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3.0 LITERATURE REVIEW AND HAZARD LEVELS FOR SOILS AND PLANTS 



Heavy metal levels in soils and plants are of concern for two 

 primary reasons: 1) decreased crop and livestock production; and 

 2) the introduction of certain toxic metals into the food chain 

 and their consumption by humans. The "soil-plant barrier" (Chaney 

 1983) reduces the risk from exposure to certain elements which are 

 either not translocated to plant foliage (lead) or produce 

 phytotox ici ty in the plant at concentrations safe for animals 

 (zinc, arsenic). Of the selected four metals evaluated in this 

 manuscript (arsenic, cadmium, lead and zinc) only cadmium readily 

 passes the soil-plant barrier. It should be noted, that ingestion 

 of soil and dust by livestock or humans bypasses the soil plant 

 barrier and increases the risk of exposure to toxic concentrations 

 of all pollutants. 



It has been shown that extractable soil levels of lead, 

 cadmium and zinc generally show better correlations with plant 

 uptake than do total soil levels (Neuman and Gavlak, 1984). 

 Chelating agents such as EDTA and DTPA have been extensively used 

 to evaluate agronomic characteristics of soils and overburden 

 materials in western states. The correlation of total or extrac- 

 table arsenic levels with vegetation uptake has been more diffi- 

 cult to define and a special discussion has been included for a 

 review of this problem. 



Numerous technical problems present themselves when universal 

 phytotoxic hazard levels for soils and plants are to be defined. 

 Some of the more important of these are: the toxic element, soil 

 pH, soil organic matter content, soil cation exchange capacity 

 (CEC) , soil texture and the plant species involved. In general, 

 there is an inverse relationship between microelement availability 

 to plants and the soil pH (Logan and Chaney 1.983). Molybdenum and 

 selenium are the only notable exceptions, both of which become 

 more available at higher pH. The Soil Survey of Broadwater County 

 Area, Montana includes a portion of the Helena Valley study area 

 and all background sites. All mapped soil units, except small 

 areas which are poorly drained, exhibit calcareous to strongly 



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