Short report on discharge during a dry season in the Neogene mudstone region

Kikugawa Basin, Shizuoka Prefecture, Central Japan

Abstract

Measurements of discharge and electric conductivity (3lsites) and water quality survey (15sites) were carried out in a dry season in the Kikugawa Basin, Shizuoka Prefecture, of which area is mostly composed of the Neogene muddy sedimentary rocks. The rocks are lithologically divided into three groups, namely the Kurami, Sagara and Kakegawa Groups in ascending order, which mainly consist of mudstone and alternating beds of sandstone and mudstone. The Neogene strata are unconformably overlain by the Quaternary Soga Group, Ogasa and Makinohara Gravels, river terrace deposits and Alluvium.
The discharge and the electric conductivity at the sites measured twice are inversely proportionate to each other (See Table 1). The inverse proportion is roughly formed between the specific discharge and the electric conductivity as shown in Fig.3. The specific discharge of the surveyed 31 sites is calculated as 5.42 1/sec/km², which is slightly less than the average specific discharge of the Kiku River during a dry season (6.71/sec/km ²) recorded at the Kamo Observation Station for the past 32 years (Ministry of Construction,1988). It seems that the specific discharge and the electric conductivity are particularly related to each other, with reference to the geological setting of catchment areas (See Fig.3).
Water samples are rich in Ca²⁺ and SO₄^2- in general and some of them contain high density of NO₃^- (See Table 2, Figs.4 and 5). These dissolved components are possibly due to artificial fertilizers and agricultural medicines used in farm lands.