. Water Supply and Irrigation Papers of the United States Geological Survey . SURFACE WELLS. 69 direction in which the ground water is moving, and sources of con-tamination above the well can therefore be foreseen. It is true, how-ever, thai the depression of the water table in the immediate vicinityof the well will permit il to draw impurities from all directions,especially in the dry months, when the depression extends considera-ble distances. In the thickly settled districts of cities and villages itis extremely difficult to locate surface wells so that they will nol bepolluted to a greater
. Water Supply and Irrigation Papers of the United States Geological Survey . SURFACE WELLS. 69 direction in which the ground water is moving, and sources of con-tamination above the well can therefore be foreseen. It is true, how-ever, thai the depression of the water table in the immediate vicinityof the well will permit il to draw impurities from all directions,especially in the dry months, when the depression extends considera-ble distances. In the thickly settled districts of cities and villages itis extremely difficult to locate surface wells so that they will nol bepolluted to a greater or less extent. Contamination from cesspools is much more dangerous than con-tamination from surface filth, for the upper few feet of the soil con-tain living organisms which purify and destroy organic impuritiesslowly seeping downward, while the organic matter in cesspools isimmediately contributed to the subsoil without the action of thenitrifying organisms. The author has noted 18 cases of typhoid feverin one district of a city, the families drawing their water from 18. Fig. 35.—Diagram illustrating danger of contamination of surface wells from cesspols. neighboring wells. The city was built upon the slope of a river ter-race, and the alternation of wells and cesspools was essentially asillustrated in fig. 35. In this particular case pure artesian water fromflowing wells could have been had by drilling about 1,000 feet. XONUNIFORMITY OF NEIGHBORING WELLS. It is a not uncommon experience to find that water can not beobtained a short distance from a good well. Such a discovery alwayscauses considerable comment, while the large regions in which groundwater is found at very uniform depths call forth no comment what-ever. Irregularities in the strata, especially in the deposits of clayin the glacial drift, account for most of the observed diversity ofsupply. Fig. 36 illustrates an extreme form of irregularity which wasobserved in East Schleswig-Holstein. By the action of the i
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