If access openings are positioned at or below the seasonal high water table, they must be?

When access openings are positioned at or below the seasonal high water table, they must be water-tight to prevent groundwater from entering the sewage system. This is crucial because if groundwater seeps into the system, it can dilute sewage and impair the treatment process, leading to the release of untreated or partially treated effluent, which can pose environmental and public health risks.

A water-tight access opening ensures that the system functions effectively and complies with regulations designed to protect groundwater and surface water quality. This adherence to water-tight requirements safeguards the integrity of the sewage management system, especially in areas where the water table fluctuates seasonally.

Each of the other options, while important for various aspects of sewage system design, does not specifically address the potential for groundwater intrusion. Ventilation may be necessary to prevent gas buildup, accessibility is critical for maintenance but does not address water ingress, and insulation might be relevant for temperature control in certain scenarios but is not a requirement related to the position of access openings relative to the water table.