doi:10.5474/geologija.2015.002 Isotopic composition of carbon in atmospheric air; use of a diffusion model at the water/atmosphere interface in Velenje Basin Izotopska sestava ogljika v atmosferskem zraku in difuzijski model na fazni meji voda/atmosfera v Velenjskem bazenu Tjaa KANDU^ Jožef Stefan Institute, Jamova cesta 39, SI–1000 Ljubljana, Slovenia; e-mail: tjasa.kanducijs.si Prejeto / Received 7. 4. 2015; Sprejeto / Accepted 5. 6. 2015 Key words: atmospheric carbon dioxide, carbon isotopes, phase boundary air/water, diffusion model, Velenje Basin, thermal power plant, anthropogenic influence Klju~ne besede: atmosferski ogljikov dioksid, ogljikovi izotopi, fazna meja zrak/voda, difuzijski model, Velenjski bazen, termoelektrarna, antropogeni vpliv Abstract COconcentrations (partial pressure of CO, pCO), and isotope compositions of carbon dioxide in air (.13C), 2 22CO2 temperature (T) and relative humidity (H) have been measured in the atmosphere in the Velenje Basin. Samples were collected monthly in the calendar year 2011 from 9 locations in the area where the largest thermal power plant in Slovenia with the greatest emission of CO2 to the atmosphere (around 4M t/year) is located. Values of pCO2 ranged from 239 to 460 ppm with an average value of 294 ppm, which is below the average atmospheric COpressure (360 ppm). .13Cranged from -18.0 to -6.4 ‰, with an average value of -11.7 ‰. These values are 2 CO2 similar to those measured in Wroclaw, Poland. We performed the comparison of .13CCO2 values in atmospheric air with Wroclaw since researchers used similar approach to trace .13CCO2 around anthropogenic sources. The isotopic composition of dissolved inorganic carbon (.13CDIC) in rivers and lakes from the Velenje basin changes seasonally from -13.5 to -7.1‰. The values of .13CDIC indicate the occurrence of biogeochemical processes in the surface waters, with dissolution of carbonates and degradation of organic matter being the most important. A concentration and diffusion model was used to calculate the time of equilibration between dissolved inorganic carbon in natural sources (rivers) and atmospheric CO2. Izvle~ek Ta {tudija opisuje rezultate analize koncentracij COv zraku (parcialni tlak CO, pCO) in izotopske sestave 2 2 2 ogljika v atmosferskem zraku (.13CCO2), temperature (T) in relativne vlažnosti (H) v atmosferi iz Velenjskega bazena. Vzorce smo vzor~ili mese~no na 9 lokacijah v koledarskem letu 2011 na obmo~ju Velenjskega bazena, kjer je locirana najve~ja termoelektrarna v Sloveniji, ki predstavlja najve~jega proizvajalca emisij CO2 v atmosfero (okoli 4 Mt/leto). Koncentracije pCO2 v zraku se v ~asu te {tudije spreminjajo od 239 do 460 ppm. Merjene povpre~ne koncentracije pCO2 v na{i {tudiji zna{ajo 294 ppm in so pod povpre~nim atmosferskim tlakom CO2, ki zna{a 360 ppm. Merjena .13Cse spreminja od -18,0 do -6,4 ‰ s povpre~no vrednostjo .13C-11,7 ‰. Vrednosti CO2 CO2 atmosferskega CO2 in .13CCO2 so v ~asu te raziskave podobne vrednostim objavljenim za Wroclaw, Poljska. Naredili smo primerjavo z .13CCO2 vrednostmi v atmosferskem zraku z Wroclawom, ker so raziskovalci uporabili podoben pristop sledenja .13CCO2 vrednosti okrog antropogenih virov. Izotopska sestava raztopljenega anorganskega ogljika (.13CDIC) v rekah in jezerih Velenjskega bazena se je v letu 2011 sezonsko spreminjala od -13,5 do -7,1 ‰. Vrednosti .13CDIC odražajo biogeokemijske procese v povr{inskih vodah, med katerimi sta najpomembnej{a raztapljanje karbonatov in razgradnja organske snovi. Izdelali smo tudi koncentracijski in izotopski difuzijski model za izra~un ~asa uravnoteženja med atmosferskim CO2 in raztopljenim CO2 na re~nih to~kah. Introduction of atmospheric CO2 have been carried out to assess their anthropogenic impact (Kuc et al., Investigation of the fate of atmospheric CO2 2003; lonGinelli & selMo, 2005; PataKi et al., is central to efforts to measure and predict 2005; ziMnoch et al., 2004). In the atmospheric global anthropogenic changes and to assess the boundary layer, the concentration and carbon impact of fossil fuel usage on environmental isotope composition of atmospheric CO2 (.13CCO2) quality (eea, 1998, 2003). Analyses of the is determined by the mixing of tropospheric concentration and anisotropic composition air with locally derived air that is affected by anthropogenic and/or biogenic CO2 sources and sinks (ziMnoch et al., 2004). Biogenic CO2 originates from plant respiration and from heterogenic soil microbes which convert soil organic matter to CO2. Because 12C is taken up preferentially by plants during photosynthesis, soils are lower in 13C than the atmosphere (BowlinG et al., 2008). Where C3 vegetation (e.g. Filipendulion (with dominant and characteristic species