to Ground Water along Coasts. 277 



table below shows the number of wells examined in sev- 

 eral successive zones parallel to the shore and the number 

 and percentage of wells contaminated by sea water in 

 each zone. Three degrees of contamination, based on an 

 arbitrary scale, are distinguished — " trace" (from 25 to 

 100 parts per million of chloride), "moderate" (100 to 

 300 parts), and "high" (more than 300 parts). More 

 than 300 parts per million of chloride is generally appar- 

 ent to the taste. For a few wells where samples could 

 not be obtained, positive statements of reliable persons 

 have been accepted as evidence of contamination. 



Contamination by sea wafer in shallow wells on the New Haven 



Coast. 



Limits of zone 



No. of 



No. of wells contaminated 



Total No. 



Percentage 



in feet from 



wells 



in 



different degi 



^ees. 



of con- 



of con- 



high-tide 



in 









taminated 



taminated 









shore line. 



zone. 



High. 



Moderate. 



Trace 



wells. 



wells. 



0- 25 



13 



4 



2 



3 



9 



69 



26- 50 



17 



1 



2 



5 



8 



47 



51-100 



35 



2 



6 



6 



14 



40 



101-200 



23 



1 



4 



2 



7 



30 



201-500 



31 











1 



1 



3 



The above table summarizes very briefly a large mass 

 of data from which the conclusion is drawn that on the 

 New Haven coast the zone in which the shallow ground 

 water is contaminated by percolation or diffusion of salt 

 water is very narrow. The greatest distance from the 

 shore at which even slight contamination is suspected is 

 250 feet. 



Similar conditions were observed at Cedar Keys, Fla., 

 where the annual rainfall is very nearly the same as at 

 New Haven and where the ground is sandy, comparable 

 to the stratified drift of Connecticut. At Cedar Keys 

 open pits are often constructed on the beaches of small 

 islands to furnish water for stock (fig. 1). The water 

 of the pit shown in fig. 1 contained 516 parts per million 

 of chloride but was usable for stock. Water in a similar 

 pit 120 feet from the reach of normal high tides contained 

 only 92 parts per million of chloride. Of six wells in a 

 zone between the limits of 125 and 350 feet from the high- 

 tide shore line, two were slightly contaminated and in 

 another salt water apparently was induced by heavy 

 pumping. 



