Very common in trace amounts. How much did they find?
I think they found enought so you would not drink the water.
Mustard gas common. I don't think so It is a man made substance. Neither cyanide nor mustard gas occurs naturally in nature. Man has to produce the stuff.
Sarin is a different story. It comes from the castor bean and can be produced in your kitchen. Also it will occur in nature without mans help.
Cyanide is a common byproduct of industry. It is found in most U.S. rivers especially where there is a gold mine up river. Mustard gas can be easily made at home by putting ammonia and chlorine bleach together. Both can combine in the water and could be introduced separately.