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Total Hg ( THg ) concentrations in northern watersnake tail tissue at the contaminated site ranged from 2 . 25 to 13 . 84 mg / kg dry weight ( mean : 4 . 85 +/- 0 . 29 ) , or 11 to 19 times higher than reference sites .
 Hg
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 Hg