POLLUTION OF UNDERGROUND WATERS

POLLUTION OF UNDERGROUND WATERS

Dr. John C. Thresh, an English scientist, in a paper read before the Association of Water Engineers, London, treated of the “Pollution of Underground Waters and Its Detection.” At starting, the author confessed that he had never yet known of a case where typhoid-fever-breeding water had been traced to the percolation of contaminating matter through several feet of compact soil before it reached the incriminated waters. The disease has been attributed to the use of shallow well water. “But in nearly every instance it is stated that sewage could practically run directly into the well; and, in the cases where there is no record of the condition of the well or of its immediate surroundings, such defects may have existed and failed to be recorded. In very many of these small outbreaks, however, the proof that the disease was due to polluted water is of a most unconvincing character.” He has never investigated a single case, sporadic or other, in which the well water was seriously liable to the suspicion of being the cause, where the well was placed or constructed that sewage had not more or less direct access. He, therefore, concludes that “water, in slowly percolating through a few feet of compact soil cannot carry with it the microbe causing typhoid fever. Where water takes days to traverse the subsoil before reaching the well or spring, the typhoid bacillus is either filtered out or loses its life. Both filtration and time are undoubtedly important factors in waterpurification by the action of the soil. The more readily and directly the polluting matter can reach the water, and the greater the danger, the longer the period which elapses between the polluting matter being deposited on, or in the soil and reaching the subsoil water, and, the more compact the previous soil, the smaller the danger. Hence, it follows that anything in solution may traverse the subsoil and reach the water therein, without being able to carry with it the living bacillus of typhoid fever. Water derived from a compact subsoil can, therefore, be readily rendered perfectly safe by the proper construction of the collecting well, and by adequate protection of a limited area round the source. Where the subsoil is fissured, as occurs in very many formations, unless the fissures are filled with deposits from a more superficial stratum, the risk of pollution of a dangerous character is much more serious. Even if the well is of great depth, or is far from any possible source of pollution, it must be remembered that, if polluting matter gets into an open fissure leading to the well, serious results may follow, even if the contaminating material has to travel a long distance before it reaches the well. Here, time may be a factor. The typhoid and cholera bacilli can live only for a limited time in water; and if these bacilli gain access to the water in these fissures, yet do not reach the well until after a lapse of several days, it is very probable that they will have died before entering the well, and will, therefore, be no longer a source of danger. Un fortunately, however, circumstances may arise greatly accelerating the rate, of flow in the fissures —such as increased pumping, heavy rainfalls, or both—and although, under normal conditions, the well may be a safe source of supply, under the exceptional conditions, it may be a dangerous source. The fact of underground water becoming polluted is too often discovered only by a sudden outburst of typhoid fever. Such would probably never be the case, were the water submitted to periodical examination.” This examination should not be limited to a mere chemical analysis, as systematic bacteriological examinations made frequently soon disclose whether the condition of the water is normal or abnormal. If the latter, when detected, it may be remediable, and thus disease may be prevented. The cause and nature of edntamination, however, are hard, sometimes impossible to discover, usuallv because of the expense involved or the unwillingness of landlord or tenant to allow’ experiments to be carried on. A careful geological survey of the district must first be made; the depth and section of the w’ell and the length and direction of any adits must be known, and the records of the systematic analyses must be considered and compared with the rainfall and with the height of the water in the nearest ponds or streams. Some information must be sought with reference to the direction of flow ot the subterranean water, and as to the cone of depression caused by pumping. The position of the outcrop of the water-bearing stratum may also require to be known, so that it may be examined, if not too far away to be a possible source of danger. If evidence can be obtained of the presence of fissures, and of their extent and direction, it may prove of the utmost value in the investigation. AH the possible sources of pollution must then be sought out and considered in connection with the knowledge so ascertained. Thus the investigator will determine which is most likely to have caused the trouble, and, therefore, the one which should first receive attention.

1 f no connection between this possible source of pollution and the spring or well is shown, attention must be directed to the next most probable source, and so on, until something definite has been determined. The best chemical to be introduced into the suspected well is common salt, lithium salt, the dye fluorescein, ammonium salts —the last, in certain cases, possessing advantages over the others. Common salts is used chiefly because of its being cheaply and easily obtained, quite harmless and easily detected and estimated in the water. If detected in the water, as all waters contain chlorides, the normal chloride must first be ascertained. Then any subsequent increase during the experiment may be attributed to the salt used. Slight variations are often observed in water from the same well. Consequently, enough salt must be added to produce a more marked effect than any recorded in the nor mal water. The quantity must be at least equal to one grain of salt per gallon (= 0.6 grains Cl.). Should it prove necessary to add this amount to a large quantity of water, the weight of salt used must be considerable. For example, it may be necessary to add sufficient salt to increase materi ally the chlorine content of 1,000,000 gals, of water. To do this would require 1,000,000 grains, or 143 lbs. of salt. Some of the salts already mentioned are decolorised by filtration through soil. Others are too expensive or are objection able in other respects. Fluorescein is unaffected by passage through chalk, sand, surface soil, etc., and, when dissolved in water by aid of an equal w’cight of caustic soda, it can be easily detected by the fluorescence, when the dilution is 1 in 100,000,000. Under very favorable conditions 1 in 200,000,000 may be detected. In other words, 1 lb. of fluorescein will distinctly color 10,000,000 gals, of water. It is the cheapest salt to employ, the easiest to apply and the easiest to detect. If used incautiously, however, it may color the whole water supply and alarm the consumers. It is, therefore, better to use salt or ammonium chloride at first; if used in moderate quantities without results, the fluorescein should be employed, which has been largely used for tinting underground streams to trace their course, and to a certain extent for determining the direction and rate of flow of subsoil water. The utility of the tests with salt, fluorescein, etc., cannot be gainsaid.

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