EXAMINATION OF WATER SUPPLIES. EXPLANATORY NOTE. The systematic examination of the water supplies of the State was begun June 1, 1887, and has been continued up to the present time. The results of the investigations made during the first two years were published in a special report of the Board upon the Examination of Water Supplies (1890), and of those made during succeeding years in the annual reports of the Board beginning with the 1890 (Twentysecond Annual) report. The special report contains a description of each of the water supplies in the State existing at the date of that report, and the annual reports contain descriptions of new works and changes in existing works. In all of these reports an alphabetical arrangement by towns has been adopted. Sources of water supply are tabulated under the name of the town supplied, other waters under the name of the town in which they are situated. The analyses of water from the larger rivers not used as sources of water supply are given in a subsequent tabulation, headed "Examination of Rivers." The method of making the chemical examinations remains unchanged, and the results are presented in the tables of this report in the same form as in the last one. The samples of water are usually received at the laboratory from twenty-four to forty-eight hours after collection. All surface water and such samples of ground water as contain suspended matter are filtered through filter-paper before determining the color, the residue on evaporation and the albuminoid ammonia in solution. Some ground waters which are perfectly clear and colorless when drawn from the ground become turbid and colored on standing, in consequence of the oxidation of the iron which they contain. In these waters the residue on evaporation is determined without filtration, since this iron is an essential and not an accidental ingredient in the water. In the changes which accompany the oxidation of the iron in waters of this character, they become first cloudy (well described by the word milky) and finally deposit a precipitate of oxide of iron. In the cloudy condition they have a distinct color, which, while it does not have the same significance as in the case of surface waters, and is only a passing phenomenon, is, nevertheless, of interest as showing a color which the water may assume while the oxidation of the iron is in progress. When the iron is all oxidized and precipitated the water may become colorless again. In some cases, however, the iron occurs in combination with organic matter, forming a much more stable body. In such cases the water is of a brown color when first drawn from the ground, and, while the iron begins to oxidize soon after the water is exposed to the air, the process goes on slowly and the water may remain colored and iron continue to precipitate for a long time. Explanatory notes will be given in connection with the tables of analyses for waters containing iron. The color of water is expressed by numbers which increase with the amount of color. The standard used is nesslerized ammonia, as described on page 531 of the Special Report upon the Examination of Water Supplies, 1890, and on page 329 of the Annual Report for 1892. Boston water, as drawn from a tap at the Institute of Technology, had an average color in 1896 of 0.49. Other water supplies in the State have had an average color of from 0 to 1.34. In cases where examinations of a source have been made with regularity for several years, the averages of the chemical analyses of each year are given. There was no change in the method employed in the microscopical examination of water between Nov. 6, 1890, and Dec. 1, 1895. The method employed between those dates is fully described in the Twenty-third Annual Report of the Board for the year 1891 (pages 395-421). Before Nov. 6, 1890, the methods employed were less perfect, so that a smaller proportion of the total number of organisms present in the water was separated from it and observed under the microscope; and, before drawing conclusions from a comparison of the microscopical examinations of waters made before and after this date, the explanatory note on page 70 of the Twenty-second Annual Report for 1890 should be read. Since Dec. 1, 1895, the method for the microscopical analysis of water has been considerably improved by discarding the ordinary glass funnels previously employed, and substituting funnels which have been constructed with stems of equal calibre, to give equal rates of filtration, and with steeper sides, so that those organisms which are of a gelatinous character will be less liable to adhere to the sides of the funnel. By the use of these funnels variations in the numbers of organisms found, which may be caused by dissimilar slopes of the sides of the funnels and unequal rates of filtration, are avoided. To indicate the amount of the so-called Zooglæa observed, the number of individual masses is not counted, but an area equal to 2,500 square microns, or .0025 square millimeters, has been adopted as an arbitrary unit. In publishing the results of the microscopical examinations the same system is followed as last year. The plants observed are classified in four groups, viz.: Diatomaceæ, Cyanophyceæ, Algæ and Fungi. The animals observed are grouped as Rhizopoda, Infusoria, Vermes and Crustacea. The names of the different genera in each group are given with the numbers of each per cubic centimeter, except that, to avoid making the tables excessively long, they are omitted when present only in very small numbers. It is not feasible to make with regard to omissions a single rule which will apply to all cases, because it is desirable to include smaller numbers of animals than of plants, and of the larger animals than of animals generally. Moreover, there are exceptional cases in which it is desirable to indicate the presence of even very small numbers of the more important plants or animals. Two general rules, however, have been adopted in printing the results, viz. : 1. All genera of plants are included in which the total number observed averages as much as 0.5 per cubic centimeter per month. 2. All genera of animals are included in which the total number observed averages as much as 0.1 per cubic centimeter per month. The larger microscopic animals, such as some of the Crustacea, are included, even when present only in very small numbers. Fractions are generally omitted from the table, the nearest whole number of organisms per cubic centimeter being given. Where the number observed is 0.5 or less, the fact that the organism was present is usually indicated by the abbreviation "pr.," but in the case of the larger organisms very small fractions are given. |