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The role of a spring river as a source of 137Cs in a lagoon environment: the case of the Stella river (Marano lagoon, Northern Adriatic Sea)
Freshwuter und Estuurine Rudioecology Edited by G. Desmet et al. 1997 Elsevier Science B.V.
97
The role of a spring river as a source of 137Csin a lagoon environment: the case of the Stella river (Marano lagoon, Northern Adriatic Sea) M. Bellia, E. Colizzab,G.P. Fanzuttib,F. Finocchiarob,R. Melisb, R. Pianiband U. Sansonea ' W P A , Via Vitaliano Brancati, 48-00144Roma, Italy bZstituto di Geologia e Paleontologia, Universita di Trieste, via Edoardo Weiss, Comprensorio di S. Giouanni, 34127 Trieste, Italy
ABSTRACT The results from some cruises made in order to determine the influence of the Stella river as a source of 13'Cs within the Lignano basin (Marano Lagoon, Northern Adriatic Sea) are presented. The suspended solid transport of this spring river is previously estimated. Furthermore, particle size and composition features of the Stella river particulate suspended matter are used to discuss the 137Csin situ Kd measured in suspended matter. The pattern of salinity of water bodies in the lagoon, nearby the Stella mouth, confirm the distribution of radiocaesium in the bottom sediments.
1. INTRODUCTION
After a radioactive fall-out on a drainage basin, soils act as temporary repository of deposited radionuclides. Radiocaesium can be then slowly removed by erosion and transported from the catchment area towards water bodies. Following the Chernobyl accident, the Friuli-Venezia Giulia region, located in north-easternmost Italy, was subjected to heavier rainfall than other Italian regions. On average, it received the highest radioactive deposition [1,21.The concentration values are particularly high on the mountain areas and then decrease towards the coast. Since 1986, concentrations of 137Csin sediment, water and suspended matter were monitored in the Marano and Grado lagoons, and in the Adriatic Sea between Trieste and the Tagliamento river delta fan [3,41, because these environments would be considered the final reception areas of radiocaesium transported from inland. This monitoring activity allowed the identification of two different areas in the lagoons: the inner part, into which spring rivers flow,
98
characterized by fine sediments and high caesium concentration; the outer part near the lagoon inlet, where marine water exchanges prevail, is characterized by sandy sediments and low caesium contamination. The high caesium concentration at the Stella mouth seems to be in contrast with the regimen of the river, which is considered a spring river. In this paper the authors provide an assessment of the radiocaesium associated with the solid transport of the Stella river, reaching the western part of the Marano lagoon, and the dispersion processes of river suspended load into the lagoon. 2. STUDY AREA
In the Friuli Plain, all the rivers with a mountain drainage basin lose their water discharge after a few kilometres due to the gravelly deposits of the high plain. Below the spring-line, in the low plain characterized by sandy and silty deposits, rivers receive waters from the ground-water table. Only during main floods, particularly in spring and autumn, do waters flow all along the riverbed, from mountain area to the sea. The Stella river (47km length) is the most important river flowing into the Lignano basin, the westernmost basin of the Marano lagoon (Fig. 1).Its source is formed by a large number of springs located some kilometres south-west of Codroipo (Udine), south of the spring-line. After a few kilometres, three main trunks can be recognized: Taglio, Stella and Torsa. Afterwards they join in a single channel. The Stella river slightly erodes the silty-clayed soils of the plain, producing a wide and flat depression, recognizable as far as the lagoon border [51. The influent trunk of the Stella river, named Corno stream, lies in the gravelly and permeable high plain; it becomes normally dry a few kilometres south of the Tagliamento morainic amphitheatre. During very important floods its waters can also flow into the Taglio river, and therefore in the Stella [6]. Sometimes it receives also the overflow water of Ledra-Tagliamento canal, diverted from the Tagliamento river for agricultural needs. From 1926 to 1950,at Casali Sacile station, an average annual discharge of 33.6 m3 s-I, was calculated [61.A similar value (32.6m3s-') was provided at the station of Ariis during the 1966-1974 period 171.The monthly average discharge of Stella is quite regular through the year: the minimum value seldom falls under 25 m3 s-l, while the maximum value is about three times the average discharge. By examining and comparing the daily discharge with the rainfall data in two barycentric stations (Fig. 21, it can be noted that the former increases for one or two days aRer heavy rains (over 25 mm), owing to a rapid drainage. In conclusion, two kinds of meteorological events are superimposed on the regularity of the spring water discharge: a short and heavy rainfall on the low plain increases the river discharge for one or two days. Moreover, when the rain is more intense and long lasting in the mountain area, the water of Corno stream and Ledra-Tagliamento canal can cross the permeable high plain and add to the Stella river waters.
