Whenever a well to be destroyed is located on a parcel one-half acre or less in area on which is located one or more dwellings, a hole shall be excavated around the well casing to a depth of six feet below the ground surface. The well casing shall be removed to within six inches of the bottom of the hole. The well shaft must be filled in the manner provided in Sections 13.16.580 and 13.16.590; except, that the depth to which impervious fill is required shall be measured from the bottom of the hole excavated pursuant to this section rather than from the surface of the surrounding ground. Such impervious fill shall be allowed to spill over into the excavation to form an effective seal. After such impervious fill has set, the excavation shall be filled with native soil. (Prior code § 7820)