Isotopic composition of precipitation at the station Portorož , Slovenia – period 2007 – 2010

The stable isotopic composition of oxygen and hydrogen (δ18O and δ2H) and the tritium activity (A) were monitored in precipitation at synoptic station Portorož (Slovenia) during the period 2007–2010. Monthly and yearly isotope variations are discussed and compared with those observed over the period 2001–2006 and with the basic meteorological parameters. The mean values for δ18O and δ2H, weighted by precipitation, are –6.28 ‰ and –41.6 ‰, and these values are 0.35 ‰ and 1.6 ‰ higher but not significantly different than for the period 2001–2006. The reduced major axis (RMA) local meteoric water line (LMWLRMA) for the period 2007–2010 is δ 2H = (8.14 ± 0.25)×δ18O + (8.28 ± 1.64), while the precipitation weighted least square regression (PWLSR) results in LMWLPWLSR δ 2H = (7.87 ± 0.28)×δ18O + (7.97 ± 1.87). The deuterium excess (δ) weighted mean value is 8.6 ‰ and is 1.2 ‰ lower than in 2001–2006, while the temperature coefficient of δ18O is 0.21 ‰/°C and is for 0.02 ‰/°C higher than for the previous period. The mean Mediterranean precipitation index (MI) for the period 2007–2010 is 2.3. Lower values of MI and deuterium excess than in the preceding period indicate stronger continental climatic character during observation period, however the differences are not statistically significant. The weighted mean tritium activity is 6.4 TU, which is 0.5 TU lower but not significantly different than in 2001–2006.


Introduction
Since initiation of the Global Network of Isotopes in Precipitation (GNIP) by the International Atomic Energy Agency (IAEA) and the World Meteorological Organisation (WMO) in 1958 the importance and need of a systematic collection of data on the isotopic composition, i.e. stable isotopes of oxygen (expressed as δ 18 O) and hydrogen (expressed as δ 2 H) and radioactive hydrogen isotope (tritium, 3 H), of precipitation across the globe to determine temporal and spatial variations in isotope ratios in precipitation, is steadily increasing. Initially GNIP was focused on monitoring atmospheric thermonuclear test fallout through levels of radioactive tritium and, after 1970, became an important observation network of stable oxygen and hydrogen isotope composition data for hydrologic investigations of water resources (Internet 1). The past decade has experienced increasing demand for accurate spatial and temporal predictions of point, regional, and continental-scale δ 18 O and δ 2 H values in precipitation, especially for some regions where little or no GNIP data existed (Terzer et al., 2013). The main reason for steadily increasing application of water isotopes remains their importance in hydrologic and hydrogeologic investigations of water resources, in verification and improvement of atmospheric circulation models and in investigation of regional, global and temporal climates. In addition, it was revealed that the δ 18 O and δ 2 H values of some plant, animal, and human tissues closely mirrored isotopic patterns of precipitation (Hobson & Wassenaar, 1997;Bowen et al., 2005) and therefore use of stable water isotopes expanded also in ecological, wildlife, food source traceability, and forensic sciences (Terzer et al., 2013).
