GEOLOGIJA 59/2, 179-192, 2016 © Author(s) 2016. CC Atribution 4.0 License http://dx.doi.org/10.5474/geologija.2016.010

Hydrogeochemical and isotopic characterization of River,

Hidrogeokemične in izotopske značilnosti reke Pesnice

Tjaša KANDUČ1*, Zoran SAMARDŽIJA1b, Nataša MORI3, Andreja JEREBIC3, Ines LEVAČIČ4, Miha KRAČUN4, Johanna A. ROBINSON1, 2, Stojan ŽIGON1, Željko BLAŽEKA4 & David KOCMAN1

1Department of Environmental Sciences, 1bDepartment for Nanostructured Materials, Jožef Stefan Institute, Jamova cesta 39, SI–1000 Ljubljana, Slovenia; e-mail: [email protected]*, [email protected], [email protected], [email protected], [email protected] 2Jožef Stefan International Postgraduate School, Jamova cesta 39, SI–1000 Ljubljana, Slovenia 3Department of Organisms and Ecosystem Research, National Institute of Biology, Večna pot 111, SI–1000 Ljubljana, Slovenia; e-mail: [email protected], [email protected] 4Institut za ekološki inženiring d.o.o., Ljubljanska ulica 9, SI–2000 Maribor, Slovenia; e-mail: [email protected], [email protected], [email protected]

Prejeto / Received 11. 10. 2016; Sprejeto / Accepted 9. 11. 2016; Objavljeno na spletu / Published online 23. 12. 2016

Key words: River Pesnica, hydrogeochemical composition, stable isotopes, carbon, biogeochemical processes Klju~ne besede: Pesnica, hidrogeokemijska sestava, stabilni izotopi, ogljik, biogeokemijski procesi

Abstract

The surface water geochemistry and carbon cycling studied here depend mainly on geological composition of the river catchment. The investigated surface waters in the river Pesnica catchment that are heavily hydromorphologicaly altered (reservoirs, channelization, land melioration systems) represent waters influenced by chemical weathering of carbonates and of mostly clastic rocks (claystone, sandstones, marlstones and siltstones). The objectives of our study were to analyze hydrogeochemical and isotopic composition of dissolved 13 13 (δ CDIC) and particulate carbon (δ CPOC) with characterization of suspended matter and evaluate biogeochemical processes in Pesnica River in Slovenia before and after the retention. Surface waters are generally close to saturation regarding calcite and dolomite, dissolved CO2 is 49 to 1000 times oversaturated relative to atmosphere. 13 δ CDIC was in the range from -14.8 to -4.2 ‰ and shows following biogeochemical processes in river system: degradation of organic matter, dissolution of carbonates and biological activity, which was confirmed with SEM microscopy and EDXS microanalysis. Results of SEM/EDXS showed that suspended matter is composed of K, Mg and Ca from locations Pesnica 1 and Pesnica 2 (above retention Perniško lake). Sample from Pesnica 3 (below retention) shows higher biological activity, while sample from Pesnica 4 (tributary of Pesnica, drainage channel Biš) contains a lot of particles of microporous structure and fiber structure, which are probably of anthropogenic 13 origin. δ CPOC is changing from -29.5 to-27.6‰ and showed different stages of degraded terrestrial material. 13 More negative δ CPOC are shown at location at drainage channel Biš and show higher terrestrial input in river 13 system as locations 1, 2 and 3 Pesnica, which have higher δ CPOC values. This first results about suspended matter in Pesnica watershed serve for evaluation of anthropogenic influences specially in relation with further influences and longer time planning of sustainable river water management.

Izvleček

Geokemija površinskih voda in kroženje ogljika v naši študiji v glavnem zavisita od geološke podlage v porečju. Raziskane površinske vode v porečju Pesnice, ki je hidromorfološko močno preoblikovano (vodni zadrževalniki, urejanje struge, melioracije), predstavljajo vode, na katere vpliva kemijsko preperevanje karbonatov in v glavnem klastičnih kamnin (glinavcev, peščenjakov, laporovcev in meljevcev). Namen naše raziskave je bil analizirati hidrogeokemično in izotopsko sestavo raztopljenega in partikulatnega ogljika skupaj s karakterizacijo suspendirane snovi in oceno biogeokemijskih procesov v porečju Pesnice v Sloveniji pred zadrževalnikom in za njim. Površinske vode so večinoma blizu nasičenja glede na kalcit in dolomit, raztopljen CO2 je od 49 do 1000 krat 13 prenasičen relativno na atmosfero. δ CDIC je bil v razponu od -14.8 do -4.2 ‰ in kaže na sledeče biogeokemijske procese v rečnem sistemu: razgradnja organske snovi, raztapljanje karbonatov in biološka aktivnost, kar je bilo potrjeno s SEM mikroskopijo in EDXS mikroanalizo. Rezultati SEM/EDXS so pokazali, da je suspendirana snov 180 T. KANDUČ, Z. SAMARDŽIJA, N. MORI, A. JEREBIC, I. LEVAČIĆ, M. KRAČUN, J. A. ROBINSON, S. ŽIGON, Ž. BLAŽEKA & D. KOCMAN iz Pesnice 1 in Pesnice 2 (nad zadrževalnikom Perniško jezero) sestavljena iz alumosilikatov s K, Mg in Ca. Vzorec iz Pesnice 3 (pod zadrževalnikom) kaže na veliko biološko aktivnost (diatomeje), medtem ko vzorec iz Pesnice 4 (pritok Pesnice, drenažni kanal Biš) vsebuje veliko delcev mikroporozne strukture in vlaknatih struktur, ki so 13 po vsej verjetnosti antropogenega izvora. δ CPOC se spreminja od -29.5 do -27.6 ‰ in kaže na različne stopnje 13 razgrajenega terestričnega materiala. Bolj negativne vrednosti δ CPOC opazimo na lokaciji v drenažnem kanalu 13 Biš in kažejo na večji terestrični vnos v rečni sistem lokacije Pesnice 1, 2 in 3, ki imajo višje vrednosti δ CPOC. Ti prvi rezultati o suspendirani snovi v porečju Pesnice služijo za evalvacijo antropogenih vplivov, posebno v povezavi s prihodnjimi vplivi in dolgoročnim načtrovanjem trajnostnega upravljanja s porečjem.

