16413
The authors evaluated Hg bioaccumulation within and among four snake species ( northern watersnakes , Nerodia sipedon ; queen snakes , Regina septemvittata ; common garter snakes , Thamnophis sirtalis ; and rat snakes , Elaphe obsoleta [ Pantherophis alleghaniensis ] ) from a contaminated site on the South River ( Waynesboro , VA , USA ) and two nearby 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