The only radioactive constituent of water molecule is tritium, 3 H, with the half-life of 12.32 yr (Lucas and unterweger, 2000; Internet 2). It is produced naturally in the upper atmosphere by reaction of cosmic-rays neutron with atmospheric nitrogen (Internet 2). The tritium with atmospheric oxygen subsequently forms tritiated water ( 1 H 3 HO) molecule that in turn reaches the Earth's surface through precipitation. However, most of the tritium present in the atmosphere and hydrosphere is of anthropogenic origin, i.e., tritium was released in atmospheric thermonuclear tests during 1960's, reaching in 1963 several orders of magnitudes higher tritium activity concentration in the atmosphere than the natural levels. After the test ban treaty in 1963, atmospheric tritium activity has been decreasing almost exponentially in the following decades, while during last 10-15 years the mean tritium activity has not been significantly changed (Krajcar BronIć et al., 2006;Vreča et al., 2014; Internet 2). Nowadays, tritium has reached in precipitation levels similar to those occurring before the thermonuclear tests, about 5 TU in central Europe (Internet 2). These low and constant concentrations limit the use of tritium in hydrogeological studies. However, tritium can be used to investigate atmospheric vapour migration under different meteorological conditions, especially if the sources of water vapour differ (KIgoshI & Yoneda, 1970;unnIKrIshnan warIer et al., 2013). Due to the water vapour exchange between the atmosphere and the sea (ocean) surface, coastal and oceanic stations show considerably lower tritium activity than the continental stations because the moisture evaporating from the sea/ocean is low in tritium content (ehhalt, 1971;Vreča et al., 2006). With the more-or-less natural tritium levels in modern precipitation, the technogenic emissions of tritium (from nuclear facilities, or from manufacturing or disposal of luminous consumer products, planned fusion demo facilities) are becoming more and more visible (hebert, 1990;rozansKI et al., 1991;MatsuMoto et al., 2013), and the knowledge of natural tritium distribution remains important from the environmental pollution point of view (thoMpson et al., 2015;ueda et al., 2015).
Regular monitoring of the oxygen and hydrogen isotopic composition of precipitation in the coastal area of SW Slovenia is important not only for studying the isotopic patterns of local meteorological conditions, but also for improvement of the knowledge of the isotopic input signal needed for hydrogeological studies of the large karstic springs used for water supply in the area (Vreča et al., 2007) and other applications. Therefore, monitoring of the isotopic composition of precipitation in Portorož (SW Slovenia) has been performed by the Jožef Stefan Institute (JSI) since 2000 (Vreča et al., 2005(Vreča et al., , 2006(Vreča et al., , 2007(Vreča et al., , 2011. To begin with, monitoring was performed in cooperation with the Slovenian Environment Agency (ARSO), the Ruđer Bošković Institute (RBI; Zagreb, Croatia) and the IAEA (Vreča et al. 2005(Vreča et al. , 2006. Since 2004 the JSI has also cooperated with Joanneum Research (JR; Graz, Austria). Details on the isotope monitoring until 2006, together with data evaluation, have been reported in Vreča et al. (2005Vreča et al. ( , 2006Vreča et al. ( , 2007Vreča et al. ( , 2011. The isotope data obtained until 2006 for Portorož (Vreča et al., 2005(Vreča et al., , 2006(Vreča et al., , 2007(Vreča et al., , 2011 are also available in the GNIP database. The data were used in different hydrogeochemical investigations of aquifers in Slovenia (brenčIč, 2009;Mezga, 2014;Mezga et al., 2014;VerboVšeK & Kanduč, 2015), Croatia (MandIć et al., 2008) andin Italy (cucchI et al., 2008), as well as in investigations of precipitation (lIotta et al., 2008;VodIla et al., 2011;MIchelInI, 2013;Vreča et al., 2014;BrenčIč et al., 2015, KoVačIč, 2015, moisture recycling in Alpine regions (FroehlIch et al., 2008) and identification and characterisation of nitrate pollution sources in Marano lagoon in Italy (Saccon et al., 2013).
The main purpose of this paper is to present new results concerning the isotopic composition of precipitation at Portorož for the period 2007-2010 and to compare them with those for the period 2001-2006(Vreča et al., 2011.

