In-stream phosphate removal by suspended sediments transported from volcanic catchments

To better understand in-stream phosphorus delivery and transformation processes, we conducted field measurements and laboratory experiments to study the natural function of phosphate removal by suspended sediments from tributary streamflows of the Abukuma River watershed. Field measurements revealed that phosphate concentrations and fluxes decreased along the Abukuma River mainstream due to inflows from a tributary catchment located in a volcanic mountain range. In the laboratory experiments, water samples collected from tributaries were mixed with those from the Abukuma River mainstream to examine the cause and effect of phosphate removal in the mainstream. Suspended sediments in the tributary samples removed up to about 70% of phosphate in the mainstream sample, depending on the amount of sediment added. X-ray fluorescence spectrometer detected a high ratio of aluminum oxide on the surface of suspended sediments and scanning electron microscopic images showed relatively uniform 0.2–0.5-μm spherical particles. These findings and the geological condition of the tributary catchments studied led us to conclude that halloysite minerals were likely to comprise the majority of suspended sediments and play a significant role in phosphate adsorption. The experimental results were up-scaled and verified using field measurements. In conclusion, natural phosphate removal in the Abukuma River can be attributed primarily to high ratios of aluminum–oxide minerals transported by volcanic tributaries, suggesting a significant hydrological contribution of volcanic catchments to in-stream phosphate transport.