Longitudinal discontinuities in riverine greenhouse gas dynamics generated by dams and urban wastewater

Surface water concentrations of inline-formulaCO₂, inline-formulaCH₄, and inline-formulaN₂O 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), inline-formulaCO₂, and inline-formulaCH₄ 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 inline-formulapCO₂ were relatively low in the middle reach (51–2465 inline-formulaµatm), particularly at the four dam sites (51–761 inline-formulaµatm), compared with those found in the largely forested upper basin with scattered patches of croplands (163–2539 inline-formulaµatm), the lower reach (78–11298 inline-formulaµatm), and three urban tributaries (2120–11970 inline-formulaµatm). The upper and middle reaches displayed generally low concentration ranges of inline-formulaCH₄ and inline-formulaN₂O, with some local peaks influenced by agricultural runoff and impoundments. By comparison, the lower reach exhibited exceptionally high concentrations of inline-formulaCH₄ (1.2–15766 nmol Linline-formula⁻¹) and inline-formulaN₂O (7.5–1396 nmol Linline-formula⁻¹), which were significantly correlated with different sets of variables such as DO and inline-formula $M18inlinescrollmathmlchem{\mathrm{normal\; PO}}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 3}-}$ 29pt17ptsvg-formulamathimgbe398bd0f0b069243c3f6f0b1b183fcd bg-15-6349-2018-ie00001.svg29pt17ptbg-15-6349-2018-ie00001.png for inline-formulaCH₄ and inline-formula $M20inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg730c3bacbcf80f0a2c8df5dbccbd0cf0 bg-15-6349-2018-ie00002.svg24pt15ptbg-15-6349-2018-ie00002.png and inline-formula $M21inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg8a28d0a06c83cc478fcb3953dfc5e6ea bg-15-6349-2018-ie00003.svg25pt16ptbg-15-6349-2018-ie00003.png for inline-formulaN₂O. 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 inline-formulaδ¹³C in surface water inline-formulaCO₂ and inline-formulaCH₄ measured during the sampling cruise along the lower reach, combined with inline-formulaδ¹³C and inline-formulaΔ¹⁴C 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 inline-formula¹³C in inline-formulaCO₂ and inline-formulaCH₄ 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 inline-formulaCH₄ 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 researchpage6350 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 inline-formulapCO₂ and in-lake production of inline-formulaCH₄ and inline-formulaN₂O.