In is not Out: Closing the Floating Macroplastic Mass Balance for The
Hong-Duong Bifurcation in The Red River, Vietnam
Abstract
Regarding transport of macroplastic (>5 mm) in rivers, its
division over bifurcations is still an understudied topic. However, this
is a critical knowledge to estimate plastic emission to oceans from
rivers. To quantify the spatiotemporal variability of plastics in the
Hong-Duong bifurcation, we executed a field campaign on a weekly basis
and applied visual counting method where we count the floating plastics
flowing through three cross sections to determine cross-sectional
distribution of floating plastic and classify polymer categories of
plastics from bridges located in the Hong-Duong bifurcation. These
bridges include Nhat Tan located in northern Hanoi in the Red River,
Long Bien is about 8km to the south from Nhat Tan in the Red River, and
Dong Tru located in the tributary Duong River (~7km from
Nhat Tan). We aim to determine the spatiotemporal changes of
macroplastics across the Hong-Duong bifurcation over the period from May
2021 to November 2021. Until July 2021, we found that the total average
macroplastic fluxes at the cross sections in Nhat Tan, Long Bien, and
Dong Tru were 698, 159, and 113 items/hour, respectively. Notably, these
values do not follow the expected plastic balance between total plastic
flux in the parent river and its tributaries, which is likely explained
by the accumulation, transport below the surface, or sedimentation of
plastics in the space between measurement locations. Additionally, over
three months of May, June, and July, the total average plastic fluxes in
all cross sections showed an increasing trend (~10%).
Furthermore, we also found that most plastics were distributed in the
right side (downstream perspective) of Nhat Tan and Long Bien, while
Dong Tru saw the opposite. Regarding the plastic classification, based
on the River-OSPAR category, we found that food wrappers, polystyrene
fragments, low density polyethylene (LDPE) bags, and polyethylene
terephthalate (PET) bottles were the top 4 items. These findings
together with the distribution of macroplastics along the cross sections
are expected to apply in correlation analyses with hydrodynamic
components in the bifurcation to determine their connectivity. This
information is crucial for improving the efforts on quantifying
macroplastic emissions from the Red River to the ocean which is still
unknown up to now.