Biogeochemistry of some selected Slovenian rivers ( Kamniška Bistrica , Idrijca and Sava in Slovenia ) : insights into river water geochemistry , stable carbon isotopes and weathering material flows

Review of biogeochemical processes studied in three Slovenian rivers (River Kamniška Bistrica, River Sava in Slovenia and River Idrijca), which represent an ideal natural laboratory for studying biogeochemical processes and anthropogenic impacts in catchments with high weathering capacity is presented. The River Kamniška Bistrica, the River Sava in Slovenia and the River Idrijca water chemistry is dominated by HCO3 -, Ca2+ and Mg2+, and Ca2+/Mg2+ molar ratios indicate that calcite/dolomite weathering is the major source of ions to the river system. The Kamniška Bistrica River, the River Sava and River Idrijca and its tributaries are oversaturated with respect to calcite and dolomite. pCO2 concentrations were on average up to 25 times over atmospheric values for River Kamniška Bistrica, 20 times for River Sava and 13 times over atmospheric values for River Idrijca. δCDIC values ranged from -12.7 to -2.7 ‰ in River Kamniška Bistrica, from -12.7 to -6.3 ‰ in River Sava in Slovenia, from -10.8 to -6.6 ‰ in River Idrijca, respectively. In all investigated rivers we found out that carbonate dissolution is the most important biogeochemical process affecting carbon isotopes in the upstream portions of the catchment, while carbonate dissolution and organic matter degradation control carbon isotope signatures downstream, except for River Idrijca where both processes contribute equally from source to outflow to River Soča.


Introduction
Systematic studies of river water geochemistry provide important information on chemical weathering of bedrock/soil and natural and anthropogenic processes that may control the dissolved chemical loads (Schulte et al., 2011;GibbS, 1972;ReedeR et al., 1972;huh et al., 1998;NéGRel & lachaSSaGNe, 2000).Since carbonate weathering largely dominates the water chemistry of river waters, characterization of rivers draining carbonate-dominated terrain is crucial to precisely identify the various contributions of the different sources of water solutes, and to estimate the weathering fluxes of the continental crust and associated CO 2 consumption (liu & Zhao, 2000).
Freshwaters cover small fraction of the Earth's surface area, inland freshwater ecosystems (particularly lakes, rivers and reservoirs) have rarely been considered as potentially important quantitative components of the carbon cycle at either global or regional scales (cole et al., 2007).Rivers are the major pathways for the transport of carbon (C) from the continents top the oceans.Global river carbon fluxes are estimated to be 0.4 Pg C/year of organic C (evenly divided between particulate and dissolved phases) and 0.4 Pg C/ year for dissolved inorganic carbon (DIC).Bulk fluxes are small components of the global C cycle but are significant compared to net oceanic uptake of anthropogenic CO 2 (SaRmieNto & SuN-dquiSt, 1992).
Concentrations of DIC and its isotopic composition of dissolved inorganic carbon (δ 13 C DIC ) are governed by processes occurring in the river system and vary seasonally.Changes of DIC concentrations result from carbon addition or removal from the DIC pool, while changes of its isotopic composition result from the fractionation accompanying transformation of carbon or mixing of carbon from different sources (atek-waNa & kRiShNamuRthy, 1998).The major sources of carbon to riverine DIC loads are dissolution of carbonate minerals, soil CO 2 derived from root respiration and from microbial decomposition of organic matter and exchange with atmospheric CO 2 .The major processes removing riverine DIC are carbonate mineral precipitation, CO 2 degassing, and aquatic photosynthesis (atekwaNa & kRiShNamuRthy, 1998).Rivers in Slovenia represents an "ideal natural laboratory" for studying biogeochemical processes and tracing the riverine carbon cycle as a result of its geologically het-erogeneous composition, relatively high specific discharge, and limited aquatic photosynthesis (GeRm et al., 1999).
The relative contributions of C3 and C4 vegetation to an ecosystem can be reconstructed using the isotopic composition of particulate organic carbon (POC, e.g.δ 13 C POC ), because of their different isotopic composition, which ranges from -32.0 to -20.0 ‰ for C3 plants and from -15.0 to -9.0 ‰ for C4 plants (deiNeS, 1980).Vegetation of the River Sava watershed in Slovenia is described in detail in Kanduč et al. (2007) and references therein.Detail evaluation of some selected sites of River Sava watershed was described with aquatic moss Fontinalis antipyretica (mechoRa & Kanduč, 2016).Hydrogeochemical and isotopic characterization of River Pesnica, Slovenia was described in detail in Kanduč et al. (2016).
