In constructing, maintaining, repairing and servicing any such sewer line the Mission township main sewer district No. 1, its agents, servants or employees: (a) Shall not use any dynamite or other explosives on or adjacent to the grounds of said state park in such a manner as may damage the buildings, trees, shrubs or plants presently existing in said state park; (b) shall do any work necessary in the construction, maintenance, repair or service of said sewer lines in such a manner as not to destroy or damage any building or tree or underground pipes, conduits or other fixtures presently existing in said state park; (c) shall after construction, maintenance, repairing or servicing such sewer lines backfill ditches or holes in a good and workmanlike manner, leaving a top layer of at least six inches of good fertile, black soil, settling the fill in such ditches with water and sodding such ditches or holes at the completion of such construction, maintenance, repair or service; (d) shall maintain any manholes necessary to such sewer line so that such manholes do not protrude above ground level in such a manner as to interfere with mowing of said grounds or as to constitute a hazard to visitors thereon.