Filipendula ulmaria (L.) Maxim.) and Bidention (species from genera Bidens L., Rorippa Scop., Chenopodium L., Polygonum L.,…), Fagus sylvatica L., Picea abies (L.) Karst., Abies alba P. Mill.) dominates, as is the case for the studied area, soil organic matter and CO2 respired by vegetation exhibit .13C values between -28 and -20 ‰ (szaran, 2002). Values of .13CCO2 derived from burning fossil fuels (anthropogenic sources) range from -40.5 (natural gas burning fumes) to -24.6 ‰ (coal burning fumes) (wiDory & javoy, 2003). Combustion of coal produces almost twice as much carbon dioxide per unit of energy as does the combustion of natural gas, while the amount from the combustion of crude oil falls in between (Energy Information administration, Emissions of Greenhouse Gases in the United States 1985­1990 (DOE/EIA-0573)). In the vegetative season the anthropogenic input is minimized and the biological input is dominant (lonGinelli & seMo, 2005). Values of .13Cand pCO in the CO2 2 atmosphere have also been used to determine pollution levels in the atmosphere (zwozaDziaK et al., 2010). Concentrations of dissolved inorganic carbon, DIC, and its isotopic composition (.13CDIC) in freshwater environments have been widely investigated (aMiotte-suchet et al., 1999; ateKwana & KrishnaMurthy, 1998; MarFia et al., 2004; KanDu^ et al., 2007) and groundwater/ surface water interactions, with evaluation of biogeochemical processes, have been reported for Velenje Basin (KanDu^ et al., 2010, KanDu^ et al., 2014). Here we report measurements of pCO2 (partial pressure) and .13CCO2 in the vicinity of the Šo{tanj thermal plant which is the biggest emitter of CO2 to the atmosphere in Slovenia. Thus, around 4 Mt of CO2 are emitted (EMEP/EEA, 2013) into the atmosphere per year. The aim of this study was 1) to measure monthly air concentrations of pCO2 and to measure .13CCO2 in air to determine the influence of the combustion of lignite on pCO2 concentrations and to define the origin of the CO2 in the air masses in Velenje Basin, 2) to compare pCO2 concentrations and .13C in air with published data (Wroclaw between 1st January and 31st December 2008) and 3) using the concentration and isotope diffusion model to calculate the time of equilibration of CO2 needed to equilibrate concentrations of pCO2 and .13CDIC values between air/water interface. Materials and methods Partial pressure of CO2 (pCO2) in the atmosphere was measured above surface water at 9 locations (Figure 1) in Velenje Basin, using an IAQ-CALC Indoor Air Quality Meter, Model 7545, Thrust Science Innovation (TSI) with an accuracy of ±3 % of reading or ±50 ppm. Air samples for measurement of the carbon isotope composition in carbon dioxide in air (.13CCO2) were sampled as follows: a Labco ampoule (4 ampoules per location) was opened in the windward direction to let it fill with air. After filling (about 2 minutes), the ampoule was immediately closed and transported to the laboratory for prompt analysis of carbon (.13C.13C isotope composition ). Air for CO2CO2 analysis was sampled 2 m above surface water. At the same locations, relative humidity (H), and Figure 1. Sampling locations (10 locations) from Velenje Basin area (river locations: 1, 2, 3, 4, 6 and 8, lake locations: 5, 7, 9). 4 Velunja, 9h03 19.5.2011 25.7 36.6 272 -11.9 sunny 4 Velunja, 9h06 16.6.2011 35.0 37.4 239 -13.0 sunny 4 Velunja, 9h07 18.7.2011 21.1 76.0 294 -13.5 showers 4 Velunja, 9h08 26.8.2011 29.1 45.0 275 -11.3 sunny 4 Velunja, 9h10 15.9.2011 25.9 28.3 247 -11.5 sunny 4 Velunja, 9h15 29.9.2011 18.9 46.0 280 -9.5 sunny 4 Velunja, 9h20 11.11.2011 8.0 55.0 270 -11.1 sunny 5 Šoštanjsko jezero, 9h20 28.1.2011 9.1 54.0 330 -10.4 sunny 5 Šoštanjsko jezero, 9h30 10.3.2011 13.6 27.2 330 -11.5 sunny 5 šoštanjsko jezero, 9h25 30.3.2011 21.5 31.2 312 -11.9 sunny 5 šoštanjsko jezero, 9h22 19.4.2011 19.6 23.0 292 -11.3 sunny 5 Šoštanjsko jezero, 9h23 19.5.2011 28.5 27.0 270 -10.5 sunny 5 Šoštanjsko jezero, 9h26 16.6.2011 29.0 47.8 255 -11.0 sunny 5 Šoštanjsko jezero,9h27 18.7.2011 21.3 77.1 303 -12.0 showers 5 Šoštanjsko jezero, 9h28 26.8.2011 28.4 38.0 280 -11.3 sunny 5 Šoštanjsko jezero, 9h30 15.9.2011 30.8 27.9 269 -10.2 sunny 5 šoštanjsko jezero, 9h35 29.9.2011 19.9 51.7 320 -10.8 sunny 5 Šoštanjsko jezero, 9h40 10.10.2011 8.3 65.9 290 -15.6 sunny, after snow 5 Šoštanjsko jezero, 9h45 11.11.2011 7.7 53.9 311 -12.3 sunny 6 Ljubela, 9h40 28.1.2011 8.6 51.8 320 -11.9 sunny 6 Ljubela , 9h50 10.3.2011 16.5 21.0 314 -12.0 sunny 6 Ljubela, 9h45 30.3.2011 20.0 30.0 299 -13.0 sunny 6 Ljubela, 9h42 19.4.2011 19.2 30.3 289 -9.5 sunny 6 Ljubela, 9h 43 19.5.2011 23.5 38.2 270 -10.5 sunny 6 Ljubela, 9h46 16.6.2011 29.4 31.4 242 -10.5 sunny 6 Ljubela, 9h47 18.7.2011 20.7 72.4 