Norio Tase

Stable isotope studies of precipitation and river water in the Lake Biwa basin, Japan.

The δ18O and δD in 17 river waters flowing into and out of Lake Biwa, the lake water itself and precipitation at 11 locations in the Lake Biwa basin were observed from October 1996 to September 1997, to evaluate the origin of river waters and the effect of irrigation water from the lake on the river waters flowing into Lake Biwa. The regional difference and altitude effect of the stable isotopes in precipitation and river water were found in the basin. The δ18O in the rivers which are located in the eastern and the southern areas in the basin is heavier than that of the other rivers, due to the effect of irrigation for the nearby paddy fields. Evaporation from Lake Biwa and the inflow rate into the lake estimated from the δ18O balance agree well with the results of previous studies which used water balance methods. The stable isotopes can enable us to evaluate not only water balance but also the origin of the water and the water flow system. Copyright

Estimation of Groundwater Residence Time Using the 36Cl Bomb Pulse

We propose a methodology for estimating the residence time of groundwater based on bomb-produced (36)Cl. Water samples were collected from 28 springs and 2 flowing wells located around Mt. Fuji, Central Japan. (36)Cl/Cl ratios in the water samples, determined by accelerator mass spectrometry (AMS), were between 43 × 10(-15) and 412 × 10(-15). A reference time series of the above-background (i.e., bomb-derived) (36)Cl concentration was constructed by linearly scaling the background-corrected Dye-3 data according to the estimated total bomb-produced (36)Cl fallout in the Mt. Fuji area. Assuming piston flow transport, estimates of residence time were obtained by comparing the measured bomb-derived (36)Cl concentrations in spring water with the reference curve. The distribution of (36)Cl-based residence times is basically consistent with that of tritium-based estimates calculated from data presented in previous studies, although the estimated residence times differ between the two tracers. This discrepancy may reflect chlorine recycling via vegetation or the relatively small change in fallout rate, approximately since 1975, which would give rise to large uncertainties in (36)Cl-based estimates of recharge for the period, approximately since 1975. Given the estimated ages for groundwater from flowing wells, dating based on a (36)Cl bomb pulse may be more reliable and sensitive for groundwater recharged before 1975, back as far as the mid-1950s.

Pollution by the fungicide pentachloronitrobenzene in an intensive farming area in Japan

Environmental pollution by pentachloronitrobenzene (PCNB) was investigated at Tsumagoi, an intensive farming area, where a great amount of PCNB has been applied. High concentrations of PCNB were detected in river water near an area of cabbage cultivation. Further, pentachloroaniline (PCA) and pentachlorothioanisole (PCTA), which are the principal biodegradation products of PCNB, were also detected, and their ratios to PCNB were higher in autumn than in summer. PCNB concentrations in soil were similar to those of river sediment, in which PCNB was concentrated 5000-10,000 times over levels in river water. The biodegradation rate of PCNB in river water was higher than in river sediment and soil. The formation rates of PCA from PCNB were higher in river sediment and soil than in river water. It is considered that PCA remains for a long time in the environment and accumulates in river sediment and soil.

Groundwater contamination in Japan

Problems on groundwater contamination in Japan are briefly summarized in this paper. Although normal physical conditions in Japan restrict the possibilities of groundwater contamination, human activities are threatening groundwater resources. A survey by the Environment Agency of Japan showed nationwide spreading of organic substances, such as trichloroethylene as well as nitrogen compounds. Synthetic detergents have also been detected even in rural areas and in deep confined aquifers, although their concentrations are not as high. Public awareness of agrichemical or pesticides abuse, especially from golf courses, is apparent. Other problems such as nitrate-nitrogen, leachate from landfills, and the leaking of underground storage tanks are also discussed.

Effects of size and shape of a region on drought coverage / Les effets de grandeur et de forme d'une région sur l'étendue des sécheresses

Abstract The stochastic components of monthly precipitation series at gauging stations over the Great Plains in the United States are used to derive time-area models from which new series were generated at a square grid of points. They served as the information for the analysis of area drought coverage by considering the effects of size and shape of a region. Drought probabilities over a region are more affected by its size than by its shape, with the shape affecting drought probabilities in the case of small regions.

Measurement of the 36Cl deposition flux in central Japan: natural background levels and seasonal variability

Essential parameters for the applications of (36)Cl as a tracer in groundwater studies include the initial (36)Cl/Cl ratio, at the time of recharge, and/or the natural background deposition flux of (36)Cl in the recharge area. To facilitate the hydrological use of (36)Cl in central Japan, this study aimed to obtain a precise estimate of the long-term average local (36)Cl flux and to characterize its seasonal variability. The (36)Cl in precipitation was continuously monitored in Tsukuba, central Japan over a period of >5 years. The (36)Cl flux showed a clear seasonal variation with an annual peak during the spring, which was attributed to the seasonal variability of tropopause height. The long-term average (36)Cl flux (32±2atoms m(-2)s(-1)), estimated from the measured data, was consistent with the prediction from the (36)Cl latitudinal fallout model scaled using the global mean production rate of 20atoms m(-2)s(-1). The initial (36)Cl/Cl ratio was estimated to be (41±6)×10(-15), which is similar to that of pre-bomb groundwater in the Tsukuba Upland. An observation period covering an 11-year solar cycle would yield more accurate estimates of the values, given the increased (36)Cl flux during the solar minimum.