60 40 -- - discharge Podroteja A 0 94-04-01 94-04-16 94-05-01 94-05-16 94-05-31 Fig. 5.25: Daily variation of discharge from the Podroteja spring. The estimated mean altitudes of the catchment areas of the given springs are in agreement with the altitudes calculated by the 5^®0-values of the yearly variation (see chapter 5.1.4). 5.2. DISSOLVED INORGANIC CARBON ISOTOPE COMPOSITION OF WATERS (J. URBANC, B. TRČEK, J. PEZD1Č, S. LOJEN) The objective of this research is to determine whether the isotope composition of TDIC in water and the chemical composition of water in the outflow from a karst aquifer can be used to interpret the carbon isotope composition and partial pressure of soil CO^ in the aquifer's recharge area. Further, an attempt was made to establish which model of carbonate rock dissolution in water can be applied to interpret initial conditions and define the degree of accuracy with which the initial conditions can be described if the only data available are those of the isotopic and chemical composition of water in the outflow from a carbonate aquifer. Thus, in our research, the carbon isotope composition and partial pressure of soil COj measured in the recharge area of a karst aquifer were compared to the values, calculated from the isotopic and chemical composition of water in the outflow from aquifer. Previous observations have shown that the formation of soil CO^ is to the greatest extent conditioned by soil temperature (BILLES et al. 1971; DORR & MUNNICH 1980; KIEFER & BROOK 1986; WOOD et al. 1993), by the quantity of organic matter in the soil (WOOD et al. 1993) and by soil moisture (KIEFER & BROOK 1986). Most of soil CO^ passes into the atmosphere, only a minor part of the total CO, is washed into the ground by precipitation (WOOD & PETRAITIS 1984; QUADE et al. 1989; HENDRY et al. 1993). Soil CO, can originate from the decomposition of organic matter or root respiration (WOOD & PETRAITIS 1984; HENDRY et al. 1993; DUDZIAK 1994). Some investigations indicate that during carbonate dissolution in soil, the solution equilibrates with soil CO^. Thus this is an open system of carbonate dissolution (REARDON et al. 1979; QUADE et al. 1989^ while indications for the closed system of carbonate dissolution in the soil were also present (DEINES et al. 1974). 5.2.1. Carbon Isotope Composition In Individual Parts Of The Researched System The carbon isotope composition of water in the outflow from aquifer can be influenced by the carbon isotope compositions of soil CO^, carbon isotope composition of the carbonate rock, and by potential changes of carbon isotope composition of water resulting from isotope exchange between the carbon from atmospheric CO^ and the carbon degassing from water CO^. 5.2.1.1. Characteristics of carbon isotope composition of soil CO^ Several sampling points underlying different vegetation covers and at different altitudes were chosen for the measurement of isotope composition and partial pressure of soil CO^. Three probes for the sampling of soil CO^ were placed in a forest with prevaihng beeches: in the Belca valley (sampling point with the lowest altitude), near Podkraj, and data obtained at Obli Vrh at an altitude of about 1000 m were also included in the investigation. The probes at Col and Grgar were located in soil underlying grass. The probe for the sampling of soil CO^ in a spruce forest was located near Podkraj. Samples of soil CO^ were taken using metal capillaries with an inside diameter of 1 mm, which were dug about 50 cm deep into the ground. Samples of soil atmosphere were transferred into preevacuated 0.7 1 glass ampoules. In the laboratory, atmospheric CO^ was isolated according to the usual procedure (CRAIG 1953) and its isotope composition was measured on the Varian Mat 250 mass spectrometer. From the quantity of isolated CO^ and the volume of the ampoule the concentration of CO, in the gas sampled was calculated. Dissolved inorganic carbon from water was extracted by adding concentrated HjPO^ acid to the water in vacuum (MOOK 1970). 2 1,8 --1,6 — 1,4 — ^1,2 --S 1 + o 0,6 --0,4 0,2 8 10 12 14 Soil temperature (°C) 16 18 20 □ Temp.-12 cm Temp. - 50 cm Fig. 5.26: Relation between soil temperature and partial pressure of soil CO^ in beech forest. 