Impervious materials used in well destruction must have a coefficient of permeability of less than one hundred (100) feet per year. Suitable materials include, but are not limited to, neat cement grout, concrete and bentonite clays. Materials which maybe used as inorganic fill include, but are not limited to, clay, sand, silt, native soils and any of the impervious materials referred to above. No fill material shall contain matter which will pollute the underground water. Used drilling mud shall not be used as a fill material. (Prior code § 7821)