99
Fig. 1. Sketch map of Friuli-Venezia Giulia plain and Marano and Grado lagoons.
The morphological evidence of the Stella river influence in the Lignano basin (surface: 50 km'; mean depth: 0.8 m) is represented by its delta, which was artificially cut and connected with a lagoon channel (Cialisia channel). The Lignano basin is characterized by wide tidal flats and by marshes of small extension. Its inlet is 500 m wide and about 11m deep; water flow through the inlet is up to 40-50 million m3 water at every tidal hemi-cycle [81. The tide is semidiurnal, with 65 cm mean tidal range; spring tides can reach range of 105 cm. The tidal stream velocity decreases &om inlet (up to 100 cm s-') to the inner part of the Cialisia channel. Here, during syzygy tides, a mean velocity of 25-30 cm s-' was measured.
100
‘O
t
I
d
7-
.-
dli
40
35
30
e
C
I
30 -
--
mrainfall discharge
Fig. 2. Relationships between daily discharge of the Stella river at Ariis station and rainfall at Talmassons station during 1973 (data from Magistrato alle Acque, Venice).
3. MATERIALS AND METHODS
In order to provide a complete set of environmental data to characterize such a complex environment, several field activities were carried out from 1991 to 1992. The physico-chemical characteristics of water and suspended matter were determined using samples collected near the surface and the bottom during seasonal surveys. Samples for 13%sdetermination were collected in May and October 1991 and November 1992. To assess the fluxes of substances from river to the lagoon environment, Total Suspended Matter (TSM) concentrations were measured by filtering 1 dm3 of water, sampled by means of a Niskin bottle, on a Whatman GF/F fibreglass filter (0.8 Fm pore size, 47 mm filter diameter) [9].The carbonate fraction was determined as difference in dry weight of TSM before and after treatment with HCl 1N. Elemental particulate organic carbon (POC) and nitrogen (PON) analyses were performed by ignition
101
in pure oxygen atmosphere, using helium as the carrier gas in a Perkin Elmer 2400 CHN Elemental Analyzer. Grain-size analyses of the suspended matter were performed on water sub-samples using a Coulter Multisizer 11, with an orifice tube of 140 pm. The TSM mineralogical composition was performed on a Siemens D 500 diffractometer, using CuKa radiation; scanning interval ranged between 2"and 35" of 20, pitch 0.1",2 seconds of computation per pitch. Semi-quantitative computation of the mineralogical phases were done using the diffractogram height of peaks. Samples of water and suspended matter for radiocaesium determination were carried out at the same depth (about 1 m from water surface) using two devices capable of filtering large amounts of water (more than 1000 dm3)using cartridge filters of 0.45 pm porosity. Each system was equipped with resin columns (ammonium hexo-cyano-cobaltferrate, NCFN) to fix radiocaesium dissolved in water. To determine the efficiency of the resins, two resins columns (diameter of 20 mm and height of 160 and 80 mm respectively) connected in series were used. During sampling, conductivity, pH and temperature were measured. 137Csconcentrations were determined in samples by gamma-spectrometry using high purity Germanium detectors (HPGe). 4.RESULTS 4 .1 . Concentration of total suspended matter in the Stella river
The solid flow of the Stella river has been always considered scarce owing to its spring characteristics [61.For this reason time series of solid measurement are lacking. Conversely, during researches on the suspended matter in the Lignano basin, it was observed that TSM concentrations at the Stella mouth could not be considered negligible, when comparing with those of the lagoon [lo].Figure 3 reports the frequency distribution of TSM concentrations in the Stella river, using all the available data (about 55 determinations at channel and mouth) collected duringvarious surveys from 1985 t o 1993 [9,10,121.The most frequent concentrations range between 5 and 10 mg dm-3; whereas concentrations between 10 and 30 mg dm-3 are observed in more than one third of cases. Concentrations higher than 60 mg dm3, measured only during flood tide, can be considered exceptional. 4.2. Characteristics of the Stella river total suspended matter
Some chemical and mineralogical analyses on TSM, referred to seasonal surveys (October 1991, February, April and July 1992) pointed out a prevailing clastic fraction over the organic one (Table 1). In fact, the mean percentage of POC is 6.9 _+ 3.1%of TSM weight and are equal or lower to these in the lagoon, The average of Stella river TSM mineralogical composition, results as follows
102
<5
5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 >60
mg/l Fig, 3.Frequency distribution of Total Suspended Matter (TSM) concentrations in the Stella river.
