CO2 outgassing from an urbanized river system fueled by wastewater

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CO2 outgassing from an urbanized river system fueled by wastewater treatment plant effluents Tae Kyung Yoon, Hyojin Jin, Most Shirina Begum, Namgoo Kang, and Ji-Hyung Park Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02344 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017

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Environmental Science & Technology

CO2 outgassing from an urbanized river system fueled by wastewater treatment plant effluents Tae Kyung Yoon†,‡, Hyojin Jin†, Most Shirina Begum†, Namgoo Kang§,∥, Ji-Hyung Park†,*

†

Department of Environmental Science and Engineering, Ewha Womans University, Seoul 03760,

Republic of Korea §

Center for Gas Analysis, Korea Research Institute of Standards and Science, Daejeon 34113,

Republic of Korea

∥Science

of Measurement, University of Science and Technology, KRISS Campus, Daejeon

34113, Republic Korea

1

ACS Paragon Plus Environment


1

ABSTRACT

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Continuous underway measurements were combined with a basin-scale survey to examine

3

human impacts on CO2 outgassing in a highly urbanized river system in Korea. While the partial

4

pressure of CO2 (pCO2) was measured at 15 sites using syringe equilibration, three cruises

5

employing an equilibrator were done along a 30-km transect in the Seoul metropolitan area. The

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basin-scale survey revealed longitudinal increases in surface water pCO2 and dissolved organic

7

carbon (DOC) in the downstream reach. Downstream increases in pCO2, DOC, fluorescence

8

index, and inorganic N and P reflected disproportionately large contributions from wastewater

9

treatment plant (WWTP) effluents carried by major urban tributaries. Cruise transects exhibited

10

strong localized peaks of pCO2 up to 13,000 µatm and 13CO2 enrichment along the confluences

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of tributaries at average flow, whereas CO2 pulses were dampened by increased flow during the

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monsoon period. Fluctuations in pCO2 along the eutrophic reach downstream of the confluences

13

reflected environmental controls on the balance between photosynthesis, biodegradation, and

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outgassing. The results underscore WWTP effluents as an anthropogenic source of nutrients,

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DOC, and CO2 and their influences on algal blooms and associated C dynamics in eutrophic

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urbanized river systems, warranting further research on urbanization-induced perturbations to

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riverine metabolic processes and carbon fluxes.

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INTRODUCTION

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As the terrestrial biosphere “breathes”,1 streams, rivers, and lakes release carbon dioxide (CO2)

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to the atmosphere during transit of organic and inorganic carbon (C) to the oceans, constituting

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an important link in the global carbon cycle.2-8 Recent global estimates of CO2 outgassing from

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inland waters range from < 1 to 3.8 Pg C y-1, which can result in a significant loss of the annual

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terrestrial sink (~2.5 Pg C y-1) of CO2 emitted from anthropogenic sources.4-7, 9 Large

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uncertainties in current estimates result from a combination of factors including sparse spatial

26

coverage of direct measurements of the partial pressure of CO2 (pCO2) in inland waters and

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difficulties in estimating gas transfer velocities.6, 7, 9 Although direct measurements of pCO2 have

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identified previously unrecognized inland water C sources such as wetlands and estuarine

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marshes,10-12 pCO2 data are still sparse for anthropogenically modified river systems worldwide,

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except for well-studied rivers in North America13, 14 and Europe.2, 15 Particularly sparse are in-

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situ measurements in rapidly urbanizing watersheds across Asia, so obtaining pCO2 data in Asian

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rivers has recently been suggested as a top priority in reducing large uncertainty in estimating the

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global riverine CO2 outgassing.16

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Anthropogenic perturbations such as dams, river channeling, and water pollution can alter

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inland water C fluxes substantially.7 Although anthropogenic changes in water quality and

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biological communities have been studied extensively,17, 18 few studies have investigated altered

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C fluxes in urbanized river systems from the perspective of human impacts on aquatic ecosystem

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processes.19 Some recent studies conducted in urbanized watersheds have identified river

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impoundment and pollution as key controls on the quantity and quality of dissolved organic

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matter (DOM).20-22 However, only a small number of studies have linked the altered flux and

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composition of DOM to direct measurements of pCO2 in urban rivers.14, 23, 24 Large spatial

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heterogeneity in pCO2 and DOM in urbanized river systems has been attributed to infrastructures

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such as wastewater treatment plants (WWTPs), dams, weirs, channels, and urban stormwater

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facilities that may represent discontinuities in the river continuum, altering natural riverine

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processes and ecosystem resilience.23, 25, 26 Discontinuities in the river continuum can either

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decrease or increase the rate of riverine CO2 outgassing via enhanced photosynthesis27 or organic

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matter biodegradation.21 Few studies have addressed these discontinuities and associated large

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spatial variations in the riverine C fluxes in urbanized watersheds using approaches with

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adequate spatial resolution.