Application of stable isotopes and biogeochemical processes in environmental studies is presented in Pezdič (1999).In this study we represent summary (review) of biogeochemical research with application of stable isotope analysis of river systems; three rivers were subject of investigation during years 2004-2011 in different time related to national research projects and program founding P1-0143 in Slovenia: River Kamniška Bistrica (Kanduč et al., 2013), River Sava in Slovenia (Kanduč et al., 2007) and River Idrijca (Kanduč et al., 2008)

Catchment and hydrological characteristics of gravel bed rivers
River Kamniška Bistrica is the left tributary of the River Sava, which is the largest river in Slovenia (Figs. 1 and 2).Kamniška Bistrica emerges at the southern foothills of Kamnik-Savinja Alps at 630 m a.s.l.elevation.The river is 32.8 km long and drains an area of 380 km 2 , with an average discharge of 15.4 m 3 /s at its confluence with River Sava.The average discharge at the mouth of the River Kamniška Bistrica measured during this study was 0.7-12.1 m 3 /s.According to discharge regimes of all rivers and streams in Slovenia, River Kamniška Bistrica has an alpine high mountain snow-rain regime (hRvatiN, 1998).The maximum discharge occurs in autumn (November) and spring (May) and minimum occurs in summer (August) and winter (February).Major tributaries of the River Kamniška Bistrica are the Črna and Nevljica rivers on the left and River Pšata on the right.In the upper reaches, from the headwater spring to Stahovica (at the confluence with River Črna), River Kamniška Bistrica changes over a short distance (6.8 km downstream) from a clean alpine river to industrially and agriculturally affected river at the confluence with the tributary River Črna, which carries sediments and waste waters from the abandoned Črna kaolin mine (RadiNja et al., 1987).
Discharge regimes of the River Sava are controlled by precipitation and the configuration of the landscape.In the upper part of the River Sava a snow-rain regime prevails and in the central and lower part a rain-snow regime (hRvatiN, 1998).Annual discharge maxima are characteristic in spring and late summer, while discharge minima occur in the summer and winter months.The mean annual long term discharge (from the years  for the gauging stations increases from 17 m 3 /s of the upper section of the River Sava at Radovljica (location 35, Table 1, Fig. 1) to 182 m 3 /s of the central section at Hrastnik (loca-tion 60, Table 1, Fig. 1) and to 290 m 3 /s in the lower section of the river at Čatež (location 68, Table 1, Fig. 1) (iNteRNet 2).Discharges are also controlled by hydropower outflows along the Sava River.The discharge conditions for the River Sava and its tributaries during the study ranged from 2 to 344 m 3 /s during spring 2004, from 1 to 144 m 3 /s during late summer 2004, and from 0.3 to 128 m 3 /s during winter, respectively.The River Idrijca joins the River Soča in the middle stretch at the village of Most na Soči. Bth rivers have torrential characteristics.Detail description of the Idrijca catchment is described in Kanduč et al. (2008).High peaks and steep mountain slopes prevent air circulation in the valley and induce severe erosion.Characteristic long-term discharge data (from the years 1949 to 2015) according to the Slovenian Environment Agency for the gauging station on the Idrijca at Hotešk, which is located above the confluence with the River Soča, are as follows: low long-term discharge varies from 3.4 to 8.5 m 3 /s, mean long-term discharge varies from 14.3 to 39 m 3 /s, and high long-term discharge varies from 113 to 644 m 3 /s (iNteR-Net 2).  1 and 2).

General geological setting of selected river watersheds
This chapter summarizes the general geological setting of the river watershed areas, as the geological composition of each river basin is very complex (Fig. 1).Therefore, the prevailing geological units are described below.Detailed general description of geological setting of investigated Slovenian rivers is described in detail in Kanduč et al. (2007Kanduč et al. ( , 2008Kanduč et al. ( and 2013)).
River Kamniška Bistrica.The upper part of the River Kamniška Bistrica is underlain by massive and stratified limestone and dolomite of middle and upper Triassic age (Fig. 2), and carbonates generally prevail in the watershed.The middle part of the river is underlain by marlstone and limestone of Miocene age (buSeR, 1987).The lower reaches of the River Kamniška Bistrica, along the right bank and before the confluence with the River Sava, is underlain by Pleistocene and Holocene age gravels, while the left bank is underlain by Permo-Carboniferous shales with a cover of Quaternary gravel.The River Kamniška Bistrica is also one of the Slovenian watersheds identified as having a high weathering capacity, due to the predominance of carbonate bedrock and high relief and precipitation (kaNduč et al., 2013).
