karst.[11] It is the high saturation of mineral waters with carbon dioxide and sodium chloride that causes the increased aggressiveness of waters to carbonate rocks.And the formation of the aggressiveness of natural waters is the main factor in the emergence and development of carbonate karst, which was considered in detail by A. A. Kolodyazhnaya in 1970.[12]

Materials and Methods
The study is interdisciplinary in nature and is based on the analysis of some articles and the synthesis of a large number of heterogeneous materials relating to certain aspects of geo-ecology.A geo-ecological assessment of the urbanized territory is given, taking into account the use of de-icing chemicals.

Results.
Subject to consumption rates, the de-icing chemical does not cause the development of voids in the soil massif and subsequent deformation of structures.However, the violation of technology contributes to the development of geological processes, which are of great danger.So, in the city center, a collapse of the soil, destruction or deformation of the building can occur due to the fact that the bearing capacity of the soil has decreased, the physical and chemical properties of the soil have changed, the rocks have been washed out, and cavities have formed in the massif.
The de-icing chemical is not the only cause of these adverse events, but it is one of the important factors, the influence of which, unfortunately, is not taken into account.Manufacturers of de-icing chemicals claim that their mixtures are environmentally friendly and will not harm with the right processing technology, and this is true.But it is not the agents themselves that cause more harm, but the fact that they enter into chemical reactions with the environment -with asphalt pavement, with car wheels, forming toxic compounds, after which they penetrate the soil and the reactions continue.The cleaning technology is also, as a rule, not followed, the housing and communal service workers scatter the reagent "by eye", as it turns out, and not according to the norms.[13] Thus, on the roads you can find handfuls of salt mixtures, the consumption of which is exceeded several times.And under such circumstances, de-icing chemicals cause great harm to the environment and safety.(fig. 1) Let us turn to the environmental aspect of the influence of the de-icing chemical on a specific example.A large-leaf linden (Tilia platyphyllos Scop) was planted on Komintern Street in Ivanteevka (Fig. 2).
The root system of the large-leaf linden is pivotal, but with well-developed lateral roots.In the first 7-8 years, the linden tree has one tap root, then it forms a powerful, deep tap root system with far diverging lateral roots and a large number of thin superficial roots.The linden was planted in 2019 at the age of 5, which means the tree is 9 years old and in addition to a deep taproot, it has many small ones that actively interact with soil components.Lindens are demanding on soil conditions; they cannot tolerate salinity, acidic and dry soils [14].After the action of sodium chloride on the growth zone of the large-leaf linden (Tilia platyphyllos Scop), it withered, which confirms the detrimental effect of the salt mixture of the agent.(Fig. 3) As a result, there are no more plants in the place with a high concentration of the agent, and even part of the salt remains on the surface.(Fig. 4).
Let us consider in more detail the chemical aspect of the interaction between carbonate rocks (CaCO 3 ) and sodium chloride (NaCl).
The process of dissolution of substances is due to the interaction of particles of the dissolved substance with solvent molecules.The solubility of solids in liquids depends on the nature of the solute and solvent, on temperature, and can vary over a very wide range.The solubility of salts is affected by temperature, pressure, solvent, common ions, acidity, hydrolysis, complexation, amphotericity, foreign salts.
Salts such as NaCl, MgCl 2 greatly increase the dissolution of carbonate rocks, which was proved by E. B. Sternina in 1952 [15] It is also worth noting that sodium chloride solution (NaCl) destroys the carbonate film (CaCO 3 ) on the pipe surface.
The solubility of all crystalline forms of calcium carbonate increases slightly with increasing temperature.The solubility of calcium carbonate in a sodium chloride solution is much greater than in water, even at a NaCl concentration close to saturation.(Table 1.2) [16] Тable 1. Solubility of CaCO 3 in water.[16] Теmperature Based on tables 1 and 2, the following graphs were compiled (Fig. 5.6)  Thus, the dissolving power of water increases sharply with the addition of NaCl salt (Fig. 8), which means that carbonate rocks dissolve more actively.Within the concentration of NaCl solution from 35.8 g/l to 150 g/l, it most actively dissolves CaCO 3 carbonate rocks.Consumption rates of DIC mainly vary from 30 to 160 g/m 2 .The average content of DIC solution will be 30-160 g/l with a water layer of 1 mm per m 2 , with a layer of 2 mm per m 2 15-80 g/l.Thus, under conditions of a small amount of solvent or an excess of the consumption rate, the concentration of DIC falls within the range of values indicated on the CaCO 3 solubility plot depending on the concentration of the NaCl solution (Fig. 8) and poses an increased danger to the process of dissolution of carbonate rocks.

Discussion
The research conducted made it possible to evaluate the effect of sodium chloride salt, which is the main component of de-icing chemicals, on the geo-ecological environment of urban areas.The fact that sodium chloride in the composition of aggressive groundwater accelerates and increases the dissolution of carbonate rocks was revealed in 1952, but in recent years this fact has become particularly relevant due to the growth of urbanized areas and a significant increase in the consumption of salt mixtures of a de-icing agent, which is a large danger to the geo-ecological environment of the city.

Conclusion.
Thus, a systematic solution to the problem of harm from the use of sodium chloride as a de-icing chemical is to develop measures to control its use based on regular geochemical monitoring of the territory.It should be noted that it is necessary to assess the content of sodium chloride in groundwater and monitor its concentration, to avoid a particularly dangerous concentration of 30-150 g/m 2 .It is also necessary to carry out a local assessment

Figure 1 .
Figure 1.Storage of de-icing chemical on the surface of the soil horizon.

Figure 2 .
Figure 2. Large-leaf linden in the area without the influence of DIC

Figure 3 .
Figure 3. Withering of a tree under the influence of DIC in 3 months.

Figure 4 .
Figure 4. Complete degradation of the soil layer at the site of local storage of the de-icing chemical

Figure 5 .
Figure 5. Solubility of CaCO 3 of various solid phases in water depending on the temperature.

Figure 6 .
Figure 6.Solubility of CaCO 3 depending on the concentration of the NaCl solution