TABLE 3. MAJOR HYDROGEOLOGI C UNITS OF THE GROUND-WATER RESERVOIR 

 ON LONG ISLAND, NEW YORK— Continued 



Physical characteristics of deposits 



Water-bearing properties 



Till (mostly along north shore and in moraines) composed of 

 clay, sand, gravel, and boulders. Outwash deposits (mostly 

 between and south of terminal moraines, but also interlayered 

 with till) consist of sand, fine to very coarse, and gravel. 

 Glaciolacustr ine deposits (mostly in central and eastern Long 

 Island) and marine clay (locally along south shore) consist of 

 silt, clay, and some sand and gravel layers; includes "20-foot 

 clay" In southern Nassau County and Queens County. 



Clay, silt, and few layers of sand and gravel. Altitude of 

 top generally is 50-80 ft below mean sea level. Occurs in 

 Kings and Queens Counties, southern Nassau County, and 

 Suffolk County; similar clay occurs in buried valleys near 

 north shore. 



Sand, fine to very coarse, and gravel; few layers of clay 

 and silt. Occurs in Kings and Queens Counties, and southern 

 Nassau County; similar deposits occur in buried valleys near 

 north shore. 



Sand, fine to medium, clayey in part; interbedded with lenses 

 and layers of coarse sand and sandy and solid clay. Gravel 

 is common in basal 50-200 ft. 



Clay, solid and silty; few lenses and layers of sand; little 

 gravel . 



Sand, fine to coarse, and gravel, commonly with clayey matrix; 

 some lenses and layers of solid and silty clay. 



Crystalline metamorphic and igneous rocks. A soft clayey zone 

 of weathered bedrock locally is more than 100 ft thick. 



Till has low hydraulic conductivity; I'commonly causes perched- 

 water bodies and impedes downward percolation of water to under- 

 lying beds. Outwash deposits have moderate to high hydraulic 

 conductivity; specific capacities of wells tapping them range 

 from about 10 to more than 200 gpm per foot of drawdown. Good 

 to excellent infiltration characteristics. G 1 ac iol acustr ine 

 and marine clay deposits have mostly poor hydraulic conduct ivi tie 

 but locally have thin layers of sand and gravel with moderate 

 hydraulic conductivities; generally retard downward percolation 

 of ground water. Contains fresh water, except near shorelines. 

 Till and marine deposits locally retard salt-water encroachment. 



Poor hydraulic conductivity; constitutes confining layer for 

 underlying Jameco aquifer. Locally, sand layers yield small 

 quantities of water. 



Moderate to high hydraulic conductivity; contains mostly fresh 

 water, but brackish water and water with high iron content 

 locally in southeastern Nassau County and southern Queens County. 

 Specific capacities of wells in the Jameco range from about 20 to 

 150 gpm per foot of d rawdown . 



Most layers have poor 

 high hydraulic conduct 

 in the Magothy general 

 drawdown, rarely are a 

 in uppermost parts, el 

 excellent quality but 

 south shores. Constlti 

 in western Long Island 

 Has been invaded by sa 

 County and southern Qu 

 shore. 



to moderate hydraulic conductivity; some have 

 "vity locally. Specific capacities of wells 

 ly range from I to about 30 gpm per foot of 

 IS much as 80 gpm per ft. Water Is unconfined 

 sewhere is confined. Water is generally of 

 has high iron content locally along north and 

 utes principal aquifef for public-supply wells 

 , except Kings County where it is mostly absent. 

 .Ity-ground water locally In southwestern Nassau 

 leens County, and in small areas along north 



Poor to very poor hydraulic conductivity; constitutes confining 

 layer for underlying Lloyd aquifer. Very few wells produce appre- 

 ciable water from these deposits. 



Poor to moderate hydraulic conductivity. Specific capacities of 

 wells in the Lloyd generally range from I to about 25 gpm per foot 

 of drawdown, rarely are as much as 50 gpm per ft. Water is confined 

 under artesian pressure by overlying Raritan clay; generally of 

 excellent quality but has high Iron content locally. Has been 

 invaded by salty ground water locally in necks near north shore, 

 where aquifer is mostly shallow and overlying clay discontinuous. 

 Called deep confined aquifer in some earlier reports. 



Hydraulic conductivity poor to virtually zero; constitutes virtually 

 the lower boundary of ground-water reservoir. Some hard, fresh 

 water is contained In joints and fractures, but is impracticable to 

 develop at most places; however, a few wells near the western edge 

 of Queens and Kings Counties obtain water from the bedrock. 



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