It could be methane or H2S. But it won't be H2S since you don't mention
an odor. You can test for methane by half filling a 2 liter plastic
soda and capping it. Then shake it hard and with a lighter or match
held by the cap, uncap the bottle as you squeeze it to make the air in
the bottle flow above/thru the flame. If there is ignition, its
methane. You can remove it with aeratin equipment.

Gary
Quality Water Associates
www.qualitywaterassociates.com
Bulletin Board www.qualitywaterassociates.com/phpBB2