On the Hydrogeological Environment of Hashirimizu Spring, Yokosuka City

Abstract

The Hashirimizu spring had been utilized for the city water supply of Yokosuka City, since one hundred years ago.
“ Where the spring water come from ?” was a mysterious question for a long time.
From 1976 to 1978, we could chtllenge to elucidate the semieternal puzzle by means of thermal-infrared imagery method (on air), measurement of temperature of sea-water (by sea), gammaray survey (carborn), geological test boring, pumping water well test and others.
It was presumed that the origin of the spring water would be came up from plural brecciated rock zones in Neogene formation.
The brecciated zones are extending from the western coast to the eastern coast of Miura peninsula and accordingly the spring has distinct characteristics in constant flowing quantity and water quality all the year round, in spite of its too-small catchment-area compared with yielding amounts from the spring.
Next, we could understand that, the yield of the spring had only 300 cubic meters a day or a little more as a surplus, beyond the daily yield of 2,700 cubic meters a day. This matters was actually certified by installation of a horizontal water collector with radially installed strainers at a suitable site to get superfluous ground-water.