TABLE 5. MAJOR HYDROGEOLOG I C UNITS OF THE GROUND-WATER 

 RESERVOIRS ON THE NORTH SHORE OF LONG ISLAND SOUND 



Geologic unit 

 Pleistocene deposits 



Hydrogeologic unit 



Stratif ied-drtft 

 aqui fer 



Triassic sedimentary 

 bedrock 



Crystalline bedrock 



Sedimentary-bedrock . 

 aqui fer 



Crystal 1 ine-bedrock 

 aqui fer 



Physical characteristics of deposits 



Unconsolidated sediments composed of inter- 

 bedded layers of gravel, sand, silt and 

 clay. Sites of former glacial lakes contain 

 predominantly very fine sand, silt and clay. 

 Deposits occur principally in valley areas. 

 Maximum known thickness is approximately 300 

 ft. 



Unconsolidated nonstratif ied sediment com- 

 posed of various proportions of sand, gravel, 

 silt and clay. D i scent inuous ly mantles bed- 

 rock and is overlain in valley areas by 

 stratified drift. Generally less than 10 ft 

 thick but maximum known thickness exceeds 

 150 ft. 



Consol idated sandstones, shales and conglom- 

 erates interbedded with basalt (traprock). 

 Underlies part of Subregion 3. 



Consolidated crystalline metamorphic and 

 igneous rocks. A soft zone of weathered 

 bedrock locally is more than 100 ft thick. 



Water-bearir.g properties 



Most saturated stratified drift has moderate 

 to high hydraulic conductivities.!^ Lacus- ' 

 trine deposits of very fine sand, silt and 

 clay, however, have low hydraulic conductiv- 

 ity. Major water-bearing unit in the area 

 capable of yielding from 50 to over 2000 

 gpm (gallons per minute) to properly construct- 

 ed individual wells. Estimated long-term 

 yields of selected stratif ied-drift aquifers 

 in Subregions 1, k and 5 range from less than 

 0.5 to over 15 million gallons per day. 



Till has low hydraulic conductivity and is 

 generally thin. Shallow wells tapping till 

 formerly used for domestic supply but most 

 proved inadequate for modern demands and 

 were replaced by drilled wells tapping bedrock. 



Hydraulic conductivity unknown. Contains 



water-bearing fractures and pores in at least 

 the upper several hundred feet. Water quality 

 locally poor. Yields of wells tapping this 

 aquifer range from less than 1 to 305 gpm. 

 The median yield is approximately 11 gpm and 

 the median specific capacity is approximately 

 1.3 gpm per foot of drawdown. 



Hydraulic conductivity unknown. Contains 

 water-bearing fractures in at least the upper 

 300 ft. Yields of wells range from less than 

 1 to 200 gpm but yields greater than 20 gpm 

 are rare. The median yield is approximately 

 7 gpm. 



]_/ Hydraulic conductivity denotes how readily water can move through porous material. 



in order to assess ground-water availability at any given location 

 the major hydrogeologic units have been subdivided or combined according to 

 their ability to yield similar amounts of water to wells. Maps prepared by 

 the U.S. Geological Survey for Subregions 1 through 5 delineate the areas capa- 

 ble of yielding selected amounts of water to individual wells (see Appendix C) . 



Circulation within each ground-water res 

 shallow. Most ground water moves through the upper 

 zone and discharges principally to streams. A deep 

 may be present in the part of Subregion 3 containing 

 rocks. Ground-water evapotranspi rat ion and subsurfa 

 Sound are the other means of natural discharge from 

 The amount of subsurface outflow has been estimated 

 gallons per year) per mile of coastline in Subregion 

 tern of ground-water circulation under natural condi 

 3. 



ervoir is relatively 



300 feet of the saturated 



circulation subsystem 



saturated sedimentary 

 ce outflow to Long Island 

 the ground-water reservoirs. 

 to be 100 mgy (million 



1 ( 37) . The general pat- 

 tions is shown on Figure 



Fresh ground water under natural conditions is generally of 

 excellent chemical quality for most uses. Dissolved solids are less than 

 500 mg/1 with the exception of some ground water in the sedimentary rocks of 



3^ 



