Hydrogeochemical Characteristics of the River Sava Watershed in Slovenia
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GEOLOGIJA 50/1, 157–177, Ljubljana 2007 doi:10.5474/geologija.2007.013 Hydrogeochemical characteristics of the River Sava watershed in Slovenia Hidrogeokemi~ne zna~ilnosti pore~ja reke Save v Sloveniji Tja{a KANDU^ & Nives OGRINC Department of Environmental Sciences, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Key words: River Sava, chemical composition, stable isotopes, sulphur, carbon, oxy- gen, deuterium Klju~ne besede: reka Sava, kemijska sestava, stabilni izotopi, žveplo, ogljik, kisik, dev- terij Abstract - 2+ 2+ The River Sava is a typical HCO3 – Ca – Mg River. Total alkalinity increases in the part of the watershed composed of carbonate and clastic rocks, which are less resistant to weathering processes. Ca2+/Mg2+ ratios are around 2 in the carbonate part of the watershed and increase in the watershed composed of carbonate and clastic rocks, indicating dis- solution of calcite with magnesium. According to PHREEQC for Windows calculations, the River Sava and its tributaries are oversaturated with respect to calcite and dolomite. 18 δ OH2O and δDH2O are related to the meteorological patterns in the drainage basin. River water temperatures fluctuate annually following air temperatures. The relationship be- 18 tween the temperature and δ OH2O and δDH2O values primarily reflects the strong depend- ence of δ18O and δD on precipitation and evaporative enrichment in heavy oxygen and hydrogen isotopes of infiltrating water recharging the River Sava from its slopes. The 13 δ CDIC values are controlled by processes in the terrestrial ecosystem and stream proces- ses such as: (1) dissolution of carbonates, (2) soil derived CO2, and (3) equilibration with 13 atmospheric CO2. Lower δ CDIC values are observed in the spring sampling season due to abundant precipitation related to soil leaching of CO2 in the river system. From discharge and concentration measurements of sulphate and according to the drainage area of the River Sava basin, the annual sulphur flux at the border with Croatia was estimated to be 7 2 2- 1.4 × 10 g SO4/km . Assuming that the sources of SO4 to the Sava are its tributaries, pre- cipitation and other sources, the contributions of these inputs were calculated according to steady state equations and estimated to be 52 : 8 : 40 %, respectively. Other sources are attributed to human influences such as industrial pollution and oxidation of sulphides. Izvle~ek - 2+ 2+ Reka Sava je tipi~na HCO3 – Ca – Mg reka. Totalna alkalnost nara{~a po pore~ju navzdol, ki ga sestavljajo poleg karbonatnih tudi klasti~ne kamnine, ki so manj odporne na procese preperevanja. Razmerje Ca2+/Mg2+ je okrog 2 v karbonatnem delu pore~ja in naraste v pore~ju, ki ga sestavljajo karbonatne in klasti~ne kamnine, kar kaže na raztap- ljanje kalcita z magnezijem. Glede na izra~une PHREEQC for Windows so reka Sava in 18 njeni pritoki prenasi~eni s kalcitom in dolomitom. δ OH2O in δDH2O vrednosti sta povezani z meteorolo{kimi spremenljivkami v drenažnem bazenu. Temperatura re~ne vode se let- 18 no spreminja glede na temperature v zraku. Povezava med temperaturo, δ OH2O odraža mo~no odvisnost δ18O v padavinah in evaporativno obogatitev na težjih izotopih kisika 13 infiltrirajo~e vode s pobo~ij, ki napaja reko Savo. δ CDIC vrednosti kontrolirajo procesi v terestri~nih ekosistemih in v reki, kot so: (1) raztapljanje karbonatov, (2) preperinski CO2 13 ter (3) uravnoteženje z atmosferskim CO2. Bolj negativne vrednosti δ CDIC so posledica ve~je koli~ine padavin in spiranja izotopsko lažjega preperinskega CO2 s terestri~nega ekosistema v spomladanskih mesecih. Iz podatkov o pretokih, koncentracijah sulfata v vodi in odvodnjevalni povr{ini smo izra~unali letni snovni tok žvepla na meji s Hrva{ko, 7 2 ki zna{a 1,4 × 10 g SO4/km . Glede nato, da smo predpostavili slede~e vire sulfata v reki Savi: pritoke, padavine in ostale vire, smo izra~unali deleže posameznih prispevkov iz ravnotežnih ena~b koncentracijske in izotopske masne bilance, ki zna{ajo 52 : 8 : 40 %. Ostale vire lahko pripi{emo industrijski onesnaženosti in oksidaciji sulfidnih mineralov. 158 Tja{a Kandu~ & Nives Ogrinc Introduction that sulphide minerals tend to be isotopi- cally lighter than sulphate minerals, and The geochemical study of river water hence the oxidation of sulphides (and that allows important information to be obtai- of organic sulphur) transposes the depleted ned on chemical weathering of rocks/soil δ34S signal to sulphate products (Pearson & and the chemical and isotopic compositions Rightmire, 1980). of the drainage basin (Gibbs, 1972; Reeder The main objectives of this study were to et al., 1972; Hu et al., 1982; Stallard & understand the hydrogeochemical charac- Edmond, 1983; Goldstein & Jacobsen, teristics of the River Sava watershed through 1987; Elderfield et al., 1990; Zhang et al., time (different seasons and flow regimes), 1995; Huh et al., 1998). Since carbonate and to quantify sources, fluxes and