What is a requirement for all septic tanks regarding their watertightness?

The correct requirement for all septic tanks regarding their watertightness is that they must be watertight above seasonal groundwater. This standard is crucial to ensure that the wastewater within the tank does not leak into the surrounding soil, especially when the groundwater level is high. Maintaining watertightness prevents the contamination of groundwater sources, which is vital for both public health and environmental protection.

When septic tanks are watertight above the seasonal high-water mark, they effectively contain waste and allow for proper treatment processes to take place without risk of infiltrating groundwater. This control helps in managing effluent properly, which is especially important in areas with fluctuating groundwater levels.

Other options presenting the idea that the septic tank must be watertight at all times, have secondary containment, or be permeable to allow drainage do not accurately reflect the typical regulatory requirements. The emphasis on being watertight specifically above the seasonal groundwater is to ensure safe and effective operation while accommodating natural changes in the groundwater level.