Chemical contaminants as a factor of soil-subsurface metagenesis*

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The soil and the subsurface region below it are two interrelated natural bodies that comprise the earth’s upper layer from land surface to aquifers. The ratio between these phases fluctuates as a function of environmental conditions, while the subsurface water content ranges from air-dried to completely watersaturated. Because the soil-subsurface water content is controlled by rainfall-irrigation, evaporation, and depth to the groundwater, a vertical water gradient always exists. The soil-subsurface zone, known also as the “critical zone” (CZ), also involves a large microbiological population, which develops under both aerobic or anaerobic conditions, and may affect the composition of the earth environment.

In general, it is considered that the structure and properties of the soil-subsurface zone are the result of natural
processes occurring over geological time scales.Despite the fact that the soil-subsurface has in many cases been affected by human impact, anthropogenic factors in defining the actual matrix and properties of the soil-subsurface have been largely neglected.

Human impacts on soil-subsurface properties have become a subject of interest only in the second part of the last century. In 1966, Yaalon and Yaron proposed that anthropogenically-induced changes in the soil formation process should be considered an integral, independent factor in addition to the five recognized natural-forming factors. In contrast to these natural factors, the rate of change of anthropogenically-induced processes is relatively rapid, with fundamental soil properties being changed within a short time. Yaalon and Yaron (1966) argue that in these changes occur within a new reference system in which the natural soil serves as a parent material at an initial state relative to the new anthropogenically-formed soil. The differences between the rates of impact on soil characteristics of anthropogenic and natural-environmental factors support the view that anthropogenically-induced changes to soil matrix and properties represent an additional, independent soil-forming factor.

Over the last decades the anthropogenic role in soil formation has received further recognition, human factor being added to the classical natural factors In a recent, comprehensive position paper on anthropogenic transformation of the earth’s soil, Richter (2007) reviewed the existing approaches on human impacts on soil formation. He distinguished between contemporary and historic influences and suggested a possible rapid change of the ecosystem on a time scale of decades as a frame for defining meta-genesis of new contemporary soils. Throughout this period, human recognized influences on soil-subsurface modification have been reduced mainly to physically- and mechanically-induced changes, with chemical impacts being essentially neglected. However, toxic chemical compounds – which are produced and used by modern society in enormous quantities – reach the land surface directly or indirectly and may contaminate the soil-vadose-aquifer zones.

While reviewing a large number of research papers on contaminant interactions in the soil-subsurface geosystem, during preparation of our book “Contaminant Geochemistry” (Berkowitz et al., 2008), we observed that most research has focused on effect of the soil-subsurface environment on the fate of contaminants; while the impact of chemical contaminants on the soil-subsurface region has been neglected. Some studies have noted contaminant-induced changes in the soil-subsurface matrix and properties, but these changes are generally considered as deviations from the “normal” situation which will disappear by natural attenuation or by restoration procedures.

However chemical contaminants often induce significant alteration of the soil-subsurface matrix and properties which may lead over a “lifetime” scale to irreversible changes. As the soil-subsurface region is an open thermodynamic system, it will never return to its initial state following exposure to chemical contamination, and new soils with different matrices and properties will be formed by changing the existing system. As a general perspective we present possible irreversible changes in the matrix and properties of soil and subsurface natural systems as a result of anthropogenic chemical contamination (Table 1).

As noted above, huge amounts of toxic chemicals are directly or indirectly released onto the land surface, which by precipitation and/or irrigation are transported through the soil layer and vadose zone into groundwater systems. Because we are interested in the irreversible impact of chemical contaminants on the soil-subsurface matrix and properties, it is important to consider their behavior in the “critical zone” (CZ). Organic contaminants, for example, may have a negative (toxic) or a beneficial (energy source) effect on biota and consequently on their persistence or natural attenuation in the CZ environment. Natural biodegradation of organic contaminants will decrease when the released contaminants have a direct toxic effect on biota, causing a decrease in biological activity. Under these conditions contaminant persistence in the soil-subsurface system will increase. When an organic contaminant represents a source of energy for the natural biota, contaminant degradation will occur together with metabolite formation. This biologicallyinduced persistence-transformation process will affect the impact of contaminants and their metabolites on potentially irreversible changes in soil-subsurface matrix and properties. Natural organic compounds may serve as ligands for inorganic contaminants such as heavy metals reaching the CZ, by forming organometallic complexes. Some forms of these compounds may be retained or transported within the CZ in a manner different than that of the original heavy metal contaminant. However within the CZ, organo-metallic compounds can be biodegraded by natural microbial populations. In this case, too, the impact of heavy
metals on the soil and subsurface matrix will be different than that of the pure heavy metal compounds. As a consequence, to consider and predict the extent of irreversible changes in soil-subsurface matrix and properties, as induced by land disposal of anthropogenic contaminants, we must examine the fate of chemicals within the CZ environment.

In conclusion we consider that under specific conditions chemical contaminants may irreversibly change the matrix and the properties of the soil-subsurface region. In many cases, a contaminated soil-subsurface system that formed under natural environmental conditions over geological time may be transformed over a short “human lifetime” into a new body with different matrix and properties. Once released on the land surface, chemical contaminants may become a forming factor of a new soil-subsurface regime.

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