1,8--1,6--1,4 ---1,2 -- s ^ + Ü '^0,8 + 0,6 --0,4 --0,2 -- a □ 10 12 14 16 18 Soil temperature (°C) 20 22 24 □ Grgar-12cm o Col-12 cm • Col - 50 cm A linear correspondence between temperature and partial pressure of soil COj can be clearly observed (Fig. 5.26, 5.27). Compared to the soil underlying beech forest, the soil overlaid with grass has lower partial pressures of soil CO^. The smaller generation of CO^ is of course conditioned by a smaller transition of organic matter into the soil overlain with grass. Fig. 5.28 shows average partial pressures measured at individual locations, compared to the average soil temperature. Values measured at sampling points located under the same type of vegetation lie along the same line, and the inclination of P^o,"^ correlation lines for beech and grass is also similar. Beech forest is the prevailing vegetation in the research area, therefore the majority of carbon isotope composition values and partial pressures of CO, were measured in soil underlying beech forest. Monthly measurements of soil CO2 carbon isotope composition showed that most of the values lie between -17 and -23 %o (Fig. 5.29), while partial pressures of CO^ range between 0.07 to 1.2 % of the total atmospheric pressure. A certain interdependence between the carbon isotope composition and partial pressure of soil CO^ is evident from Fig. 5.28. Such correlation could result from the mixing of biogenic carbon, originating from the decomposition of organic matter in the soil, with the carbon from atmospheric CO,. In this case, the ratio between biogenic carbon and atmospheric carbon can be expressed by the mixing equation: . P, 5'-'C . F b b a a § »C. = --(11) P. P ... partial pressure a ... atmospheric b ... biogenic t ... total In the mixing model, the following 5"C concentrations were adopted: -23 %o for organic carbon, -8 %o for atmospheric carbon (KEELING et al. 1979), and 0.03 % for the partial pressure of atmospheric CO^. The curve of the mixing model is given in Fig. 5.29, indicating that modelled results are in good correlation with the values measured. Equation 11 was statistically proved, the test statistic F = 3.7-t-lO-' (TRČEK 1996). Thus it can be concluded that fluctuations in carbon isotope composition of soil CO^ can be to a great extent attributed to the mixing of biogenic and atmospheric carbon. In the above case, sampling points were located under the same type of plant, namely under beech. A different carbon isotope composition of biogenic carbon is to be expected in soil zones underlying other types of vegetation. Figure 5.30 shows the relation between the carbon isotope composition and 1.8 - 1,6 t I 1,4 -.1,2 - S o 0,6 — 0,4 - 0,2 -- Belca-beech (460 m) / iPodkraj-beech (860 mV-^ , ' ^ MGrgar-grass (390 m) ■ Podkraj-spruce (850 m) ,^ Col-grass (620 m) ■ Obli vrh-beech (1000 mY 8 10 12 14 Soil temperature - 50 cm (°C) 16 18 20 Fig. 5.28: Mean soil temperatures of different locations related to the mean partial pressure of CO^. -23 -25 H-^-1-f-1-^-^-1-h 0 0,2 0,4 0,6 0,8 -H-^-h- 1,4 1,6 1 1,2 pC02 (%) Belca-b o Podkraj-b x Obli vrh-b 1,8 partial pressure of soil CO^ for all types of vegetation covers sampled. It can be seen that samples from spruce forest show no significant deviation from the properties of CO^ in beech forest, while more considerable differences are observed in soils under grass covers. The samples taken from soil underlying grass near Col have a more negative isotope composition of biogene soil CO, of about -24 %c, while the samples from Grgar show a distinctly more positive isotope composition of the biogene component, about -21.5 %o. This difference in the carbon isotope composition of soil CO^ under the same type of vegetation is attributed to the different altitude and the different mean soil temperatures. The sampling point near Grgar is situated at an altitude of 390 m in rather Mediterranean climatic conditions, and that near Col is at 620 m above the sea level, where chmatic conditions are much harsher. The prevailing types of grass in warmer areas are those with the Hatch-Slack (C4) cycle which generates more positive 5"C values in plant tissues. On the other hand, the Calvin cycle (C3) prevails in grass from colder areas, giving more negative 5"C values of plant tissues (GERLING 1984). Thus it can be concluded that in the area of the Trnovsko-Banjška Planota, CO^ with varying isotopic properties enters the ground: the most negative values of biogenic carbon isotope composition are to be expected in CO^ from soil underlying grass and from higher-lying and colder areas (about -24 %o), soils underlying beech or spruce forests tend to have somewhat more enriched 5"C values of biogenic carbon (about -23 %o), and grass from warmer and