Abstract:
The aim of this study was to determine if
Arase dam gate removal and flushing elevated concen trations of any trace elements in Kuma River and
Yatsushiro Bay sediments or caused riverine environ mental change. The Arase dam gate on the Kuma River
was opened in April 2010. Surface and bottom sedi ments were compared using 10-cm-long cores (2011)
and two grain size fractions. Surface sediment data from
2002, 2012, and 2013 from the Kuma River and
Yatsushiro Bay were also compared. The sediments
were analyzed using XRF for 23 elements, and the grain
size analysis was done. The short core surface and
bottom sediments do not show major chemical changes,
and therefore, may not represent post-and pre-dam sed iments. Results based on 2011 samples show that the
removal of the Arase dam gates in 2010 has been
geoenvironmentally beneficial due to the decrease of
environmentally related trace elements Pb and Zn in
2013. However, a slight increase in the levels of Cr,
Cu, Zr, and Nb in 2013 indicates that periodic flushing
in winter leads to elevation in these elements due to an
increase in the fine fraction. Metal enrichment factors
(EF) in 2002 are higher and these have decreased by
2013. Some elements exceed environmental guidelines,
but this is due to natural background values, and there is
no anthropogenic contamination. Thus, the environment
of the river and bay has been significantly improved due
to the dam opening. This result suggests that assessment
and environmental monitoring studies are very impor tant for dam management and future decision making
