predictions of the quantity of water that can be withdrawn from the ground- 

 water reservoir at a given site and the hydrologic response to man-made or 

 natural changes. A summary of information on the major hydrogeologic units 

 is shown on Table 3. 



There are two general systems of circulation within the ground- 

 water reservoir. A shallow circulating subsystem, generally under water-table 

 conditions, that discharges principally into streams and a deep circulation 

 subsystem, generally under artesian conditions, that discharges into the bays, 

 the Atlantic Ocean and Long Island Sound. Seepage to springs and ground-water 

 evapotranspi ration are the other principal means of natural discharge from 

 the ground-water reservoir. The pattern of ground-water circulation under 

 natural conditions is shown on Figure 2. 



A water budget analysis quantitatively expresses the inflow to, 

 outflow from and changes in storage within a hydrologic system. Table k is 

 a water budget for approximately half of Long Island (refer to Fig. 11) pre- 

 pared by Franke and McClymonds (J_0) . This table is an approximate summary of 

 relations between the components of the hydrologic system under natural condi- 

 tions. 



Fresh ground water on Long Island, under natural conditions, has 

 a dissolved solids content of less than 50 mg/1 (milligrams per liter) and 

 IS of excellent chemical quality for most uses. The low dissolved solids 

 concentration is due to the generally low solubility of materials in the 

 saturated zone. The pH is generally low (commonly less than 6) which makes 

 the water corrosive to metals and in places the iron content is considerably 

 above the 0.3 mg/1 limit recommended for public supply use (^5) . 



The ground-water quality in the upper glacial, Magothy and Lloyd 

 hydrogeologic units has been summarized on maps by the U.S. Geological Sur- 

 vey (see Appendix C) . The chemical and physical quality of water at any 

 point within the ground-water reservoir results from the natural geochemical 

 conditions and the effects of man's activities. 



The average natural ground-water temperature varies from about 

 50° to 70°F (10-21°C) depending on depth below land surface. The average tem- 

 perature to a depth of about UOO feet is relatively constant and thereafter 

 increases at a rate of about 1°F per hundred feet of depth (5). Seasonal 

 variation in ground-water temperatures in the upper part of the saturated 

 zone ranges from 2° to 20°F. The range in seasonal variation decreases with 

 depth. 



Man's activities have to various degrees affected the natural 

 hydrologic system by depleting the ground-water reservoir, artifically recharg- 

 ing the ground-water reservoir and changing the ground-water quality. The 

 ground-water reservoir is depleted by (1) irrigation (2) increased direct 

 runoff to streams from urbanized areas (3) disposal of sewage effluent to the 

 ocean and (h) export of water to New York City. The total loss is estimated 

 at 125 mgd in 1965^ 60 percent of which (75 mgd) is discharge of sewage 

 effluent to the ocean ( 10) . 



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