Introduction Geological and geomorphological setting

It is well known that rivers reflect the biogeo- River Pesnica is located in northeastern Slove- chemical processes occurring in their catchment nia (Fig. 1). It is a left tributary of River , and areas (Meybeck, 1981; Meybeck, 1987; Popescu et is the main watercourse in Slovenske gorice. The al., 2015; Li et al., 2016). Understanding the or- river originates above Pesnica (in ) at an ganic and inorganic carbon cycles in river sys- altitude of 300 m; at town Ormož (altitude 190 m) tems is important because it helps to understand it flows out to the Drava River (Fig. 1). Its length is the biogeochemical processes in a river and its 69 km, of which 65 km being in Slovenia (Statisti- catchment basin (Karim & Veizer, 2000; Barth cal Office of Republic of Slovenia, 2002). Its catch- et al., 2003; Sameer et al., 2016). Several studies ment area comprises 539 km 2. Its mean average dis- evaluating biogeochemical processes in Slovenian charge between 1926 and 1965 was 6.72 m 3/s. It has a rivers have been carried out in the past ten years, mixed snow/rain river regime with high water lev- contributing substantially to a general under- els in March and November and low in August and standing of their interactions with their catch- September. In Pesnica catchment area, 3 gauging ments (Kanduč & Ogrinc, 2007; Kanduč et al., stations are monitored by the Slovenian Environ- 2008; Zavadlav et al., 2013; Kanduč et al., 2013). mental Agency (SEA): Ranca, Gočova, Zamušani I Kobold, 2012). Suspended organic matter is known to be complex mixture of molecules such as carbo- Land cover in Pesnica catchment is dominated hydrates, amino acids, fatty acids and phenols, by agriculture areas (~75 %), followed by forests particles from wastewaters and industrial water, and semi-natural areas (~23 %) and with less soil organic matter and biological material like than 2% of artificial surfaces C( LC, 2012). phytoplankton and plant parts (Hope et al., 1994). Suspended matter in rivers is derived mostly Since Pesnica flows through a lower bushy en- from soil and plant material, so suspended or- vironment, it has a low specific runoff (10.6 l/s/ 13 2 ganic matter (δ CPOC) in rivers can be used to de- km ; gauging station Zamušani) and a runoff co- termine the contribution of terrestrial vegetation efficient of 32.2 % (gauging station Zamušani). and soil matter in the river ecosystem (Hedges, 1992). In past years it was found, that mainly in summer, the discharge of Pesnica is mostly below Pesnica is a river that was extensively regu- 0.5 m3/s (Urbanek, 1994). Since it flows through lated in the 60′s when it was channeled and when an intensively agricultural area, it contains large multipurpose retention basins were built to miti- amount of atrazine and steroid organic sub- gate floods(L eitinger, 2012). Nowadays, droughts stances, together with a high concentration of in combination with severe loads of nutrients and nitrates in past decade. Pollution with sewage other pollutants from households and agriculture sludge was moderate, and in recent years the adversely affect the flora and fauna in the area river’s ecological status has improved (Urbanek, (Markoli, 2012). 1994).

The main objective of our study was to un- Pesnica River is heavily hydromorphologically derstand the hydrogeochemical characteristics altered (from Zgornja Kungota to outflow). For and prevailing biogeochemical processes in the the purposes of flood prevention, several reser- Pesnica catchment waters using geochemical voirs have been constructed to retain the water: and stable carbon isotopes analysis, respec- Pernica and tributaries were artificial lakes were tively. constructed for water retention. Hydrogeochemical and isotopic characterization of Pesnica River, Slovenia 181

2 Drainage channel at settlement Biš-the mu- M2 –Miocene sediments are composed of nicipality Trnovska vas, is a right hand tributary gravel, sand, marl, limestone and sandstone. of Pesnica River (P4 in Fig. 1). 1 M3 – In the lower part of the catchment Geologically, Pesnica catchment area belongs sandy marl sediments prevail and within them to the Panonian basin and is of Tertiary and are mica, sandstone, claystones and marlstones. Quaternary ages. The catchment is composed Gravels are composed of siliciclastic and other of Miocene and Pliocene sediments (siltstones, metamorphic and magmatic rocks. marlstones, claystones, dolomite/calcite) and 2 quaternary sediments. The area of the Panonian M3 - Miocene sediments are located in the basin was split into more tectonic units with the eastern part of Pesnica catchment and are com- Slovenske gorice as the largest tectonic unit. In posed of sandy clays, sandy clay marls, sands the Pesnica catchment more faults were found in and sandy gravels. different directions, belonging to the category of tectonic faults. The most important are: Globov- Pliocene sediments are located in the nica, Pesnica, faults (named after the southern part of the River Pesnica catchment tributaries that flow through them). Surface wa- (Pliocene) and are composed of clay, sand, con- ter creates channels, in which the rocks are the glomerate and clayey marl (Mioč & Žnidarčič, most fractured (Mioč & Žnidarčič, 1989; Perko et 1989). al., 2001). Following Miocene and Quaternary sed- iments are present in the River Pesnica watershed: Quaternary sediments are located along surface water flow in the valley and are com- Miocene sediments (M21) that outcrop up posed of clastic material (gravels, sands, marls, in the northeastern part of the catchment are clays). composed of conglomerate, sandstone and marl.

