Fresh Water 



297 



some loss of sodium and a pickup of cal- 

 cium and magnesium by the water (Fig. 

 234). Contact with organic matter in the 

 sediments may result in reduction of sulfate 

 ion to sulfide and its partial disappearance 

 from the water. Chloride is the only com- 

 mon anion of sea water that is unlikely to 

 be affected during movement through an 

 aquifer. Bicarbonate (with carbonate) is 

 usually the most abundant anion in ground 

 water, particularly in southern California. 

 Accordingly, an increase in the chloride/ 

 bicarbonate ratio may signal imminent 

 serious contamination by sea water. When 



Figure 234. Change of water composition in Manhattan 

 Beach well no. 7 (70 IE) — about 1.5 km inland — as a re- 

 sult of sea water intrusion. Computations made before 

 1949 are from Kramsky and others (1952); analyses for 

 later computations are from files of Los Angeles County 

 Flood Control District, courtesy E. J. Zielbauer. Between 

 May 1950 and February 1951 the well served as an injec- 

 tion point for recharging the Silverado zone. The solid 

 line represents observed values; the dashed line represents 

 values that would have existed if the native ground water 

 had simply become mixed with sea water in proportions 

 such as to produce the observed chloride contents. Note 

 that observed values are higher than simple mixing ones 

 for calcium and magnesium, but lower for sodium, sul- 

 fate, and usually bicarbonate ions. 



the latter occurs, the total salt content rises 

 to unmistakable concentrations that make 

 the water unfit for domestic, agricultural, 

 and most industrial uses. Serious contami- 

 nation has occurred in many of the coastal 

 ground-water basins (Richter and Marhave, 

 1952; Slater and Richter, 1952), including 

 Mission Valley, San Luis Rey Valley, Santa 

 Margarita Valley, Orange County Coastal 

 Plain, Los Angeles County Coastal Plain, 

 Malibu Creek Valley, and Trancas Creek 

 Valley. Contamination is threatened at 

 many other areas and can eventually occur 

 at still more. 



The useful life of a well may be prolonged 

 by less intensive pumping at the first sign of 

 approaching sea water intrusion. Reduc- 

 tion of pumping resulted in a rise of water 

 table of as much as 5 meters in much of the 

 Los Angeles Coastal Plain between 1955 

 and 1958. However, water needs in this 

 area are so great that more drastic measures 

 are needed to prevent intrusion. An exper- 

 imental fresh-water barrier along 3 km of 

 the coast of Santa Monica Bay was con- 

 structed in 1950 by injection of fresh water 

 into wells spaced about 150 meters apart 

 600 meters inland from the shore (Banks, 

 Richter, and Harder, 1957). By this method 

 the water table in the aquifer was built 

 above sea level as a long ridge, reportedly 

 with little loss of fresh water to the sea. 

 Additional fresh-water barriers are to be 

 constructed in the Long Beach and the 

 Redondo Beach areas in 1960. Other 

 methods of preventing intrusion of sea water 

 that have been used or considered here and 

 elsewhere in the region are recharge of 

 aquifers by spreading or pumping into re- 

 charge wells large quantities of treated or 

 raw fresh water or of treated sewage ef- 

 fluent, development of a pumping trough 

 adjacent to the coast to remove landward- 

 flowing sea water, and construction of arti- 

 ficial semipermanent barriers of sheet piling, 

 asphalt, cement grout, or puddled clay. 



The need for ever larger supplies of fresh 

 water in southern California, coupled with 

 the insufficiency of local ground water and 

 the high cost and political problems of im- 

 portation from Lake Mead, Owens Valley, 



