GC_2-2013_KB.indd AB STRA CT The activity concentrations and the distribution of manmade 137Cs and naturally occurring radio- nuclides 40K, 238U and 232Th in surface and core sediments of the semi-enclosed, river-dominated ma- rine environment of the Neretva Channel were investigated in relation to the sedimentological charac- teristics and the total organic carbon (TOC) content. The activity concentrations of radionuclides were determined by gamma spectrometry. Distinct interrelationships between sediment properties and the spatial distribution of radionuclides were observed. The highest accumulation of 137Cs occurs close to the river mouth, in the region of intensive deposition of organic matter of terrestrial origin. This dis- covery implies that the river-borne organic material and its deposition processes should be considered as the most important factor controlling distribution of 137Cs in this transitional terrestrial-marine en- vironment. Sediment accumulation rates, estimated from the distribution of 137Cs in core sediments, were approximately 6 mm y–1 in front of the Neretva River mouth and 4 mm y–1 in the channel area. The spatial distribution of natural 40K and 232Th radionuclides indicates their distinct association with fi ne-grained sediments. The interrelationship of 238U with fi ne-grained particles was somewhat weaker but still present. The results obtained indicate that the accumulation pattern of natural radionuclides in the Neretva Channel sediments is mainly governed by the deposition of fi ne-grained material. This study scrutinizes the baseline level for the occurring radionuclides and should be used for monitoring and assessing the radionuclide pollution record in the investigated transitional terrestrial-marine envi- ronment of the Adriatic. Keywords: radiomiclides, sedimentation rate, total organic carbon, fi ne-grained sediments, Neretva Channel  Geologia Croatica 66/2 143–150 4 Figs. Zagreb 2013 Geologia CroaticaGeologia Croatica Activity concentrations and distribution of radionuclides in surface and core sediments of the Neretva Channel (Adriatic Sea, Croatia)  Irena Jurina1, Maja Ivanić1, Tamara Troskot-Čorbić2, Delko Barišić1, Neda Vdović1 and Ivan Sondi3 1 Division for Marine and Environmental Research, Ruđer Bošković Institute, Zagreb, Croatia 2 E&P Research Laboratory Department, INA-Oil Industry Plc., Zagreb, Croatia 3 Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Zagreb, Croatia; (corresponding author: ivan.sondi@rgn.hr) doi: 10.4154/gc.2013.11 1. INTRODUCTION Environmental radioactivity has become a subject of scien- tifi c interest in recent decades, not only due to possible risks to human health, but also because radionuclides have been recognized as tracers of many complex biogeochemical pro- cesses (PORCELLI & BASKARAN, 2011). Sediment ero- sion and accumulation rates or transport of elements to the oceans are only a few examples of processes where radio- nuclide monitoring has been applied. Geologia Croatica 66/2Geologia Croatica 144 The mobility of radionuclides in aquatic environments is governed by their geochemical characteristics, such as solubility, complexation ability and affi nity for adsorption onto mineral surfaces (CHABAUX et al., 2003). Natural ra- dionuclides, such as 238U, 232Th and 40K, are released into the environment through erosion and chemical weathering of the radionuclide-bearing rocks, and are subsequently transported through surface and ground water systems (VI- GIER et al., 2001). The fate of radionuclides in transitional terrestrial-marine and marine environments is mostly infl u- enced by their interaction with clay minerals, organic matter and colloidal iron and manganese oxides and hydroxides (ANDERSSON et al., 2001; McKEE, 2008). River-domi- nated coastal areas of the Adriatic region are characterized by complex physico-chemical interactions and transforma- tions of dissolved and particulate organic and inorganic com- pounds (SONDI et al., 1994; SONDI et al., 1995; SONDI et al., 2008). In such environments, the transport and deposi- tion of radionuclides is governed by dispersal processes af- fecting fi ne-grained sediments, direct precipitation of oxides and oxyhydroxides and coagulation of inorganic and organic colloidal materials (ANDERSSON et al., 1998; LIGERO et al., 2001). There are several anthropogenic sources of radionu- clides whose contribution to overall environmental radioac- tivity cannot be neglected. Major sources of radioactive con- tamination include the nuclear weapons program, nuclear power plants, uranium mining and milling, commercial fuel processing and nuclear accidents (HU et al., 2010). The an- thropogenic release of radionuclides into the environment includes both natural and manmade isotopes, such