Sampling
Monthly composite precipitation has been sampled since October 2000 at the synoptic meteorological station Portorož (45°28'N, 13°37'E; 2 m a.s.l.) which is located at the airport and is part of the Slovene national meteorological network maintained by the ARSO (Internet 3). The station is included in the GNIP under the station name Portorož and has the GNIP code 1410500.
Water samples were collected by the staff of the meteorological station from standard precipitation gauge three times daily. After measuring the amount of precipitation the sample was poured into a 5-litre plastic bottle with a well tight cap to avoid evaporation and kept in a dark place. After bringing the samples to the laboratory the impurities (e.g. dust, particles) were removed from the sample by filtration (Whatman Grade 589, Black Ribbon) before taking aliquots for different isotope analyses. Then a 50 mL glass bottle was filled for the stable isotope analysis of oxygen and hydrogen and the rest of the water was transferred to 0.5 or 1 L high density polyethylene (HDPE) bottles for tritium analysis. During the sampling period April 2007 was very dry with only 2.2 mm precipitation which was not sufficient for isotope analysis.
Meteorological data (amount of precipitation and air temperature) are available from the ARSO (Internet 3).

Stable isotope analysis
The oxygen isotopic composition (δ 18 O) was measured by means of the water-CO 2 equilibration technique (epsteIn & MaYeda, 1953) on a dual inlet isotope-ratio mass spectrometer Finnigan DELTA plus by means of the fully automated equilibration technique at JR until February 2007 (see also Vreča et al., 2011). Since then, a continuous flow isotope-ratio mass spectrometer IsoPrime (GV Instruments, UK) coupled to an automatic water-CO 2 equilibration system MultiFlow was used at the JSI. The isotopic composition of hydrogen was determined on a continuous flow Finnigan DELTA plus XP mass spectrometer with a HEKAtech high-temperature oven, by reduction of water over hot chromium (MorrIson et al., 2001) at JR. Results are reported as δ values in per mill (‰) relative to the Vienna-SMOW standard (Coplen et al., 2002). All measurements were carried out together with laboratory standards that were calibrated periodically against international standards, as recommended by the IAEA. Measurement precision was better than ±0.1 ‰ for δ 18 O and ±1 ‰ for δ 2 H. Tritium activity in three samples (February, March and April 2010) was determined by the Group for radiochemistry at the Department of Environmental Sciences of the JSI following electrolytic enrichment by liquid scintillation counting (LSC-EE) on a Tri Carb 3170 TR/SL Liquid Scintillation Counter (LSC, Canberra Packard) in accordance with method accredited by Slovenian Accreditation since 2009 (accreditation certificate LP-090). The LOD was 2.9 TU.

Data reduction
The approach of data reduction described by Vreča et al. (2008Vreča et al. ( , 2011Vreča et al. ( , 2014 where P X-XI is the sum of precipitation in October and November (in mm), P V-VI is the sum of precipitation in May and June (in mm) and P year is the annual precipitation (in mm).
Furthermore, mean values weighted by the amount of precipitation were calculated from all monthly data, and then summed over all collected samples, per month, over each season: winter (December, January, February), spring (March, April, May), summer (June, July, August) and autumn (September, October, November), and per year. The minimum required number of data fulfilled the requirement of eight monthly measured samples per year and more than 70 % of total precipitation amount collected per year (IAEA, 1992).
Linear correlations between δ 2 H and δ 18 O were calculated by methods usually applied in stable isotope studies -the ordinary least squares regression (OLSF) and the reduced major axis (RMA) regression (IAEA, 1992;hughes & crawFord, 2012). In addition, we applied a precipitation weighted least square regression (PWLSR) method, introduced by hughes & crawFord in 2012, which takes into account the precipitation amount in particular month. The same approach was used by Vreča et al. (2014) for the 2007-2010 isotope record at Ljubljana. The lines are defined as local meteoric water lines (LMWL OLSF , LMWL RMA and LMWL PWLSR ).
Finally, oxygen-temperature correlation was calculated using air temperature data provided by ARSO from station Portorož for all monthly data and for the mean monthly data from the studied period.

