234 The Philippine Jounial of Science i»i6 



A majority of the wells drilled by the Bureau of Public Works 

 have penetrated these classes of material. A majority also of 

 the successful wells have obtained their water from sands and 

 gravels of littoral or alluvial or combined littoral and alluvial 

 origin. It cannot be said, however, that littoral and alluvial 

 deposits are uniformly productive of artesian water. Such 

 deposits are irregular, and individual beds do not extend over 

 large areas. On the contrary, the formation is characterized by 

 narrow lenses of sand or gravel or clay, such as would be ex- 

 pected in the beds of modern streams or along beaches. It is 

 due to this feature of littoral and alluvial deposits that so often 

 a flowing well will be secured adjacent to a drilled hole which 

 has obtained no water or only pumping water. A striking ex- 

 ample is the case- of two wells drilled within 50 meters of each 

 other between the Philippine General Hospital and the Bureau 

 of Science in Manila. The first well reached a depth of 178 

 meters and obtained only 113 liters of water per minute. The 

 second well obtained 322 liters per minute at a depth of only 137 

 meters. On the completion of the second well work was resumed 

 on the first well in an attempt to get water at the 137-meter 

 horizon, but all efforts to this end failed. Neither of these wells 

 flowed, but numerous experiments have demonstrated that in this 

 class of deposits flowing wells, nonflowing wells, and dry wells 

 may be situated side by side. 



As has been said, littoral and alluvial deposits are made up 

 of loosely consolidated sands, clays, and gravels. The loose 

 character of the formation is responsible for the commonly noted 

 phenomenon that in wells situated near the coast line the level 

 of the water in the well varies with the stage of the tide in the 

 adjacent sea. The fresh water in the upper part of the land 

 mass is always percolating through porous beds toward the sea, 

 and in the region of the seashore it is in some measure in a condi- 

 tion of hydrostatic equilibrium with the sea water, which satu- 

 rates the porous beds outcropping on the sea floor. The rising 

 tide actually increases the hydrostatic pressure on the ground 

 water in the adjacent porous beds. This effect is especially 

 marked where an old coral reef has been included between the 

 deeper beds of the formation, because the loose structure of the 

 coral reef affords unusually free passage for water. 



Another factor which must be considered in connection with 

 littoral and alluvial formations is the possibility of obtaining 

 salt water in wells adjacent to the coast. Littoral deposits are 

 contaminated by the salt water in which they were formed. 

 Close to the coast line percolation of the fresh ground water 