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The Han River basin is a highly modified river system draining the middle of the Korean

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Peninsula including the densely populated Seoul metropolitan area with a population > 25

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million along the lower reach (Figure 1). Longitudinal variations in dominant land use, along

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with a cascade of dams along the middle reach, make the Han River basin an ideal venue to

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study anthropogenic perturbations to hydrologic and biogeochemical flows. A basin-scale survey

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of pCO2, DOM, and nutrients from the forested headwater stream to the estuary of the Han River

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was combined with continuous underway measurements of pCO2 employing an automated

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equilibrator system8 along the downstream reach in the Seoul metropolitan area to explore how

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river impoundment and pollution alter riverine CO2 outgassing and organic matter transport in

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the highly urbanized watershed. We hypothesized that urban tributaries delivering high loads of

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CO2, DOM, and nutrients derived from WWTP effluents to the downstream river could not only

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enhance CO2 outgassing in the receiving water but also alter the rates of the primary production

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and organic matter biodegradation along the downstream reach. We expected that enhanced

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primary production, particularly during algal blooms, could significantly lower the level of pCO2

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via planktonic uptake of CO2, resulting in large longitudinal variations in pCO2 along the

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eutrophic downstream reach.

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Figure 1 Study sites in in the Han River basin: (a) a basin-scale transect (~300 km); (b) an

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underway transect (30 km) and an urban tributary transect (JN; ~35 km) in the Seoul

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metropolitan area. Three major urban streams (AY: Anyang River, JN: Jungnang River, TC: Tan

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River), four wastewater treatment plants (WWTPs), and two submerged weirs are marked on the

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Seoul metropolitan area map (b).

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MATERIALS AND METHODS Site description. The 515-km long Han River consists of the North Han and South Han

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branches and the lower Han River. The river drains an area of 26,142 km2 along the middle of

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the Korean Peninsula and flows westward into the Yellow Sea (Figure 1; more detailed

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descriptions of the study site and methods provided in SI).28 The dominant land cover in the Han

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River basin varies longitudinally from the highly forested upstream reach to increasingly

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urbanized areas along the mid- to downstream reaches. The Han River flowing through the Seoul 5



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metropolitan area is channelized with two submerged weirs at the east (RKM 39) and west end

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(RKM 70) of Seoul (Figure 1b). Three major urban tributaries feed into the lower reach along the

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city of Seoul: the Tan River (TC), Jungnang River (JN), and the Anyang River (AY) enter the

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mainstem at RKM 68, 64, and 49, respectively (Figure 1b). Three WWTPs discharge at the rate

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of 1.5 million m3 d-1 to JN, with the bulk (1.3 million m3 d-1) discharged from the WWTP located

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near the mouth. Four WWTPs located within Seoul release effluents from tertiary treatments

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including modified Ludzack Ettinger (MLE) and anaerobic-anoxic/oxic process (A2O) at the rate

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of 4.3 million m3 d-1.29

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Basin-scale survey. A basin-scale survey along the North Han branch and the lower reach

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was conducted at 15 locations on 4–6 May 2015 at average river flow (183 m3 s-1 at RKM 57;

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Figure S1). The sites were selected at intervals from a forested headwater stream (RKM 299)

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through the middle and lower reaches impounded by dams or weirs to the estuarine reach (RKM

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23) (Figure 1a, Table S1). A 35-km transect of the urban tributary JN, together with WWTP

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effluents near the confluence, was surveyed at eight sampling points from the headwater to the

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confluence on 13 November 2015 and 12–13 May 2016. In addition to air temperature and

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barometric pressure measured with a portable sensor (Watchdog 1650 Micro Station, Spectrum

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Technologies, USA), we measured in situ water temperature, pH, electrical conductivity, and DO

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using a portable pH meter (Orion 5-Star Portable, Thermo Scientific, USA). The surface water