Meteorological data: Precipitation and temperature
Temporal variations in precipitation and air temperature for the period 2007-2010 are presented in Figure 1. The lowest precipitation was recorded in April 2007 (2 mm) and the highest in September 2010 (249 mm) (Figure 1), which was much greater than the mean September precipitation (118 mm) for the reference period 1981-2010 (Internet 3). In the same months during the period 2007-2010 were minimum and maximum records observed also for Ljubljana (Vreča et al., 2014). Besides in 2010, very high precipitation was recorded also in September 2007 (234 mm, Figure 1), although the total amount of precipitation in 2007 was below the long-term mean (  Vreča et al., 2011) and also higher than in 1981-2010 (968 mm, Internet 3). In 2010 a record amount of precipitation was registered (1,394 mm), which is 40 % more than on the long-term average (cegnar, 2011) (Table 1). This value is much lower than the annual mean during the periods 2001-2006 (5.5), 1981-2010 (5.9) and 1971-2000 (6.1) (Table 1). The mean air temperature for the whole period was 13.9 °C, on average 0.3 °C and 0.7 °C higher than that for 2001-2006 and 1981-2010 (Table 1) Long-term mean monthly temperatures for periods 1971-2000 and 1981-2010 show winter minimum in January and summer maximum in July (Internet 3). The difference between maximum and minimum air temperature, which may be taken as a measure of the continentality of the climate (i.e. the larger the difference, the more continental the type of climate) was 18.4 °C and 18.6 °C for 1971-2000 and 1981-2010. During the 2007-2010 the maximum monthly temperature was observed always in July while the minimum was recorded in December (2007), February (2008) or January (2009 and 2010). The difference between maximum and minimum varied from 17.9 °C to 20.4 °C, with a mean of 19.2 °C, and indicates more continental climate at Portorož in investigated period than in the preceding long-term climatological periods. Therefore, both indices of continentality, the MI that takes into account seasonal distribution of precipitation, and the range of temperatures, indicate higher continental influences on the climate in Portorož in the investigated period. These facts, together with the increase in mean annual air temperatures and general decrease in amount of precipitation may indicate modern climatic changes in the area but have to be investigated into more detail in future.
Stable isotope data (δ 18 O, δ 2 H and d) Results of monthly isotopic composition of precipitation parameters, δ 18 O, δ 2 H and d, are summarized in Table 2  Monthly variations of deuterium excess (d) are presented in Figure 3c. The lowest value was obtained for the sample collected in April 2010 (-0.7 ‰, Table 2). The highest d values were observed in July 2010 (14.9 ‰) and November 2010 (14.8 ‰). The mean value is 7.4 ‰ (n = 47) ( Table 2) and is 1.6 ‰ lower than the mean value of 9.0 ‰ from the period 2001-2006 (Vreča et al., 2011). Most d values (72 %, i.e. 34 out of total 47) range between 5 and 15 ‰. No value higher than 15 ‰, typical for Mediterranean-derived precipitation (Cruz-San et al., 1992;rozansKI et al., 1993), was determined indicating different climate conditions as in the period 2001-2006 when more than 10 % of d values exceeded 15 ‰ (Vreča et al., 2011). It is known, that d values can alter as the air mass moves inland, due to secondary processes such as evaporation from an open surface water body which returns moisture to the air (craIg & gordon, 1965;MerlIVat & jouzel, 1979;gat et al., 1994;gat, 1996). In addition, d values can change as precipitation falls through the atmosphere (gat, 1996; araguas-araguas et al., 2000; peng et al., 2004) or as the precipitation sample sits in the precipitation collector (harVeY, 2005). It was estimated that the initial d values should not be less than 3 ‰ and that lower values should be used with caution unless the source of their evaporative enrichment is known (harVeY, 2005). A high proportion (15 %) of d values was <3 ‰ (Table 2). These values probably indicate secondary evaporation processes and correspond mainly to months with low precipitation, related to local stormy and windy events in the area or to months with light rainy events, therefore we did not exclude them from evaluation. It is interesting that high d values (>13 ‰) were obtained for July 2010 and also July 2008. For both months very intense thunderstorms were characteristic with prevailing south and southwest winds (Internet 3).
Monthly mean δ 18 O, δ 2 H and d values, weighted by precipitation amount are summarized in Table  3 and presented in Figures 4 and 5a, where they are compared with the 2001-2006 values (Vreča et al., 2011). The higher δ 18 O and δ 2 H values in spring and summer months, due to higher air temperatures, constitute a typical seasonal variation which is less pronounced than in Ljubljana (Vreča et al., 2014). Seasonal variations of δ 18 O and δ 2 H of 5.47 ‰ and 43.1 ‰ (Table 3), respectively, were observed and are smaller than for continental stations like Ljubljana (8.81 ‰ and 53.7 ‰, respectively, Vreča et al., 2014). The lowest δ 18 O and δ 2 H values are observed in January (Table 3) (Table 5), while the weighted mean d values show an overall decrease by 1.2 ‰. However, changes are not statistically significant different. The maximum annual weighted mean δ 18 O and δ 2 H values were observed in 2008, when the smallest difference between maximum and minimum air temperature (i.e. 17.9 °C) was recorded and precipitation was higher during the spring-summer season. Consequently spring-summer precipitation with higher isotopic composition contributed more to the annual weighted mean. Similar pattern was observed in Ljubljana (Vreča et al., 2014). The lowest annual weighted mean δ 18 O and δ 2 H values were observed in 2010 and can be attributed to the lowest mean annual air temperature (13.1 °C) during 2007-2010, lower temperature than the long-term records (Table 1), a cold January and December and the highest precipitation (1,394 mm). The highest annual weighted mean d value was observed in 2010 (Table 5) and is related mainly to the Mediterranean-derived precipitation in the autumn ( Table 2). The lowest annual weighted mean d value was observed for 2008.

