Abstract
Water chemistry and hydrology of tufa-bearing stream of Shirokawa Town, Ehime Prefecture were studied. The streamwater is originated from a limestone underground water system and contains a large amount of calcium carbonate (2.1∼3.1 millimol/l in alkalinity). The Pco2 of the water quickly decreases from a high value (reaches 3, 000 μ atm in summer) and saturation index for calcite (SIC) increases to values exceeding 0.5 after issuing from the spring. Throughout the year, CaCO3 was deposited along the stream, as indicated by the general decreasing trends of alkalinity and Ca-contents to the downstream direction. According to the results of calculation, daily deposition of calcite was largest in late July, whereas the water in winter was inefficient in depositing tufas. In this tufa-bearing stream, 5.8 tons of calcite were deposited, and 72.6 tons of calcium carbonate flowed out in a year.Water chemistry and hydrology seasonally changed under the controls of weather conditions and underground processes. Water temperature and flow in the stream varied generally following the changes in temperature and rainfall at the nearest weather observation point. Subsurface processes were discussed based on spring water data. Alkalinity of the spring water was high in summer and autumn that is probably associated with evaporation of water in underground and active dissolution of limestone in summer due to high Pco2 in soil air. The amount of flow is negatively correlated to SIC of the spring water. Probably, CO2-degassing in underground is an important control to the SIC. During the periods of larger water flow, the high underground water level may have restricted gas exchange across the issuing point and the underground CO2-degassing.