290 -11.8 showers 6 Ljubela, 9h48 26.8.2011 -12.0 sunny 6 Ljubela, 9h 50 15.9.2011 27.6 34.5 252 -12.0 sunny 6 Ljubela, 9h 55 29.9.2011 21.3 47.3 283 -12.3 sunny 6 Ljubela, 10h00 10.10.2011 8.2 66.3 280 -7.7 sunny, after snow 6 Ljubela, 10h05 11.11.2011 9.9 54.0 250 -11.1 sunny 7 Velenjsko jezero, 10h00 28.1.2011 9.2 54.5 328 -9.4 sunny 7 Velenjsko jezero,10h10 10.3.2011 15.3 20.2 316 -11.1 sunny 7 Velenjsko jezero, 10h05 30.3.2011 22.3 31.5 299 -12.8 sunny 7 Velenjsko jezero, 10h12 19.4.2011 19.2 31.9 295 -11.7 sunny 7 Velenjsko jezero, 10h13 19.5.2011 26.6 36.5 271 -10.5 sunny 7 Velenjsko jezero, 10h16 16.6.2011 35.0 37.4 239 -10.7 sunny 7 Velenjsko jezero,10h08 18.7.2011 22.1 71.8 289 -12.0 showers 7 Velenjsko jezero, 10h20 15.9.2011 27.3 31.5 249 -12.5 sunny 7 Velenjsko jezero, 10h40 29.9.2011 10.2 54.1 300 -10.7 sunny 7 Velenjsko jezero, 11h00 10.10.2011 10.2 54.1 300 -9.5 sunny, after snow 7 Velenjsko jezero,11h20 11.11.2011 9.5 55.2 260 -18.0 sunny 8 Lepena,10h20 28.1.2011 12.9 44.2 335 -12.4 sunny 8 Lepena,10h30 10.3.2011 16.2 18.0 316 -11.5 sunny 8 Lepena,10h25 30.3.2011 24.8 28.8 309 -13.5 sunny 8 Lepena,10h32 19.4.2011 20.0 21.6 290 -10.7 sunny 8 Lepena,10h33 19.5.2011 25.6 36.8 262 -10.3 sunny 8 Lepena,10h36 16.6.2011 31.4 35.2 244 -12.5 sunny 8 Lepena, 10h28 18.7.2011 21.0 77.2 285 -12.4 showers 8 Lepena, 11h00 26.8.2011 31.1 31.0 271 -12.0 sunny 8 Lepena, 11h20 15.9.2011 27.3 28.7 246 -11.9 sunny 8 Lepena,11h40 29.9.2011 20.6 50.5 282 -11.6 sunny 8 Lepena,12h00 10.10.2011 11.3 50.4 298 -9.0 sunny, after snow 8 Lepena, 12h20 11.11.2011 9.2 52.2 316 -12.2 sunny 9 p Škalsko jezero, 10h40 28.1.2011 6.3 66.2 328 -15.0 unny sunny 9 Škalsko jezero, 10h50 10.3.2011 17.8 23.2 326 -11.7 sunny 9 Škalsko jezero, 10h45 30.3.2011 22.6 27.7 298 -12.7 sunny 9 Škalsko jezero,11h05 19.4.2011 19.0 29.8 291 -14.5 sunny 9 Škalsko jezero,11h25 19.5.2011 26.5 37.6 262 -10.1 sunny 9 Škalsko jezero, 11h45 16.6.2011 31.6 35.8 251 -10.5 sunny 9 Škalsko jezero,12h05 18.7.2011 21.3 74.5 283 -11.9 showers 9 Škalsko jezero, 12h25 26.8.2011 30.2 45.6 270 -11.5 sunny 9 Škalsko jezero,12h45 15.9.2011 29.7 27.7 272 -12.7 sunny 9 Škalsko jezero, 13h05 29.9.2011 22.0 47.1 285 -11.6 sunny 9 Škalsko jezero, 13h25 10.10.2011 7.4 63.3 293 -8.9 sunny, after snow 9 Škalsko jezero,13h45 11.11.2011 8.9 53.6 317 -17.9 sunny 10 Paka, at 8h10 28.1.2011 5.6 70.0 330 -12.1 sunny 10 Paka, at 8h20 10.3.2011 11.6 31.0 360 -11.4 sunny 10 Paka, at 8h10 30.3.2011 20.1 29.8 323 -13.8 sunny 10 Paka, at 8h05 19.4.2011 19.7 25.6 305 -9.5 sunny 10 Paka, at 8h02 19.5.2011 24.2 42.3 293 -11.9 sunny 10 Paka, at 8h03 16.6.2011 28.8 40.1 266 -10.5 sunny 10 Paka, at 8h06 18.7.2011 20.0 63.5 333 -13.5 showers 10 Paka, at 8h07 26.8.2011 28.1 53.0 460 -14.9 sunny 10 Paka, at 8h08 15.9.2011 25.7 39.0 267 -13.0 sunny 10 Paka, at 8h15 29.9.2011 17.2 65.9 333 -12.0 sunny 10 Paka, at 8h18 10.10.2011 12.6 37.6 318 -6.4 sunny, after snow 10 Paka, at 8h19 11.11.2011 10.0 53.4 314 -12.3 sunny Air temperature ranged from 5.6 to 35.0 °C during 2011 (Figure 2A). Relative humidity ranged from 18.0 to 78.0 % with an average value of 43.6 % (Figure 2B). CO2 concentration in the atmosphere, expressed in ppm as pCO2 and carbon isotope signatures of carbon dioxide in the atmosphere (.13CCO2) from the Velenje Basin indicate seasonal variation (Figures 3A and B). Partial pressures (pCO2) in the atmosphere from 9 different locations range from 239 to 460 ppm – average 294 ppm. The lowest pCO2 value was recorded at Velunja location and the maximum value at Paka River (Figure 3A). The values of .13CCO2 range from -18 to -6.4 ‰, depending on the source (Figure 3 B). The .13CCO2 values that approach -6.4 ‰ (location Paka, South Preloge mine) could reflect bacterial CO2 and/or endogenic CO2 from underground coalmine activity (lazar et al., 2014), while values approaching -18 ‰ (Škalsko and Velenjsko jezero in November 2011) could be attributed to anthropogenic Figure 2A. Air temperature in the calendar year 2011. Figure 2B. Humidity in the calendar year 2011. Numbers from 1–10 refer to sampling locations. At location 3 only surface water was sampled. pollution and natural sources (Figure 3 B). For comparison, the concentration of atmospheric CO2 at the pristine river Kamni{ka Bistrica source was 355 ppm and .13CCO2 value -9 ‰ in different sampling seasons in 2011 (KanDu^, unpublished .13C data). The concentrations of pCO2 and CO2 values reported in this study for Velenje basin are similar to those reported for southern Poland (Kuc et al., 2003; ziMnnoch et al, 2004) (Figure 4). Comparison with Wraclaw, Poland was performed since their study was focused on investigation of isotopic composition of carbon in air (.13CCO2) around anthropogenic