as 137Cs. Once introduced into the environment, these radionuclides are affected by the same processes as other components of the natural systems. However, due to their known source, they often provide a recognisable imprint in the environment which facilitates the identifi cation of processes and changes occurring in natural ecosystems. In that context, the natural radionuclide 40K and man- made 137Cs were used as tracers of sediment transport and sedimentation dynamics in a river-dominated karstic estuary in the north-eastern Adriatic (SONDI et al., 1995). It was shown that the highest activity concentrations of these ra- dionuclides occur at the river mouth in accordance with the prevalent sedimentation of clayey particles associated with terrestrial organic matter. PETRINEC et al. (2012A) inves- tigated the activity concentrations of manmade 137Cs and natural radionuclides 40K, 226Ra, 228Ra and 238U in seawater and sediments at several locations along the eastern Adriatic coast – from Piran Bay to the Otranto Strait. They discov- ered signifi cant correlations between the activity concentra- tions of 40K, 228Ra and 238U and sedimentological character- istics, suggesting that variability in radionuclide activity concentrations can be explained by differences in the min- eral composition of the sediments. Distribution of 137Cs ac- tivity concentration in core sediments was used to estimate sediment accumulation rates at several locations in the mid- dle and south Adriatic Sea. A sedimentation rate of ~ 1.8 mm y–1 was reported for deposits from the South Adriatic Pit, while Albanian offshore shelf sediments had a much higher accumulation rate of ~ 4 mm y–1 (PETRINEC et al., 2012B). Sediment accumulation rates of approximately 1 to 5 mm y–1 were also reported at different locations in the Krka River estuary (CUKROV et al., 2007). In addition, CUKROV et al. (2009) illustrated that diverse natural sources of sediments can only partially explain the variations in activity concen- trations of 238U and 226Ra, and that signifi cant anthropogenic input of radionuclides can be observed in the area close to the town of Šibenik. This paper reports on a study of the activity concentra- tions of 137Cs, 40K, 238U and 232Th radionuclides in the sed- iments of the Neretva Channel. In particular, it reports on the accumulation patterns of these radionuclides in surface and subsurface sediments and the sediment accumulation rates based on vertical distributions of the 137Cs activity concen- tration. Finally, it addresses the geochemical behaviour of the investigated radionuclides through their association with organic and inorganic compounds in a semi-enclosed and river-dominated coastal environment of the Adriatic region. 2. STUDY AREA The Neretva Channel is an isolated, narrow part of the Adri- atic Sea, semi-enclosed by the Pelješac peninsula on the southern side. The surrounding area is mainly karstic terrain, consisting of Jurassic and Cretaceous limestones and dolo- mites with some occurrences of Triassic limestones, Eocene limestones and fl ysch (Fig. 1). The channel itself is a mic- rotidal, low-wave energy environment characterised by river-dominated sedimentation processes (JURINA et al., 2010). The Neretva River enters the channel on its northern side; it is the largest river on the Croatian part of the eastern Adriatic coast, and the only one forming a deltaic system. The river is approximately 255 km long with a catchment area of about 13000 km2. In the upland part of the river ba- sin, the river drains a heterogeneous terrain characterised by several geological units: Triassic and Miocene clastic sedi- mentary rocks, Triassic volcanic rocks and volcano-sedimen- tary series, a Mesozoic carbonate succession, and Cretaceous fl ysch (MOJIĆEVIĆ & LAUŠEVIĆ, 1973a; MOJIĆEVIĆ & LAUŠEVIĆ, 1973b; SOFILJ & ŽIVANOVIĆ, 1980; MOJIĆEVIĆ & TOMIĆ, 1982). Most of the sediment load carried by the river originates from the fl ysch and clastic de- posits exposed at the surface in this area. The estimated an- nual sediment discharge in 2000 was 13.58 x 106 tons (EU- ROSION, 2004). The lowland part of the Neretva River is only 36 km long and fl ows through Quaternary alluvial de- posits. Average annual water discharge is 332 m3s–1 with peaks in December and April (ORLIĆ et al., 2006). 3. METHODS 3.1. Sampling and sampling preparation The fi eld surveys in the Neretva Channel were conducted in October 2009 and June 2010. Three undisturbed sediment Jurina et al.: Activity concentrations and distribution of radionuclides in surface and core sediments of the Neretva Channel (Adriatic Sea, Croatia) Geologia Croatica 145 cores up to 35 cm long were retrieved at stations K1–K3 (Fig. 1). At an additional 28 stations (marked as dotted grid in Fig. 1) only the uppermost 5 cm of core sediments (sur- face sediments) were collected. All sediment samples were retrieved using a gravity corer (Uwitec, Austria), were fro- zen immediately after sampling and kept at –20°C. Prior to analysis, frozen sediment cores from stations K1–K3 were cut into 5 cm segments, after which the sediments were freeze dried (FreeZone 2.5, Labconco, USA) and homoge- nized. 