Selenium Enriched Biogeochemical Food Webs

The spatial heterogeneity of selenium content in rocks, various soils, and especially the peculiarities of its translocation from soils to plants, lead to the formation of so-called selenium biogeochemical provinces (see Chapter 7 Biogeochemical mapping ). These provinces are characterized by a deficient or excessive concentration of this trace metal in all links of biogeochemical food webs. Selenium deficiency in fodder crops is related to the less than 30 ppm content of this element (Table 8). This leads to myopatia (white colored animal tissues), necrotic degeneration of kidney, exude diathesis. The addition of sodium selenite is used for prevention of these diseases. 

The excessive accumulation of selenium, more than 1 ppm, in crops and grasses is related to soils formed on volcanic ash and sedimentary rocks such as shale and sandstone. This leads to chronic selenium toxicity (Table 7). The acute toxicity is at Se content 25 ppm by dry weight. 

The characteristic example of Se-enriched regions is the San loaquin Valley in 

The irrigation systems were constructed in this valley in 1930-1960s. It was found afterward that irrigation drainage water from parts of the San Joaquin Valley contains levels of selenium and other trace elements that have been implicated in bird deformations in the Kesterson Reservoir (Tanji etal, 1986). Depending on location and season, the drainage water contains 10()-14()0/rg/L Se, predominantly as selenate (SeOa, Se-IV), the most soluble form, whereas the California State Water Resources Control Board has recommended an interim maximum mean monthly selenium concentration of 2-5 fig/L in receiving waters and wetlands. 

Irrigation using surface and ground water in this region has substantially altered the physical and chemical nature of the groundwater flow system and finally the total biogeochemical food web. 

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We can see that Se deficit is often monitored in the biogeochemical food webs in the Chita region, Russia, and Latvia and the Se enrichment in the Fergana valley of Uzbekistan. The Se deficit is also connected with the lower values of Cb in the first two regions in comparison with the latter, where selenium is more mobile due to... 

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