sources in relation with other air parameters. The unpolluted .13CCO2 value (around -8 ‰) is taken from Baltic Sea values (white & vauGhn, 2009) and the .13CCO2 values of respiration of C3 plants from PataKi et al., 2003. In a coal burning chimney, .13CCO2 values are -24.1 ‰, exhaust from a gasoline propelled car has values of .13CCO2 of -31.7 ‰, from a diesel car -31.9 ‰ and from a liquid petroleum gas car -33.5 ‰ (GórKa et al., 2011). The characteristic value of .13CCO2 for a coal-burning chimney is -24.1 ‰ and is much lower in comparison to .13CCO2 values in our study, where .13C CO2 ranges from -18.0 to -6.4 ‰ (Table 1). No correlation was obtained between the following parameters measured in the atmosphere for different locations and in different seasons in Velenje Basin: pCOvs. .13C(R2=0.0292), H vs. 2 CO2 pCO (R2=0.0324), pCO vs .13C (R2=0.0292), T vs. 22CO2 pCO2 (R2=0.2644), T vs. .13CCO2 (R2=0.0008). Similarly no significant regression was obtained between measured quantities in air (daily temperature vs. humidity, CO concentration, CO2 concentration, .13C CO2) for Wroclaw (GórKa et al., 2011). Seasonal variations of total alkalinity, .13CDIC and pCO2 (ppm) in surface waters, with pCO2 (closed system, measurements with cardboard box) measured and pCO2 measured just above surface water during year 2011 are presented in Table 2. Discharge data (Q) were obtained from the Slovenian Environmental Agency gauging stations for the year 2011 for locations Velunja, Lepena and Paka. Alkalinity in surface waters changes seasonally from 2.2 to 5.7 mM in January 2011, from 2.6 to 5.5 mM in May 2011, from 2.5 to 6.1 mM in August 2011 and from 2.5 to 5.7 mM in October 2011. .13C DIC changes seasonally from -11.0 to -8.8 ‰ in January 2011, from -11.8 to -7.7 ‰ in May 2011, from -13.5 to -7.1 ‰ in August 2011 and from -12.8 to -9.1 ‰ in October 2011 (Table 2). Higher .13CDIC values would be expected in lake water (standing water) since it equilibrates more quickly than surface water (running water), but it is only the case in lake Velenje (.13CDIC = -7.7 ‰ in spring season). The opposite trend is observed between .13CDIC and alkalinities (Figure 5A), with the lowest .13CDIC value and the highest alkalinity being observed at location Pe~ovnica (location 2) in January 2011. Since surface water is an open system, its equilibration with the atmosphere is important. Equilibration lines (Figure 5A) were calculated according to possible biogeochemical processes influencing .13CDIC value as follows: Line 1. Given the isotopic composition of atmospheric CO2 of -7.8 ‰ (levin et al., 1987) and the equilibration fractionation with DIC of +9 ‰, DIC in equilibrium with the atmosphere should have a .13CDIC of about +1 ‰. Line 2. Considering the average isotopic (.13C composition of carbonates ) with a CaCO3 value of -2 ‰ (KanDu^ & PezDi^, 2005) and isotopic fractionation (and enrichment in 12C) due to dissolution of carbonates, which is 1.0±0.2 ‰ (roManeK et al., 1992), .13CDIC would be -3.0±0.2 ‰. DIC22 data (m3/s) and surface water temperature (°C) in the year 2011. Numbers Locations Date of sampling Q (m3/s) T [° C] Alkalinity (mM) d13CDIC (‰) . pCO2 air, opened system (ppm) pCO2 water/air, closed system (ppm) 1 Toplica January, 2011 8.6 3.6 -10.4 355 357 2 Pečovnica January, 2011 2.1 2.2 -10.2 360 356 3 Klančnica January, 2011 4 Velunja January, 2011 0.633 2.0 3.0 -8.8 357 357 5 Šoštanjsko jezero January, 2011 1.2 2.7 -11.0 6 Ljubela January, 2011 3.0 5.4 -10.1 361 7 Velenjsko jezero January, 2011 2.2 3.2 -9.8 363 8 Lepena January, 2011 0.063 3.0 5.7 -10.4 364 355 9 Škalsko jezero January, 2011 0.7 5.4 -11.0 363 10 Paka January, 2011 2.79 2.6 4.4 -10.1 360 363 Numbers Locations Date of sampling Q (m3/s) T [° C] Alkalinity (mM) d13CDIC (‰) . pCO2 air, opened system (ppm) pCO2 water/air, closed system (ppm) 1 Toplica May, 2011 16.5 3.6 -9.9 362 365 2 Pečovnica May, 2011 14.0 3.0 -10.4 404 425 3 Klančnica May, 2011 16.5 2.9 -11.8 370 388 4 Velunja May, 2011 0.431 15.8 2.9 -8.9 362 362 5 Šoštanjsko jezero May, 2011 20.2 2.6 -9.1 368 368 6 Ljubela May, 2011 16.0 5.1 -10.3 358 370 7 Velenjsko jezero May, 2011 19.6 3.4 -7.7 361 351 8 Lepena May, 2011 0.052 17.1 5.5 -10.3 387 353 9 Škalsko jezero May, 2011 20.9 4.9 -8.4 350 356 10 Paka May, 2011 2.05 14.1 4.1 -9.9 402 389 Numbers Locations Date of sampling Q (m3/s) T [° C] Alkalinity (mM) d13CDIC (‰) . pCO2 air opened system (ppm) pCO2 water/air, closed system (ppm) 1 Toplica August, 2011 16.2 4.8 -10.4 360 362 2 Pečovnica August, 2011 16.7 6.1 -13.1 358 360 3 Klančnica August, 2011 23.3 2.5 -7.0 355 367 4 Velunja August, 2011 0.37 16.7 3.4 -7.8 400 408 5 Šoštanjsko jezero August, 2011 16.7 4.6 -12.6 395 396 6 Ljubela August, 2011 23.3 4.2 -7.7 