Which type of openings should be cast-in for a septic tank?

The correct choice emphasizes the importance of watertight openings in a septic tank, which are crucial for preventing contamination of groundwater and ensuring the system functions effectively. Watertight openings help to contain all effluent and prevent leaks, which can lead to environmental and public health hazards. This is particularly essential in septic systems, as any failure in containment could cause untreated sewage to escape into the surrounding soil and water sources, potentially leading to significant health risks.

In contrast, drainage openings are designed for the exit of water or effluent, which is not suitable for the containment required in a septic system. Ventilation openings, while necessary for proper air circulation within a tank, do not provide the watertight seal essential for sewage containment. Service access openings, although important for maintaining and inspecting the system, must also be designed to be watertight to prevent any leakage. Thus, the requirement for watertight openings is central to maintaining the integrity and safety of the septic system.