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pCO2 at 20 cm depth was measured by a manual headspace equilibration method.8, 30, 31 The

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equilibrated headspace air sample, as well as an ambient air sample collected for equilibration,

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was injected into a GC (7890A, Agilent, USA) fitted with a Supelco Hayesep Q 12 ft 1/8”

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column for the measurement of CO2 concentration. Grab water samples collected 20 cm below

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the surface were filtered (GF/F, Whatman) at the laboratory and then analyzed for dissolved

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organic C (DOC; TOC-VCPH, Shimadzu, Japan), fluorescence excitation-emission matrices

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(EEMs; F7000, Hitachi, Japan), UV absorbance (8453, Agilent, USA), major ions (883 Basic IC

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plus, Metrohm, Switzerland), and chlorophyll a (Chl-a).32 Fluorescence and UV absorbance data

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were used to calculate fluorescence index (FI), as an optical index that can distinguish the origin

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of DOM between terrestrial/allochthonous sources (1.3–1.5) and microbial/autochthonous

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sources (1.7–2.0).33-35

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Continuous underway measurements. Continuous underway measurements of pCO2 were

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performed on three cruises along the lower reach (RKM 70 to 39; Fig. 1), twice at average flow

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(212 m3 s-1 on both 11 May 2015 and 10 June 2016,) and once at high flow (757 m3 s-1 on 28

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July 2015). The boat speed was maintained at ∼10 km h-1 based on the prior tests and other

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studies.8, 36 To compare spatial differences in pCO2 during a selected day period between three

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cruise transects, the measurements were conducted within four hours in the afternoon when the

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mean pCO2 is close to the daily mean value based on our year-long continuous pCO2

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measurements at RKM 53 (unpublished data). pCO2 was continuously measured using a spray-

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type equilibrator8, 10, 37, 38 connected to an infrared gas analyzer (IRGA; LI820, Li-Cor, USA).

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The equilibrated headspace air in the equilibrator chamber was circulated through a channel

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including an air filter, a desiccant (Drierite) column, and the IRGA. The data were logged every

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1 s on a laptop. Basic water quality parameters were measured in the water from an additional

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water pump channel onboard, using a portable multiparameter meter (Orion 5-Star Portable,

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Thermo Scientific, USA or 6820 V2, YSI Inc., USA). Air temperature and barometric pressure

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were recorded in a micro-logger (Watchdog 1650 Micro Station, Spectrum Technologies, USA).

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The cruises were tracked by a GPS tracking unit (Montana 650, Garmin Ltd., USA). The gas

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samples of the manual headspace and the water samples were collected onboard and at the urban

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streams near the confluences, and later analyzed in the laboratory with the same methods used

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for the basin-scale field survey. In the June 2016 transect, stable C isotope ratios of CO2 (δ13CO2)

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in the gas samples were analyzed at the UC Davis Stable Isotope Facility using a GasBench-

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IRMS (ThermoScientific, Bremen, Germany). The actual water saturated pCO2 in the

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equilibrator was calculated from the pCO2 measured in dry air, barometric pressure, water

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temperature, and equilibrium water vapor (refer to the supporting information; Eq. S1).39

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Aqueous pCO2 in the equilibrator was then corrected to in situ temperature using a temperature

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effect coefficient. To compensate the time lags of the measured pCO2 resulting from the

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residence time of the water circuit and the response time of the sensors, the logged values were

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corrected according to Eq. S2.36

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CO2 outgassing estimation and downstream mass balance of CO2 and DOC. The CO2

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flux at the water-air interface (fCO2) was estimated for the mainstem section along the city of

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Seoul based on the differences in measured pCO2 between water and air and an estimated gas

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transfer velocity (K). Our observations and available data were insufficient to apply empirical K

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models,40 so a K value was adopted from a study conducted in the Hudson River (5 cm h-1)41

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based on the similarities in river hydrology and climatic conditions. The downstream evolution

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of CO2 and DOC was simulated using a simple mass balance model adapted from the modeling

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approach used for estimating CO2 fluxes from the Amazon river and wetlands.12 The model

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consists of inputs of CO2 and DOC from the upper reach (RKM 69) and from the urban

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tributaries (TC and JN); and CO2 outgassing from the section from RKM 69 to 48 (Eq. S3, S4).