350 353 7 Velenjsko jezero August, 2011 16.4 3.1 -11.0 365 370 8 Lepena August, 2011 0.04 16.4 3.6 -13.5 375 378 9 Škalsko jezero August, 2011 17.5 3.8 -11.3 360 364 10 Paka August, 2011 1.86 24.0 2.5 -7.1 350 355 Numbers Locations Date of sampling Q (m3/s) T [° C] Alkalinity (mM) d13CDIC (‰) . pCO2 air opened system (ppm) pCO2 water/air, closed system (ppm) 1 Toplica October, 2011 9.3 4.0 -12.5 408 420 2 Pečovnica October, 2011 6.3 2.5 -12.1 421 450 3 Klančnica October, 2011 3.2 4 Velunja October, 2011 0.37 7.8 3.1 -11.3 388 390 5 Šoštanjsko jezero October, 2011 9.1 3.3 -10.3 386 395 6 Ljubela October, 2011 7.5 5.1 -12.3 386 400 7 Velenjsko jezero October, 2011 11.6 2.9 -9.1 386 400 8 Lepena October, 2011 0.03 7.5 5.7 -12.8 396 402 9 Škalsko jezero October, 2011 10.4 5.2 -12.1 386 400 10 Paka October, 2011 1.55 6.7 4.5 -11.0 460 480 Line 3. An average .13C value of -26.6 ‰ for particulate organic carbon (POC) was assumed to represent the isotopic composition of POC that was transferred to DIC by in-stream respiration. Open system equilibration of DIC with CO2 enriches DIC in 13C by about 9 ‰ (MooK et al., 1974), which corresponds to a value of -17.6 ‰. Line 4 represents open system equilibration of DIC, with soil CO2 originating from degradation of organic matter with .13CCO2 of -26.6 ‰. From Figure 5A it is observed that most of the samples fall between lines 2 and 3: dissolution of carbonates with an average .13C= -2 ‰ CaCO3 and non-equilibrium carbonate dissolution with carbonic acid produced from soil zone with .13C of -26.6 ‰. The highest pCO is observed CO2 2 at location Paka (location 10) with a value of 460 ppm (open system), pCO2 measured value is 480 ppm (measured as a closed system) in October 2011 probably due to higher degradation of organic matter at the end of the summer season. Elevated pCO2 concentrations are also recorded at Figure 3A. pCO2 (partial pressure in air) in the calendar year 2011. atmosphere DICex based on a diffusion model (two layer model in which the molecules are transported through a gas film and a liquid layer adjacent to the surface) can be calculated according to the following equation (BroecKer, 1974): (1) Figure 3B. .13CCO2 in the calendar year 2011. Numbers from 1–10 refer to sampling locations. At location 3 only surface water was sampled. where D is the CO2 diffusion coefficient in water with value of 1.26 x 10-5 cm2/s at a temperature of 10 °C and 1.67 x 10-5 cm2/s at a temperature of 20 °C (jähne et al., 1987), z is the empirical thickness of the liquid layer cm, CO and CO are the concentrations of 2eq 2 dissolved CO2 at equilibrium with the atmosphere and with the studied water mol · cm-3, respectively. The thickness of the boundary layer z, a thin film existing at the air-water interface, depends largely on wind velocity (BroecKer et al., 1978) and water turbulence (holley, 1977). D/z, therefore, is the gas exchange rate, which gives the height of the water column that will equilibrate with the atmosphere per unit time. Using a mean wind speed of 4 m/s in all sampling seasons (jähne et al., 1987), D/z was estimated to be 8 cm/h under low turbulence conditions, 28 cm/h under moderate turbulence conditions and 115 cm/h under high turbulence conditions. Unpolluted CO2 Velenje Basin .13C2 2011 Wraclaw d13CCO2 2008 Respiration Vegetation period Coal combustion Wood combustion Natural gas combustion Car gasoline combustion Car LPG combustion Car petroleum combustion .13 CCO2 (‰) 18.11.2010 7.1.2011 26.2.2011 17.4.2011 6.6.2011 26.7.2011 14.9.2011 3.11.2011 23.12.2011 11.2.2012 1 -4 -9 -14 -19 -24 -29 -34 Date Figure 4. .13CCO2 levels in the calendar year 2011 compared with those at Wroclaw (GórKa et al., 2011). Bold lines indicate the potential anthropogenic sources analyzed in Wroclaw (GórKa et al., 2011). The .13CCO2 value characteristic for the absence of pollution is taken from Baltic Sea values (white & vauGhn, 2009) and .13CCO2 values characteristic for C3 plants respiration from PataKi et al., 2003. Calculation of the CO2 flux between the river water surface and the atmosphere at the Paka River gauging station, according to equation (1), gives values ranging from 2.6 x 10-8 to 9.0 x 10-8 mol/cm2h in spring 2011, from 6.0 x 10-8 to 20 x 10-8 mol/cm2h in late summer 2011 and from 2.7 x 10-8 to 9.4 x 10-8 mol/cm2h in winter 2011. Taking into consideration the river surface area of 0.40 km2 (mean width of 10 m and length of 40 km), the total loss of inorganic carbon through its surface in the spring ranges from 6.0 x 104 mol/day during periods of low wind speeds to 2.0 x 105 mol/day during high turbulence storm events. The predicted total loss of inorganic carbon to the atmosphere in the late summer ranges from 1.0 x 