The board of trustees of any such conservancy district shall have power and authority and it shall be their legal obligation and duty to prevent the pollution of any waters from which a water supply may be obtained by any city, incorporated town, individual or village within said district, provided that the authority of the Pollution Control Board of the State of Illinois or its successor as may be fixed by law shall not be superseded and said board of trustees shall have the right and power to appoint and support a sufficient police force, the members of which may have and exercise police powers for the purposes of this Act only over the territory within such conservancy district, and over the territory outside of said district included within a radius of fifteen miles from the intake of any such water supply in any such waters, for the purpose of preventing the pollution of said waters and any interference with any of the property of such conservancy district; but such police officers when acting within the limits of any such city, town or village, shall act in aid of the regular police force thereof, and shall then be subject to the direction of its chief of police, city or village marshals, or other head thereof; provided, that in so doing, they shall not be prevented or hindered from executing the orders and authority of said board of trustees of such conservancy district.