2:25 PM - 2:40 PM
[AHW23-03] Cause of strong acidity in non-volcanic area inferred from light element isotopic compositions and rock-water reaction
Keywords:helium isotope, carbon isotope, oxygen and hydrogen isotope ratios, leaching test, water-rock interaction, highly acid water
It is important to understand the causes of acidic water discharge in the natural environment, since it may contribute to heavy metal loadings on water resources. Previous studies have inferred that volcanic hydrothermal systems and rock-water interaction phenomena affected acidic water quality formation (Sato et al., 2010). Another previous study focusing on the strong acidity of the Dokuzawa mineral spring in the Shimo-Suwa region, Japan, analyzed the isotopic ratio of sulfur in the dissolved sulfate ion, suggesting that pyrite was also involved in the unique water quality formation (Muramatsu et al., 2014). However, it is inadequate to evaluate whether volcanic activities affects based solely on the isotopic ratio of specific elements; hence, the causes and discharge processes of acidic water in the natural environment have not been well clarified. The identification of the rocks and minerals that contribute to the formation of acidic water and their interaction with water are also incompletely understood. Therefore, the objectives of this study focusing on the Shimo-Suwa region including the Dokuzawa mineral spring are 1) To discuss the involvement of volcanic activities in hot springs based on light element isotopic compositions, 2) To identify the rocks and minerals that contribute to the formation of acidic water quality by petrological study in the Dokuzawa area, 3) To make clear causes of the acidic water discharge by grasping characteristics of elemental leaching from rocks.
Water and rocks were collected from the Dokusawa mineral spring and its surrounds for the following analyses. The water samples were analyzed for major dissolved ion concentrations, oxygen and hydrogen (δ18O, δ2H), carbon (δ13C), and noble gas (4He/20Ne, 3He/4He) isotope ratios. The rock samples were observed under stereo microscope and polarization microscope, and measured X-ray powder diffraction patterns to identify minerals of the rock samples. In addition, the rock samples were used for water-rock reaction experiments. The findings of this study were as follows.
The water from the Dokuzawa mineral spring and the vicinity hot springs had similar isotope ratios (δ18O, δ2H) as the precipitation at the Lake Suwa area. The helium and carbon isotope ratios of the hot springs, which were determined to be less than 50% atmospheric mixing from 4He/20Ne, were as follows. The helium isotope ratios (R/RA) showed from 2.8 to 3.7 and carbon isotope ratios (δ13C) showed from -24.42 to -63.18‰. These results indicate that the hot spring waters including the Dokuzawa mineral spring mainly consist of meteoric water (precipitation) and would not be directly contributed by volcanic fluids. Therefore, the influence of volcanic activity on the meteoric water recharged as groundwater is small, and there could be other factors causing strong acidity water quality.
In water-rock reaction experiments using rocks around the Dokuzawa mineral spring, a rapid decrease in pH and an increase in ORP of the experimental solution were observed. The EC of the solution increased as pH decreased. In addition, the highest sulfate ion concentration was observed in the solution after experiments, which water quality is slightly similar to the Dokuzawa mineral spring. Based on mineral observations and results of XRD analyses, the amounts of pyrite and clay minerals in the whole rock are estimated. The more pyrite-rich and clay-mineral-rich rocks were used, the greater the decrease in pH of the solution was, inferring those minerals were likely to be involved in the reaction. These results suggest that strong acidic water in the study region is produced by the reaction of meteoric water with pyrite and clay minerals in rocks underground or at the surface of the ground.
Water and rocks were collected from the Dokusawa mineral spring and its surrounds for the following analyses. The water samples were analyzed for major dissolved ion concentrations, oxygen and hydrogen (δ18O, δ2H), carbon (δ13C), and noble gas (4He/20Ne, 3He/4He) isotope ratios. The rock samples were observed under stereo microscope and polarization microscope, and measured X-ray powder diffraction patterns to identify minerals of the rock samples. In addition, the rock samples were used for water-rock reaction experiments. The findings of this study were as follows.
The water from the Dokuzawa mineral spring and the vicinity hot springs had similar isotope ratios (δ18O, δ2H) as the precipitation at the Lake Suwa area. The helium and carbon isotope ratios of the hot springs, which were determined to be less than 50% atmospheric mixing from 4He/20Ne, were as follows. The helium isotope ratios (R/RA) showed from 2.8 to 3.7 and carbon isotope ratios (δ13C) showed from -24.42 to -63.18‰. These results indicate that the hot spring waters including the Dokuzawa mineral spring mainly consist of meteoric water (precipitation) and would not be directly contributed by volcanic fluids. Therefore, the influence of volcanic activity on the meteoric water recharged as groundwater is small, and there could be other factors causing strong acidity water quality.
In water-rock reaction experiments using rocks around the Dokuzawa mineral spring, a rapid decrease in pH and an increase in ORP of the experimental solution were observed. The EC of the solution increased as pH decreased. In addition, the highest sulfate ion concentration was observed in the solution after experiments, which water quality is slightly similar to the Dokuzawa mineral spring. Based on mineral observations and results of XRD analyses, the amounts of pyrite and clay minerals in the whole rock are estimated. The more pyrite-rich and clay-mineral-rich rocks were used, the greater the decrease in pH of the solution was, inferring those minerals were likely to be involved in the reaction. These results suggest that strong acidic water in the study region is produced by the reaction of meteoric water with pyrite and clay minerals in rocks underground or at the surface of the ground.