3.2. Analyses Sediment samples were granulometrically characterized by the laser diffraction particle size analyzer (LS 13 320, Beck- man Coulter, USA). One gram of dry sample was suspended in 50 ml of redistilled water and placed in an ultrasound bath for 3 minutes. Approximately 5 to 10 ml of the suspension obtained was placed in a dispersion unit cell and analysed. No surfactants were used. Particle size was calculated on a volume basis using the Mie theory. Samples were measured in triplicate and the average particle size distribution spectra were taken as results. The clay-silt-sand ratios of the sedi- ments were determined according to the modifi ed WENTH- WORTH scale (1922) with the clay-silt boundary at 2 μm. The total organic carbon (TOC) content of the sediments was determined by combustion of acid insoluble matter in a Leco IR–212 carbon analyser (USA), after treatment with hot 1:1 diluted 36.5% HCl. Gamma-spectrometry measurements were conducted using a low-background hyperpure germanium (HPGe) “Canberra” semiconductor detector system coupled to an 8196-channel analyzer (Meriden, USA). Prior to measure- ment, sediment samples were oven dried at 105° to constant weight. The expanded uncertainty of measurements are stated as the standard uncertainty of measurement multiplied by the coverage factor k = 2, which for a normal distribution corresponds to a coverage probability of 95%. 3.3. Calculation Pearson correlation coeffi cients were calculated using Sta- tistica for Windows Ver. 7.0 (StatSoft Inc., USA). Contour maps were constructed using SURFER 8 (Golden Software, USA) with kriging as an interpolation method. Figure 1: Geological map of the Neretva Channel showing the study area and sampling stations. Legend: 1– Quaternary alluvial deposits; 2 – Eocene fl y- sch; 3 – Eocene limestones; 4 – Cretaceous limestones and dolomites; 5 – Jurassic limestones and dolomites 6 – Triassic limestones. Geologia Croatica 66/2Geologia Croatica 146 4. RESULTS AND DISCUSSION 4.1. Granulometric and mineralogical characteristics of sediments Considering the ratio of different grain size fractions (SHEPARD, 1954), surface sediments from the Neretva Channel were classifi ed as clayey silts (Fig. 2). The signifi - cant amount of sand fraction (64 %) was only observed in the sample collected in the vicinity of the small islet located in the southern part of the investigated area (Fig. 1). There were no signifi cant changes in granulometric characteristics in the core sediment samples. The spatial distribution of the fi ne-grained particles, i.e. mud (< 63 μm), in the surface sed- iments is presented in Figure 3. It is important to note that the predominant accumulation of fi ne-grained particles oc- curs in north-western part of the investigated area, toward the exit from the Neretva Channel. Such an accumulation pattern is in accordance with the observed water circulation, where a hypopycnal river plume formed at the mouth dis- tributing fi ne-grained particles over the channel area and to- ward the open sea. The mineral composition of all the inves- tigated surface sediments was similar. Samples were mainly composed of calcite, quartz, feldspars, dolomite and a sig- nifi cant amount of clay minerals, particularly illite (JURINA et al., 2010). 4.2. Distribution of radionuclides in surface sediments The results of the present study provide a good example of the distribution of radionuclides in a semi-enclosed fl uvio- marine environment of the Adriatic region. The two domi- nant processes identifi ed as the most important factors gov- erning the deposition of radionuclides in surface sediments of the Neretva Channel, were the sedimentation of fi ne- grained material and deposition of the river-borne terrestrial organic matter. The distribution of 137Cs in surface sediments of the Ner- etva Channel is shown in Figure 3. The activity concentra- tions varied from 3.7 ± 1.1 to 13.7 ± 2.1 Bq kg–1. The high- est values were found close to the river mouth and decreased seaward. A similar distribution pattern was observed for TOC, with values ranging from 0.29 to 0.98 % and the high- est values being determined in the area close to the Neretva River inlet (Fig. 3). This suggests a close association of 137Cs with sedimentary organic matter. The obvious unknown is the mechanism governing the depositional pattern