TABLE 1 Main characteristics of suspended matter at the mouth of the Stella river October 1991 Parameters
Surf.
February 1992 April 1992
Bottom Surf.
Bottom Surf.
July 1992
Bottom Surf.
Bottom
2.95 10.13 5.04 7.30 6.17 36.89 Concentration (mgdm3) 9.13 43.8 48.0 47.4 30.5 40.5 Carbonate (%I 39.3 39.0 33.5 38.8 Organic carbon (mg dm-3) 506.2 2138.5 383.3 562.6 284.0 340.7 698.6 1667.1 5.6 Organic carbon (%I 5.5 4.9 13.0 5.6 4.7 11.3 4.5 11.6 13.1 7.3 8.9 9.8 6.7 7.7 9.5 C M ratio 7.2 8.7 8.6 12.2 Mean diameter (pn) 11.9 16.1 10.0 11.7 2.5 15.0 1.1 9.4 3.6 31.3 Salinity (%d 1.6 5.2 4.4 11.4 8.7 11.5 Modal diameter (pn) 13.6 21.8 11.4 14.2 22.6 25.6 23.9 33.5 5th percentile (pn) 29.3 38.3 21.7 28.5
(in percent): calcite (14 f 31, dolomite (50 f 91, quartz (18 f lo), feldspar (3 f l), illite (7f 41, kaolinite (2 It 21, chlorite (6 f 3).Dolomite is the most important mineral, followed by quartz and calcite. The sum of clay mineral percentages can reach 15%;illite and chlorite content are almost equal, kaolinite is lower. Montmorillonite or other expandable clay minerals were not detected.
103
~
-
.... .. ...... ... . . . ~
.... ....._
.........
i
.-.-..... ....
Diameter (yrn)
Fig. 4. Grain size frequency curve of the Stella river suspended matter (solid) compared with a lagoon basin one (dotted).Sampling of October 1991.
The grain size distribution of TSM shows a modal diameter ranging from 21.8 to 4.4 pm. The highest values relate to the measurements of October, during which a high water discharge was recorded; the lowest to April, in which the lowest tidal range was recorded. The mean diameter has a narrow range from 7.2 to 16.1pm. The diameter of the 5th (coarser)percentile does not exceed 40 pm. Figure 4 shows an example of grain size frequency curve of the Stella river particulate matter compared with one from the lagoon basin. 4.3. Water salinity
Salinity data (Fig. 5) show that brackish water (salinity <6%0) is confined to the northernmost part of the Cialisia channel, up to 1.5-2.0 km from the Stella mouth during ebb tide. During flood tide the area is further on restricted in a narrow strip along the lagoon border. 4.4. Radiocaesium content
Figure 6 reports the mean 137Csconcentrations measured in the superficial layer of bottom sediment samples collected in the Marano lagoon. The distribution of 137Csconcentrations range from 4 to 60 Bq kg-'. Table 2 indicates the mean 137Csconcentration measured in water and suspended matter of the Stella river. From May 1991 to November 1992 the 137Csconcentration in the Stella river water was quite uniform, ranging between 4.1 x 10"' and 12.0 x 10"' with a mean of 8 x lo4 Bq 1-'. A low variability it is also observed in the 137Cs concentrations measured in the suspended matter collected in the same period. Table 2 summarizes also the results of 13'Cs measurements in water and
104
Fig. 5. Surface salinity patterns in the Lignano basin near the Stella river mouth (ebb tide hemicycle):(a) October 8, 1991; (b) February 18, 1992; (c) April 28,1992; (d) July 30,1992.
suspended matter. I3'Cs concentrations in brackish water are from 2 to 4.5 times higher than those measured in fresh water. The concentrations of 137Cs in suspended matter are very close to that determined for samples taken in the Stella river.
