Stream water chemistry of forested headwater areas in the Shimanto River basin

Abstract

In this study, the spatial distribution of stream water chemistry was investigated in headwater areas within the Shimanto River basin to evaluate the effect of forested headwater on the quality of downstream waters under ordinary flow condition. The study area was mainly underlain by Paleozoic/Mesozoic sandstone and shale, with partially exposed limestone and granite. The difference in the bedrock geology of the catchments strongly affected the solute composition of the stream water. The catchments underlain by limestone were characterized by high pH, EC, and Ca²⁺ and HCO₃^- concentrations, and the ones underlain by granite were characterized by low pH, EC, and concentrations of major ions as compared to the catchments underlain by sandstone and shale. The concentrations of Na⁺ and Cl^- negatively corresponded to a distance from the coastline of the Pacific Ocean, thereby suggesting the difference in the deposition of sea salt during storm events that frequently occur from June to October. High runoff rate during rainy season affected the decreasing Na⁺, Mg²⁺, Ca²⁺, and SO₄^2- concentrations in the stream water by dilution with the rainwater. Warm and humid summer climate may further accelerate the dissolution of silicate minerals in the bedrock and soils, resulting in an increase in the Si concentrations. The mean concentration of dissolved total nitrogen in the headwater was 0.17 mg L^-1 in summer and 0.26 mg L^-1 in winter. These results satisfy one of clear stream standards of the Shimanto River (i.e., 0.3 mg L^-1 for dissolved total nitrogen) established by Kochi Prefecture Government and indicate that the forested headwaters would contribute to the clear downstream water environment.