Natural radioactivity of groundwater in Serbia

Activity concentrations of radionuclides 40K, 228Ra, 226Ra, 238U and Th232 and gross alpha and beta activities were analyzed in more than 100 samples of groundwater in Serbia. The highest gross alpha activity was recorded at 1.33 Bq/L (average 0.12 Bq/L), while the highest beta activity was 5.43 Bq/L (average 0.68 Bq/L). The potassium isotope 40K exhibited the highest active concentration (2.6 Bq/L) and was the largest contributor to the gross natural beta activity. Among the analyzed samples, 28 were found to have elevated beta activity concentrations, of which five samples also measured elevated alpha activity. All the groundwater samples that exhibited elevated radioactivity were of the HCO3-Na type and were genetically associated with granitic rocks. Their TDS levels and CO2 gas concentrations were also elevated.


Introduction
The discovery of radioactivity and its impacts was a turning point in the evolution of geological sciences and largely affected the development of geochemistry, including isotope geochemistry, and geochronology that plays an important role in terrestrial geology.A special discipline is environmental geology that studies the impact of human activity on the environment, as well as that of natural radioisotopes (OMALJEV & ANTO-NOVIĆ 1996).Radioactivity is an important parameter of geophysical measurements in gamma prospecting, logging of natural gamma radioactivity (gamma logging) and spectral gamma logging.
Prior to the discovery of radioactivity, it was believed that the main cause and source of heat flow within the Earth was cooling of its previously heated body (MAROVIĆ 2005).Apart from the heat content of the Earth immediately after formation, the radiogenic de-cay of the unstable isotopes of uranium ( 238 U, 235 U), thorium ( 232 Th), and potassium ( 40 K) provides the largest internal source of heat (CLAUSER 2011;HAZEN et al. 2009).The release of heat was likely more intense in the Earth's distant past than today, because the amount of radioactive elements constantly decreases as a result of decay (MAROVIĆ 2005).Uranium, thorium and 40 K, are no longer trapped in the planet's core, they migrated to the Earth's surface in the early stages because of the crystalline and chemical properties of the compounds into which they were incorporated (ANTONOVIĆ 1989).cles, helium cores or ions comprised of two protons and two neutrons, make up gross alpha activity.Gross beta activity is comprised of negatively charged particles (electrons) or, rarely, positively charged particles (positrons).Nearly all radionuclides are beta emitters, except for some of the heaviest cores, while the major β emitting radionuclides are 210 Pb and 228 Ra as well as 40 K (TURHAN 2013).Beta emitters are generally also gamma emitters, with the exception of some pure beta (β -) emitters such as 14 C, 45 Ca, 63 Ni, 90 Sr, 106 Rh and 147 Pm (ICRP 1991).
Environmental radiation originates from a number of naturally occurring and man-made sources.Radioactive materials occur naturally anywhere in the environment (for example uranium, thorium and potassium) (WHO 1993).
Elements 84 through 92 on the periodic table have no stable isotopes at all and can be grouped into one of three radioactive series or families.These are the uranium-radium, uranium-actinium and thorium series.Natural radioactive series are a result of decay of three radioisotopes: 235 U, 238 U and 232 Th, ending with isotopes 206 Pb, 208 Pb, 207 Pb and 209 Bi (USGS, 1998).The radiation generated though the decay of these naturally-occurring radioactive isotopes is the natural radioactivity found in our environment (ICRP 1991).
The radioactivity in groundwater comes mainly from radionuclides of the natural decay chains 238 U and 232 Th, and 40 K in soil and bedrock.Some radionuclides can migrate easily in water, depending on the mineralogical and geochemical composition of the soil and rock, redox conditions and the residence time of ground water in the soil and bedrock, as result of the reaction of the ground water with soil and bedrock (VESTERBACKA 2007).
The territory of Serbia is rich in thermal and mineral waters.Because of the volcanic and plutonic activity in the geological past, most of the mineral or thermal water originates from these rocks (PROTIĆ 1995).Groundwater occurrences mark different regional geological-structural features and the largest number of mineral groundwater is related to the granite intrusions and volcanic rocks (MARINKOVIĆ et al. 2013).A major portion of uranium and thorium in igneous rocks is concentrated in accessory minerals such as zircon, sphene and apatite.Other highly radioactive minerals (e.g.monazite, alanite, pyrochlore, xenotime and thorite) are found only occasionally.In general, uranium and thorium concentrations in igneous rocks increase with increasing rock acidity.The major radioactive minerals are shown in Table 1.
Numerous radioactive elements and isotopes occur in natural waters: uranium, radium, radon (descendants of uranium and thorium), while thorium is virtually absent because of its low mobility in geochemical systems where groundwater reservoirs develop (DANGIĆ 1995).