Fig. 1. Land use map of River Pesnica catchment area (Corine Land Cover 2012) 182 T. KANDUČ, Z. SAMARDŽIJA, N. MORI, A. JEREBIC, I. LEVAČIĆ, M. KRAČUN, J. A. ROBINSON, S. ŽIGON, Ž. BLAŽEKA & D. KOCMAN

Materials and Methods Samples for characterization of suspended In order to determine how artificial reservoirs matter from each location (Fig. 1) were collected influence biogeochemical processes in the river (September 2016 sampling) using standard rep- system, water was sampled at the inflow and out- resentative sampling techniques (Schuster & flow of one of the accumulation lakes (Perniško Reddy, 2001) in HDPE bottles. From each lo- jezero) during the crop growing season. In addi- cation, 3 l of surface water were sampled for tion, a small drainage channel nearby Biš village microscopic (SEM/EDXS-scanning electron was chosen to investigate how an area dominated microscopy/energy - dispersive X-ray spectros- by intensive agriculture contributes to the bio- copy) analyses (suspended matter characteriza- geochemical processes of Pesnica River. tion) and for isotopic composition of carbon in 13 suspended matter (δ CPOC). Surface water sam- Discharge data (Miha Supek, personal commu- ples were filtered through a Whatman GF/F nication) were obtained from the Environmental (pore size 0.7 µm) glass fiber filter. The isotopic Agency of the Republic of Slovenia (Tab. 1). Sam- composition of carbon in suspended matter was ples were collected four times in each month from determined using a Europa 20-20 continuous February to September (except July) 2016 at four flow IRMS-SL preparation module. The filter gauging stations: 1 (before dam), 2 (before dam), was treated with 1M HCl to dissolve any car- 3 (after dam), 4 (drainage channel Biš). Tempera- bonate present. Approximately 1 mg of sample ture and dissolved oxygen (DO) of surface waters was scrapped from the GF/F filter and dried at were measured with a WTW Multi 340, CellOx 60 °C. The isotopic composition of carbon was 325. pH was measured with a Seven Compact pH/ determined after combustion of the capsules in ion Metler Toledo and electroconductivity with a hot furnace at 1000 °C. IAEA CH3 and IAEA an Ultrameter II MYRON L COMPANY 6 PFCE, CH6 standards were used to relate the analytical serial 6218180, both in the laboratory after sam- results to the VPDB. Sample reproducibility for pling. carbon was ±0.2 ‰.

Sample aliquots collected for cation, anion All stable isotope results for carbon are and alkalinity analyses were passed through a expressed in the conventional delta (δ) notation, 0.20 µm nylon filter into HDPE bottles (30 ml) defined as per mil ‰ ( ) deviation from the and kept refrigerated until analyzed. Samples for reference standard VPDB. Precision of working 13 13 13 δ C DIC analysis were passed through a 0.45 µm standards was ±0.2 ‰ for δ CDIC and δ CPOC. nylon filter and stored in glass serum bottles filled with no headspace and sealed with septa caps. Samples of suspended matter from Pesnica catchment area from September sampling were Alkalinity was measured using Gran titra- examined to study the morphology of parti- tions (Clesceri et al., 1998). Cations (Ca2+, Mg2+, cles and their element composition, using a + + 2- - - Na , K ) and anions (SO4 , Cl , and NO3 ) were field-emission-gun scanning electron micro- measured by ion chromatography (Metrohm, 761 scope (FEGSEM) JEOL JSM 7600-F equipped Compact IC). with Oxford Instruments INCA energy – dispersive X-ray spectroscopy (EDXS) with 20- The stable isotope composition of dissolved in- mm2 silicon-drift detector (SDD). SEM/EDXS 13 organic carbon (δ CDIC) was determined with a analyses were performed at 20kV accelerating Europa Scientific 20-20 continuous flow IRMS voltage, 15 mm working distance and 100 s ac- (isotope ratio mass spectrometer) with an AN- quisition time. The internal INCA EDS standard CA-TG preparation module. Phosphoric acid database was used for the analyses. (100 %) was added (100-200 µl) to a septum-sealed vial which was then purged with pure He. The Thermodynamic modeling was used to eval- water sample (5 ml) was injected into the septum uate pCO2 (partial pressure) and the saturation tube and headspace CO2 was measured (modified state of calcite (SIcalcite) and dolomite (SIdolomite) after Kanduč, 2006). In order to determine the using pH, alkalinity and temperature as inputs optimal extraction procedure for surface water to the PHREEQC speciation program (Park- hurst ppelo samples, a standard solution of Na2CO3 (Carlo & A , 1999). Version 2.16.03 Phre- 13 Erba) with a known δ CDIC of -10.8±0.2 ‰ was eqc for Windows was used to calculate SIcalcite, prepared with a concentration of 0.66 g/l. SIdolomite and pCO2. Hydrogeochemical and isotopic characterization of Pesnica River, Slovenia 183

Results and discussion for example the congruent dissolution of cal-

Theoretical background of dissolution of cite (CaCO3) in limestone: CaCO3+ CO2(g) + H2O 2+ – minerals = Ca + 2HCO3

