Pollution between Stations 117 and 2$Li 



In order to determine the extent of pollution southward 

 from. Station 117, a series of four stations was established on a line 

 running toward Station 25U. The data obtained from the single samples 

 taken at the four stations are shown in Table 93. As far as they go 

 they indicate that pollution extended southward about 3 miles. At k and 

 6 miles from Station 117 there were no Tubificidae, It is not clear 

 why there should be so few Hexagenia larvae at Station 267, since the 

 bdttom material was mud and the station is about the same distance from 

 the channel as Station 2^6, where they were abundant (Table 88) . At 

 Station 268, only 3/8 mile from Station 25ii, there were l62 tubificids 

 per square meter. 



Pollution inshore 



In order lo aet-ermine the extent of pollution along the shore 

 north and south of River Raisin, samples were taken at ten stations 

 situated on a nearly straight line through Station 200. A summary of 

 the results obtained is shown in Table 9U, It may be seen that 

 Station 200, at the river's mouth, was the only one showing heavy pollu- 

 tion, although at Station 2lh, l/U mile south, the number of tubificid 

 worms in the single sample was at the upper limit of the range for 

 moderate pollution. In the next l/ii mile the number dropped sharply to 

 2l6 per square meter, indicating light pollution. No samples were taken 

 between Stations 2l5 and 116F because of the presence of sand-bars which 

 made navigation hazardous. The data from the latter s tation shows that 

 light- pollution extended southward from the river at least two miles. 

 That it did not extend much farther is known from the records of samples 

 taken at Stations 116d, 216, 217, and 218, which are not shown in the 

 table. Station 116D is located two miles southwest of Station 116F, and 

 the others are located at half-mile intervals on a line running from 

 Station 116D toward the shore (Figure 23) • At all of these stations the 

 bottom was composed of hard gray clay overlain by clean sand which con- 

 tained very few animals. Hence the southern limit of the polluted area 

 may be placed about 3 miles from the mouth of the river. 



On the north side of the river we find another example of a 

 station near the river (Station 201) having a lower tubificid count than 

 a station farther away (Station 202), Since the latter is represented 

 by three samples as compared with a single sample at Station 201, we are 

 justified in regarding the zone of moderate pollution as extending to a 

 point between Station 202 and 203. Judging by the record of three samples 

 at Station 20U, pollution extended northward to a distance of less than 

 one mile. At distances of two and three miles, the bottom was hard, 

 composed of clay or solid rock, and there were no accumulations of organic 



2% 