Oxygen -temperature correlation
The linear correlation between δ 18 O in monthly samples and mean monthly air temperature at Portorož, T, for the period 2007-2010 ( Figure 6) is: 21×T -8.94; r = 0.61, n = 47 (6) The linear correlation between monthly weighted δ 18 O values and mean monthly air temperature at Portorož, T monthly , for the period 2007-2010 ( Figure 6) is: δ 18 O = 0.20×T monthly -9.03; r = 0.86; n = 12 (7) The correlations obtained differ slightly in their intercept values and have the higher slope as that for the records 2001-2003and 2001-2006(Vreča et al., 2006. The slope of about 0.20 ‰/°C is between those typical of continental and maritime stations (RozansKI et al., 1993) but indicates a more continental climate influence than during the period 2001-2003 (Vreča et al., 2006), lower than the one estimated for the same period for Ljubljana (Vreča et al., 2014) and close to an average isotope temperature gradient of 0.25 ‰/°C for groundwater in Slovenia estimated by Mezga et al. (2014).

Tritium activity
Results of monthly tritium activity (A) of precipitation are summarized in Table 2. We present also two numerical results below the LOD (September and November 2007) for the purpose of statistical analysis. Amount of precipitation weighted mean monthly, seasonal and annual values are summarized in Tables 3,  4  2014). In May 2009 high air temperatures were recorded, precipitation was lower than the longterm mean and most precipitation fell during two thunderstorms (Internet 3). Therefore, the relatively high tritium activity may indicate the influence of continental effect and re-evaporation of tritium from the continental land surfaces. The minimum (1.8 TU, <LOD) was recorded in September 2007. Seasonal fluctuations typical of stations of the Northern hemisphere (RozansKI et al., 1991) are observed for 2007 and 2010, with lower tritium activities in autumn and winter and higher ones in spring and summer ( Figure  3d), while in 2008 we observed maximum in October and in 2009 three peaks were recorded in January, May and October. Similar patterns of 3 H were observed also for Ljubljana (Vreča et al., 2014) and Zagreb (Krajcar bronIć, unpublished data). The seasonality is evident also by about 3 TU difference between autumn-winter and spring-summer precipitation (Table 4). Monthly weighted mean A values are presented in Table 3 and in Figure 5b. Maximum value was observed in June (9.7 TU) and the minimum in September (4.0 TU) and December (4.3 TU) (Table 3), which is slightly different than in the period 2001-2006 ( Figure 5b). The differences are related to different climate conditions (see Meteorological data: Precipitation and temperature) during periods 2001-2006 and 2007-2010. The mean tritium activity in the studied period is 7.2 TU (Table 2) and is the same as in the period 2001-2006(Vreča et al., 2011. The weighted mean annual A is 6.4 TU and is 0.5 TU lower than for the 2001-2006 record (Table 5) but not significantly different.
The weighted mean tritium activity at the Portorož station (6.4 TU) is 2.1-2.6 TU lower than the mean tritium activity for the same period at the continental stations Ljubljana and Zagreb (8.5 TU and 9.0 TU, Vreča et al., 2014) and is related to the coastal position of the station. Similar difference (1.7-2.5 TU) between Portorož and continental stations was also observed for a short-term period 2001-2003(Vreča et al., 2006. A good correlation between mean monthly deuterium excess (d) and tritium activity (A) has been obtained with a slope of -0.35 ‰/TU (r = 0.52, n = 12) indicating that during the period 2007-2010 higher d values were associated with lower A values ( Table 3) that correspond to precipitation that has been formed by moisture evaporated from the Adriatic Sea (FroehlIch et al., 2008).

Conclusions
The results of the water isotopic composition (δ 18 O, δ 2 H, d and A) of precipitation collected at Portorož in the period 2007-2010 are presented and compared with the data from the period 2001-2006. Isotope data corroborate conclusions derived from the analysis of the meteorological data about the change in climate setting in the Portorož area: more positive δ 18 O and δ 2 H mean values reflect an increase in temperature, while lower deuterium excess (d) values together with the lower MI values indicate a change in air mass trajectory towards more continental influence. However, the time period of isotope data records is relatively short and is influenced by year-toyear differences in weather conditions.
The results presented are important for further scientific and practical applications in hydrology and hydrogeology, and in climatology. Taking into account the characteristic geographic diversity of Slovenia, which influences considerably the climate and the isotopic composition of precipitation, a more detailed investigation of the complete 10-year isotope data set (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) for Portorož needs to be performed. The LMWLs obtained for 10-year isotope record will be useful above all in investigating the karst hydrological systems in Slovenia that are fed directly by precipitation. Finally, a mathematical model that enables estimation of δ 18 O and δ 2 H values for the precipitation generated at distinctive atmospheric circulation patterns proposed by BrenčIč et al. (2015) will be applied to separate data with different air mass origins.