[4]. Under these conditions, sewage discharged at a depth of 1,000 feet 

 would be less likely to contaminate surface waters than would sewage 

 discharged near or above the thermocline. Bacterial survival time would 

 be a less important consideration at the greater depth, so far as man is 

 concerned. 



The brief survival period of E^ coli cells in containers floated on 

 the surface of the ocean and exposed to sunlight (0- , 1-, and 4-hour 

 samples. Table 2) was not surprising. It is well known that sunlight 

 kills E^ coli in shallow dishes of water [5,6]. 



The apparent germicidal properties of sunlight on the samples 

 exposed several feet below the ocean surface for 24 and 48 hours (Tables 

 1 and 2) was somewhat unexpected. These samples were tethered to the 

 nylon line at a distance of approximately 10 feet from the surface. The 

 short cord by which they were attached to the line may have permitted 

 the samples to rise a few feet but not to the surface. The samples, no 

 doubt, oscillated in depth with the waves; but, in any event, the sunlight 

 had to penetrate several feet of water to reach the E^ coli cultures. 



It is generally believed that, at the most, the germicidal radiation 

 in sunlight penetrates 1 meter in seawater [5]. Zobel [6] gives 3 

 meters as the limiting depth for the penetration of abiotic radiation, 

 but he concludes that at greater depths than 10 to 20 centimeters the 

 radiation is too feeble to sterilize seawater. 



In the last of the sunlight experiments at CEL, germicidal radiations 

 were able to penetrate both pyrex glass and polyethylene plastic. Pyrex 

 glass is opaque to wavelengths shorter than 2,800 Angstroms. Its trans- 

 parency gradually increases with longer wavelengths, and it transmits 

 greater than 90% of radiation from 3,600 to 7,000 Angstroms [7]. 

 Polyethylene is opaque to wavelengths shorter than 2,270 Angstroms. Its 

 translucency gradually increases with longer wavelengths, and it transmits 

 a relatively high percentage of ultraviolet and visible radiation longer 

 than 2,800 Angstroms [8]. Finally, that part of the sun's radiation 

 that is composed of wavelengths shorter than 2,920 Angstroms is cut off 

 completely by the atmospheric ozone layer and by oxygen [9]. The 

 results of the CEL experiment indicate that the intensity of sunlight 

 radiation with wavelengths greater than 2,920 Angstroms must be great 

 enough to kill E. coli cells in seawater. 



An extensive investigation by Lukiesh [7] at General Electric 

 Laboratories, Cleveland, OH, indicated that the maximum germicidal 

 effectiveness in the killing of E^ coli in shallow dishes of water is 

 exhibited by radiant energy with wavelengths of 2,537 to 2,575 Angstroms. 

 However, with high enough intensities and prolonged exposures, all wave- 

 lengths in the ultraviolet, and even in the visible spectrum, were 

 germicidal. At a wavelength of 4,000 Angstroms, 10,000 times as much 

 radiant energy, and at a wavelength of 7,000 Angstroms, 100,000 times as 

 much radiant energy were required to kill E. coli cells as was required 

 at a wavelength of 2,540 Angstroms. Hence, even though surface sunlight 

 contains no radiation shorter than 2,920 Angstroms, it is germicidal, 

 apparently because it contains some ultraviolet radiation with wavelengths 