of the TOC and how this process infl uences the distribution of 137Cs in the surface sediments of the Neretva Channel. We may speculate that the coagulation and deposition of terrestrial dissolved and particulate organic matter in the freshwater- seawater mixing zone results in scavenging of this radionu- clide from the water column and its fi xation in the seabed sediments. The importance of organic matter for 137Cs ac- cumulation in marine sediments was previously reported by RUBIO et al. (2003) and LIGERO et al. (2001). Correlation between the 137Cs activity concentrations and the TOC con- tent (r = 0.45, p < 0.05) in the Neretva Channel sediments is in agreement with those observations. River-borne organic material is mainly composed of humic and fulvic acids which are shown to coagulate even at low salinities (SONDI et al., 1996; SONDI et al., 1997; SONDI et al., 1998). Conse- quently, pronounced deposition of terrestrial organic matter occurs in mixing zones, close to the river mouth, as observed in the Neretva Channel sediments. The TOC content de- creases seaward indicating the reduced fl uvial infl uence and probably more pronounced accumulation of marine organic matter in the sediments. The activity concentrations of 137Cs in the surface sediments of the Neretva Channel were found to be higher than those in sediments from the southern Adri- atic region (0.8 – 3.8 Bq kg–1, PETRINEC et al., 2012A). This can be attributed to the considerable load of 137Cs in the Neretva River discharge processes. Previous measurements of the 137Cs activity concentrations in the Raša River estu- ary also revealed a high accumulation at the river mouth, indicating that fl uvial discharge should be considered as the main source of 137Cs in the coastal environment of the Adri- atic region (SONDI et al., 1995). The lowest measured activity concentrations of 40K and 232Th were 350.3 ± 44.1 Bq kg–1 and 19.8 ± 5.0 Bq kg–1, re- spectively. The spatial distribution of these two radionuclides displayed a similar accumulation trend. In addition, a strong positive correlation (r = 0.76, p < 0.001) occurs between these two radionuclides, indicating their close association. High activity concentrations, up to 647.6 ± 74.6 Bq kg–1 for 40K and 34.2 ± 6.3 Bq kg–1 for 232Th, were observed in sur- face sediments containing a high percentage of fi ne-grained particles (Fig. 3). The correlation coeffi cients between 40K and 232Th activity concentrations and mud content in surface sediments of the Neretva Channel are 0.41 (p < 0.05) and 0.48 (p < 0.01), respectively, suggesting the important role of fi ne-grained particles in the transport and deposition of Figure 2: Ternary diagram for the classifi cation of sediments (SHEPARD, 1954) with samples from the study area denoted on the basis of the ob- tained sand/silt/clay ratios. Jurina et al.: Activity concentrations and distribution of radionuclides in surface and core sediments of the Neretva Channel (Adriatic Sea, Croatia) Geologia Croatica 147 232Th and 40K in the sediments of the Neretva Channel. Re- garding the 40K behaviour, this radionuclide is very soluble and easily incorporated into the clay mineral crystal lattice, particularly in the interlayer sites of an illite-type structure. Thorium isotopes are considered insoluble in cation form (Th4+) but can be mobilised through complexation with or- ganic and inorganic ligands (LANGMUIR & HERMAN, 1980). After entering the surface waters, Th is easily ad- sorbed onto mineral surfaces, particularly Fe oxyhydroxides (ANDERSSON et al., 1995). Accordingly, the close asso- ciation of 232Th and 40K in the surface sediments of the Ner- etva Channel could be a consequence of their mutual inor- ganic carriers. The transport of both radionuclides is governed by their binding to the clay mineral surfaces and/ or co-precipitated Fe oxide and oxyhydroxide coatings. Therefore, the accumulation of these radionuclides in the Neretva Channel area is governed by fi ne-grained sediment deposition processes. The activity concentrations of 238U in surface sediments from the Neretva Channel varied from 17.5 ± 9.8 to 46.1 ± 14.2 Bq kg–1. Association of this radionuclide with sedimen- tary organic matter in the Neretva Channel sediments was not observed, although many studies report organic phases as being important carriers of 238U in river and brackish wa- ters (ANDERSSON et al., 1998). Correlation between 238U and 232Th activity concentrations (r = 0.51, p < 0.01) sug- gests that the distribution of these radionuclides in the Ner- etva Channel sediments is at least partially governed by the same processes. Indeed, an increase of 238U activity concen- trations was observed in the area characterized by the accu- Figure 3. Distributions of mud and TOC