The concept of a geochemical barrier is associated with man-made pollution of the environment as a result of the migration of chemicals along with precipitation, underground or surface water flows. The concentration of harmful compounds can reach hazard class 1, and their maximum allowable values can be exceeded several times, which leads to the occurrence of geochemical anomalies in groundwater and reservoirs even at great distances from the source of pollution. Studies of geochemical barriers have provided new information about the possibility of reducing the mobility of toxic compounds.
Definition
The term "geochemical barrier" was first introduced by the Russian scientist AI Perelman. Its essence lies in the designation of the area of the earth's crust, where there is a sharp decrease in the intensity of migration and the concentration of chemicals. As a result, they pass from the state of technogenic dispersion to stable mineral associations. These barriers are used toprotect the environment from industrial pollution.
This theory is most widely used in ecology, geology, geochemistry of landscapes, oceans and seas. A simple example of a barrier is the migration of groundwater saturated with iron ions. Under the ground, this element is almost completely dissolved in the liquid. Upon reaching the surface, iron is oxidized under the influence of oxygen, and the metal precipitates in the form of a s alt, that is, it passes into the mineral phase. The same phenomenon is observed when iron solution is transported through water pipes. In this case, they speak of a man-made barrier.
Geochemical barriers and their classification
Barriers are distinguished by several features:
- By origin (genetic classification): natural; technogenic (arising in the process of human activity); natural-technogenic.
- By size: macrogeochemical barriers, in which a decrease in migration processes occurs at distances of the order of thousands of meters; mesobarriers (from several meters to 1 km); micro-barriers (from a few millimeters to several meters).
- By the nature of the movement of substances: bilateral - migration of flows from different sides, different types of associations can be deposited in the barrier (shown in the figure below); lateral (subhorizontal); mobile; radial (sub-vertical).
- According to the way substances enter: diffusion; infiltration.
Natural and man-made types
Among the above types of geochemical barriers, the following classes are distinguished:
- Mechanical. During the migration of substances, their phase does not change, but they move (most often within the biosphere). An example is the rolling of debris along the slopes of mountains.
- Physico-chemical. Barriers arise as a result of changes in the physicochemical environment. At present, this class of phenomena is the most studied and systematized (its description is given below).
- Biogeochemical (phytobarriers and zoobarriers). They are characterized by a change in the form of the state and a small path of migration. Most often, such a barrier is associated with the accumulation of chemical elements as a result of the vital activity of animals and plants. This class includes both natural and man-made geochemical barriers (waste migration on agricultural land and pastures).
Complex barriers
When several classes of these phenomena are superimposed in space, a complex geochemical barrier arises, which is isolated into a separate independent category. Scientists believe that in natural conditions such barriers occupy one of the leading places. An example is the combination of oxygen and sorption barriers in mountainous areas:
- springs rising to the surface of the earth in gley horizons are saturated with dissolved ferric hydroxides, which are oxidized under the influence of atmospheric air (oxygen barrier);
- precipitating colloids are good sorbents for otherschemical compounds;
- as a result, a second sorption barrier is formed.
The large role of complex barriers is also evidenced by the fact that many mineral deposits were formed due to them.
Varieties of physical and chemical barriers
The following types of physical and chemical barriers are distinguished:
- Oxygen. Oxidation occurs in the presence of a large amount of free oxygen in the waters approaching the barrier.
- Sulfide (hydrogen sulfide). Precipitation of substances in reaction with H2S.
- Gley. This barrier is characterized by a reducing reaction (without free oxygen and hydrogen sulfide).
- Alkaline. As a result of the decrease in acidity, the formation of hydroxides and carbonates, which precipitate into an insoluble precipitate.
- Acid. With a decrease in pH, the formation of sparingly soluble s alts is observed.
- Evaporative. The concentration of migratory substances increases due to water evaporation and s alt crystallization.
- Sorption. There is an extraction of certain substances due to natural sorbents (clay, humus and others).
- Thermodynamic. Increasing the concentration and precipitation of substances with a sharp fluctuation in pressure and temperature. This process is most pronounced in waters containing carbonic acid.
Subclasses
Among the group of physical and chemical barriers, there is also a gradation by subclasses. Tot althere are 69 of them. They differ in acid-base characteristics for each type of barriers.
Among the mechanical barriers, there are subclasses depending on the state of aggregation and other characteristics of the substance in the migration flow:
- minerals and isomorphic impurities;
- dissolved gases (steam);
- colloidal systems;
- compounds of synthetic origin;
- animals and plant organisms.
Examples
Simple examples of geochemical barriers of the physicochemical class are as follows:
- In a humid climate in the forests, a powerful litter of fallen leaves is formed. A distinctive feature of groundwater under such conditions is that it is poor in oxygen. As a result, chemical elements are leached from the soil, including manganese and iron. When they reach the surface, their oxidation begins with the formation of insoluble hydroxides (oxygen barrier). This mechanism leads to the formation of native sulfur deposits.
- If there are deposits of minerals containing sulfides of iron and other metals on an elevated plot of land, then their washing out by natural precipitation contributes to the formation of groundwater with an acid reaction of the environment. In lowlands, under high humidity and anaerobic (oxygen-free) conditions, sulfates are reduced to sulfides (sulfide barrier). Deposits of copper, selenium and uranium are often confined to such a mechanism.
- If the soil is composed of limestonerocks, then in a humid climate, under the influence of decaying organic residues, iron, nickel, copper, cob alt and other elements are leached. Limestones create an alkaline geochemical barrier that helps neutralize acidic groundwater and form insoluble hydroxides.
Social barriers
In modern geochemistry, a new subclass is also distinguished - social geochemical barriers. Their distinguishing feature is that they have not previously arisen in natural conditions for those compounds that are concentrated on them. Barriers of this subclass are considered only in the context of man-made or complex geochemical barriers.
Among them there are 4 subclasses:
- household (landfills of solid or liquid household waste);
- construction;
- industrial;
- mixed barriers (landfills for construction, industrial and household waste).