Dissolved Ca2+ and Mg2+ are largely supplied and the incongruent reaction with K–spar: by the weathering of carbonates, with smaller contributions from silicate weathering, as in- (KAlSi3O8): 2KAlSi3O8+ 2CO2+ 11H2O = - + – dicated by the relatively high HCO3 and low Si Al2Si2O5(OH)4+ 2K + 2HCO3 + 4H4SiO2 (5) concentrations. Most of the water samples in our - study deviate from a 2:1 molar ratio of HCO3 The amount of CO2 dissolved according to (Fig. 2A) and are the consequence of weathering reaction (3) depends mainly on the tempera- of clastic rocks composed of clay minerals as fol- ture and its partial pressure. For example, for lows (1-5): an atmospheric carbon dioxide pressure of 10–2 atmosphere and for a soil atmosphere of nearly

CaAl2Si2O8 + 3H2O + 2CO2 (Anorthite) → pure carbon dioxide, the amount of calcium that 2+ - Al2Si2O5(OH)4 + Ca + 2HCO3 (Kaolinite) (1) can be dissolved (at 25 °C) until saturation is 65 and 300 ppm, respectively. The calcium and hy- In the case of weathering anorthite, only Ca2+ drogencarbonate ions released into soil water and is present in carbonate reactions (precipitation of groundwater eventually reach the river system. calcite). The water resulting from reaction (5) contains hydrogencarbonate, potassium, and dissolved

NaAlSi3O8 + CO2 + 11/2H2O (Albite) → silica in the ratios 1:1:2, and the new mineral, + - Na + ½ Al2Si2O5(OH)4 + 2H4SiO4 + HCO3 kaolinite, is the solid weathering product. The (Kaolinite) (2) dissolved constituents of reactions (calcite) and (K-feldspar) eventually reach the river systems. + In case of weathering of albite, Na and HCO3- The global silicate weathering fluxes and asso- + are in 1:1 stoichiometry ratio. The origin of K can ciated CO2 consumption fluxes have been freshly be mica shist. estimated on the basis of data on the 60 largest rivers of the world (Gaillardet et al., 1999). Only Weathering of calcite and dolomite is presented active physical denudation of continental rocks below: was found to be able to maintain high chemical

weathering rates and significant CO2 consump- Calcite : tion rates. 2+ - CaCO3 + CO2(g) + H2O ⇔ Ca + 2HCO3 (3) General geochemical characteristics of Dolomite : river water 2+ 2+ - Ca0:5Mg0:5(CO3) + CO2 + H2O ⇔ Ca + Mg + 2HCO3 (4) Mean daily discharge data (Q), parameters measured in the field (T-temperature, D.O.- The carbon dioxide content of rain and soil dissolved oxygen (mg/l)), geochemical and water is particularly important in weathering isotopic data of water (Pesnica catchment area) processes (Gaillardet et al., 1999). The chemi- are presented in Table 1. cal composition of rainwater changes markedly after entering the soils (Gaillardet et al., 1999). Most of the studied rivers in Slovenia anduč 2+ 2+ - The upper part of the soil is a zone of intense (K et al., 2008) have a Ca +Mg : HCO3 biochemical activity. One of the major biochem- ratio 2:1 indicating that geological composition ical processes of the bacteria is the oxidation of (carbonates prevail) controls the composition of organic material, which leads to an increase of river waters. In Pesnica River catchment (studied carbon dioxide in the soil gas (Gaillardet et al., locations Pesnica 1, Pesnica 2, Pesnica 3 and 1999). Above the zone of water saturation, the Pesnica 4) clastic rocks prevail (Fig. 1) meaning soil gases may contain 10 to 40 times as much that most of the cations (Ca2+, Mg2+, Na+, K+) as carbon dioxide as the free atmosphere (ref- are originated in clastic rocks (e.g. sandstones, erence value for partial pressure in atmosphere claystones, marls) and also limestone, but to a lark ritz is 316 ppm (C & F , 1997)). This CO2 minor extent. gives rise to a variety of weathering reactions, 184 T. KANDUČ, Z. SAMARDŽIJA, N. MORI, A. JEREBIC, I. LEVAČIĆ, M. KRAČUN, J. A. ROBINSON, S. ŽIGON, Ž. BLAŽEKA & D. KOCMAN

Table 1. Field and laboratory measurement data (T, pH, conductivity, DO), discharge data (Q), major geochemistry data 2+ 2+ + 2- - 13 (Ca , Mg , Na , total alkalinity after Gran, SO4 , NO3 ), stable isotopic composition of dissolved inorganic carbon (δ CDIC), 13 stable isotopic composition of particulate organic carbon (δ CPOC).

