the mean surface turbidity was only 4. 5 p. p. m. The 

 probable error was greatest for the bottom also in July. 



Another outstanding feature of the turbidity graph 

 is that it indicates a decided clearing of the surface 

 waters month by month, the mean decreasing steadily 

 from 7. p.p. m. in June to 2. 5 p. p. m. in Septem- 

 ber. When the surface cleared in late June there was 

 an increase in the bottom turbidity. However, for 

 the rest of the summer there was no evidence of such 

 an inverse relationship, for after July there was a 

 marked decrease in the mean turbidity at both the 

 surface and bottom. 



The chemical analyses of water samples collect- 

 ed in harbors and near potential sources of pollution 

 have yielded positive evidences of contamination of 

 the inshore waters. The effects of these various lo- 

 cal domestic and industrial wastes are felt but little 

 in the offshore waters. As a typical example we may 

 mention for discussion the series of determinations 

 made near Ashtabula. Ohio, in August 1929. 



A series of samples was examined from the sur- 

 face, beginning in the Ashtabula River above the 

 bascule bridge and running out past the ore docks 

 through the harbor entrance to the open lake. The 

 results of these determinations are given in the 

 appended table 14 and in figure 39. 



The graph indicates pollution at station 01 in the 

 river. All values plotted are abnormal when com- 

 pared with the conditions in the open lake waters. 

 The oxygen and pH were low and the carbon dioxide, 

 chloride, and turbidity were high. 



It is significant that conditions altered so quick- 

 ly in a distance of 750 meters downstream to the 

 next station. 02. at the ore docks. Here the oxygen. 

 pH. carbon dioxide, and chloride content were rapid- 

 ly 'approaching normal surface values. The recovery 

 from pollution was quite rapid and stations 03 and 04. 

 just inside and outside the harbor breakwall, were 

 nearly normal. 



Station 06. 37 was offshore about 5, 000 meters 

 from Ashtabula. Here conditions were found to be 

 practically normal on each of the 4 summer cruises. 

 The curves going &om the source of pollution in 

 Ashtabula harbor out to this normal station show 



clearly the effect of dilution upon the abnormal lo- 

 cal conditions. Recovery appears to be rapid at this 

 time of the year, at least in the surface waters. If 

 this may be taken as a typical example, it is easily 

 seen how the inshore abnormalities are diluted, dis- 

 sipated, and eventuaUy lost in the lake. That the 

 effects of certain centers of pollution can be detected 

 far out into the lake is nevertheless true, as is brought 

 out in the discussion of that subject in another paper. 



92 