River Sava in Slovenia.The valley of the Sava River extends in a NW-SE direction comprising almost half the surface area of Slovenia and has a very heterogeneous geological composition.Both branches of the Sava River (Sava Bohinjka and Sava Dolinka rivers) emerge in the Julian Alps, composed mostly of Triassic limestones and dolomites.Leaving the Alps approximately at the confluence of the Sava Bohinjka and Sava Dolinka rivers, river then flows on the Holocene and Pleistocene fluvioglacial sediments (terraces) (ŽlebniK, 1971).Eastwards from the city of Ljubljana, the watershed in Sava folds is mainly composed of Permo-Carbonian clastic sediments, which alternate with some Triassic carbonates, with Miocene sandstones, clays and gravels in some of the valleys.Leaving the Sava folds, the watershed in the Krško-Brežice area mainly consists of terraced Holocene and Pleistocene sediments -sands and gravels.The catchments of the River Sava's tributaries are composed of Triassic and Jurassic carbonates, Permo-Carbonian sandstones and siltstones, Oligocene clay and volcanic rocks, Miocene clastic rocks and Pleistocene sediments (buSeR, 1987).
River Idrijca.The beds in the upper part of the River Idrijca are composed of various sedimentary and volcanic rocks, predominantly massive and stratified Triassic limestones and dolomites.Along Idrijca, Middle Permian mica quartz sandstone and red sandstone with conglomerate are exposed as the oldest rocks.In the lower part of the flow, before the confluence with the River Soča, stratified and massive Upper Triassic dolomites and Cretaceous limestones with marls appear (buSeR, 1987).In general, the Idrija region has a very complex tectonic structure (mlakaR & čar 2009; čar, 2010) with several major faults dissecting the area and tectonic nappes overlying several units.

Sampling and used methods
Surface water sampling locations (Fig. 1, Tables 1 and 2) were selected based on their relationship to confluence of the major and minor streams, typically sampled before and after the confluence.Sampling of river water and tributaries was performed at different sampling seasons according to discharge regimes (hRvatiN, 1998;FRataR, 2005).Temperature, conductivity, dissolved oxygen (DO), and pH measurements were performed in the field.The precision of dissolved oxygen saturation and conductivity measurements was ±5 %.The field pH was determined on the NBS scale using two buffer calibrations with reproducibility of ±0.02 pH unit.Total alkalinity was measured within 24h of sample collection by Gran titration (GieSk-eS, 1974) with a precision of ±1%.Carbonate rocks from hinterland of river watershed were ground to powder in an agate mortar and then approximately 2 mg of sample was first flushed with He and then transformed to CO 2 by H 3 PO 4 acid treatment.NBS 18 and NBS 19 were used as reference materials.The isotopic composition of carbonate (δ 13 C CaCO3 ) was measured with a Europa Scientific 20-20 continuous flow IRMS ANCA-TG preparation module.All methods are described in detail in Kanduč et al. (2007Kanduč et al. ( , 2008Kanduč et al. ( and 2013)).
All stable isotope results for carbon are expressed in the conventional delta (δ) notation, defined as per mil (‰) deviation from the reference standards VPDB.Precision was ±0.2 ‰ for δ 13 C DIC , δ 13 C POC and δ 13 C CaCO3 .
Major and minor cation chemistry was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) technique.The precision of the method was ±2% for major (Ca 2+ , Mg 2+ , Na + and K + ) and ±5% for minor elements (Sr and Si).The stable isotope composition of dissolved inorganic carbon (δ 13 C DIC ) was determined with a Europa Scientific 20-20 continuous flow IRMS ANCA-TG preparation module.Phosphoric acid (H 3 PO 4 , 100 %) was added (100-200 ml) to a septum-sealed vial which was then purged with pure He.The water sample (6 mL) was injected into a septum tube and headspace CO 2 was measured (modified after miyajima et al., 1995;Spötl, 2005).In order to determine the optimal extraction procedure for surface water samples, a standard solution of Na 2 CO 3 (Carlo Erba) with a known δ 13 C DIC of -10.8 ‰ ± 0.2 ‰ was prepared with a concentration of either 4.8 mol/L (for samples with alkalinity above 2 mmol/L) or of 2.4 mmol/L (for samples with alkalinity below 2 mmol/L).The carbon stable isotope composition of particulate organic carbon (δ 13 C POC ) was determined with a Fig. 2. General geological map of Slovenia with selected three rivers: Kamniška Bistrica, Sava in Slovenia and Idrijca.Geological data were obtained from the 1: 5 Million International Geological Map of Europe and Adjacent Areas (IGME 5000) dataset (iNteRNet 3).
