On the basis of the concept of the groundwater basin (Freeze and Witherspoon,1967), which is the three dimensional closed system that contains the entire flow paths followed by all water recharging the system, the evaluation of the natural basin yield (Freeze and Witherspoon,1967) is attempted in Ichihara region, Chiba Prefecture. Ichihara region is situated to the east of Tokyo Bay and covers an area of about 200 square kilometers. Topography of the area mainly consists of diluvial uplands called Shimosa Upland and alluvial lowlands. The basin is bounded on the bottom by the kasamori Formation which is composed of very muddy material and is considered as the impermeable basement, on the top by the water table which is the replica of the topography, and all sides by imaginary impermeable boundaries which simulate the groundwater divides. Amount of recharge to the groundwater basin is estimated from the following surface water balance equations, R= (1 -f) P-EtPEt where R is the recharge to the groundwater basin, f is the direct runoff coefficient, P is the precipitation and Et is the evapotranspiration. Meteorological data at Ushiku and f=20% lead to the annual recharge to the basin 606mm/year. About 600mm/year of recharge can be expected in the investigated area. As the development of the Keiyo industrial zone in the coastal area, the amount of withdrawal of groundwater had been increased. At the same time, groundwater level had declined and many undesired phenomena had been arisen.
For the purpose of groundwater investigation, relating to the dry dock construction, two types of wells with 31m length having different screen diameters,100mm and 200mm, were constructed in the sea bed at the depth of 11∼ 12m. These wells must have the cut-off structure to eliminate the sea water and unexpected strata water. Bentonite-cement mixture was utilized for a triple-tube 100mm diameter well to cut off water above the aimed strata. For the same reason, mortar was utilized for a double-tube 200mm diameter well. The wells were sustained by platform and piles to avoid the damage of cut-off structure due to oscillatory motions produced by tides and ocean waves. Pumpages were stable during full-pumping after the well construction. The observed water levels in these wells without pumping were higher than tidal levels, and the peaks of well water level were observed later than those of tide level. According to these facts, and observations during the construction, it was concluded that these wells were appropriately planed and constructed to investigate the groundwater at the sea bed.