On Site Sewage Systems Practice Test

Setback distance from water sources is determined by how quickly soil can move and dilute effluent. When the soil beneath the leaching bed has a slower percolation rate, the effluent stays in the soil longer and has more opportunity to reach groundwater that could feed a reservoir. The percolation time of 25 minutes per centimeter indicates a relatively slow absorption, so a larger minimum distance is required to protect the reservoir.

With this rate, the minimum safe distance is 15 metres. That means the bed cannot be located within 15 metres of the reservoir; it must be at least 15 metres away. A 20-metre setback would also satisfy the protection needs, but the question asks for the minimum distance that cannot be exceeded in terms of proximity.