2+ +Mg 2+ Ca (mM) 2.77 2.83 2.82 1.69 4.07 4.17 3.91 2.50 3.87 3.96 2.53 2.78 2.29 3.25 1.60 1.65 3.75 3.60 1.44 2.79 1.89 1.12 0.88 1.42 2.54 2.52 1.24 1.52

2 pCO (ppm) 109648 104713 123027 81283 144544 125893 158489 27542 190546 199526 169824 147911 102329 91201 79433 15488 83176 69183 39811 58885 204174 117499 56234 120226 141254 120226 316228 120226

2 pCO (bar) -1.96 -1.98 -1.91 -2.09 -1.84 -1.90 -1.80 -2.56 -1.72 -1.70 -1.77 -1.83 -1.99 -2.04 -2.10 -2.81 -2.08 -2.16 -2.40 -2.23 -1.69 -1.93 -2.25 -1.92 -1.85 -1.92 -1.50 -1.92 SIdol. 0.5 0.48 -1.26 0.3 -1 -1.77 -2.06 -0.74 0.13 0.01 -2.14 -0.77 -1.26 -1.26 -1.44 -2.61 -0.24 -0.53 -0.23 -1.6 -0.5 -0.5 -0.79 -0.78 -0.95 -0.6 -0.64 -1.89 SIcalc. -0.2 -0.2 -0.29 -0.94 -0.13 0.14 -0.15 -0.48 0.05 0.06 -0.2 -0.18 -0.2 0.02 0.0 -0.66 0.47 0.54 0.04 0.35 -0.19 -0.66 -0.98 -0.19 0.39 0.32 -0.85 -0.22 POC ) C 13 ‰ δ ( n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. -27.6 -27.8 -28.5 -29.5 DIC ) C 13 ‰ δ ( -13.7 -14.2 -13.7 -13.5 -11.4 -14.8 -12.3 -14.2 -11.6 -11.4 -7.3 -13.1 -13.7 -13.4 -7.1 -13.9 -12.3 -12.1 -4.2 -13.1 -11.8 -7.2 -8.1 -13.4 -13.0 -12.8 -9.8 -13.4 TA (mM) 3.88 3.79 3.66 1.77 4.85 5.65 5.20 1.49 5.10 5.20 4.50 3.80 3.40 3.50 3.20 1.24 5.05 5.09 2.81 4.34 4.80 2.70 4.40 1.60 6.69 5.74 2.97 4.19 2+ Mg (mM) 0.50 0.50 0.51 0.38 0.79 0.80 0.77 0.65 0.83 0.83 0.79 0.77 0.53 0.64 0.31 0.38 0.73 0.70 0.06 0.78 0.35 0.28 0.38 0.41 0.43 0.43 0.28 0.46 2+ Ca (mM) 2.27 2.33 2.31 1.31 3.28 3.37 3.15 1.86 3.04 3.13 1.74 2.01 1.76 2.61 1.29 1.27 3.02 2.90 1.38 2.01 1.54 0.84 0.50 1.01 2.11 2.08 0.96 1.05 + K (mM) 0.05 0.06 0.06 0.02 0.05 0.05 0.05 0.01 0.06 0.06 0.06 0.011 0.05 0.06 0.03 0.01 0.10 0.09 0.08 0.01 0.06 0.05 0.07 0.00 0.05 0.04 0.06 0.00 + Na (mM) 0.42 0.44 0.51 0.16 0.54 0.57 0.53 0.20 0.86 0.89 0.65 0.19 0.41 0.52 0.25 0.12 0.93 0.99 0.46 0.17 0.48 0.34 0.27 0.08 0.47 0.47 0.29 0.09 2- 4 SO (mM) 0.25 0.25 0.29 0.14 0.34 0.34 0.33 0.13 0.33 0.32 0.32 0.04 0.28 0.29 0,16 0.114 0.27 0.27 0.21 0.03 0.27 0.21 0.17 0.02 0.41 0.38 0.27 0.09 - 3 NO (mM) 0.19 0.14 0.13 0.46 0.05 0.05 0.05 0.06 0.04 0.05 0.00 0.00 0.06 0.07 0.03 0.25 0.07 0.07 0.00 0.00 0.07 0.01 0.00 0.00 0.07 0.10 0.05 0.00 - Cl (mM) 0.33 0.42 0.9 0.27 0.35 0.36 0.35 0.28 0.46 0.47 0.40 0.20 0.29 0.33 0.19 0.19 0.47 0.48 0.29 0.12 0.47 0.40 0.32 0.10 0.4748 0.4687 0.3690 0.1509 Cond (µS/cm) n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 500 512 314 398 pH 7.21 7.22 7.14 7.01 7.15 7.20 7.30 7.40 7.12 7.11 7.13 7.15 7.23 7.29 7.31 7.35 7.50 7.52 7.58 7.59 7.40 7.30 7.20 7.3 7.41 7.45 7.22 7.25 2 O (mg/l) n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 17,60 18,70 21,20 17,00 21,50 23,30 26,40 19,70 47.20 47.00 60.20 15.50 T (°C) 5.25 5.25 5.20 5.10 4.35 4.35 4.35 6.00 13.60 13.60 13.60 15.90 14.00 14.00 14.00 14.00 17.10 17.10 17.10 17.10 17.3 17.5 17.6 17.7 18.80 19.60 22.40 17.30 /s) 3 Q (m 6.20 6.20 6.20 6.22 0.91 0.91 0.91 2.99 0.25 0.25 0.25 1.47 1.69 1.69 1.69 4.16 0.18 0.18 0.18 0.64 0.634 0.634 0.058 n.a. 1.086 1.086 0.179 n.a. Date 16.2.16 16.2.16 16.2.16 16.2.16 18.3.16 18.3.16 18.3.16 18.3.16 14.4.16 14.4.16 14.4.16 14.4.16 16.5.16 16.5.16 16.5.16 16.5.16 13.6.16 13.6.16 13.6.16 13.6.16 15.8.16 15.8.16 15.8.16 15.8.16 2.09.16 2.09.16 2.09.16 2.09.16 Location 1 Pesnica 2 Pesnica 3 Pesnica 4 Pesnica (Biš) 1 Pesnica Pesnica2 3 Pesnica 4 Pesnica (Biš) 1 Pesnica 2 Pesnica 3 Pesnica 4 Pesnica (Biš) 1 Pesnica 2 Pesnica 3 Pesnica 4 Pesnica (Biš) 1 Pesnica 2 Pesnica 3 Pesnica 4 Pesnica (Biš) 1 Pesnica 2 Pesnica 3 Pesnica 4 Pesnica (Biš) 1 Pesnica 2 Pesnica 3 Pesnica 4 Pesnica (Biš) No. 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 n.a.-not analyzed Hydrogeochemical and isotopic characterization of Pesnica River, Slovenia 185

