134 



approach was a combination of extraction and injection wells. The aquifer 

 length (parallel to the Delaware River) considered was 1,520 m and 

 5 injection wells were located parallel to and 300 m from the Delaware 

 River and the 5 extraction wells were located 600 m from the river. Only 

 0.014 m-^/s was pumped from each extraction well and reintroduced into the 

 aquifer through the adjacent injection well. This resulted in a freshwater 

 "mound" that performs as a barrier to saltwater inflow. Following the 

 drought, the injection wells are used as extraction wells (0.014 m-'/s) with 

 the water returned to the river, thereby reducing the salinity in the 

 aquifer further. 



8.4.5 Okinawa- jima Island 



The construction of underground flow barriers through pumping of 

 cement grout near the coastline can create underground impoundments similar 

 to surface reservoirs. In 1978, such an experimental subsurface barrier 

 was constructed in a small buried valley on Miyako-jima Island (Sugio et 

 al., 1987). The barrier was constructed in a very porous limestone aquifer 

 and is 16.5 m high, 5 m wide and 500 m long. Based on field monitoring 

 studies conducted over a four year period, the installation was judged a 

 success; the hydraulic conductivity and porosity were reduced from 

 0.17 cm/s and 20% to 5 x 10'^ cm/s and 6%, respectively. Plans are under 

 way to construct a much larger barrier at Komesu on Okinawa- jima Island 

 where the annual rainfall is 2,400 mm and occurs during a nine month 

 period. With the present high permeability, much of this fresh water flows 

 to the sea and saltwater intrusion tends to occur during the remaining 

 three months when heavy pumping is conducted for irrigation purposes. 



Based on numerical modeling with a particular barrier design of 5 m 

 thickness, it is concluded that with the barrier no salinity intrusion will 

 occur for a 60 day period during the drought and that if a longer period is 

 desired, the barrier thickness must be increased or the pumping rate 

 (6,000 m-^/day) must be decreased. 



8.5 RESEARCH NEEDS 



Given the ambient flow conditions in a coastal aquifer, the 

 transmissive and porosity properties of the aquifer and various scenarios 



