Hydrologic study of volcano Fuji and its adjacent areas

Abstract

Since 1917, especially after the World War II, many investigations has been carried on to clarify the hydrology of Fuji Goko (five lakes) and hydrogeology of volcano slope with relation to groundwater development. Mt. Fuji, the highest peak of 3776 meters in Japan, is one of the largest and the most typical strato-volcano of basaltic rocks with gentle slope on all sides. It is noticed that this volcano has many large springs, five lakes with abundant water and some characteristic rivers on and around volcano. Within past ten years, groundwater has been developed or is about to be developed in quantities ranging from 1, 400, 000m3 per day on southern part to 24, 000m3 per day on northern part. An extensive development of ground water for industrial use as well as for agricultural one has caused many troubles, such as abrupt decline of groundwater pressure, stopping or decreasing of spring discharge and groundwater contamination by sea water invasion. Volcano body is a big reservoir which absorb and store rain water in its body as ground-water and release it over a long enough period. Hydrologic knowledges involving mode of occurrence of water, storage capacity of groundwater and water balance on volcano make it possible to evaluate the water potentiality of volcano as reservoir. The author reviewed the results of past studies of hydrology of this area and collected all data useful for further study. (Tab. 1, 2, 3) (Fig. 2, 3) Mt. Fuji consist of three parts, Komitake, Old Fuji and New Fuji, errupted successively in descending order. (Tsuya 1940) The author and his collaborator, assumed that Komitake was impermeable in the previous studies. But, Machida's (1964) tephrochronological study of the pyroclastic fall deposit of this area supplied them new knowledges about permeability problemes. (Tab. 5) He collected all data about lakes (Tab. 6), river discharge (Tab. 7), spring discharge (Tab. 8) (Fig. 4) and permeability coefficient of different rocks. (Tab. 10) He also cheked data of geophysical explo-ration such as electric, seismic and radio active prospecting (Fig. 7) and that of deep well. (geologic column, electric logging, water level and specific discharge) Among these Ochiai's (one of his collaboraters) quantitative data about the velosity of groundwater through lava and porosity, using iso-topic materials By these data above mentioned, he simulated groundwater body in Mt. Fuji as shown in Fig. 8 for mathematical analysis. He proposed the differential equation (1), transforming this by Bessel's function and obtained following equation which showed discharge amount of springs around Mt. Fuji. Q=-2παTmAJ1(ma)e-m2T/S t Here, a:radius T:transmmissibility coefficient S:storage coefficient v:time Time dependant decresing factor is expressed as τ=τ0e-m2T/S t. The result obtained from this calculation has fairy good coincidence with actual measurements. He also proposed a diagramatic division on hydrologic structure of Mt. Fuji, classifying volcano body into three zones, as upper recharging zone, middle recharging zone, spring zone and gave brief explanation about these. (Fig. 9)