Which of the following is defined as a receptor in relation to onsite wastewater?

In the context of onsite wastewater management, a receptor refers specifically to any entity that can potentially receive or be affected by the discharge of wastewater or effluent. In this case, a water well or surface water is considered a receptor because it can be impacted by contaminants if the wastewater system is not properly designed, managed, or maintained. These natural resources are vulnerable to contamination from nearby wastewater systems, making their protection crucial.

The other options listed do not fit the definition of a receptor in the same way. A septic tank is a component of the wastewater treatment system that collects and treats wastewater; an aeration system is a method used to enhance treatment by adding oxygen to the wastewater; and a wastewater treatment plant is the infrastructure designed to treat wastewater before it is released back into the environment. None of these options receive or are directly impacted by wastewater in the same manner as a water well or surface water, which are directly susceptible to contamination from onsite treatment systems. Thus, a water well or surface water most accurately aligns with the definition of a receptor in this context.