Table 1.Sampling locations and geochemical data for spring sampling season (sampling years: Idrijca: 2006-2007, Kamniška Bistrica: 2010-2011, Sava: 2004-2005).ID numbers correspond to the locations in Figure 1.Europa-Scientific 20-20 continuous flow IRMS ANCA-SL preparation module.For POC 1 L of the water sample was filtered through a Whatman GF/F glass fiber filter (0.7 mm).Filters and soil were treated with 1 M HCl to remove carbonate.
Thermodynamic geochemical modeling was used to evaluate CO 2 partial pressures (pCO 2 ) and the saturation state of calcite and dolomite (SI calcite and SI dolomite ) using pH, alkalinity, and temperature as inputs to the PHREEQC speciation program (paRkhuRSt & appelo, 1999).

Aquatic geochemistry of selected gravel bed rivers in Slovenia
The temperature of surface water in River Kamniška Bistrica, pH and conductivity ranged from 1.7 to 26.6 °C, 7.1 to 8.8, and 160.7 to 497.4 mS/cm.DO saturation varied seasonally from 59.6 to 76.8 % in the winter and from 68 to 140 % (Kanduč et al., 2013).In River Idrijca water temperature was 7.3 to 13.0 °C, conductivity ranged from 181 to 465 mS/cm, pH ranged from 7.77 to 8.82 (Kanduč et al., 2008).Temperature in River Sava water ranged from 0.4 to 15.7 °C, conductivity ranged from 62.3 to 632 mS/cm and pH ranged from 7.24 to 8.99, respectively (Kanduč, 2006;Kanduč et al., 2007).All results are described in detail in Kanduč et al. (2007Kanduč et al. ( , 2008Kanduč et al. ( and 2013)).
The major solute composition of selected gravel-bed rivers was dominated by HCO 3 -, Ca 2+ and Mg 2+ .Concentrations varied seasonally according to discharge, with higher concentrations observed in autumn at lower discharge and lower concentrations during the spring sampling season.Dissolved Ca 2+ and Mg 2+ are largely supplied by the weathering of carbonates (Fig. 3), which are the most dominant rocks in the watersheds, and prone to chemical dissolution, with smaller contributions from silicate weathering, as indicated by the relatively high HCO 3 -and low Si concentrations (Kanduč, 2006;Kanduč et al., 2007Kanduč et al., , 2008Kanduč et al., and 2013)).
Figure 3 presents Ca 2+ + Mg 2+ versus alkalinity for all three selected gravel bed rivers in Slovenia.Most of the samples have a 2:1 mole ratio of HCO 3 -to Ca 2+ + Mg 2+ following the reactions (GaillaRdet et al., 1999): Some samples deviate from 2:1 line due to weathering of other minerals in river watershed, like albite and anorthite: The pH, temperature and pCO 2 of a watershed determine the carbonate speciation, controlling the HCO 3 -carrying capacity.In Slovenian watersheds, total alkalinity comprises carbonate alkalinity (Kanduč, 2006;Kanduč et al., 2007), and therefore the total alkalinity is assumed as HCO 3 -, which is also the main DIC species at the pH of 7.0 to 9.0 measured in all investigated watersheds.Concentrations of HCO 3 -in main channel of River Kamniška Bistrica (Fig. 3A) vary seasonally from 1.93 to 4.19 mM in autumn 2010, from 1.88 to 4.99 mM in winter 2011, from 1.55 to 4.39 mM in spring 2011 and from 1.70 to 5.57 mM in summer 2011, respectively.Concentrations of HCO 3 -(alkalinity) in tributaries vary seasonally and range from 3.25 to 4.58 mM in autumn 2010 (Fig. 3A).The alkalinity concentrations in the main channel sampling sites varied seasonally in River Sava (Fig. 3B) in the main channel from 2.60 to 3.75 mM in spring, from 2.63 to 4.79 mM in late summer 2004, and from 2.67 to 4.17 mM during winter.The upper alpine headwater catchments of the River Sava have thin soils developed on carbonate bedrock.In the central and lower part of the River Sava watershed, tributary streams have more variable alkalinity concentrations, ranging from about 0.39 to 6.02 mM (Kanduč et al., 2007).River Idrijca (Fig. 3C) had alkalinities in range from 3.88 to 4.66 mM in autumn 2006 and from 4.12 to 4.43 mM in spring 2007, while in tributaries alkalinities range from 3.09 to 5.10 mM in autumn 2006 and in spring 2007 from 3.15 to 5.04 mM (Kanduč et al., 2008).
Differences in alkalinities in carbonate-bearing watersheds are related to the geological composition of the watershed (Fig. 2), the relief (Fig. 1), the mean annual temperature, the depth of the weathering zone, the soil thickness and the water residence time in the system.Weathering rates in-crease in thicker soils like shales due to the higher residence time of shallow groundwater in contact with minerals in comparison to watersheds composed of carbonate minerals.
