Hydrogeological and geochemical investigations are required when the deep underground environment is explored or when the water from these depths are to be used as a resource. This study demonstrates the methodology on geochemical sampling of groundwater to evaluate the redox condition in a deep borehole with large downward hydraulic gradient. In the case of losses of drilling fluids during drilling, groundwater must be sampled using single or double packer equipment at the time of the fluid loss. When there was no loss of drilling fluids groundwater samples were successfully obtained using a long-term multi-packer system.
Unpaved irrigation canal of total extension 21km exists in the central part of river Narusegawa alluvial fan, Akita Prefecture, Japan. The canal is still utilized for paddy field in 2005. A part of the unpaved irrigation canal (39°14´ 38´´ N, 140° 31´ 49´´E), of which full length is in 220m and average width is in 0.98m, was chosen for measurement of infiltration. The measurement has been performed in the period from May to August, 2005. As a result, average infiltration rate during the observation period was 15.4 l/s, and average infiltration capacity was estimated at about 18.8cm/h. As this numerical value, unpaved irrigation canals of this district have a superior groundwater recharge function. This is a function approximately equal to artificial recharge facility for groundwater.
Water quality, stable isotope ratios of hydrogen and oxygen (δ18O and δ D) and temperature-depth profile in the Chikushi Plain were examined by field observations and chemical analysis in 2004 and 2005. Water samples for chemical analysis were collected from observation wells, pumping wells, rivers and springs.
Groundwater quality in Shiroishi and Saga areas are divided into two types of Ca-HCO3 and Na-HCO3. In the mountain region and peripheral plain, groundwater quality are characterized by Ca-HCO3 type. Groundwater quality, on the other hand, in the central and coastal areas are Na-HCO3 type. The ranges of δ18O and δD are similar to those of spring water collected from mountain region. Subsurface thermal gradient in the central and coastal area is larger than that in the peripheral plain. These results show that groundwater is recharged in the mountain region and peripheral plain and flows toward the coastal area.
Groundwater quality in Chikugo area is characterized by Ca-HCO3 type without reference to sampling location and depth. The values of δ18O and δD are higher than those of spring water collected from the mountain region. Subsurface thermal gradient in this area is very small and it seems that downward heat transport from the surface may take place. These results show that groundwater recharge area is not only mountain region but also low land and rivers in Chikugo area.
In order to clear the origin of thermal water in fractures in deep granite bedrocks, stable isotopic ratios of hydrogen and oxygen were analyzed for thermal waters from deep boreholes through bedrocks as well as hot spring waters, cold spring waters and shallow groundwaters in Okayama and Tottori Prefectures, western Japan.
Although most thermal waters from deep boreholes can be regarded as meteoric water origin from the viewpoint of the isotopic ratio, part of the thermal waters has clearly δD and δ18O values lower than those of the local meteoric waters. On the other hand, the isotopic ratios higher than those of the meteoric water are found in the thermal waters with high chlorine in the coastal area. If such the high chlorine water is to be formed by mixing of fresh water with seawater, the isotopic ratio of the fresh water must be lower than that of the local meteoric water. Water with isotopic ratios lower than the present meteoric water can be considered to exist definitely in fractures in the granite bedrocks in the district.
The isotopic ratio of meteoric water in the Frigid Zone is lower than that in the Torrid Zone, because the isotopic ratio of meteoric water is influenced by the temperature of surface water of sea in the source area. The thermal waters with very low isotopic ratio in this area may be considered to originate from the meteoric water in a climate condition colder than the present. According to the isotopic records of deuterium and oxygen-18 in the ice cores bored through ice sheets in Greenland and Antarctica, the low isotopic ratios of ice are corresponded to be in ages more than 10,000 years old at least, suggesting that the isotopic ratios of old meteoric water more than 10,000 years before the present are lower than those of the present meteoric water over the earth. The low isotopic ratios in deep thermal water from the district might be thus considered to be very old water of the glacial epoch 10,000 years at least before the present.
On the other hand, the transit time of water through dense granite rocks may be estimated to be roughly tens of thousands years under the condition of hydraulic gradient 0.01 for a horizontal distance of 50 km, by using the permeability of 10-6 cm/s or so and the porosity of 0.001 to 0.01 on the basis of literature.
Thus a deep water circulation with a time scale of tens of thousands years can be inferred to be formed through the granite bedrocks in the district.
Relationship between water quality represented by major inorganic ion concentrations and land use was discussed for the Mamagawa River which runs in the eastern part of the Tokyo Metropolitan Area. Principal component analysis for land use in the drainage basin yields 3 principal components. The first principal component expresses the degree of coverage with green, the second one indicates the degree of occupation by residential area, and the third one shows the degree of non-industrial development. The green area has a positive correlation with Ca2+, Mg2+ and NO3-, and a negative correlation with Na+, K+, Cl-, EC, and TMI (Total Major Ions). It shows a weak negative correlation with SO42- and HCO3-. Especially, the forest has a high positive correlation with NO3- due to chemical process of production of inorganic matter from humus. The agriculture area has also a high positive correlation with Ca2+, Mg2+ and NO3- due to fertilizer and soil conditioner. The residential area has a positive correlation with SO42- and a negative one with NO3-. Sulfate ion, SO42-, is originated from sulfuric salt in waste water in residential areas. Nitrate ion, NO3-, is considered to be decreased due to increase of the forest and agriculture areas which supply NO3-. Development of industrial area induces an increase in concentration of almost all ions. Water quality of the main stream changes at junctions with tributaries. The tributary with a different land use type from that of main stream strongly affects the water quality of main stream, because the water quality of the tributary is also different from that of the main stream.