Fig. 2A. Ca2++Mg2+ ratio versus alkalinity with lines 6.00 February 2016 1:2 indicating weathering of carbonates; deviation March 2016 from the 1:2 line indicates 5.00 April 2016 weathering of clay minerals (albite, anortite) and mica May 2016 + (mM) schist (origin of K ), which

2+ June 2016 4.00 mostly contribute to alka- August 2016 linity rather than to weath- +Mg

2+ - 2+ 2+ ering of calcite/dolomite. September 2016 2HCO3 = Ca +Mg Ca 3.00

2.00

1.00

0.00 0.00 2.00 4.00 6.00 8.00 10.00 12.00 Total alkalnity (mM)

2+ 2+ 7.00 Fig. 2B. Mg and Ca concentrations indicating the dominance of dolomite or calcite in the Pesnica 6.00 catchment 1

5.00 Dolomite only 0.75 February, 2016 4.00 March, 2016 (mM)

2+ April, 2016 3.00 Mg 0.5 May, 2016

June, 2016 2.00 0.33 August, 2016

1.00 September, 2016 0.1

Calcite only 0.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 Ca2+ (mM)

2+ 2+ The relation of Mg versus Ca indicates the Thermodynamic modelling (pCO2, SIcalcite, SIdolmite) relative contributions of calcite and carbonate to weathering intensity in the Pesnica catchment. From Fig. 3 it can be seen that the highest pCO2 Most of the samples indicate that weathering of was observed in September 2016 with a value of calcite is dominant over the entire Pesnica River 316227.8 ppm (-1.50 bar) at location Pesnica 3 a nd t he catchment (Fig. 2B). A Mg2+/Ca2+ ratio of around lowest value of 15488.2 ppm (-2.81 bar) at location

0.33, typical for weathering of calcite with Pesnica 4 (Fig. 3 A, Table 1). pCO2 concentrations magnesium, is characteristic at all investigated in the river system depend mainly on conditions, locations in River Pesnica, only some samples such as temperature, pH and alkalinity. High 2+ 2+ 2+ 2+ having Mg /Ca of around 0.5. A Mg /Ca ratio concentrations of CO2 can arise from higher of 0.33 is also characteristic of rivers comprising groundwater discharge and/or respiration in the catchment, e.g., River and its streams because of the input of organic matter. All tributaries, Tisa River, Ilz River and Inn River the samples were supersaturated with CO2 with (Kanduč et al. 2007; Szramek et al. 2007; Kanduč respect to the atmosphere (10−3.5 bar = 316 ppmv) et al, 2013). according to thermodynamic conditions in the investigated period in 2016 (Fig. 3A). 186 T. KANDUČ, Z. SAMARDŽIJA, N. MORI, A. JEREBIC, I. LEVAČIĆ, M. KRAČUN, J. A. ROBINSON, S. ŽIGON, Ž. BLAŽEKA & D. KOCMAN

Fig. 3A. pCO2 versus pH pH in river water system. All

samples have pCO2 values 6.90 7.00 7.10 7.20 7.30 7.40 7.50 7.60 7.70 greater than atmospheric 0.00 (10 −3.5 bar = 316 ppmv) caus- february 2016 ing increase in calcite satu- -0.50 March 2016 ration B. Saturation index of calcite (SI ) versus dolo- April 2016 calcite mite (SIdolomite) in river water -1.00 May 2016 system. Saturation for cal- June 2016 cite/dolomite is defined as August 2016 log IAP/K=0. -1.50 September 2016 CO2 -2.00 log p

-2.50 Oversaturation with CO2

-3.00

-3.50

-4.00

1 Fig. 3B. Saturation indi- ces of calcite (SIcalcite) versus oversaturation dolomite (SI ) in a river February 2016 dolomite 0.5 with calcite water system. Saturation for March 2016 calcite/dolomite is defined as log IAP/K=0. SIcalcite 0 April 2016 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 May 2016

-0.5 June 2016

August 2016 dolomite

-1 SI September 2016 oversaturation with dolomite -1.5

-2

-2.5

-3

All samples in River Pesnica catchment were Isotopic composition of dissolved inorganic 13 undersaturated with calcite and dolomite, only carbon (δ CDIC) and particulate organic carbon 13 some samples in September 2016 season were (δ CPOC) oversaturated with calcite (Fig. 3B). SIcalcite ranged from -0.98 to 0.54 and SIdolomite ranged Values of the isotopic composition of dis- 13 from -2.61 to 0.48. It has been clearly shown that solved inorganic carbon (δ CDIC) can help most of the investigated rivers in the world con- decipher the contributions of organic matter ay stitute sources of CO2 to the atmosphere (M - decomposition, carbonate mineral dissolution, orga et al., 2005). Oversaturation with SIcalcite is equilibration with atmospheric CO2 to DIC and traced only in April at locations 1 and 2 before in-stream biological processes (Kanduč et al., 13 dam and oversaturation with dolomite only in 2007). Figure 4 shows δ CDIC versus alkalinity June. for Pesnica river water from February 2016. 13 δ CDIC in Pesnica catchment ranges from -14.8 Hydrogeochemical and isotopic characterization of Pesnica River, Slovenia 187