Mg 2+ versus Ca 2+ relations indicate the relative contribution of calcite/dolomite to carbonate weathering intensity in gravel bed rivers (Fig. 4).Most of the samples indicate that weathering of calcite is dominant over the entire River Kamniška Bistrica, especially in the upper and central reaches (Fig. 4A).A Mg 2+ /Ca 2+ ratio around 0.33 is typical for weathering of calcite for the entire length of the River Kamniška Bistrica as well as for rivers comprising Danube watershed (kaNduč et al., 2013).In contrast, rivers comprising St. Lawrence watershed (North America) have ratios Mg 2+ /Ca 2+ greater than 0.33 (SZRamek et al., 2007).Most of the samples in River Sava (Fig. 4B) fall below 0.22 line, indicating weathering of calcite, only some samples in River Sava tributaries fall above 0.5 Mg 2+ /Ca 2+ line indicating weathering of dolomite.From Figure 4C it can be observed that most of the samples indicate that weathering of dolomite is dominant over the entire River Idrijca,  especially in the upper and central flow of the river.A Mg 2+ /Ca 2+ ratio around 0.33 is characteristic only in the lowland tributaries of the River Idrijca composed mainly of limestone.
The major control on carbonate weathering intensity is runoff (amiotte Suchet & pRobSt, 1993).Carbonate weathering intensity normalized to drainage area, quantifies HCO 3 -produced from mineral weathering.Figure 5 compares carbonate weathering intensities as a function of specific runoff for the River Idrijca watershed, combining new data from this study with published official data for the River Sava, River Kamniška Bistrica and data from berner & berner (1996) for world rivers and the River Danube.Global theoretical models of CO 2 consumption in carbonate watersheds show an alkalinity value around 3 mmol/L determined from a best-fit line (amiotte Suchet & pRobSt, 1993).The climate and topographic relief in Slovenian watersheds importantly influence the carbonate weathering intensity and specific runoff.Roy et al. (1999) noted that linked factors such as lithology, residence time of water, mechanical erosion, etc., have more influence together than they do separately.The watershed of the River Idrijca is typically an environment where enhanced mechanical weathering increases chemical weathering (FaiRchild et al., 1999;aN-deRSoN et al., 2000;jacobSoN et al., 2000) and causes a high carbonate weathering intensity, since the river is a steep mountain river with torrential character, e.g.River Idrijca with 80 mmol/l•km 2 •s (Fig. 5) and Kamniška Bistrica with the highest weathering intensity of 150 mmol/l•km 2 •s (Fig. 5).
The world average value for carbonate weathering intensity is 7 mmol/l km 2 s (beRNeR & beRNer, 1996).For the River Sava and its tributaries, the mean long term weathering intensity is from 37 to 140 mmol/l km 2 s.Also carbonate weathering intensity (HCO 3 in mmol/l km 2 s) of some other world rivers (Mississippi, World, Danube) is presented on Figure 5. From Figure 5 it can be observed that Slovenian gravel bed rivers have higher HCO 3 -weathering intensity in comparison to world rivers.

Thermodynamic modeling and isotope geochemistry with emphasize on carbon cycle
Thermodynamical modeling software PHREEQC for Windows was used to calculate pCO 2 and saturation indices for calcite and dolomite (SI calcite and SI dolomite ) along the main water channel and tributaries.In all investigated bed rivers a high value of pCO 2 was observed during all sampling seasons, meaning that rivers represent sources of CO 2 into air.from 234.4 to 9,120 ppm in April and from 223.9 to 4,074 ppm in January 2005 (Kanduč, 2006).In autumn all sampling locations on the River Idrijca watershed are above equilibrium with atmospheric CO 2 .These higher partial pressures in autumn are probably due to higher degradation of organic matter in the river and due to lower discharge (deveR et al., 1983).Lower pCO 2 (below normal atmospheric pressure at locations 5 and 6, Table 1, Fig. 1) in the spring are observed due to the higher pH of the water, which lowers the evasion of CO 2 from water.
The calcite saturation index (SI calcite =log([-Ca 2+ ]*[CO 3 2-])/K calcite ; where K calcite is the solubility product of calcite and was generally well above equilibrium (SI calcite =0)), indicates that calcite was supersaturated and precipitation was thermodynamically favoured along most of the course of all selected gravel bed rivers in Slovenia (Fig. 6).Calcite and dolomite were supersaturated and carbonate precipitation was thermodynamically favoured along most of the course of River Kamniška Bistrica (Fig. 6A).SI calcite and SI dolomite seasonally change in River Sava and their tributaries and reach oversaturation in central and lower flow of the river, while in upper part of the river rarely reach saturation (Fig. 6B).Low SI calcite and SI dolomite are observed at tributary location of River Sava (Fig. 6B).In most of the samples of River Idrijca and its tributaries calcite and dolomite are oversaturated, only one sample in River Idrijca is undersaturated with respect to dolomite (Fig. 6C).

