This study examined the geochemical properties and the distribution of mineral spring water, river water and associated discharge gases in Tsuwano Town, Shimane Prefecture, Western Japan. In Tsuwano, high Na-Cl groundwater emits gas bubbles composed mainly of CO₂. The geochemical properties of mineral spring water, gas, hydrogen and oxygen isotopes and noble gas isotopes suggest that mineral spring water at Tsuwano possibly contains deep-seated fluid. In addition, the largest deep-seated fluid flux occurs along active faults, suggesting that active faults are pathways for fluid upwelling from depth. Furthermore, in Tsuwano, deep-seated fluid discharge occurs along the Lake Ohara-West Yauneyama fault system and surface fractures.

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[in Japanese]

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Three years of field observations conducted of the groundwater level, nitrate concentration, and isotope ratios (δ¹⁵N_NO3 • δ¹⁸O_NO3) of groundwater in the Sata River alluvial fan, Asakura, Fukuoka revealed that groundwater levels fluctuated seasonally and that nitrate concentration fluctuation is affected both by paddy irrigation and timing of fertilization activities. To evaluate nitrogen behavior and its budget in the groundwater aquifer system, a three-dimensional groundwater flow coupled with convection-dispersion model was developed, incorporating detailed nitrogen load information including the fertilizing rate, timing, and effects of nitrogen uptake rate by planted crops at the ground surface for each agricultural land use. Results suggest that nitrate concentration in the studied groundwater shows clear seasonal fluctuation annually with the lower nitrate concentration around 3 mg/L. Because the nitrogen budget under the present condition shows a quasi-steady state with nitrate concentrations that cannot decrease below 3 mg/L, this concentration limit has been created by the long-term cumulative effect of the nitrogen load. The nitrogen discharge rate through the discharge flow of the groundwater from the surface of the studied alluvial fan to the lowland area outside of the study area was estimated using this model as 106 kg/ha/y.

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The potential map for nitrate load in groundwater was produced using statistical data from agriculture and forestry for Shimabara City, Nagasaki Prefecture, Japan. Observed distribution of high nitrate loads corresponded well with the locations of high nitrate loads from the detailed potential map using land-use from aerial photographs. It was confirmed that polluted groundwater is typically located in the downstream of the high potential nitrate load area in this map. The findings indicate a high accuracy of the potential map. We also examined the distribution-change of nitrate loads using the potential map based on the old municipalities. The distribution of high potential areas was drastically changed after 1985.

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A comprehensive 3D groundwater flow simulation was performed by using results of accumulated hydrological observed data and nitrogen load statistical data to understand the 3D behavior of NO₃^－ in groundwater in the Miyakonojo Basin. We firstly constructed 3D groundwater flow model which shows high correlation with the observed data of river discharge and groundwater level in this area. By using this model, nitrate transportation simulation was applied to the study area. The nitrogen infiltration amount was estimated based on statistical data for agriculture and forestry. The simulated NO₃^－ distribution and historical change of NO₃^－ concentration showed good correlation with the observed data under the condition where denitrification were not occurred. This result suggests that NO₃^－ pollution attenuation in groundwater is largely attributed to dilution and diffusion in this area.

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NO₃-N concentrations, δ¹⁵N and SF₆, in groundwater were measured at 39 observation wells in 2009 and 2012 in a reservoir area of the Sunagawa underground dam, completed in 1994, to evaluate the environmental impacts of construction of the dam in Miyako Island, Okinawa Prefecture, Japan. Groundwater level was lower than the top of the cut off wall and NO₃-N concentrations were high in 2009. On the other hand, groundwater level was higher than the top of the cut off wall and NO₃-N concentrations relatively low in 2012. The reason was presumed that the heavy rain with a dilution was fewer in 2009 than 2012. The distribution of NO₃-N concentrations was roughly the same as the distribution of nitrogen loads in surface because of the operation of the composting plants. δ¹⁵N was smaller than reported in previous studies. Amounts of livestock wastes spread onto fields may have decreased. SF₆ concentrations suggest that a part of the groundwater circulates every several years and old groundwater remains stagnant in other parts. These results show that partial circulation of groundwater results in insufficient mixture in the aquifer. It is necessary to select the monitoring well of the water quality in the reservoir area of underground dam in consideration of these ununiformities.

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