This paper presents some hydrogeological features of Ryomo area in Tochigi Prefecture, based on the investigation for an appropriate usage of groundwater (1975,1979,1981) and the integrated investigation of hydraulics and hydrogeology in Tokyo urban area (1974) which were carried out by Tokyo Regional Bureaw of International Trade and Industry.
This paper is summarized as follows;
1. The investigated area consists of mountains, hills, plateaus and lowlands, which are put in order from surrounding mountainous area towards a center of Kanto-plane.
2. The mountains comprise consolidated hard rocks such as granitic rocks, volcanic rocks and Paleozoic formations. The area except the mountains is composed of unconsolidated formations such as Tertiary and Ouaternary system.
3. The above unconsolidated formations results unconformably upon the impervious basement comprising the consolidated rocks and are divided into six formations such as formation“F” to formation “A”.
4. A form of the impervious basement indicates existence of two waste-filled valleys at both sides of the Ota observation well. One of them was called “Kinugawa waste-filled valley” by Dr. Akio Suzuki. And it is estimated that a hill lies in the gound at the extention of the Utsunomiya hill.
5. The thermal gradient of groundwater is divided into three groups in the investigated area. The thermal gradient is 1°C/100 m in south of Tone river and is 3° C/100 m in north of Tone river. Further, it is 5°C/100 m in the Watarase basin.
6. Groundwater quality varies from CaSO₄ type to Ca (HCO₃) type on the Key-diagram correspondent with a depth of respective formations. And it is likely thet there is a higher concentration in sodium ion in the area where is affected by groundwater of the basement.
Although above mentioned results were led from the study, further, a palaeogeographicai study is required on difference of the thermal gradient of groundwater and on a type of groundwater.