Many thousands of septic tanks are located across the GGH, servicing individual residences or businesses. Some communal septic systems serve groups of houses and other buildings. In areas far from wastewater treatment plants, subdivision developers may propose large-scale septic systems (known as Large Subsurface Sewage Disposal Systems).
MOECC collects information on large subsurface systems, but information on individual septic systems (where they are located, how old they are, and what their capacity may be) is scattered and hard to assemble. At present, only septic systems that have been identified as significant drinking water threats in Sourcewater Protection Plans are monitored and regularly inspected.
 Individual septic systems for single houses are regulated under the Ontario Building Code Act, 1992. Multi-residential septic systems and systems handling more than 10,000 litres a day are regulated under the Ontario Water Resources Act, 1990.