1:45 PM - 2:00 PM
[AHW28-12] Continuation Process of Strong Acidic Water through Dissolution and Precipitation Cycles of Sulfide and Sulfate Mineral
Keywords:water–rock interaction, strong acid water, leaching experiment, 3H–3He method
The formation and continuous discharge of strong acidic springs are considered problematic because they are a source of water pollution. Water acidification can occur for various reasons, including acidification caused by the reaction between water and minerals, which is a typical example. However, there are many unknown factors because the reaction differs depending on the contact time between water and minerals, the mineral species, the mineral assemblage, and the mineral formation environment. The objective of this study is to clarify the fundamental process by which minerals cause strong acidification, focusing on the Dokuzawa mineral spring, where strongly acidic water springs forth from a hydrothermal alteration zone, by examining the relevant environmental factors and mineral effects.
Rocks and water were collected from the Dokuzawa mineral spring. The constituent minerals of the collected rock samples were observed for their microstructure, and their chemical composition was analyzed. The water samples were analyzed for major dissolved ion concentrations and noble gas isotope ratios (3He/4He, 4He/20Ne). Additionally, the residence time of the groundwater in the Dokuzawa mineral spring area was estimated using the 3H-3He method. A precipitation experiment was conducted, in which the water samples were air-dried, and the precipitated materials were observed, qualitatively analyzed, and the minerals were identified. Furthermore, a 10-minute leaching experiment was performed using rock samples, during which pH, EC, and the concentrations of dissolved elements were monitored.
The collected rock samples consist of pyrite, clay minerals, jarosite, leucoxene, and rare silver oxide. These minerals might have occurred due to past hydrothermal alteration. Among these minerals, pyrite, and jarosite, are known as minerals for causing acidification. In addition, the rock's surface is colored yellow, and unidentified fine minerals, which might be sulfate minerals, are present in the grain boundaries. The metavoltine and alunogen precipitated from the water sample of the Dokuzawa mineral spring. This suggests that the fine sulfate minerals on the rock surface are most likely metavoltine and alunogen, precipitated from strong acidic water. Moreover, in the leaching experiment, the pH of the water rapidly decreases, and the EC increases, suggesting that the dissolution of sulfate minerals, which are more soluble than pyrite, is the primary cause in the initial stage of strong acidification that occurs when rainwater permeates rocks.
These results suggest that the dissolution of pyrite and sulfate minerals, which occurred during past hydrothermal alteration, causes the water to become strongly acidic. Subsequently, the sulfate minerals are re-precipitated by evaporation of strong acidic water, and the sulfate minerals are again dissolved by rainwater. The dissolution of sulfate minerals is probably promoting the dissolution of sulfide minerals. The formation of strong acidic water at the Dokuzawa mineral spring is considered to be sustained by repeated cycles of mineral dissolution and precipitation.
Rocks and water were collected from the Dokuzawa mineral spring. The constituent minerals of the collected rock samples were observed for their microstructure, and their chemical composition was analyzed. The water samples were analyzed for major dissolved ion concentrations and noble gas isotope ratios (3He/4He, 4He/20Ne). Additionally, the residence time of the groundwater in the Dokuzawa mineral spring area was estimated using the 3H-3He method. A precipitation experiment was conducted, in which the water samples were air-dried, and the precipitated materials were observed, qualitatively analyzed, and the minerals were identified. Furthermore, a 10-minute leaching experiment was performed using rock samples, during which pH, EC, and the concentrations of dissolved elements were monitored.
The collected rock samples consist of pyrite, clay minerals, jarosite, leucoxene, and rare silver oxide. These minerals might have occurred due to past hydrothermal alteration. Among these minerals, pyrite, and jarosite, are known as minerals for causing acidification. In addition, the rock's surface is colored yellow, and unidentified fine minerals, which might be sulfate minerals, are present in the grain boundaries. The metavoltine and alunogen precipitated from the water sample of the Dokuzawa mineral spring. This suggests that the fine sulfate minerals on the rock surface are most likely metavoltine and alunogen, precipitated from strong acidic water. Moreover, in the leaching experiment, the pH of the water rapidly decreases, and the EC increases, suggesting that the dissolution of sulfate minerals, which are more soluble than pyrite, is the primary cause in the initial stage of strong acidification that occurs when rainwater permeates rocks.
These results suggest that the dissolution of pyrite and sulfate minerals, which occurred during past hydrothermal alteration, causes the water to become strongly acidic. Subsequently, the sulfate minerals are re-precipitated by evaporation of strong acidic water, and the sulfate minerals are again dissolved by rainwater. The dissolution of sulfate minerals is probably promoting the dissolution of sulfide minerals. The formation of strong acidic water at the Dokuzawa mineral spring is considered to be sustained by repeated cycles of mineral dissolution and precipitation.