How to construct a water tight and gas proof enclosure in a highly contaminated area safely?

Dear Friends, We are going to built a 20 m inside diameter and 13 m deep enclosure using secant pile system/caissons to construct a sewage pumping station in a highly contaminated area. Presence of toxic gases such as methane, Hydrogen sulphide etc combined with water/oil are expected from the surrounding soil. In order to arrest the flow of gases combined with water to the excavations we need your advise to adopt a suitable technique. Did anyone have any options other than secant piling and well point system  to be used for safe enclosure to construct a sewage pumping station inside? The enclosure should be water tight and gas proof. The  Project is a sewerage network project in Bahrain and the pump station is to be constructed in a highly contaminated Bay area. Land reclamation works has already commenced. We have sent inquiries to several specialists in this field and waiting for their feed back now. Please share  your ideas with me. Regards, Davis Project Manager JASCO, Bahrain Mob: 00973 39212572 Email:akdavis71@yahoo.com