400 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



no odor, while vanilla has a strong odor but no taste. The tastes of 

 some substances are really odors due to gases perceived by the ol- 

 factory nerves through the nose or the posterior nares as the substance 

 is placed on the tongue. The majority of the disagreeable tastes 

 observed in water supplies are caused by organic matter in suspension, 

 which produces both odors and tastes. 



Water obtained from ground sources when used immediately is 

 generally odorless. If it is kept in storage under certain conditions, 

 or allowed to stagnate in parts of the distribution system, there is 

 a resulting unpleasant odqr. There are deep well waters that have 

 an odor of sulfur and wells that may be polluted from surface 

 drainage, which smell and taste moldy. The surface waters most 

 frequently develop objectionable odors due to the presence of or- 

 ganic matter caused by decayed or living organisms (usually plank- 

 ton), or to the chemicals used for disinfection of the water supply. 



Complaints are made to the waterworks engineers by a few per- 

 sons who are unusually sensitive to odors when microscopic organ- 

 isms are present in 500 to 1,000 units (a standard unit is an area of 

 20 by 20 microns on a slide, or 400 square microns). When the organ- 

 isms are present in 1,000 to 2,000 units, complaints become more 

 numerous, and when above 2,000 units, complaints are general. Usu- 

 ally algicides are used when the concentration of algae reaches 1,000 

 units. 



The chief odors noticed are aromatic (like geranium), caused by 

 diatoms and a few Protoza; grassy, caused by blue-green algae 

 principally, and by a few green algae ; and fishy, caused by diatoms, 

 green algae, and, chiefly, Protozoa. Among the diatoms, the most 

 common offenders are Asterionella (pi. 3, fig. 5) and Tdbellaria. 

 They first make known their presence by an earthy odor which is 

 generally characteristic of diatoms. As they increase in number, 

 they produce an odor very similar to that of the geranium, then as 

 they continue to multiply in great numbers, their odor is distinctly 

 fishy. Most numerous offenders among the blue-green algae are 

 Anahaena (pi. 4, fig. 2) , Aphanizomenon, Coelosphaerium, Clathrocys- 

 tis, and Microcystis. Their odor of fresh-cut grass changes to that of 

 pungent nasturtium as they increase, until finally when they start de- 

 caying in large numbers, they smell like a vile pigpen. Uroglena and 

 Synura are the worst offenders among the Protoza. TJroglena gives the 

 water an oily, fishy taste and odor. In larger amounts, the flavor 

 becomes more disagreeable, resembling cod-liver oil. A very small 

 amount of Synwu'Cb causes difficulty, as it gives a cucumber, musk- 

 melon, or fishy taste that leaves a bitter after-taste at the back of 

 the tongue. When the Protozoa are alive, the bitter taste is delayed 

 a few moments, but when they are dead and decaying, the bitter 



