『Abstract
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
9.2-9.5-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.
Keywords: Abukuma River; Halloysite; Phosphate removal; Suspended
sediments; Volcanic catchment』
1. Introduction
2. Study area
3. Method
3.1. Field measurements
3.2. Laboratory experiments
4. Results and discussions
4.1. Concentration and flux of phosphorus along the mainstream
4.2. Effect of suspended sediments in tributary water
4.3. Comparison of three tributaries
4.4. Chemical composition of suspended sediments
4.5. Scanning electron microscopic observation of sample surfaces
5. Estimating phosphate removal by suspended sediments under field
conditions
6. Conclusions
References