105 to 5.0 x 105 mol/day and from 6.0 x 104 to 2.1 x 105 mol/day in winter. Concentration diffusion model In addition, values of the time evolution of stream pCO2 and .13CDIC were calculated using available diffusion models (e.g. BroecKer 1974; richey et al. 1990; aucour et al., 1999). These calculations yield the amount of time needed for CO2 evasion and for stream – atmosphere isotopic exchange relative to the transit time of stream waters. Such calculations were performed only for two main tributaries: Velunja River (location 4) and Paka River (location 10) for all sampling seasons (Figure 1, Table 2). The estimated rate of change of DIC concentration due to CO2 evasion are calculated by: (2) and the DIC concentration in water is expressed as a function of time by: (3) where h is the mean depth of the river cm and t is the time needed for equilibration min, all other parameters having been determined by equation (1). The calculations assume a value of 8 cm/h for D/z (low turbulent conditions due to low discharge) for both locations (4 and 10) (MooK, 1970) and h values of 10 cm. The computed results, according to equation (3), show that between 0.6 and 2.6 hours (January, 2011), 8.8 and 9.2 hours (May, 2011), 5.7 and 6.4 hours (August, 2011), and from 5.7 to 6.4 hours (October, 2011) would be required for equilibrium between atmospheric CO2 and dissolved riverine CO2 to be approached. Isotopic diffusion model Additionally, the rate of change of .13CDIC resulting from CO2 exchange between the river and the atmosphere was also estimated by the equation (aucour et al., 1999): (4) Again, the DIC concentration (DIC) is expressed as a function of time (t) by: (5) In equations (4) and (5), .13Ca and .13CDIC are the .13C values of atmospheric CO2 (-7.8 ‰; levin et al., 1987) and DIC, .13C0 is the initial value of DIC and . is the equilibrium fractionation factor - between CO2 and HCO3 (zhanG et al., 1995). Starting with the .13CDIC value of –12.5 ‰ (aucour et al., 1999) and h value of of 10 cm, calculated time of equilibration ranged from 26.2 to 132.6 hours, which would be needed to equilibrate .13C and .13Cvalues. This DICCO2 time interval was calculated for Velunja River Figure 5A. .13CDIC values of surface water samples as a function of alkalinity, with lines indicating processes occurring in surface waters in Velenje Basin. Arrows show expected trends for a variety of biogeochemical processes (coetsiers & walraevens, 2009). Figure 5B. Seasonal variation of pCO2;comparison between pCO2 air (open system) and pCO2 water/air (closed system) at 9 locations from Velenje Basin. Normal pCO2 in air is considered to be 360 ppm. (location 4) and Paka River (location 10) and suggests that stream – atmosphere isotopic exchange alone cannot explain the 13C enrichment of DIC in this carbonate/clastics catchment. Stream – atmosphere isotopic exchange alone cannot explain the 13C enrichment of DIC since longer time is needed for equilibration than expected. Both models (concentration and isotopic) should provide same values of time of equilibration, but in our case they do not. However, it has been shown that equilibration of CO2 between water/air boundaries is more significant in impermeable silicate drainages (KanDu^ et al., 2007). Therefore equilibration of atmospheric CO2 does not influence the value of .13CDIC in surface waters significantly, which is a consequence of low discharge conditions in the catchment area. Conclusions Values of the carbon isotope composition of atmospheric CO2 (.13CCO2), at locations in the vicinity of the thermal power plant in Velenje Basin, have been measured in the calendar year 2011. Based on measurements of alkalinity and .13Cfor surface water, values of .13Cof air DIC CO2 samples taken just above water (opened system) and from a closed cardboard box (closed system) it is concluded that combustion of lignite in thermal power plant has little influence on the .13Cvalue in the atmosphere. Measured CO CO2 2 concentrations (average pCO2 value of 294 ppm) and .13CCO2 in the atmosphere in the vicinity (few kilometers) of the thermal power plant are in the normal range in the atmosphere (360 ppm) and the influence of lignite combustion is negligible at the locations investigated in this study. The values of .13CCO2 in air range from -18 to -6.4 ‰, with an average value of -11.7 ‰, indicating the absence of influence of coal combustion, since the characteristic value of coal combustion is -24.1 ‰. .13CCO2 values in our study (observations during year 2011) are similar as obtained for Wroclaw, Poland (observation during year 2008). The total alkalinity in surface waters ranged from 2.2 to 6.1 mM. Dissolution of carbonates and degradation of organic