Longitudinal discontinuities in riverine greenhouse gas dynamics generated by dams and urban wastewater
Authors: Jin Hyojin , Yoon Tae Kyung , Begum Most Shirina , Lee Eun-Ju , Oh Neung-Hwan , Kang Namgoo , Park Ji-Hyung 
Published: October 2018

Abstract. Surface water concentrations of CO2, CH4, and
N2O have rarely been measured simultaneously in river systems
modified by human activities, contributing to large uncertainties in
estimating global riverine emissions of greenhouse gases (GHGs). Basin-wide
surveys of the three GHGs were combined with a small number of measurements
of C isotope ratios in dissolved organic matter (DOM), CO2, and
CH4 in the Han River basin, South Korea, to examine how
longitudinal patterns of the three gases and DOM are affected by four cascade
dams along a middle section of the North Han River (hereafter termed “middle
reach”) and treated wastewater discharged to the lower Han River (“lower
reach”) traversing the Seoul metropolitan area. Monthly monitoring and
two-season comparison were conducted at 6 and 15 sites, respectively, to
measure surface water gas concentrations and ancillary water quality
parameters including concentrations of dissolved organic carbon (DOC) and
optical properties of DOM. The basin-wide surveys were complemented with a
sampling cruise along the lower reach and synoptic samplings along an urban
tributary delivering effluents from a large wastewater treatment plant (WWTP)
to the lower reach. The levels of pCO2 were relatively low
in the middle reach (51–2465 µatm), particularly at the four dam
sites (51–761 µatm), compared with those found in the largely
forested upper basin with scattered patches of croplands
(163–2539 µatm), the lower reach (78–11298 µatm), and
three urban tributaries (2120–11970 µatm). The upper and middle
reaches displayed generally low concentration ranges of CH4 and
N2O, with some local peaks influenced by agricultural runoff and
impoundments. By comparison, the lower reach exhibited exceptionally high concentrations of CH4
(1.2–15766 nmol L−1) and N2O (7.5–1396 nmol L−1),
which were significantly correlated with different sets of variables such as
DO and PO43- for CH4 and NH4+ and
NO3- for N2O. Downriver increases in the levels of DOC
and optical properties such as fluorescence index (FI) and protein-like
fluorescence indicated an increasing DOM fraction of anthropogenic and
microbial origin. The concentrations of the three GHGs and DOC were similar
in magnitude and temporal variation at a WWTP discharge and the receiving
tributary, indicating a disproportionate contribution of the WWTP effluents
to the tributary gas and DOC exports to the lower reach. The values of
δ13C in surface water CO2 and CH4 measured
during the sampling cruise along the lower reach, combined with
δ13C and Δ14C in DOM sampled across the basin,
implied a strong influence of the wastewater-derived gases and aged DOM
delivered by the urban tributaries. The downstream enrichment of
13C in CO2 and CH4 suggested that the spatial
distribution of these gases across the eutrophic lower reach may also be
constrained by multiple concomitant processes including outgassing,
photosynthesis, and CH4 oxidation. The overall results suggest that
dams and urban wastewater may create longitudinal discontinuities in riverine
metabolic processes leading to large spatial variations in the three GHGs
correlating with different combinations of DOM properties and nutrients.
Further research is required to evaluate the relative contributions of
anthropogenic and in-stream sources of the three gases and DOM in eutrophic
urbanized river systems and constrain key factors for the contrasting
impoundment effects such as autotrophy-driven decreases in
pCO2 and in-lake production of CH4 and
N2O.