Fig. 4. Seasonal varia- Alkalinity (mM) 13 tion of δ CDIC values versus 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 alkalinity in Pesnica river 4.0 water system, with lines 4. Open system equilibration with the atmosphere with an average value of 1.6‰ indicating biogeochemical 2.0 processes in river water. These include (see equili- δ13 0.0 3. Dissolution of carbonates with Ccarb of +2.4‰ February 2016 bration lines): 4. Open sys- March 2016 tem equilibration with the atmosphere with an average -2.0 April 2016 valueof 1.6 ‰ 3. Dissolu- Pesnica 3 May 2016 -4.0 tion of carbonates accord- 13 June 2016 ing to the average δ CCaCO3 (+2.4 ‰, Kanduč et al., 2013) ( ‰) -6.0 August 2016 Pesnica 2 Pesnica 3 of all measured samples DIC Pesnica 3 September 2016

C composing the recharge -8.0 13 Pesnica 3 area of river water, caus- δ 2. Nonequilibrium carbonate dissolution by carbonic acid produced from soil zone ing +1‰±0.2 ‰ enrichment -10.0 12 13 in C in DIC (Romanek et with a δ CCO2 of -26.6‰ Pesnica 3 al., 1992), 2. Nonequilib- -12.0 rium carbonate dissolution by carbonic acid produced -14.0 from soil zone CO2, 1. Open- 1. Open system equilibration of DIC originating from degradation of organic matter system equilibration of DIC -16.0 13 with δ CCO2 of -26.6‰ with soil CO2 originating from degradation of organic -18.0 13 matter with δ Csoil = -26.6 ‰.

13 13 to -4.2 ‰ (Table 1). An average δ CPOC value of 2016) indicating that respiration can alter δ C- anduč 13 -26.6 ‰ (K et al., 2013) was assumed to DIC to more positive δ CDIC values in-stream, es- represent the isotopic composition of DOC that pecially in summer months (Fig. 4). Controls of 13 was transferred to DIC (Dissolved Inorganic longitudinal variation in δ CDIC in rivers have Carbon) by in-stream respiration. Open-system been studied by Roach et al., 2016. At location equilibration of DIC with CO2 enriches DIC in Pesnica 2 (before dam) and Pesnica 3 (after 13 ook 13 C by about 9 ‰ (M et al., 1974), which corre- dam) δ CDIC values are less negative indicating sponds to the value of -17.6 ‰ shown in Figure 4. CO2 reduction in river system and enrichment Non-equilibrium dissolution of carbonates, with 12C isotope (Fig. 4). with one part of DIC originating from soil CO2 (- 26.6 ‰), and the other from carbonates with an According to the model of Devol & Hedges 13 average δ CCa of +2.4 ‰, leads to an intermedi- (2001), suspended matter (particulate in contin- 13 anduč an ate δ CDIC value of -12.1 ‰ (K et al., 2012). uation) may be divided into three fraction (K - Given the isotopic composition of atmospheric duč, 2010 & references therein). Isotopic com- evin CO2 (-7.8 ‰, L et al., 1987) and the equilib- position of carbon in particulate organic matter 13 13 rium fractionation with δ CDIC of +9 ‰, DIC in (δ CPOC) was measured only in September and equilibrium with the atmosphere should have ranged from -29.5 to -27.6 ‰. At Biš location 13 13 a δ CDIC of about +1 ‰ (Fig. 4). Biogeochemical more negative δ CPOC (-29.5 ‰) was observed in processes such as dissolution of carbonates and comparison to characteristic degraded organic open-system equilibration with the atmosphere matter (-26.6 ‰). For comparison, the δ13C value 13 can be neglected in the Pesnica catchment since of particulate organic carbon (δ CPOC) in the river water is composed mostly of clastic rocks main channel of the Kamniška Bistrica River, (sandstones, marls, claystones). a pre-alpine gravel-bed river with prevailing intensive agriculture in the catchment, varies 13 Higher values of δ CDIC are observed at from -28.6 to -25.4 ‰ and from -29.9 to -25.2 ‰ Pesnica 3 location (March, 2016) and slightly in the tributaries (Kanduč et al., 2013). Phyto- 13 more negative values δ CDIC at Pesnica 2 (above plankton binds CO2 from DIC in water leading the Pernica lake). From Figure 4 it can be seen to a 13C fractionation of around -20 ‰, produc- 13 that most of the samples from Pesnica River ing biomass with δ CPOC ranging from -32.0 to 13 catchment fall around the line of nonequilib- -45.0 ‰. At the River Pesnica catchment δ CPOC rium carbonate dissolution by carbonic acid ranges from -27.6 to -29.5 ‰, meaning that pho- 13 produced from the soil zone with δ CCO2 of -26.6 tosynthetic activity was not so significant at our ‰. Samples from Pesnica 3 (retention) have investigated locations. 13 δ CDIC values less negative up to -4.2 ‰ (June 188 T. KANDUČ, Z. SAMARDŽIJA, N. MORI, A. JEREBIC, I. LEVAČIĆ, M. KRAČUN, J. A. ROBINSON, S. ŽIGON, Ž. BLAŽEKA & D. KOCMAN