Sampling
During the course of the study: Radioactivity of Groundwater in the Republic of Serbia (PAPIĆ et al. 2008(PAPIĆ et al. -2011)), gross alpha and beta activities and the radionuclides ( 40 K, 228 Ra, 238 U, 226 Ra and 232 Th) in more than 100 groundwater samples were analyzed and represented in this paper.Sampling was conducted applying standard closed-source methods and suitable polyethylene containers were filled with 10-15 L of water.

Determination of gross alpha/beta activity concentrations
The gross alpha and beta activities of groundwater samples and gamma spectrometry measurements of natural radionuclides ( 238 U, 228 Ra, 226 Ra, 40 K, 232 Th) have been carried out in the Institute of Occupational and Radiological Health "Dr Dragomir Karajović".The gross alpha/beta analyses were performed according to a routine procedure outlined in ISO9696 and ISO9697 (ISO 9696, 1992;ISO 9697, 1992).
The gross alpha/beta activity determination method was based on the evaporation of 3 L of the water sample under UV lamps and calcining at 550 °C to a constant mass.The mineral residue was used to prepare a thin-film test sample.Suitable-geometry measurements were made on a low level proportional counter Thermo-Eberline FHT 770T, featuring a 21% efficiency for alpha radiation and 33% for beta radiation.The instrument was calibrated using standard sources.The sample measurement time was 3600 s.The measurement results were used to determine gross alpha and beta activities of the water samples in Bq/L.

Determination of radionuclide activity concentrations
The method for the determination of gamma-emitter radionuclide activity was based on the evaporation of 8-10 L of the water sample to a volume of 200 mL and quantitative transfers to polyethylene vessels.The vessels were closed and left to stand for about 40 days, to prevent radon emanation and establish a radioactive balance between members of the natural radioactive series of 238 U.The sample was then measured on a gamma spectrometer with a HP Ge detector, whose relative efficiency was 23%.The detector was calibrated using standard radioactive reference material, MIX-OMH-F.The duration of sample measurements was 1.3 days, depending on the concentrations present.The gamma radiation spectrum was analyzed to determine the concentrations of specific radionuclides ( 40 K, 228 Ra, 226 Ra, 238 U and 232 Th) in Bq/L.
Chemical analyses were performed at the Hydrochemistry Lab of the University of Belgrade Faculty of Mining and Geology to define hydrogeochemical conditions and determine groundwater types.The following parameters were analyzed: sodium, potassium, calcium, magnesium, chlorides, hydrocarbonates, carbonates, sulfates, TDS, hardness, pH, specific conductivity and CO 2 .
The results were statistically processed using IBM software SPSS 17.0 (Inc SPSS 2009) and graphically interpreted by ESRI ArcGIS 9.3 (ESRI 2012).

