Change of alkalinity and inorganic ions with rock weathering in a small river

Abstract

Alkalinity and major dissolved inorganic components contained in river water were measured, and weathering in the drainage area was evaluated. Owasawa river, a branch of Hirakawa river in the suburbs of Hirosaki City, was chosen for the survey. The upper reaches are mainly mountainous areas with few human impact, and the lower reaches have an influence from sewage contamination. Survey was started in January of the snow season. It has been continued through the snow melt season and up to now. As a result, it was found that the average concentration of each dissolved component has a clear difference between the upper and lower reaches and the concentrations of many components increased with the flow. In the upper reaches, the sum of equivalent concentrations of Ca and Mg ions and the alkalinity showed a good correlation. The sum can account for 48% as the counter ions of hydrogen carbonate at the most upstream, while increased to 79% at the most downstream of the upper reaches. Furthermore, when sea salt components in the inorganic ions was corrected, the contribution of the sum of Ca and Mg ions as the counter ions of hydrogen carbonate was increased. It suggest that the dissolution of rock increases the river water alkalinity in mountain stream.