Composition of suspended matter differentiates nosilicates (e.g. anorthite, albite, mica schist). by SEM/EDXS From Fig. 5B with a corresponding spectrum, a pattern similar to that in Figure 5A is seen. The composition of suspended matter differ- Figure 5C-a with a corresponding graph shows entiates from Pesnica catchment area at inves- that suspended matter is composed of diatoms; tigated locations: Pesnica 1, Pesnica 2 (above spherical structures are also observed. The spec- Pernica lake), Pesnica 3 (below the Pernica lake) trum shows that material is composed of Si, Al, and Pesnica 4- drainage channel Biš (Fig. 1). At O, Fe, K, Ca, Na, Mg (Fig. 5C-c). Diatoms are Pesnica 1 and Pesnica 2 sampling sites (Figs. 5A, composed of Si-oxide, while spherical struc- B) we can observe that aluminosilicates (K, Al, tures with corresponding spectrum have a mi- Si) prevail in suspended matter, while at sam- croporous structure and are composed of Fe, Si, pling location Pesnica 3 we observe high biolog- O, Ca. Fig. 5D with corresponding spectrum rep- ical activity (diatoms) (Fig. 5C-b) and at Pesnica resents suspended matter composed of Si, O, Fe, 4 (drainage channel Biš) inorganic fibers prevail Mg, Na, K, Ca with traces of Zn and Fe (Fig. 5D). (Fig. 5D), which could be of anthropogenic ori- Fibrous and microporous particles that were ob- gin. All SEM Figures have corresponding spec- served and analysed under higher magnification tra, meaning that elements: Al, Si, O, K, Na, Mg are composed of Al, Si, Fe and P, which could prevail with traces of Ti and Fe representing be attributed to anthropogenic input in the river detrital geological material composed of alumi- system.

Fig. 5A. SEM micrograph and corresponding EDXS analysis/spectrum of samples of suspended matter at Pesnica 1.

Fig. 5B. SEM micrograph and corresponding EDXS analysis/spectrum of samples of suspended matter at Pesnica 2 (above reservoir). Hydrogeochemical and isotopic characterization of Pesnica River, Slovenia 189

Fig. 5C-a. SEM micrograph and corresponding EDXS analysis/spectrum of samples of suspended matter at Pesnica catchment area from location Pesnica 3 (below the reservoir, retention).

Fig. 5C-b. SEM micrograph and corresponding EDXS analysis/spectrum of samples of suspended matter at Pesnica catchment area from location Pesnica 3 (below the reservoir, retention).

Fig. 5C-c. SEM micrograph and corresponding EDXS analysis/spectrum of samples of suspended matter at Pesnica catchment area from location Pesnica 3 (below the reservoir, retention). 190 T. KANDUČ, Z. SAMARDŽIJA, N. MORI, A. JEREBIC, I. LEVAČIĆ, M. KRAČUN, J. A. ROBINSON, S. ŽIGON, Ž. BLAŽEKA & D. KOCMAN

Fig. 5D. SEM micrograph and corresponding EDXS analysis/spectrum of samples of suspended matter at Pesnica catchment area from location Pesnica 4 (drainage channel Biš).

Conclusion suspended material at investigated locations. Pesnica River water solute chemistry is Microscopic SEM/EDXS analyses revealed - 2+ 2+ dominated by HCO3 , Ca and Mg . Seasonal that at Pesnica 1 and 2 detrital clastic material (spring, summer and autumn) concentrations of prevails in the samples, while at location Pesnica total alkalinity ranged from 1.24 to 6.69 mM. 3 phytoplantonic and/or phytobenthic algae From a geochemical point of view, we found out prevail, presumably originating from Pernica that dissolution of clastic material influences the Lake, while at location near drainage channel geochemical composition of river water. Most Biš, particles originating from wetland plants locations of surface water were near equilibrium are observed in addition to degraded organic with respect to calcite (SIcalcite from -0.98 to material.

0.54) and dolomite (SIdolomite from -2.61 to 0.48) in all sampling seasons or, otherwise, generally These first results on suspended matter in the undersaturated. According to the calculated River Pesnica catchment provide first data, that pCO2 values, surface water is a source of CO2 will help in evaluation of anthropogenic impacts, to the atmosphere. pCO2 in Pesnica catchment especially in relation to future impacts and for ranged from 15488 to 316228 ppm. Alkalinity long term planning for sustainable management 13 and isotopic composition of δ CDIC values were (e.g. regulation of river channel, deforestation and used to trace carbon cycling in surface waters. agriculture activity). The application of stable 13 Seasonal δ CDIC values (from February 2016 to isotopes in river management studies is useful September 2016) ranged from -14.8 to -4.2 ‰ tracer for determining the origin of dissolved indicating different biogeochemical processes in solutes and evaluation of biogeochemical 13 Pesnica River catchment. δ CDIC values around processes, and has been widely used in other and below -12 ‰ indicate that the most important catchments in Slovenia (e.g. Sava River, Kamniška process in Pesnica River catchment in our study Bistrica River, River, River). is degradation of organic matter. At a location below the reservoir Pernica lake (Pesnica 3 13 location), higher δ CDIC values have been observed Acknowledgements 13 since April 2016. The highest value of δ CDIC of -4.2 ‰ was observed in June 2016. This was due This research was conducted in the framework to the presence of large amounts of algae from of the project L2-6778 and program research foun- phytoplankton and phytobenthos in reservoir ding P2-0084 and P1-0143 funded by the Slovenian upstream of the sampling location, as confirmed Research Agency (ARRS). Sincere thanks to Roger 13 using SEM microscopy. δ CPOC ranged from -29.5 Pain for improving the English of the manuscript. to -27.6 ‰ and indicated different degree of Hydrogeochemical and isotopic characterization of Pesnica River, Slovenia 191

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