Mass balance calculation with evaluation of biogeochemical processes in selected gravel bed rivers in Slovenia
Mass balance calculations were performed in previous studies (Kanduč et al., 2007(Kanduč et al., , 2008(Kanduč et al., and 2013)).
The δ 13 C DIC value can determine the contributions of organic matter decomposition, carbonate mineral dissolution, and exchange with atmospheric CO 2 to DIC in selected gravel bed rivers in Slovenia.The δ 13 C DIC values of the main channel of the river varied seasonally (year 2010-2011) from -10.9 ‰ (River Kamniška Bistrica, location 20, Table 1, Fig. 1) to -2.7 ‰ (River Kamniška Bistrica Spring, location 14, Table 1, Fig. 1) while δ 13 C DIC in tributaries ranged from -12.7 ‰ (Rača, location 27, Table 1, Fig. 1) to -6.9 ‰ (Kamniška Bistrica Spring, location 14, Table 1, Fig. 1) (kaN-duč et al., 2013).The δ 13 C DIC in River Sava varied seasonally from -12.7 to -8.6 ‰ in spring 2004, from -11.8 to -7.3 ‰ in late summer 2004 and from -10.6 to -6.3 ‰ in winter 2005.The River Sava tributaries had δ 13 C DIC values that varied from -13.5 to -5.8 ‰ in spring 2004, from -12.8 to 3.3 ‰       in late summer 2004, and from -11.9 to -4.2 ‰ in winter 2005 (Kanduč et al., 2007).δ 13 C DIC varied seasonally in River Idrijca watershed from -10.8 to -9.0 ‰ in autumn 2006 and from -10.6 to -8.3 ‰ in spring 2007.The δ 13 C DIC value of the river water is controlled by the geological composition of the watershed.Along the River Idrijca flow the dissolution of carbonates is the major contributor to δ 13 C DIC values, but some parts of the watershed also drain shales, mudstones, and sandstones (Kanduč et al., 2008).Thus, in those parts δ 13 C DIC is much lower (central part of the River Idrijca, lower reaches of River Kamniška Bistrica and central and lower flow of River Sava in Slovenia) since the thickness of soil is on this bedrock much higher and soil CO 2 contributes much more to DIC than on carbonate bedrocks.δ 13 C DIC was also generally lower during spring season at higher discharge (Fig. 7).The average δ 13 C value of Mesozoic carbonate rocks (δ 13 C CaCO3 ) in the hinterland of River Kamniška Bistrica is +2.4 ‰ (Kanduč et al., 2013).The δ 13 C of Mesozoic carbonate rocks (δ 13 C CaCO3 ) from the River Sava watershed ranged from -1.4 to +2.7 ‰, with an average of +1.4±1.3 ‰ (N=12) (Kanduč et al., 2007).The δ 13 C value of Mesozoic carbonate rocks (δ 13 C CaCO3 ), which forms the slopes in the watershed of the River Idrijca is on average +2.0±0.7 ‰ (N = 8) (Kanduč et al., 2008).
Figure 7 shows a plot of δ 13 C DIC versus alkalinity in different sampling seasons for selected gravel bed rivers in Slovenia.Changes over the course of the rivers indicate processes affecting δ 13 C DIC , e. g. degradation of organic matter (line 3), carbonate mineral dissolution (line 2), and equilibration with atmospheric CO 2 (line 1) (baRth et al., 2003).
At River Kamniška Bistrica source carbonate dissolution prevails, while in central and lower part of the river degradation of organic matter and dissolution of carbonates prevails (Fig. 7A).The δ 13 C DIC values from the River Idrijca watershed (Fig. 7C) indicate that nonequilibrium carbonate dissolution predominates along the flow of river, since the watersheds are mainly composed of carbonate rocks with inclusions of clastic rocks, approaching a δ 13 C DIC value of -12.3 ‰.In tributaries of the River Idrijca watershed (Fig. 7C), River Kamniška Bistrica (Fig. 7A) and River Sava in Slovenia (Fig. 7B) dissolution of carbonate minerals prevails, which leads to higher δ 13 C DIC values.Mineralization of organic matter appears to be the dominant source of δ 13 C DIC along the Idrijca flows (Fig. 7C), where the greater soil thickness enables accumulation of soil CO 2 due to the greater degree of silicate rock weathering, which leads to more a negative δ 13 C DIC .
