During the last two years extensive investigations have been made in of the water tunnel system to locate possible sources of pollution, especially by leakage from sewers into adjacent shafts. This work was prompted by a marked difference in the bacterial quality of water samples collected from intakes at the lake cribs and those from intermediate points in tunnels between the lake intakes and the pumping stations served. As the result of studies made since 1924 a program for protecting water tunnel shafts has been adopted, the development of which will be presented in this paper. In order to distribute water from its six intakes in Lake Michigan to the ten pumping stations, the City of has in operation about 58 miles of tunnels, 22 miles of which are under the lake and 36 miles under the city. A detailed description of these tunnels and the connecting land shafts was given by one of the authors in a paper in The Journal, Vol. 14, page 175, entitled Experiences with Large Land Tunnels in Chicago and, therefore, will not be repeated here Unlike tunnels serving most of the large cities of this country those in are not under pressure. This fact, together with the loss of head resulting from flow through the tunnel, presents unusual complications which make the protection of the tunnel system against pollution from surface sources, and in particular from adjacent sewers, a serious problem. In any level district the depth to which sewers and underdrains are laid usually determines the ground water level due to infiltration. In the ground water level in the vicinity of a tunnel shaft may dip to approximately the elevation of the water in the shaft, which is usually many feet lower than the adjacent sewers. Figure 1