Activity concentrations and distribution of radionuclides in surface and core sediments of the Neretva Channel (Adriatic Sea, Croatia)

Abstract

The activity concentrations and the distribution of manmade '^'Cs and naturally occurring radionuclides'^''K, ^^'U and -^-Th in surface and core sediments of the semi-enclosed, river-dominated marine environment of the Neretva Channel were investigated in relation to the sedimentological characteristics 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 '^^Cs occurs close to the river mouth, in the region of intensive deposition of organic matter of terrestrial origin. This discovery implies that the river-borne organic material and its deposition processes should be considered as the most important factor controlling distribution of '"Cs in this transitional terrestrial-marine environment. Sediment accumulation rates, estimated from the distribution of '^^Cs in core sediments, were approximately 6 mm y ' in front of the Neretva River mouth and 4 mm y"' in the channel area. The spatial distribution of natural ''"K and -^^Th radionuclides indicates their distinct association with fine-grained sediments. The interrelationship of ^^^U with fine-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 fine-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 environment of the Adriatic.

Sediment Dynamics of the Neretva Channel (Croatia Coast) Inferred by Chemical and Physical Proxies

“…Estimated sediment accumulation rates (SARs; Figure 6) range from 1.9 to 8.5 mm/yr. These values differ partially from those previously estimated by other authors [16], ranging from 4 to 6 mm/yr and based only on the distribution of 137 Cs in core sediments. In particular, the lowest SAR value of 1.9 mm/yr was found at site 06BC ( Figure 6), located to the NW of the channel at the boundary with the Korcula Channel, a location comparable to that of sample K3 of [16] where a SAR of 4 mm/yr was estimated.…”

“…In particular, accumulation of fine-grained particles (mean grain size > 7.5 ϕ [38]) occurs mainly in two areas: in front of the NR mouth and in the central/northeastern part of the NC (Figure 2). This accumulation pattern is in agreement with the observed water circulation, where a hypopycnal river plume formed at the mouth distributes fine-grained particles over the entire channel area [16,17]. These observations suggest a different nature of the two observed coarser-grains depositional zones.…”

“…The southernmost zone may only receive river sediments sporadically during events of strong hydro-dynamism in the channel and may be considered a non-depositional area characterized by relict coarser sediment. This silt-sandy accumulation zone was previously reported by [16]. Instead, the northernmost sandy zone may be related to the influence of high energy currents from the open Adriatic Sea that remove the finest part of the sediments.…”

“…Unfortunately, as mentioned above, the 1986 137 Cs peak (Chernobyl accident) is not cleatracer [47,48]rly identifiable in the investigated sedimentary intervals. This is probably due to the strong bioturbation and sediment mixing (see X-ray images in Figure 3f,g) as suggested before by [16]. Moreover, since the half-life of 137 Cs is 30.17 years, it could not be excluded that the concentration is below the experimental sensitivity.…”

“…Several studies were carried out on sediment dynamics and geochemistry in the Adriatic Sea, but they were mainly focused on its western side (Italian Economic Water Zone; see [8][9][10][11][12][13][14] and reference therein). Hence, few geochemical data are available from sedimentary deposits of the eastern coast [15][16][17] and potential input of contaminants is still little known.…”