Results and discussion
Basic statistical processing, including active concentration ranges, median and standard deviation (Table 2), was conducted on the basis of 125 analyses of groundwater samples for gross alpha and beta activities and a certain number of active concentrations of radionuclides 40 K, 228 Ra, 238 U, 226 Ra and 232 Th.
The World Health Organization, following the recommendations of the International Commission on Radiological Protection (ICRP), has studied the radiological aspects of drinking water quality and recommended reference values for α-unstable radionuclides in drinking water of 0.5 Bq/L, and β-unstable radionuclides of 1 Bq/L (ICRP 1991; WHO 1993; OFFICIAL GAZETTE OF RS 2011).When the average and median values were compared to regulated MPC (maximum permissible concentration) levels, the groundwater samples did not exhibit elevated radioactivity.However, when the maximum values of the individual parameters were evaluated, gross alpha and beta activities and the concentrations of 228 Ra and 226 Ra were found to exceed permissible concentrations.Histograms of gross alpha and beta activities and active concentrations of the tested radionuclides were produced to determine the cumulative distributions of the analyzed parameters (Fig. 1).
The histograms of gross alpha and beta activities and active concentrations of unstable radionuclides showed that all distributions exhibited positive skewness.Only five samples measured gross alpha activity in excess of the MPC of 0.5 Bq/L, while 28 samples Table 2. Maximum permissible concentration (MPC), average, minimum and maximum activity concentrations of radionuclides, standard deviation and median values and gross alpha and beta activities in groundwater samples collected across Serbia.were above the beta activity threshold of 1Bq/L.Radionuclides 226 Ra, 228 Ra and 232 Th exceeded MPC levels only in a few isolated cases, deemed to be extremes or outliers for statistical interpretation purposes and therefore disregarded.There is no regulated MPC level for 40 K in Serbian regulations;the average value of the samples is 0.56 Bq/L.The distribution of 40 K was found to be similar to that of gross beta activity and this radionuclide was the greatest contributor to gross beta activity, corroborated by high coefficient of correlation (R 2 = 0.844) between gross beta activity and the potassium isotope 40 K.The concentration of 40 K was found to be consistent with the geochemistry of potassium, which is one of the main elements of magma (MITTLEFELDHT 1999).
The most frequently encountered radioactive potassium minerals are orthoclase and microcline feldspars (KAlSi 3 O 8 ) and muscovite (Na 2 KAl(SiO 4 ) 3 ), which are the main minerals of acidic igneous rocks and pegmatites.Alunite (KAl 3 (SO 4 ) 2 (OH) 6 ) occurs though alteration in acidic volcanites, while sylvite and carnallite (KCl, MgCl 2 •6H 2 O) are deposits found in salt se-diments (ANTONOVIĆ 1989). 40K also exhibited a high coefficient of correlation (R 2 = 0.43) with gross alpha activity but this result was not unexpected given that the TDS levels of the tested groundwater samples were up to 6650 mg/L.These groundwaters were formed in deep hydrogeological structures.Figure 2 shows that gross active concentrations of alpha/beta emissions increased with increasing TDS.
Both correlations between α/β activity and the concentrations of TDS (Total dissolved solids) with a coefficient of R 2 =0.36 for gross alpha activity (Fig. 2A) and R 2 =0.42 for gross beta activity (Fig. 2B).
The samples were divided into two groups to better understand the hydrogeochemical conditions in which the groundwaters that exhibited elevated radioactivity were formed.One group was comprised of 28 groundwater samples whose active alpha or beta concentrations exceeded MPC, and the other group was made up of 97 samples that measured below MPC.Select chemical composition parameters are presented in box-plots to show the general differences between these two groups (Fig. 3).
All the tested samples that exhibited elevated beta activity concentrations belonged to the Na-HCO 3 type of groundwater, suggesting that the groundwater traced to an aquifer in granitoid rocks with elevated concentrations of dissolved solids and CO 2 .The samples whose radioactivity was elevated measured CO 2 concentrations up to 1510 mg/L.The water samples whose mineral content (TDS) was 500 mg/L, typical of groundwater that did not show elevated radioactivity, were dominated by Ca 2+ and Mg 2+ ions, while with increasing TDS these ions were replaced with the Na + ion.
Gas CO 2 in groundwater may be derived from a variety of sources, including metamorphic devolatilisation, magmatic degassing, oxidation of organic mat-Natural radioactivity of groundwater in Serbia 67 Fig. 2. A, Correlation between gross alpha activity and dissolved minerals.B, Correlation between gross beta activity and dissolved minerals.ter and interaction of water with sedimentary carbonates (CARTWRIGHT et al. 2001).Research conducted to date has shown that the most important radioactivity anomalies are found in calc-alkaline igneous rocks (JELENKOVIĆ 1991), which are rich in alkaline oxides and metals and make up most of the continental crust.Past studies of carbonated groundwater have shown that groundwater with elevated CO 2 concentrations tends to occur in areas of large tectonic faults, in zones of Tertiary and Quaternary volcanism and within regional metamorphism.These formation must have been related to the part of the lithosphere in which the CO 2 gas was generated.(STEVENS et al. 2001;MARINKOVIĆ et al. 2012MARINKOVIĆ et al. , 2013)).Spatial distribution of gross beta activity is apparent in Figure 4, and shows that most of the groundwater samples that exhibit elevated beta activity are located in the Vardar Zone or at the interface between the Vardar Zone and the Serbo-Macedonian Massif (SMM) and the Inner Dinarides (ID).This studys include 11 occurrences of groundwater from the area of the Pannonian Basin (PB) and all samples have beta activity concentrations less than 1 Bq/L.Also the area of the Carpatho-Balkanides (CB) covered by the 10 groundwater samples is characterized by low values of total beta activity.
Previous research has shown that certain deposits of radioactive elements can be spatially associated with volcanic complexes, being favorable environments for the creation of such deposits, but that the deposits themselves are linked with igneous reservoirs.It has been established that these deposits, and consequently the places where groundwater with elevated concentrations of radioactive elements is found, can occur in areas associated with large regional geotectonic zones, usually following their direction (RISTIĆ 1969).The main CO 2 generators are located within the Vardar Zone that is characterized by a highly complex geological/tectonic makeup and Tertiary magmatism and associated with the occurrence of carbonated groundwater (MARIN-KOVIĆ et al. 2012).This groundwater also exhibits naturally elevated radioactivity.

Fig. 1 .
Fig. 1.Histograms of gross alpha and beta activities and active concentrations of radionuclides 40 K, 226 Ra, 228 Ra, 232 Th and 238 U.

Fig. 3 .
Fig. 3. Box plots of CO 2 , HCO 3 -and Na + concentrations and TDS for two groups of groundwater samples (the group of 97 samples is shaded).