matter are the most important biogeochemical processes affecting .13C DIC. They range seasonally from -13.5 to -7.1 ‰ in the surface waters (lakes, rivers) investigated in this study. pCO2 in the air immediately above water (open system) and in the air above the water, measured in the cardboard box (closed system), is similar at all measured locations. The highest pCO2 in an open system – immediately above water– and in a closed system (measured in a box) were measured at Paka (location 10) and Pe~ovnica (location 2) in May 2011 and in October 2011, respectively. Both locations are located in the vicinity of the thermal power plant. Based on thermodynamic modelling and on previous studies reported for Slovenian watersheds (rivers and lakes), surface waters acted like sources of CO2 (oversaturated more than 10 times) released to the atmosphere. However, the measurements of pCO2 reported here were made just above the surface water, where normal values of pCO2 (around 360 ppm) are present. Two diffusion models (concentration and isotopic) were applied to obtain the time of equilibration at two locations. Between 0.6 and 6.4 hours were required to equilibrate atmospheric CO2 and dissolved riverine DIC (concentration diffusion model), and 26.2 to 132.6 hours to .13C.13C equilibrate and values (isotopic DICCO2 diffusion model) if equilibration with atmospheric CO2 was the only factor influencing DIC values of surface waters. Even though Velenje Basin is a natural analogue with very large amounts of endogenic and bacterial CO2 (with the characteristic value of .13C 2 ‰) and with large amounts of CO emitted CO2 2 (around 4 Mt/year) from lignite combustion from the thermal power plant, we conclude from this study that pCO2 concentrations in air around the thermal power plant are not elevated. Acknowledgements This study was conducted in the frame of national research projects funded by Slovenian Research Agency (ARRS) Z1-2052, L1-5451 and Programme research group P1-0143. Special thanks are given to Mr. Stojan Žigon for technical support in laboratory. Thanks also to Prof. Roger Pain for linguistic corrections. References aMiotte-suchet, P., auBert, D., ProBst, j.l., Gauthier-laFaye, F. ProBst, a., anDreux, F. & viville, D. 1999: .13C pattern of dissolved inorganic carbon in a small granitic catchment: the Strengbach case study (Vosges mountains, France). Chemical Geology, 159: 129–145. ateKwana, e:a. & KrisnaMurthy, r.v. 1998: Seasonal variations of dissolved inorganic carbon and .13C of surface waters: application of modified gas evolution technique. Journal of Hydrology, 205/3: 265–278. aucour, aM., shePParD, s.M.F., GuyoMar o. & wattelet, j. 1999: Use of 13C to trace the origin and cycling of inorganic carbon in the Rhône river system. Chemical Geology, 159: 87–105. BowlinG, D. r., PataKi, D.e. & ranDerson, j.t. 2008: Carbon isotopes in terrestrial ecosystem pools and CO2 fluxes. New. Phytol., 178: 24–40. BroecKer, w.s. 1974: Chemical Oceanography. Harcourt Brace Jovanovich, New York. BroecKer, h.c., PeterMan, j. & sieMs, w. 1978: The influence of wind on CO2 – exchange in a wind – wave tunnel, including the effects of monolayers. Journal of Marine Research, 36: 595–610. coetsiers, M. & walraevens, K. 2009: A new correction model for 14C ages in aquifers with complex geochemistry-application to the Neogene Aquifer, Belgium. Applied Geochemistry, 24: 768–776. EMEP/EEA 2013: Air pollutant emission inventory guidebook. Available online: http:// www.eea.europa.eu/publications/emep-eea­guidebook-2013. enerGy inForMation aDMinistration, eMissions oF Greenhouse Gases in the uniteD states 1985-1990. DOE/EIA-0573 (Washington, DC, September 1993), p. 16. euroPean environMent aGency, 1998: Assessment and Management of Urban Air Quality in Europe, EEA Monograph no. 5. http://www. eea.europa.eu/publications/environmental_ monograph_2006_5. euroPean environMent aGency, 2003: Air pollution in Europe 1990-2004. EEA Report No 2/2007. http://www.eea.europa.eu/publications/eea_ report_2007_2. GiesKes, j.M. 1974: The alkalinity-total carbon dioxide system in seawater. In: GolDBerG, E.D. (ed.): Marine chemistry of the sea, 5: 123–151. GórKa, M., sauer, P.e., lewicKa-szczeBaK, D. & jeDryseK, M.o. 2011: Carbon isotope signature of dissolved inorganic carbon (DIC) in precipitation and atmospheric CO2. Environmental Pollution, 159: 294–301. holley, e.h. 1977: Oxygen transfer at the air – water interface. In: GiBBs, r.j. (ed.): Transport Processes in Lakes and Oceans, Proc. Symp. On Transp. Processes in the Ocean held at the 82nd Nat. Meet of the AICE, Atlantic City, N. J. Aug. 29. – Sep. 1, 1976:17–150. Plenum Press. jähne B., heinz G., & Dietrich w. 1987: Measurements of the diffusion coefficients