lower areas give the most positive initial isotopic signal (about -21.5 %o). -22 -23 -24 0,8 1 1,2 1,4 1,6 1,8 pC02 (%) □ Grgar-g v Col-g Beica-b Podkraj-b Obli vrh-b x Podkraj-s 5.2.1.2 Carbon isotope composition of carbonate rocks The research area is for the most part composed of Mesozoic limestones and dolomites and flysch rocks. In all, 24 rock samples were taken for isotopic analyses. Measurements of carbon isotope composition of observed samples showed a range in 5"C values between 0 and +4 %o (Fig. 5.31). It can be observed on the graph that dolomites have mostly more enriched and 5'®0 values compared to limestones or flysch. o CO □ o 1 D -2 O 180-karb. limestone dolomite flysch Fig. 5.31: Carbon and oxygen isotope composition of the rocks on investigated area. 5.2.1.3. Isotope composition of the total dissolved inorganic carbon in the outflow The yearly curve of 5"C values in the outflow shows certain seasonal variability (Fig. 5.32 to 5.39). General characteristics are most clearly evident from the spring Hotešk (Fig. 5.32). The Figure shows that the most depleted S'-^'C values of DIG in the outflow are detected in late fall, usually in November. Then a rather rapid change towards more positive 5"C values takes HOTEŠK -16 Jan-93 Apr-93 Jul-93 Oct-93 Jan-94 Apr-94 Jul-94 Oct-94 Jan-95 Apr-95 Jul Date 13C-DIC © Alkalinity Fig. 5.32: Carbon isotope composition and alkalinity of Hotešk spring. place until spring, and a gradual depletion in values follows over the entire year to the late fall minimum. Such seasonal isotopic variability can be explained in the following way: Due to the correlation between pedotemperature and partial pressure of soil CO^, the gradual increase of soil temperature over the year results in an increased production of soil CO^. In the fall, soil temperatures are the highest, organic matter enters the ground due to lost leaves, which results in a maximum in soil CO^ partial pressure. A higher content of the biogenic component in soil CO^ is reflected also in the carbon isotope composition, which reaches its most depleted S'^'C values, and in the DIC isotope composition in the outflow, where the most depleted 5"C values were also detected. Lower soil temperatures in winter considerably hinder the processes of soil COj generation, consequently the soil atmosphere contains a higher percentage of atmospheric CO^ with more positive 5"C values. When the water from melted snow penetrates the soil in spring recharge area, a larger quantity of atmospheric carbon enters the system, resulting in enriched 6"C values in the outflow. HUBEU -13 -14 -15 -16 Jan-93 Apr-93 Jul-93 Oct-93 Jan-94 Apr-94 Jiil-94 Oct-94 Jan-95 Apr-96 Jul-95 Date -i^ 13C-DIC o Alkalinity rS -4,5 -4 -3,5 CT -3 P -2,5 .C -2 "S -1,5 < -1 -0,5 -0 Fig. 5.33: Carbon isotope composition and alkalinity of Hubelj spring. A comparison of carbon isotope composition variation amplitudes in the outflow is also very interesting. The springs Vipava (Fig. 5.34), Podroteja (Fig. 5.35), Hotešk (Fig. 5.32) and Prelesje (Fig. 5.38) show a fairly similar range of yearly 5"C values at about 2.5 %o . Yearly fluctuations of the carbon isotope signal are much larger in-the Hubelj, where the amplitude of 8"C values from spring 1993 to spring 1994 was over 6 %o (Fig. 5.33). This indicates that Hubelj has a higher aquifer water exchange rate, resulting in a less pronounced dampening of the isotope signal. A fairly large range in the isotope signal was measured also in samples from Mrzlek spring, however, the large variability can in this case be attributed to a stronger influence of the Soča river and the fact that Soča river water is more enriched in the heavier carbon isotope (Fig. 5.36). During surface flow, isotope exchange between the DIC and atmospheric carbon takes place. Because atmospheric carbon is enriched in the heavier carbon isotope, a change towards the more positive values can be expected. Most of the samples for the 7"' SWT project were taken from springs, yet some of them were also taken from lower course of surface flow, e.g. from streams on flysch rocks and from river Soča. In order to evaluate the scope of isotope exchange, sampling was carried out along the Bela stream above VIPAVA Jan-93 Apr-93 Jui-93 Oct-93 Jan-94 Apr-94 Jul-94 Oct-94 Jan-95 Apr-95 Jul-95 Date 13C-DIC o Alkalinity Fig._ 5.34: Carbon isotope composition and alkalinity of Vipava spring. PODROTEJA -6 -7 -- -9 --£-10 -- o 1, I CO -12 + -13 -- 1 -14 ^ -15 - -16 Jan-93 -4,5 -4 __^ CT -3