If you are in the States, it would be the department of Health. Call for building codes and planning department, ask them for the health department of building and planning. You should really contact the people in these places first. they may have a grandfather clause that excuses them. You may want to ask them to install composting toilets rather than insist on septic, which could be a real mess, leaching all that broken earth into your lake, and the leach fields for septic still leach into the lake too, OR maybe these people would be forced to sell their homes due to an inability to pay for septic and a troubles soul may come and kill you for destroying their lives.
Just because you think you know best, you might cause a lot of pain and grief; I really recommend communication FIRST. If these people are dumping shit straight into your fishing/ swimming area and don't want to stop, and don't care, then you know you have the law to back your rights. that's what i think.