Groundwater quality in the relatively small Yamagata Basin exhibits considerable variation. The water obtained from artesian wells in the downstream of the Midaregawa Fan in Hairi district is referred to as donko-sui, and is one of the selected 100 exquisite and well-conserved waters. The donko-sui is used for fish farming and daily life water in this district. On the other hand, kanake-mizu from the Sagae district is not potable due to the high Fe content. The kanake-mizu is often used for snow melting in winter, and the iron hydroxide changes the color of road surfaces to redblown. In the Hirashio district on the western margin of the basin, salt is produced in a salt pan fed by a saline spring with a Cl- concentration of more than 10,000 ppm. To clarify the hydrogeological reasons why such various interesting groundwaters exist in the Yamagata Basin, the authors examined a three dimensional distribution of chemistry of groundwater including in Tertiary sediments, which has been assumed to a hydrological basement, by field surveys and reviews of previous studies. The results are summarized as follows.
(1) The hydrogeology of the Hairi district is characterized as the discharge zone of very active groundwater flow system in the Midaregawa watershed. Donko-sui is confined and has low electric conductivity and oxidative potential due to the very short residence time.
(2) Kanake-mizu is found not only in the Sagae district but in the lowland area where it becomes reductive and Fe-rich after a long residence time.
(3) Groundwater from the deep zone in Tertiary sediments (>400 m) could be classified into two groups based on hydrochemical characteristics. One group had lowδ¹⁸O, δD and d values, and was obtained from the eastern, southern, and northern parts of the basin. The δ values and Cl^- concentration suggested that the groundwater from this group was formed by mixing with paleo sea-water and precipitation in an ice age, which has low δ and d values. The other group had high δ value and low d values, respectively. The properties are similar to saline spring at the Hirashio district but differed markedly from the water samples of the first group. The second group would be originated from paleo seawater but needs some mechanism to form the high δ value.
(4) Groundwater from the shallow zone in Tertiary sediments (<400m) has low Cl^-concentration, and which suggested that this zone should not be assumed to a hydrological basement.

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