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The proportion (termed “residual”) of the total C measured at RKM 62 and 48 that was not

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accounted for by the upstream and/or tributary inputs of CO2 and DOC and CO2 outgassing from

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the given section was assumed to represent either CO2 production (i.e., gain) or consumption (i.e.,

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loss) resulting from organic matter biodegradation or photosynthesis, respectively. All data

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processing, statistical analyses, and modeling were conducted using R.42

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RESULTS AND DISCUSSION

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Spatial pattern of pCO2 and DOC across the Han River basin. The basin-wide survey

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revealed distinct downstream increases in pCO2, DOC, and FI, particularly from the point where

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the river flows into the city boundary of Seoul (Figures 2a–c). The surface water pCO2 ranged

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from 100 to 730 µatm across the upper and middle reaches and began to exceed 1,500 µatm in

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the upper part of the lower reach (Figure 2a). Relatively low pCO2 values in the vicinity of the

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dams (288 µatm; N = 7) may indicate an enhanced algal uptake of CO2 in the impounded reaches.

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Compared to the upstream values, DOC and FI increased 1.5–2-fold in the highly urbanized

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lower reach (Figure 2b, c). The three major urban tributaries exhibited distinctively higher values

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of pCO2, DOC, and FI than all the mainstem sites. This finding suggests that the urban tributaries

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and WWTP effluents, in combination with in-stream metabolic processes in the downstream

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reach, can significantly alter the fluxes of CO2 and DOC in the urbanized river system.

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The transect study along the urban tributary JN revealed longitudinal increases in CO2, DOC,

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and inorganic nutrients resulting in large exports into the confluence (Figures 2e-h). The

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effluents from a WWTP located 4 km upstream of the JN confluence were highly enriched in

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CO2, DOC, and inorganic nutrients (Figures 2e-h, Table S2). pCO2, DOC, and FI measured at the

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most downstream location of the tributary were similar to those measured at the WWTP effluents,

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indicating that the WWTP effluents account for the bulk of the tributary export of carbon and

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other nutrients to the Han River. Along the tributary upstream of the WWTP, spanning from the

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upper, rural area (> 28 km upstream from the confluence) to the middle, urbanized area (4–28

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km), pCO2 ranged from 1,000–3,000 µatm. Upon receiving the WWTP effluent, the pCO2 in the

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receiving water increased 4-fold, approaching 8,000 µatm.

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Figure 2 Spatial variations in pCO2, dissolved organic carbon (DOC), and fluorescence index (FI)

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along a basin-scale transect (a–d) and an urban tributary (Jungnang River; JN on Figure 1)

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transect (e–h). RKM refers to the river km from the mouth. The measurements in the wastewater

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treatment plant (WWTP) effluents are indicated by red box plots (e-g).

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The spatial pattern of DOC along the JN transect was similar to the pCO2 pattern (Figure 2f).

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The FI values indicated a gradual shift from the dominance of terrestrial DOM (FI < 1.5) in the

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upper reach (Figure 2g) to the greater contribution of autochthonous DOM (FI > 1.7) in the

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WWTP effluent and its downstream reach.21, 34 High loads of CO2 and DOC in the WWTP

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effluents likely originate from the wastewater treatment processes including microbial oxidation

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and biomass respiration that transform organic compounds to CO2 and DOM.43, 44 The WWTP

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effluents strongly influenced other water quality variables, especially inorganic nutrients (Table

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S2).

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Continuous underway measurements of pCO2 along the lower reach. Three cruise

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surveys along the lower reach of the Han River exhibited unique spatial patterns of pCO2, DOC,

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and inorganic nutrients that reflect the material inputs from tributaries and WWTPs (Figures 3, 4;

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Table S3). Mean pCO2 values of 1,066 µatm, 2,518 µatm, and 2,566 µatm, for May 2015, July

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2015, and June 2016 transects, respectively, were all above the atmospheric saturation level (409

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µatm), pointing to the role of the lower reach as a net source of CO2. The levels of

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supersaturation are similar to or higher than the reported values for some temperate rivers,14, 45

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but lower than those for subtropical and tropical rivers.23, 46, 47 Two transects at average flow

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(May 2015 and June 2016) showed strong localized pulses of pCO2 along the tributary JN

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confluence (Figures 3a,c) and noticeable downstream increases in DOC, FI, and inorganic N and

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P concentrations (Figure 4; Table S3). pCO2 increased from < 100 µatm at a location upstream of

