SEA WATER FROM GROUND SOURCES 



181 



inch in diameter) encountered at this 

 depth, 



Kesults of testing confirmed, in general, 

 the tlieories of Barksdale et al. (1936) re- 

 garding deptli of the fresh-water lens. 

 This is sho^Yn as follows (average ground- 

 water level 11 feet below surface: salinity 

 of the bay 26 to 27 parts per thousand) : 



Salinity of 

 well water 

 in parts 

 At depth of — per thousand 



11 feet 



27 feet 



52 feet 20 



58 feet 25 



65 feet 26 



70 feet 23 



However, the zone of diffusion was not 

 as narrow as indicated by Barksdale et al. 

 (1936). Another unexpected result was 

 the reduction in salinity below 65 feet, in- 

 dicating that fresh water was intruding 

 from below, perhaps under artesian pres- 

 sure resulting from the increase in eleva- 

 tion of the stratum toward the adjoining 

 land mass. The effect of these factors is 

 to confine 'the zone of higher salinity wa- 

 ter to a relatively narrow band. The 

 thickness of this band can be expected to 

 change in response to changes in the sa- 

 linity of the bay and the height of the 

 fresh- water head above sea level. 



On this basis, we decided to establish 

 the well at 60 to 65 feet, but in raising the 

 plastic well pipe from the point of maxi- 

 mum penetration (70 feet) to a shallower 

 depth, the well screen became dislodged. 

 In order to replace tliis screen, the casing 

 was removed. Unfortunately, it could not 

 be reinstalled to the same depth, and the 

 well screen finally was established at a 

 maximum depth of 571/2 f^et, where the 

 salinity was only 1 part per thousand less 

 than that of the bay (fig. 4) . 



After the initial flushing, the water, as 

 it came from the well, was quite clear and 

 appeared at first to meet all quality re- 

 quirements, but within 4 hours following 



exposure of the water to air, a brownish- 

 red precipitate appeared rendering the 

 water unsuitable for our use without 

 treatment. 



The precipitate proved to be iron at a 

 concentration of 4.8 parts per million, ap- 

 pearing as ferric hydroxide, Fe(OH)3, 

 which resulted from oxidation of soluble 

 ferrous hydroxide, Fe(0H)2. 



In the Monmouth County area of New 

 Jersey, iron contamination of potable 

 ground-water supplies of fresh water is a 

 familiar problem. In order to eliminate 

 excessive amounts of iron in domestic wa- 

 ter supplies, local communities commonly 

 drill wells to depths of over 700 feet. 

 Fresh-water supplies for Army installa- 

 tions at Sandy Hook are presently taken 

 from wells of 500 to 900 feet in depth to 

 avoid iron contamination, which is very 

 often associated with sands of the English- 

 town formation.'^ From logs of wells 

 drilled at Sandy Hook, sands of the Eng- 

 lishtown formation appear to range from 

 120 to 170 feet below land surface and are 

 underlain by sand and sandy clay to a 

 depth of about 230 feet, which in turn is 

 underlain by clay strata. In places, this 

 clay is a solid, impermeable layer 10- feet 

 thick. This indicates that the source of 

 iron contamination of the sea water sup- 

 ply is from the Englishtown formation 

 above the 230-foot clay layer. Since it is 

 unlikely that iron would occur in the shal- 

 low, recent sands (in which the well screen 

 is located) , it may be that iron-bearing wa- 

 ter is being forced up from the English- 

 town formation. The pressure could re- 

 sult from a standing head of water devel- 

 oped over the clay in the adjoining land 

 mass because the strata slope rapidly up- 

 ward towards the shore. 



This is the only explanation of the sa- 

 linity reversal which appears compatible 



■^ The Englishtown formation of late Cretaceous age 

 consists of tan and gray quartz, fine- to medium-grained 

 sand, locally containing beds of clay (Seymour Su- 

 bitzky, personal communication). 