content and 137Cs, 40K, 238U and 232Th activity concentrations in the surface sediments of t he Neretva Channel. Geologia Croatica 66/2Geologia Croatica 148 mulation of fi ne-grained particles, although there was no signifi cant correlation between 238U and the mud content of sediments. The somewhat different distribution of 238U in the Neretva Channel surface sediments could be explained by desorption of this radionuclide from its organic and inor- ganic carriers. In the coastal mixing zones, fl uvial sedimen- tary material enters an environment of increased alkalinity. This enhances the formation of uranyl carbonate complexes and results in remobilization of uranium (LANGMUIR, 1997). The high activity concentration of 238U (490 Bq m–3) discovered in the seawater in Ploče harbour (PETRINEC et al., 2012A) is in agreement with the proposed desorption behaviour of uranyl species. The mean activity concentration of 40K (526 Bq kg–1) in the surface sediments of the Neretva Channel is signifi cantly higher than the world average value of 370 Bq kg–1 (UN- SCEAR, 1988). The high content of fi ne-grained particles in the Neretva Channel sediments, the abundance of clay min- erals, particularly illite, can partially explain the elevated activity concentrations of 40K. However, the Neretva River delta plain, which is adjacent to the study area, is an impor- tant agricultural region. Considering that 40K activity con- centrations as high as 6500 Bq kg–1 have been reported in phosphate fertilizers (KHATER & AL-SEWAIDAN, 2008), their extensive use in the delta plain may be a signifi cant source of 40K radionuclide in the Neretva Channel sediments. The specifi c activities of natural radionuclides 238U and 232Th in surface sediments of the investigated area did not vary signifi cantly. In comparison, the study in the nearby Krka River estuary reported activity concentrations from 14.1 ± 2.5 to 485 ± 16 Bq kg–1 for 238U (CUKROV et al., 2009). The town of Šibenik, located in the Krka River estu- ary, is the main Croatian port for phosphate ore transhipment, so localized anthropogenic input of 238U explains the wide range of values observed across the estuary. In the Neretva Channel, anthropogenic input of 238U and 232Th radionu- clides appears to be less pronounced. The mean values of 238U and 232Th activity concentrations in surface sediments of the Neretva Channel are 31 Bq kg–1 and 26 Bq kg–1, re- spectively. These values are comparable to the world aver- age value of 25 Bq kg–1 reported for these two radionuclides (UNSCEAR, 1988). Similar activity concentrations for 238U (28 Bq kg–1) and 232Th (21 Bq kg–1) were also reported in the Venice lagoon (DESIDERI et al., 2001). The use of fer- tilizers in the adjacent agricultural region of the Neretva River delta plain may contribute to slight enrichment in the Neretva Channel sediments of the 238U and 232Th radionu- clides. 4.3. Distribution of radionuclides in core sediments The distribution of 137Cs in core sediments from stations K1–K3 is presented in Figure 4A. At station K1, activity concentrations of 137Cs were constant (~ 9 Bq kg–1) in the uppermost 12.5 cm of the sediment strata. The values started to decrease in deeper sediment layers and reached 0 Bq kg–1 at 32.5 cm depth. The nuclear weapon testing and the sub- sequent atmospheric fallout of the 137Cs radionuclide reached its maximum around 1963. The quantities of 137Cs deposited in sediments prior to this were small and have been continu- ously reduced by radioactive decay up to the present day. For example, the amount of 137Cs deposited in 1954 was re- duced to half by 1984, and is reduced to about one quarter of the initial quantity by 2013. The detection limit of the gamma spectrometer used in this study was 0.3 Bq kg–1 for 137Cs. RITCHIE & McHENRY (1990) argued that the deter- mination of 137Cs near the detection limits can cause inac- curacies in establishing the onset of accumulation. The quan- tity of this radionuclide deposited in the Neretva Channel sediments prior to the fallout maximum in 1963 was prob- ably insuffi cient to be still reliably detectable. Therefore it is assumed that 1963 is taken as the onset for accumulation of 137Cs in sediments and the sedimentation rate at station K1 is estimated to ~ 6 mm y–1. At stations K2 and K3 the activity concentration of 137Cs became undetectable at 22.5 cm depth. An estimated sedimentation rate at both stations is ~ 4 mm y–1. The difference in the sedimentation rate be- tween stations K1–K3 is due to their varying distances from the Neretva River mouth. It is expected that more sediment is deposited at station K1 located in front of the Neretva in- let, than at stations K2 and K3 which are in the central chan- nel area. Nevertheless, considering the