The evasion of CO 2 from the River Kamniška Bistrica, River Sava in Slovenia and River Idrijca can be calculated (equation 5) based on the thin-film diffusive gas exchange model (bRoeck-eR, 1974;RaymoNd et al., 2012): where D is the CO 2 diffusion coefficient in water of 1.26 *10 -5 cm 2 /s at a temperature of 10 °C and 1.67 *10 -5 cm 2 /s at a temperature of 20 °C (jähNe et al., 1987), and z is the empirical thickness of the liquid layer [cm].
A simple isotopic mass balance calculation was performed in order to quantify different sources of DIC in all three selected gravel bed rivers: at River Kamniška Bistrica mouth (location 20, Table 1, Fig. 1), at the River Idrijca mouth (location 6, Table 1, Fig. 1), at River Sava in Slovenia mouth (location 70, Table 1, Fig. 1) considering the sum of tributary inputs and biogeochemical processes in the watershed.The major inputs to the DIC flux (DIC RI ) and δ 13 C DIC originate from tributaries (DIC tri ), degradation of organic matter (DIC org ), exchange with the atmosphere (DIC ex ), and dissolution of carbonates (DIC ca ) can be estimated by Eqs.(6 and 7): The contribution of rainwater to riverine DIC is considered to be minimal as it contains only a small amount of DIC (yaNG et al., 1996).
DIC RI and DIC tri were calculated from the concentrations of alkalinity and water discharge, with the corresponding measured δ 13 C values for δ 13 C RI and δ 13 C tri.The average diffusive flux of CO 2 from the river to the atmosphere, DIC ex , estimated from Eq. ( 5), was taken into account.In Eqs.
(5 and 6) the minus sign indicates outgassing of CO 2 , which is observed in autumn, but not in the spring season.The δ 13 C ex value was calculated according to the equation for equilibrium isotope fractionation between atmospheric CO 2 and carbonic acid in water (ZhaNG et al., 1995), where a δ 13 C value of -7.8 ‰ for atmospheric CO 2 was used (leviN et al., 1987).The isotopic composition of the contribution of equilibration between atmospheric CO 2 and DIC (δ 13 C ex ) would then be +1.4 ‰ in the autumn and +1.8 ‰ in the spring sampling season, considering atmospheric CO 2 as the ultimate source of CO 2 in the River Sava in Slovenia, River Idrijca and River Kamniška Bistrica drainage system.For δ 13 C POC and δ 13 C CaCO3 average values of -26.6 ‰ and +2.0 ‰ were used in the mass balance equations.
Contributions of DIC from various biogeochemical processes were determined using steady state equations for different sampling seasons at the mouth of the River Kamniška Bistrica; results indicate that: (1) 1.9-2.2% of DIC came from exchange with atmospheric CO 2 , (2) 0-27.5 % of DIC came from degradation of organic matter, (3) 25.4-41.5 % of DIC came from dissolution of carbonates and (4) 33.0-85.0% of DIC came from tributaries (Kanduč et al., 2013).In both sampling seasons the most important biogeochemical process is weathering of carbonates, while degradation of organic matter is more expressed in the spring sampling season.A less significant process in both sampling seasons is exchange with atmospheric CO 2 and is not marked in the spring sampling season due to the pCO 2 value (at location 28, Table 1, Fig. 1), which is near equilibrium with atmospheric CO 2 pressure.In River Sava mouth among biogeochemical processes dissolution of carbonates contributes the highest proportion in both sampling seasons, which moves δ 13 C DIC to more positive values.Mass balances for riverine inorganic carbon suggest that carbonate dissolution contributes up to 26 %, degradation of organic matter ~17 % and exchange with atmospheric CO 2 up to 5 %.The concentration and stable isotope diffusion models indicated that atmospheric exchange of CO 2 predominates in streams draining impermeable shales and clays while in the carbonate-dominated watersheds dissolution of the Mesozoic carbonate predominates (Kanduč et al., 2007).The calculated contributions to the average DIC budget from DIC tri :DIC ex :DIC org :DIC ca at the River Idrijca mouth were 61 :-11:19:31 % in autumn 2006:-11:19:31 % in autumn and 35:0:26:39 % in spring 2007:-11:19:31 % in autumn (Kanduč et al., 2008)).

