My installers have my instructions to respectfully decline to work in such a site and have the client contact me. Same goes for spaces with scorpions, venemous snakes, black widows, skunks, unterminated/non-enclosed A/C wire ends, broken glass, etc...Injuries that can be prevented should. Muddy kind of goes with the job some days, but leaking septic pipes are not acceptable. Don't charge more, just don't do the job until whatever the problem is gets cleaned up. Most clients will appreciate you bringing such problems to their attention, and understand your installer's concerns.