Here is an article explaining possible exposures to arsenic.

http://www.atsdr.cdc.gov/csem/arseni..._pathways.html

and

http://www.hc-sc.gc.ca/hl-vs/iyh-vsv...rsenic-eng.php

If you are using MJ bought from unknown sources, the arsenic may be from smoking that. (arsenical pesticides)
I have seen documentaries where law enforcement has encountered camps in Mexico where MJ was grown, and many jugs of pesticide were found in the camps. Those who supply this way only to make money don't care what they use to keep the bugs off the plants...they don't care what happens to the people who buy the stuff.
Local supply by licensed growers for medicinal MJ (hydroponic) are less risky.

The next question I guess would be:
Are you being exposed daily to something with arsenic? Or is this the excretion of a previous dose? To give you an example, a playground in my area poisoned a woman who played softball near a arsenic treated play climbing set. The whole park was closed. Lately parks have abandoned this type of wood, and use other materials to contruct climbing equipment.

You might investigate a quality hair lab test. This can show timing of a poisoning. But if all your hair is short, it might have been cut off by now. I think they can do the test off pubic hair however.

If you do not show Beau's lines on your fingers, then the levels are not critically high. Beau's lines are horizontal across the nails that reveal when the arsenic was ingested. The nail stops growing for a while, then restarts, and the line forms.