sinks of weathering largely dominates the water carbon and sulphur from the isotopic cha- chemistry of river waters, characterization racteristics of their dissolved species. of the water chemistry of rivers draining carbonate-dominated terrain is crucial to precisely identify the various contributions Characterization of the River Sava of the different sources to water solutes, and drainage basin to estimate weathering rates of the continen- tal crust and associated CO2 consumption The River Sava, the largest river in Slo- (Fairchild et al., 1994, 1999, 2000; Gail- venia and a tributary of the Danube, origi- lardet et al., 1999; Liu & Zhao, 2000). nates in the Triassic carbonate hinterland Rivers also reflect biogeochemical pro- at Zelenci (Figure 1, location 1) as the Sava cesses occurring in their catchment areas Dolinka and the karst spring Savica (Figure and help to quantify material transport from 1, location 6) as the Sava Bohinjka. The con- land to ocean (Palmer et al., 2001). Within fluence of these two sources is at Radovljica. this context, understanding of the carbon cy- From there the river is named Sava and fi- cle is particulary important because it helps nishes its course at Belgrade, merging with to evaluate the health of the river and its the Danube. From the source of the Sava catchment basin (Telmer & Veizer, 1999). Dolinka to the national border with Croatia Investigations of major elements and stable its length is 219 km. Along its studied course carbon isotopes of dissolved inorganic car- of 219 km, as well as the Sava Bohinjka 13 bon (δ CDIC) are useful for such studies and source, the river flows through agricultu- to evaluate environmental influences (Ka- ral areas and the urban centres of Jesenice, rim & Veizer, 2000; Barth et al., 2003). Kranj, Ljubljana, the mining area of Zasavje Rivers represent a linkage between pre- and Kr{ko (Figure 1). cipitation and groundwater, which all to- Discharge regimes along its flow are con- gether form the hydrogeological cycle. trolled by precipitation and the configura- Stable isotopes of oxygen and hydrogen are tion of the landscape. In the upper part of used to trace processes of evaporation, con- the Sava a snow – rain regime prevails and densation, snow melting, mixing of waters turns over in the central and lower part to a of different origin and recharge conditions rain – snow regime (Hrvatin, 1998). Annu- in the studied watersheds (Clark & Fritz, al maxima are characteristics in spring and 1997). late summer, while minima occur in summer 2- The natural sources of SO4 in river water and winter months. In the years 1961–1990 are rainfall, groundwater, and weathering of the mean annual amount of precipitation S-rich minerals. Human activities (e.g. air in the River Sava watershed was 1500 mm pollution, mining, smelting of sulphide ore, (Zupan~i~, 1998). Long term (from the refining of petroleum, and chemical indus- years 1960–1991) the mean annual discharge 2- tries) also contribute to the SO4 content of varies at the gauging stations of Radov- river water. These multiple sources of sul- ljica (in the upper part of the watershed), phate can frequently be distinguished by Hrastnik (in the central part of the water- their specific isotopic signatures. For exam- shed) and ~atež (in the lower part of the ple, river sulphate derived from dissolution watershed) from 18.6, 181.7 and 290 m3/s, of evaporates has positive δ34S values where- respectively (ARSO, 2004). Discharge ra- as sulphate from oxidation of sulphides or tios between high and low waters are from biogenic emissions may have strongly 1 : 100 (extreme 1 : 250) and are also contro- negative δ34S. The latter is due to the fact lled by hydroelectric power plant outflows. Hydrogeochemical characteristics of the River Sava watershed in Slovenia 159 Figure 1. Detailed location map of the numbered sampling sites in the River Sava watershed. Sample sites are described in Table 1. The drainage area of the River Sava in Slo- confluences of major and minor streams, at venia is 10881 km2 (ARSO, 2004). points before and after the confluence. Sam- The valley of the Sava extends in a NW– pling locations of the Sava watershed are SE direction and comprises almost half the presented in Figure 1. Sampling was per- Slovenian territory with variegated geolo- formed at 41 locations in different seasons gical composition. At the confluence of the (April – spring 2004, September – late sum- Sava Bohinjka and Sava Dolinka it accumu- mer 2004 and January – winter 2005), accor- lates Pleistocene fluvioglacial sediments and ding to the discharge regimes of the Sava formed terraces, from Ljubljana the water- and its tributaries. shed is mainly composed of Permo-Carbo- Temperature, conductivity, dissolved nian clastic sediments, which alternate with oxygen (DO), and pH measurements were Triassic carbonates in the Zasavje area and performed in the field. Sample aliquots col- pass over to Miocene sandstones, clays and lected for chemical analysis were immedia- gravels on the left bank of the river.