68 



LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



with sea water disperses the bacteria, and this effect must be accounted for 

 in order to observe the death rate. In complicated estuarine situations, 

 where there are several sources of contamination, it becomes especially 

 difficult to differentiate between mortality and dilution. The recent develop- 

 ment of an instrument for the continuous recording of salinity and tem- 

 perature, however, has made it possible to conduct more rapid and accurate 

 surveys of hydrographic conditions. From the data thus obtained the 

 rate of dilution of the introduced contaminated water can be estimated. 

 It is this possibility which has stimulated our interest in the field. 



A comparatively simple picture of the fate of sewage bacteria in sea 

 water may be found at the Sewer Outfall at New Bedford, Massachusetts. 

 Here the sewage is introduced from a seven-foot diameter outfall pipe at 

 a depth of 30 feet, 1100 yards offshore into water that is relatively uncon- 

 taminated. During the rising tide the flow of sewage is decreased and 

 sometimes stops completely. During the falling tide the sewage wells 



Fig. 3. The salinity and the indicated 

 numbers of organisms producing gas 

 from lactose broth at various distances 

 from the New Bedford Sewer outfall. 



out of the pipe in sufficient volume so that it is readily detectable. It is, 

 indeed, difficult to obtain samples directly in the upwelling column since 

 the boat's course is deflected by the rising and spreading current. A 

 tidal current flow of about half a knot sweeps past the location of the 

 outfall, and, on the ebb, carries the polluted water seaward. Several 

 series of observations on the distribution of salinity and coliform bacteria 

 have been made at this location. 



The surface distribution of salinity and bacteria along the axis and 

 across the axis of the tidal current are both shown in Figure 3. As 

 would be expected, the salinity of the outfall water is low com- 

 pared to the surrounding sea water of Buzzards Bay. The introduced 

 sewage is diluted with 13-14 volumes of sea water by the time it appears 

 at the surface. In crossing the axis of current in the neighborhood of the 

 outfall, the bacterial numbers increase to a maximum at a location near 

 the outfall and then decrease again on the other side. The distribution of 

 salinity and of bacteria along the axis are similar except that the down- 

 stream distances required to reach a given concentration are greater. 