of sparingly soluble gases in water. Journal of Geophysical Research, 92: 10767–10776. KanDu^, t., szraMeK, K., oGrinc, n. & walter, l.M. 2007: Origin and cycling of riverine inorganic carbon in the Sava River watershed (Slovenia) inferred from major solutes and stable carbon isotopes. Biogeochemistry, 86: 137–154. KanDu^, t., jaMniKar, s. & Mcintosh, j. 2010: Geochemical characteristics of surface and groundwaters in the Velenje basin (Slovenia). Geologija, 53/1: 37–46, doi:10.5474/ geologija.2010.003. KanDu^, t., Grassa, F., Mcintosh, j., stiBilj, v., ulrich-suPovec, M., suPovec, i., jaMniKar, s. 2014: A geochemical and stable isotope investigation of groundwater/surface-water interactions in the Velenje Basin, Slovenia. Hydrogeology Journal, 22/4: 971–984, doi:10.1007/s10040-014-1103-7. KanDu^, t. & PezDi^, j. 2005: Origin and distribution of coalbed gases from the Velenje Basin, Slovenia. Geochemical Journal, 39:397– 409. Kuc, T., roŽansKi, K., ziMnoch, M., necKij, M. & Korus, a. 2003: Anthropogenic emissions of CO2 and CH4 in an urban environment. Applied Energy, 75/3-4: 193–203, doi:10.1016/S0306­2619(03)00032-1. lazar, j., KanDu^, t., jaMniKar, s., Grassa, F. & zavšeK, s. 2014: Distribution, copmposition and origin of coalbed gases in excavation fields from the Preloge and Pesje mining areas, Velenje Basin. International Journal of Coal Geology, 131: 363–377, doi:10.1016/j. coal.2014.05.007. levin, i., KroMer, B., waGenBacK, D., Minnich, K.o. 1987: Carbon isotope measurementsof atmospheric CO2 at a coastal station in Antartica. Tellus, 39B: 89–95. lonGinelli, a. & selMo, e. 2005: Seasonal and diurnal variations of .13C and concentration of atmospheric CO2 at Parma, Italy. Geol. Quarter., 49/2: 127–134. MarFia, a.M., KrishnaMurthy, r.v., ateKwana, e.a. & Panton, w.F. 2004: Isotopic and geochemical evolution of ground and surface waters in a karst dominated geological setting: a case study from Belize, central America. Applied Geochemistry, 19: 937–946, doi:10.1016/j.apgeochem.2003.10.013 MooK, w.G. 1970: Stable carbon and oxygen isotopes of natural water in the Netherlands. In Isaotopic Hydrology, IAEA: 163–190. MooK, w.G., BoMMerson, j.c. & staverMan, w.h. 1974: Carbon isotope fractionation between dissolved bicarbonate and gaseous carbon dioxide. Earth Planetary Science Letters, 22: 169–176. PataKi, D.e., BowlinG, D.r. & ehlerinGer, j.r. 2003: Seasonal cycle of carbon dioxide and its isotopic composition in an urban atmosphere: anthropogenic and biogenic effects. Journal of Geophysical Research, 108/D23: 4735, doi:10.1029/2003JD003865. PataKi, D.e., tyler, B.j., Peterson, r.e., nair, a.P., steenBurGh, w.j. & ParDyjaK, e.r. 2005: Can carbon dioxide be used as a tracer of urban atmospheric transport? Journal of Geophysical Research, 110/ D15102, doi:10.1029/2004JD005723. richey, j.e., heDGes, j.i., Devol, a.h. & Quay P.D. 1990: Biogeochemistry of carbon in the Amazon River. Limnology Oceanography, 35/2: 352–371. roManeK, c.s., GrossMan, e.l. & Morse, j.w. 1992: Carbon isotopic fractionation in synthetic aragonite and calcite: effects temperature and precipitation rate. Geochimica Cosmochimica Acta, 56/1: 419–430. sPötl, c. 2005: A robust and fast method of sampling and analysis of .13C of dissolved inorganic carbon in ground waters. Isotopes in Environmental and Health Studies, 41/3: 217–221, doi:10.1080/10256010500230023. szaran, j., niezGoDa, h. & treMBaczowsKi, a. 2002: Respiration and assimilation process and isotopic composition of atmospheric carbon dioxide. Nukleonika, 47 (Suppl. 1): S59–S61. white, j.w.c. & vauGhn, B.h. 2009: University of Colorado, Institute of Arctic and Alpine Research (INSTAAR), Stable Isotopic Composition of Atmospheric Carbon Dioxide (13C and 18O) from the NOAA ESRI. Carbon Cycle Cooperative Global Air Sampling Network, 1990-2008, Version: 2010-01-08. Path: ftp.cmdl.noaa.gov/ ccg/co2c13/flask/event/. wiDory, D. & javoy, M. 2003. The carbon isotopic composition of atmospheric CO2 in Paris. Earth Planetary Science Letters, 215: 289–866. zhanG, j., Quay, P.D. & wilBur, D.o. 1995: Carbon isotope fractionation during gas-water exchange and dissolution of CO2. Geochim Cosmochim Acta, 59/1: 107-1146. ziMnoch, M., FlorKowsKi, t., necKi, j.M. & neuBert, r.e.M. 2004: Diurnal variability of .13C and .18O of atmospheric CO2 in the urban atmosphere of Kraków, Poland. Isotopes in Environmental and Health Studies, 40/2: 129–143. zwozaDziaK a., GórKa M., sówKa i., lewicKa­szczeBaK D., zwozaDziaK j. & orion jeDryseK M. o. 2010. Air polluion origins using PM10 Data and CO2 Isotopic Analysis. Polish J. of Environ. Stud., 19/6: 1345–1352. Internet resources: internet:http://vode.arso.gov.si/hidarhiv/pov_ arhiv_tab.php (cited online May 2015).