A GEOHYDROLOGICAL STUDY OF COASTAL GROUND WATERWITH SPECIAL REFERENCE TO THAT OF SAND BAR AREA

Abstract

This study suggests that an area of sand bar is originally an area abundant in ground water resource, but in developing this resource, there uill caused such troubles as scarcity of well yields and salt water encroachment. The findings and conclusions in Hachirogata area (Akita Pref.), Miho area (Shizuoka Pref.) and Yumigahama area (Tottori Pref.) are summarized as follows: At the north-western side of Hachirogata, there extends a sand bar, which is 10 km long, 2 km wide and 5 m at altitude on the south, 20 m in the middle and several ten meters on the north. Lake Hachirogata is separated from the Japan Sea by this bar covered by sand dune. According to the results from topographical, geological, electrical resistivity survey, seismic prospecting and test borings, the following layers are distingushed: Formation Age Permeability (K) Depth to the lower cm/sec limit (below s. 1.) Wakimoto F. Pliocene l×10-7 - Shibikawa F. Pliocene_??_1×10-4-1×10-6 100-250m Katanishi F. Pleistcene_??_clay 1×10-7 Hachirogata F. Hollocene_??_sand 1×10-4-1×10-5 sand dune 1×10-3 max. 70m Judging from ρ-a curve, sea water invades through pervious buried valley on Pleistocene deposits. At Miho Sand Spit which extends from Udosan (Kunosan) Hill eastward is considerd as a prolonged forked sand spit. A sand and gravel layer which makes principal aquifer of this region is underlaid with less permeable sand and silt layers of plaistocene age which compose Udosan Hill. The ground water body in this aquifer is separated into three parts, Kuno area, old spit area and new spit area. At inner bay area which has a thicker sand and gravel layer or younger spit area, influence of tidal fluctuation on the ground water level is conspicuous and there forms Herzberg's lens. At Yumigahama sand bar, alluvial deposit of 70m (max. value) is found on the pleistocene basalt or tertialy formation by electrical resistivity survey and test borings. The alluvium is devided into three, upper fine sand layer, next silty layer containing volcanic ashes and basal gravel layer. The silty layer is saturated with saline water and acts an cap layer to the underlying gravel layer which contains methan gas. Such condition above mentioned modifies the form of Herzberg's lens of this region. Fresh grouned water which is believed to be furnished mainly by artificial irrigation canal, Yonekawa, during irrigation period, flows in the uppermost fine sand layer and the motion of this water is restricted by the shape of an underlying silt layer.