Household wastes move from the internal plumbing system to the septic tank. In the tank solids settle, forming a "sludge" while grease and other light materials float to the surface and form a "scum." The sludge and scum must be removed from the system periodically, typically every one to four years, while the liquid portion of the waste leaves the tank and is evenly dispersed into the leaching field where it percolates through gravel then reenters the soil.

A potential water quality concern is the domestic subsurface sewage disposal system. Failure of such a system will result in an unsightly and odorous discharge and could create a serious health hazard by harboring bacteria, viruses and pollutants which may cause disease (e.g. polio, hepatitis, dysentery).

The Connecticut and New York Public Health Code sets standards for the design, installation, and location of subsurface sewage disposal systems. No part of a septic system may be located within specific distancesof a drinking water supply reservoir or its tributary stream or watercourse, or within a specific distance from a well, depending on soil and pumping conditions.

A septic system that is properly maintained and pumped should not pose great risk to the watershed.

You can find a list of the most common hazardous chemicals and household wastes here.