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the TC confluence (RKM 69) to ~3,000 µatm in the downstream reach of the JN confluence

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(RKM 62). Strong localized pulses of pCO2 up to ~13,000 µatm were observed in the JN stream

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plume mixing with the mainstem along RKM 62–65, reflecting large inputs of CO2 delivered by

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the tributary (Figures 3, S2). The observed localized pCO2 peaks along the tributary confluences

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have rarely been observed by the previous continuous underway measurements conducted in

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urbanized rivers and estuaries14, 45 or systems with lower proportions of urban land use.46-48 In a

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rare case, Zhai et al.49 found exceptionally high pCO2 values approaching 5,000 µatm along an

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upper reach of the Pearl River estuary with a polluted tributary.

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Figure 3 Underway measurements of pCO2 along the 30-km lower reach of the Han River

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dissecting the Seoul metropolitan area. The gray lines indicate the measurements along the

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confluences receiving the urban tributary discharge (a, c). The May 2015 measurements along

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the mainstem were modified from a prior publication.8

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Figure 4 Urban tributary impacts on downstream water quality: pCO2 (a), dissolved organic

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carbon (DOC; b), fluorescence index (FI; c), chlorophyll-a (Chl-a; d), inorganic nutrients (e–g),

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and dissolved oxygen (DO; h). The upstream (76–68 km; N = 6) and downstream (62–48 km; N

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= 12) reaches were compared with two urban streams (TC; N = 2–3) and (JN; N = 5–8). Asterisks

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(*) and daggers (†) indicate significant (* P < 0.05, ** P < 0.01, and *** P < 0.001) differences

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in space (before and after the confluence; α) and time (sampling season; β), respectively (Yijk = µ

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+ αi + βj + (αβ)ij + εijk).

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DOC concentrations increased by 45% and 8% across the transition from the up- to the

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downstream location of the TC and JN confluences, respectively (Figure 4b; Table S3). FI,

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inorganic N and P, EC, and Chl-a also showed general longitudinal increases, consistent with the

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basin-scale survey. The longitudinal increases along the reach downstream of the two tributaries

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TC and JN were greater at average flow than at high flow, indicating a homogenizing effect of

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fast and high river flow during the monsoon period. In the May 2015 transect, high

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concentrations of PO43- and NH4+ peaked at RKM 53 near the JN confluence and rapidly

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decreased along the downstream reach, likely due to a combination of dilution and rapid algal

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uptake of labile nutrients during the algal bloom.

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Observed longitudinal patterns can be explained by the mixing of the mainstem and tributary

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waters, as indicated by downstream changes associated with distinctively higher pCO2, DOM,

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and inorganic nutrients and lower DO and Chl-a in the tributaries (Figure 4). For example, pCO2

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at TC (6240 ± 794 µatm) and JN (9699 ± 1442 µatm) was 80- and 124-fold higher, respectively,

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than the value (78 µatm) measured at an upstream location (RKM 69). DOC concentrations were

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2‒3 times higher at the two urban tributaries than the upstream value. The May 2015

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measurements had particularly high values of pCO2 along the JN confluence and gradually

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decreased moving downstream from ~3,000 µatm at RKM 62 to ~260 µatm at RKM 53.

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The measurements of δ13CO2 during the June 2016 cruise exhibited a gradual longitudinal

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enrichment of 13C along the reach downstream of two tributaries TC and JN (Figure 3c). δ13C

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increased from −20.9 ‰ at RKM 76 to −16.7 ‰ at RKM 49. The observed values of δ13C fall in

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the lower range of δ13CO2 and δ13DIC measured for various inland waters (−25 – 0‰).23, 46, 50, 51

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Interpreting the observed variations in δ13C is challenging because δ13CO2 reflects the interplay

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between the complex processes involving gas exchange, organic matter decomposition, and

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dissolution of carbonates of either natural or anthropogenic origin.23, 50 The observed range is

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similar to those for CO2 originating from the decomposition of organic matter (−25 – −15‰),

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implying the dominant role of biogenic processes in the river or anthropogenic sources such as

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WWTPs. 13C enrichment along the reach downstream of the tributaries also suggests that there

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was a contribution from urban tributaries enriched in 13C (TC: –18.3 ‰, JN: –18.2 ‰, AY: –14.7

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‰) relative to the upstream value (–20.9 ‰). As suggested by Zeng et al.23, 13C enrichment in

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DIC can result from a combination of processes including the preferential algal uptake of CO2

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depleted in 13C, organic matter degradation, and the enhanced air-water gas exchange in the

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impounded, slow-moving water facilitating the invasion of atmospheric CO2 enriched in 13C

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(δ13C: –8 ‰) as observed at a location upstream of the TC confluence (RKM 69 and 78), in

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which pCO2 was undersaturated.