Conclusions
The major solute composition of the River Kamniška Bistrica is dominated by HCO 3 -, Ca 2+ and Mg 2+ .Concentrations of HCO 3 -ranged from 1.6 mM to 5.6 mM in main channel and from 2.6 to 5.5 mM in tributaries.The majority of River Kamniška Bistrica system was supersaturated or near equilibrium with respect to calcite/dolomite in all sampling seasons.According to the calculated pCO 2 values, the river is source of CO 2 to the atmosphere during all sampling seasons, higher pCO 2 is observed during summer season.Lower alkalinities and higher δ 13 C DIC values of -2.7 ‰ were observed in the upper carbonate part of the watershed, while higher alkalinities and more negative δ 13 C DIC values of -12.7 ‰ were observed in the central and lower part of the Kamniška Bistrica system.
The major chemical composition of River Sava water is HCO 3 -, Ca 2+ and Mg 2+ .Seasonal (spring 2004, late summer 2004 and winter 2005) concentrations of HCO 3 -range from 2.63 to 4.79 mM, while its tributaries have concentrations of HCO 3 -ranging from 0.39 to 6.02 mM.The majority of the River Sava system is supersaturated or at equilibrium (in the upper part of the river flow) with respect to calcite in all sampling seasons.δ 13 C DIC values range from -12.7 to -6.3 ‰ and were observed in late summer.The observed differences in pCO 2 , alkalinities and δ 13 C DIC between the carbonate rock drainages versus mixed lithology watersheds (carbonate and clastic rocks) at downstream locations are the consequence of the soil thickness since carbonate rocks are more resistant to mechanical weathering processes.The partial pressure is lower in the carbonate part of the watershed and higher at downstream locations.Lower alkalinities and higher δ 13 C DIC values are observed in the upper carbonate part of the watershed, while higher alkalinities and lower δ 13 C DIC values are observed in the central and lower part of the River Sava watershed.
The biogeochemical processes affecting DIC and δ 13 C DIC values were quantified by concentration and isotope mass calculations and it can be concluded that the most important biogeochemical processes at the River Sava mouth in order of significance in different sampling seasons are: (1) carbonate dissolution comprising 19.4 % in spring to 25.9 % in late summer, (2) degradation of organic matter comprising 10.8 % in winter to 16.7 % in late summer, while (3) atmospheric exchange comprises 0.8 % in spring to 4.9 % in late summer.
The River Sava in Slovenia has high discharge, low stream photosynthetic activity and represents a river system where among the biogeochemical processes geological factors prevail (carbonate dissolution).Construction of hydroelectric power plants in the central and lower Sava flow in the next five years will affect the carbon cycle, e.g accelerated primary production, degradation of organic matter, degassing of CO 2 from the river.This investigation will also help to evaluate the biogeochemical state of the river after dam constructions.
The major solute composition of River Idrijca water is dominated by HCO 3 -, Ca 2+ and Mg 2+ .Seasonal alkalinity concentrations ranged from 3.88 to 4.66 mM, while its tributaries had concen-trations of HCO 3 -ranging from 3.09 to 5.10 mM.The majority of the River Idrijca system was supersaturated or near equilibrium with respect to calcite.The biogeochemical processes affecting DIC concentrations and δ 13 C DIC (in the range from -10.8 to -6.6 ‰) calculated by mass balance equations showed that the most important biogeochemical processes at the River Idrijca mouth are: carbonate mineral dissolution, degradation of organic matter and atmospheric exchange.The River Idrijca is a river with torrential character, has a high specific discharge and therefore high weathering intensity.
In all three investigated rivers carbonate dissolution and degradation of organic matter are the most important biogeochemical processes in river system, while exchange with atmosphere could be negligible according to mass balance equations.
presented in the Figures 1 and 2.

Fig. 1 .
Fig. 1.General topographic map of the major river network in Slovenia, with a detailed location map of the numbered sampling sites for the rivers Kamniška Bistrica, Sava (Slovenian part) and Idrijca.Digital elevation model (DEM) was obtained from the Shuttle Radar Topography Mission (SRTM) dataset (iNteRNet 1).Numbers correspond to the sampling points IDs (see Tables1 and 2).

Calculated pCO 2
Fig. 5. Carbonate weathering intensity (HCO 3 in mmol/l km2 s) versus specific runoff (1/km 2 s) indicating high carbonate weathering intensity in selected rivers in Slovenia (River Kamniška Bistrica, River Sava in Slovenia, River Idrijca) and in the world.Data include mean long-term data of discharge and alkalinity from the Slovenian Environment Agency (2004-2011) for the Slovenian rivers, and beRNeR & berner (1996) for world rivers, River Danube and Mississipi River.

Fig. 6 .
Fig. 6.SI calcite versus SI dolomite in different sampling seasons in different periods for the selected rivers (A: Kamniška Bistrica, B: Sava in Slovenia, C: Idrijca).