268 



yards outside the entrance to the canal (station C). As the depth of the water at 

 the upper end of the canal was only about 1 foot at low tide, and about 6 feet at high 

 tide, one sample was considered sufficiently representative of both surface and bottom 

 conditions. At stations B and C, where the depths at low tide were, respectively, 

 30 and 40 feet, separate surface and bottom samples were taken. The results are 

 shown in the accompanying table: 



CONDITIONS AT THE MOUTH OF THE CHANNEL STREET SEWER 



*Sample taken at low water slack, August 17, 1923. **Iodometric method. 



These data show a very low content of dissolved oxygen at station A, nearest 

 the sewer outlet, and a correspondingly low Sorensen value. The value for hydrogen 

 sulfide is extraordinarily high at low tide, owing probably to the presence of a great 

 deal of organic matter in suspension, in addition to the actual amount of hydrogen 

 sulfide in solution. The lowered salinity, as compared with stations B and C, is of 

 course due to dilution with sewage water. 



At station B the dissolved oxygen and pH values are still notably low, though 

 higher than at station A. The dissolved hydrogen sulfide has dropped to a nearly 

 negligible figure. The fact that the surface values for dissolved oxygen are lower 

 than the bottom values, which is contrary to the general rule, indicates that the sewage 

 contaminated water tends to flow out on top of the more saline bay water. This is 

 also shown in the pH values at high tide. 



At station C, fifty yards outside the entrance to the canal, the chemical effects 

 of pollution have almost or altogether disappeared. The salinity and dissolved oxygen 

 values are higher, and the hydrogen sulfide values lower, than is the average for this 

 part of the bay (judging by our results at the Ferry Building; see fig. 104). The 

 Sorensen values are lower than the average, but this may only doubtfully be attributed 

 to the effect of pollution. 



The conditions prevailing at station B indicate approximately the limit of pol- 

 lution that the marine borers are able to survive. Although Teredo navalis has not 

 been found at this locality, Bankia setacea is known to occur here, and to have done 

 considerable damage in the spring of 1922 to temporary piling driven during the 

 construction of the new China Basin Terminal. No Limnoria were found at that time; 

 and it is believed that neither Bankia nor Limnoria are able to survive much farther 

 up the channel than this point. 



The foregoing data indicate that the efl'ect of sewage in San Francisco Bay is 

 quickly dissipated by the strong tidal currents prevailing; sewage pollution cannot be 

 expected to afford protection from borers except actualh' at the mouths of large sewers. 



