The concept of a geochemical barrier is associated with anthropogenic 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 permissible values can be exceeded several times, which leads to the occurrence of geochemical anomalies in groundwater and water bodies even at large distances from the source of pollution. Studies of geochemical barriers have provided new information on the possibility of reducing the mobility of toxic compounds.
Definition
The term "geochemical barrier" was first introduced by the Russian scientist A. I. Perelman. Its essence is to indicate the area of the earth's crust, on which there is a sharp decrease in the intensity of migration and the concentration of chemicals. As a result, they transfer from a state of technogenic dispersion to stable mineral associations. These barriers are used to protect 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 a liquid. Upon reaching the surface, iron oxidizes under the influence of oxygen, and the metal precipitates in the form of salt, that is, it passes into the mineral phase. The same phenomenon is observed when transporting an iron solution through water pipes. In this case, they talk about the technogenic barrier.
Geochemical barriers and their classification
Barriers are distinguished by several signs:
- By origin (genetic systematization): natural; technogenic (arising in the process of human activity); natural and technogenic.
- In size: macrogeochemical barriers in which a decrease in migration occurs at distances of the order of thousands of meters; mesobarriers (from several meters to 1 km); microbarriers (from several millimeters to several meters).
- By the nature of the movement of substances: bilateral - migration of flows from different directions, different types of associations can be deposited in the barrier (shown in the figure below); lateral (subhorizontal); movable; radial (subvertical).
- By the method of receipt of substances: 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.
- Physicochemical. Barriers arise as a result of changes in the physical and chemical environment. Currently, 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 shape of the state and a small migration path. 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 technogenic 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 distinguished 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 that reach the earth’s surface in gley horizons are saturated with dissolved ferric hydroxides, which are oxidized by atmospheric air (oxygen barrier);
- precipitated colloids are good sorbents for other chemical 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 have been formed thanks to them.
Varieties of physico-chemical barriers
Among the physical and chemical barriers, the following types are distinguished:
- Oxygen. Oxidation occurs in the presence of a large volume of free oxygen in the waters approaching the barrier.
- Sulfide (hydrogen sulfide). Precipitation of substances in reaction with H 2 S.
- Gley. This barrier is characterized by a reduction reaction (without free oxygen and hydrogen sulfide).
- Alkaline. As a result of a decrease in acidity, the formation of hydroxides and carbonates that precipitate in an insoluble precipitate.
- Acid. With a decrease in pH, the formation of poorly soluble salts is observed.
- Evaporative The concentration of migratory substances increases due to evaporation of water and crystallization of salts.
- Sorption. The extraction of certain substances occurs due to natural sorbents (clays, humus and others).
- Thermodynamic. Increased concentration and precipitation of substances with a sharp fluctuation in pressure and temperature. Most clearly, this process proceeds in waters containing carbonic acid.
Subclasses
Among the group of physicochemical barriers, there is also a gradation by subclasses. There are 69 in total. They differ in acid-base characteristics for each type of barrier.
Among mechanical barriers, subclasses are distinguished, 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;
- animal and plant organisms.
Examples
The following can be given as simple examples of geochemical barriers of the physicochemical class:
- In humid climates, a thick litter of fallen leaves forms in the forests. A distinctive feature of groundwater in such conditions is that they are poor in oxygen. As a result, chemical elements are leached from the soil, including manganese and iron. Upon reaching the surface, their oxidation begins with the formation of insoluble hydroxides (oxygen barrier). This mechanism leads to the formation of deposits of native sulfur.
- 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 acidic environment. In lowlands, with 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 limestone, then in a humid climate, under the influence of decomposing organic residues, leaching of iron, nickel, copper, cobalt and other elements occurs. Limestones create an alkaline geochemical barrier that helps neutralize acidic groundwater and the formation of insoluble hydroxides.
Social barriers
Modern geochemistry also distinguishes a new subclass - social geochemical barriers. Their distinctive feature is that they did not previously occur under natural conditions for those compounds that concentrate on them. Barriers of this subclass are considered only in the context of technogenic or complex geochemical barriers.
Among them, 4 subclasses are distinguished:
- domestic (landfills of solid or liquid household waste);
- construction;
- industrial;
- mixed barriers (landfills for construction, industrial and household waste).