0141G75 



calcareous conditions (U.S. Soil Conservation Service, 1977). Mean 

 pH values of surface soils (0-4 inch) for the background sites and 

 the project area are 8.0 and 7.2 respectively. The pH values in 

 the project area ranged from 4.7 to 8.2 and, except for an area in 

 and near the City of East Helena, were generally >6.5 (EPA, 1986). 

 A pH level of >6 . 5 is considered to be effective in reducing the 

 availability of metals (Chaney 1973, CAST 1976). The selected 

 phytotoxic soil criteria are generally based on soil pH levels 

 greater than 6.5 when these data were available. Other parameters 

 are discussed in the following sections on specific element 

 levels . 



All elemental levels for plants and soils are reported in 

 parts per million (ppm) dry weight basis unless otherwise noted. 



3.1 Arsenic in soils and plants 



3.1.1 Arsenic literature review 



Arsenic is present in all soils, with typical values ranging 

 from 0.1 to 40 ppm total arsenic. In plants, background concen- 

 trations vary from 0.01 to 5 ppm ( Kabata-Pend ias and Pendias 

 1984). Natural elevated soil values of up to 8000 ppm have been 

 noted in a few rare cases (Kabata-Pendi as and Pendias 1984). 

 However, such excessive levels are usually due to soil application 

 of arsenic-containing pesticides, or less frequently, from 

 smelting operations. Inorganic arsenate of low solubility makes up 

 the largest fraction of soil arsenic. The availability of this 

 arsenic to plants and the potential for plant toxicity is depend- 

 ent upon many factors, some of the major ones being: soil pH, 

 texture, and fertility level; and plant species (Wauchope 1983). 

 The interactions possible from these factors complicate the 

 interpretation of phytotoxic soil and plant arsenic levels. In 

 general, soils with higher levels of easily soluble arsenic will 

 increase the risk of reducing plant growth (Walsh et al . 1977). 

 The results of a number of studies regarding toxic levels of 

 arsenic in soils and plants are summarized in Tables 30, 31 and 

 32. 



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