uncertainties of the measurements and estimated sedimentation rate, the ob- served difference of 2 mm y–1 is small. These results imply the signifi cance of the fl uvial input of sediments to the entire investigated area. Figure 4. Activity concentrations distributions in the sediment cores from stations K1−K3: A) 137Cs, B) 40K, C) 238U, D) 232Th. Jurina et al.: Activity concentrations and distribution of radionuclides in surface and core sediments of the Neretva Channel (Adriatic Sea, Croatia) Geologia Croatica 149 The vertical distribution of 137Cs at station K2 was sim- ilar to the vertical distribution of this radionuclide at station K1; the maximum activity concentration was found at the surface (7.2 ± 1.5 Bq kg–1) and values continuously de- creased with depth. At station K3, the maximum activity concentration of 137Cs (5.9 ± 1.0 Bq kg–1) was observed at 12.5 cm sediment depth. This increase was attributed to the Chernobyl nuclear accident which occurred in 1986. The sedimentation rate, calculated based on this peak position, is ~ 4 mm y–1, which is in agreement with the previously es- timated sedimentation rate based on the onset of 137Cs ac- cumulation. The reason why this peak was only observed at station K3 is somewhat unclear. It is possible that disturbance of the sediments by bioturbation or some other process was less pronounced at Station K3 than in other areas of the Ner- etva Channel which allowed preservation of this distinct 137Cs enrichment. The vertical distribution of 40K, 238U and 232Th activity concentrations are presented in Figure 4B–D. The activity concentrations of these three radionuclides in the core sedi- ments were similar at all three sampling stations. Values ranged from 537.7 ± 62.6 to 689.6 ± 74.6 Bq kg–1 for 40K, from 21.9 ± 6.7 to 41.5 ± 11.0 Bq kg–1 for 238U and from 25.3 ± 5.3 to 43.2 ± 6.4 to 43.2 ± 6.4 Bq kg–1 for 232Th. The vertical profi les of 40K, 238U and 232Th activity concentra- tions revealed a similar accumulation pattern in core sedi- ments for all three radionuclides, particularly at station K3. Considering the observed similarities in the temporal accu- mulation pattern of 238U, 232Th and 40K, depth variations in these radionuclides activity concentrations can be attributed to differences in grain-size and mineralogy of the deposited sediments. The only signifi cant difference in the vertical accumula- tion trend of these radionuclides is an increase in 238U activ- ity concentration observed in surface sediments at station K2. This increase could be due to the penetration of seawa- ter enriched with soluble 238U in the surface sediment layers at this sampling station (PAPAEFTHYMIOU et al., 2007). An alternative explanation is differential bioturbation rates between the investigated stations. There are two mechanisms through which bioturbation processes can promote uranium release from sediments to the water column (ZHENG et al., 2002). Bioturbation can oxygenate sediment and/or stir them up, closer to the water-sediment interface. Since uranium is subject to remobilisation in oxidising conditions, either of these scenarios can be the explanation for the lower activity concentration of 238U in surface sediments at stations K1 and K3. If bioturbation is less pronounced at station K2, removal of 238U radionuclide from surface sediments layers should not occur. 5. CONCLUSIONS 1. The highest accumulation of 137Cs occurred close to the river mouth, in the region of intensive deposition of or- ganic matter of terrestrial origin, with amounts decreasing progressively seaward. 2. The sedimentation rates from 4 – 6 mm y–1 were estimated based on 137Cs distribution in the core sediments. 3. The distribution of natural radionuclides 40K, 238U and 232Th follows the accumulation pattern of the fi ne-grained sediment particles. This implies fi ne-grained sediment dis- persal processes as the dominant factor governing the ac- cumulation trend of these radionuclides. 4. Signifi cant enrichment of 40K in the Neretva Channel sed- iments could be explained by the use of fertilizers in the Neretva delta agricultural region, but further investiga- tions are required. Anthropogenic input of 238U and 232Th appears to be negligible. ACKNOWLEDGMENT This work was supported by the Ministry of Science, Education and Sports of the Republic of Croatia (Grant 098-0982934-2742). The au- thors gratefully acknowledge the assistance of Momir MILUNOVIĆ and Srečko KARAŠIĆ during fi eld surveys. REFERENCES ANDERSSON, P.S., PORCELLI, D., GUSTAFSSON, Ö., INGRI, J. & WASSERBURG, G.J. 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Acta, 66, 1759−1772. doi:10.1016/S0016-7037(01)00886-9 Manuscript received March 19, 2013 Revised manuscript accepted April 25, 2013 Available online June 19, 2013