gloss tubing 



No. 3 rubber 

 / stopper 



-standpipe 



water surfoce 



sample 

 viol 



-glass tubing 



apporatus in operating 

 position 



Figure 3.--Apparatus for collecting water samples from standpipes. 



for three men. An automatic burette with a 

 three-way stopcock was used for making 

 titrations. 



Reliability of Samples 



Two precautions are necessary to insure 

 collection of water samples that are repre- 

 sentative of points sampled. First, it is 

 essential to leave standpipes in the stream- 

 bed about 24 hours before sampling to allow 

 conditions within the gravel to stabilize. Sec- 

 ond, the withdrawal of large water samples 

 should be avoided to prevent water originating 

 at other levels from entering a standpipe. 



With regard to the first precaution, Wickett 

 (1954) reported that points normally having 



very low dissolved oxygen values required 

 several days after driving a standpipe for 

 their dissolved oxygen levels to return to 

 their normal levels. In the present study, 

 consecutive readings w^ere made at six points 

 over a period of 95 hours after placement 

 of standpipes. At points where oxygen level 

 was relatively low at time of the first de- 

 termination, oxygen levels declined for at 

 least 24 hours. At points having relatively 

 high oxygen values at time of the first deter- 

 mination, consecutive readings did not show 

 any trend in their variation. Data on oxygen 

 levels are given in table 1. Temperature of 

 intragravel water was not uniform at all points 

 sampled. Temperature increased about 4° F. 

 at each point during the 95-hour sampling 

 period. 



