384 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



County, Long Island, is fresh and contains less than 100 p.p.m. of 

 chloride. In parts of Ivings Comity and southern Manhattan, how- 

 ever, the chloride content of the ground water has become as high 

 as 15,000 p.p.m. This indicates contamination of the fresh ground 

 water by encroachment of sea water (Perlmutter and Arnow, 1953, 

 p. 37). Pumping of fresh ground water may cause a reversal of 

 the gradient of the fresh ground water, which usually moves from 

 the high land areas to the ocean. The reversal of the gradient allows 

 the sea water to enter the fresh ground-water reservoir. 



A large body of salty ground water beneath southwestern Nassau 

 County and southeastern Queens County, Long Island, probably is 

 encroaching landward at a rate somewhat less than 100 feet per year 

 (Perlmutter, Geraghty, and Upson, 1959, p. 417). 



Salty ground water cannot be used for municipal water supply, 

 but it can be used by industry, principally for cooling and air con- 

 ditioning. The problem of continued encroachment of sea water 

 can be and is being solved either by reducing the total amount of 

 ground water pumped or by pumping the water out of the ground, 

 using it for cooling, and pumping it back into the gromid. This 

 recirculation is a water-conservation measure that can reduce the 

 total amount of water removed from the ground-water reservoir 

 and thus slow down the rate of salt-water encroachment. 



The sediment carried by streams and deposited in reservoirs also 

 affects its quality. Although some waters have small concentrations 

 of sediment, they may, because of the great total volume of water, 

 carry a large total sediment load. A reservoir built on a river that 

 has a large sediment load may eventually be filled with silt and 

 other sediment and become unusable for further storage. Sediment 

 is also a major problem to the operators of municipal and industrial 

 waterworks, which must have facilities capable of removing sediment 

 from water before the water can be used. 



Some major reservoirs have lost sizable parts of their capacity 

 owing to accumulation of sediment carried by streams flowing into 

 the reservoirs. The sediment load of the Colorado River near the 

 United States-Mexico boundary was 180 million tons per year before 

 construction of dams on the main stem of the Colorado. After 

 Hoover Dam and other dams were built, the sediment was impounded 

 in the reservoirs instead of being transported to the delta at the 

 mouth of the river. Lake Mead, the lake behind Hoover Dam, has 

 filled with sediment at a rate slower than originally estimated, but 

 reservoir capacity needed for storage of water has nevertheless been 

 lost. In addition, the loss of the normal sediment load of the river 

 below Hoover Dam lias changed the regimen of the stream and 

 allowed it to cut its channel deeper than if it carried its normal 

 sediment load. 