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In the July 2015 transect during the summer monsoon period, spatial variations in pCO2,

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DOM, and inorganic nutrients were less pronounced than the measurements during the other

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transects at lower flows (Figure 3b; Table S3). The higher river flow might have weakened the

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effects of urban tributary inputs and the spatial variability along the mainstem. Most

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measurements except pCO2 were lower in magnitude in the monsoon season than at lower flows

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(Table S3). The lower FI values observed during the monsoon transect (1.41) than those obtained

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during the low-flow transects (1.55 in 2015 and 1.52 in 2016) can be explained by the greater

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contribution of DOM from terrestrial sources in the upper reaches. The decrease in FI resulting

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from an increasing contribution of terrestrial DOM has been observed during the monsoon

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season across major rivers in Korea.52

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Downstream transformation of DOC and CO2. The mass balance model consisting of the

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measurements of pCO2 and DOC in the mainstem and the two urban tributaries (TC and JN) and

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the estimated rate of CO2 outgassing were used to track changes in major C components along

268

the sections from RKM 69 (upstream of TC confluence) through RKM 62 (downstream of JN

269

confluence) to RKM 48 (upstream of AY confluence) (Figure 5). In May 2015, two tributaries

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TC and JN accounted for 108% and 47% of pCO2 and DOC concentrations measured at RKM 62,

271

respectively, with the WWTP effluent near the JN confluence alone contributing to 15



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approximately one-half of CO2 and one-fifth of DOC carried by the two tributaries. Although we

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cannot rule out potential overestimation resulting from cumulative errors in estimating the

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upstream and tributary contributions and CO2 evasion downstream of the confluence, the

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extremely high tributary pCO2 relative to the very low upstream value indicates the dominant

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role of the tributary carrying WWTP effluents in increasing pCO2 along the downstream reach at

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relatively low flow (May 2015 and June 2016) compared to the monsoon period (July 2015). In

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the monsoon season (July 2015), however, the contribution of the same WWTP effluents was

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much smaller than at lower flows because of the dilution effect of high river flow. Outgassing

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from the 14-km downstream transect was estimated to release 38% and 12% of CO2 measured at

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RKM 62 at average and high flow, respectively. When the rate of outgassing is estimated for the

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lower reach up to the weir preventing the tidal water intrusion (RKM 39), 55% and 23% of the

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CO2 inflow from the WWTP near the JN confluence may be released within the 23-km

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downstream transect (RKM 62 to 39) at average and high flow, respectively.

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When the fraction associated with CO2 outgassing (fCO2) from the RKM 62-48 section was

286

subtracted from the total C concentration measured at RKM 62, the difference (DOCRKM62 +

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CO2RKM62 – fCO2RKM48–62) was either higher (May 2015) or lower (July 2015 and June 2016) than

288

the combined concentration of DOC and CO2 measured at RKM 48 (DOCRKM48 + pCO2RKM48).

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The residual found in May 2015 indicates the consumption of CO2 via planktonic uptake in the

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14-km transect. In contrast, the increased C concentration at RKM 48 observed in July 2015 and

291

June 2016 can be explained by the production of CO2 in the same section. At high flow (July

292

2015) the concentration of Chl-a varied little across the transect, but increased at average flow

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between RKM 62 and 48 by 31.4 and 11.8 µg L-1 in May 2015 and June 2016, respectively

294

(Table S3).

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Figure 5 Changes in CO2, dissolved organic carbon (DOC), chlorophyll-a (Chl-a) from a

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mainstem location (RKM 69) upstream of the first tributary (TC) through a downstream location

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(RKM 62) of the second tributary (JN) to a location (RKM 48) upstream of the third tributary

299

(AY). Estimates of the tributary input and CO2 outgassing, and residuals are indicated between

300

the measurements at three locations. Note that fCO2 from RKM 69 to RKM 62 was too low (

CO2 outgassing from an urbanized river system fueled by wastewater

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