105
Fig. 6. '37Cs in the superficial layer of Marano lagoon bottom sediments (Bq kg-'1.
In the same table the in situ Kdvalues are given for suspended matter from the Stella river and from the westernmost part of the Marano lagoon. The partitioning of 137Csbetween soluble and particulate phases is defined here as the ratio of 137Cssorbed in the particulate phase to the concentration of this radionuclide in solution.
106
TABLE 2 '37Cs concentrations and in situ Kd values
Location
Sampling periods
Number samples
Water (Bq l-'x104)
Suspended matter
(Bqg - ' ~ l O - ~ )
in situ Kd (mlg-1x10-5)
Stella River
May 1991 octo. 1991 Nov. 1992 May 1991 Oct. 1991
2 2 4 2 2
9f3 9f2 6f4 20f2 28f4
8.2M.2 11.35of0.001 9fl 7f2 7fl
1.of0.3 1.3f0.3 1.8f0.8 0.4m.i 0.25f0.01
Marano Lagoon
5. DISCUSSION AND CONCLUSIONS
The sediment 137Csconcentrations in the inner part of the lagoon, in which the influence of the Stella river prevails, are significantly higher than those measured in sediments taken near the lagoon inlet-zone (Fig. 6). As already seen, the salinity, together with the compositional and grain-size characteristics of the Stella particulate matter confirm that the influence of the river is restricted to the northern part of the Cialisia channel (Fig. 5). The bottom of the inner part of the lagoon is therefore a reservoir of fine sediments [ l l l , and consequently a sink for 13%s, transported by the Stella river. The decrease of caesium values from the Stella mouth towards the lagoon inlet can be due to the dispersion processes due to tidal stream or wind-induced resuspension on bottom sediments [121. As can be seen from the data given in Table 2, the in situ & values are very variable and highly dependent on the composition of water. The & values for suspended matter collected in the Stella river (mean conductivity value = 0.6 mS cm-') range between 1.0 and 1.8 ml g-' x 10'. The values determined for suspended matter collected in brackish water, where the mean conductivity value is fifty times higher (30 mS cm-'), are about five times lower (from 0.25 to 0.4 ml g-' x lo5)than those determined in the Stella river. K ' and Na' are the major ions competing with radiocaesium in brackish environments and there is a considerable literature 113,141 reporting values of & lower in the marine than in the fresh water environments. The differences in behaviour could be simply explained considering that when different water types interact, changes in the major chemical composition of the resulting water can alter the availability of 13'Cs adsorption sites on particulate matter. The desorption processes of 13%s on particles of the Stella river are favoured by an ion exchange mechanism due to the higher presence of ions such as K ' in the lagoon environment. The & presented in this paper cannot be evaluated in terms of K' and Na' variability in fresh and salt water because these data were not
107
measured during our samplings. Only for the Stella river literature values are available [Elfor ; the sampling area of the river considered in this paper, K+ values range from 2 to 3 ppm and Na’ from 3 to 40 ppm. Finally the in situ Kdvalues reported in Table 2 are higher (about two order of magnitude) if compared with measurements carried out by other authors [16]in similar environments. Our values could be explained considering the presence of significant concentrations of clay minerals (about 15%) in particles transported by the Stella river. Besides the TSM concentration values normally measured in Stella water can be compared to those in lagoon water (5-10 mg dm3), the flux of particulate, mainly inorganic matter from the Stella river cannot be considered negligible. Firstly, because the liquid discharge of the river is steady in time and its influence can be registered inside a conservative environment as the lagoon. Secondly, because during heavy rains the river solid discharge widely increases, thus providing an enrichment of caesium content in suspended matter coming from the Tagliamento mountain catchment area through the Ledra and Corn0 hydraulic systems. Combing the TSM data with the average annual discharge of Stella river, it is possible to evaluate the amount ofTSM and 137Cstransported into the lagoon. Assuming a TSM mean concentration value of 10 mg dm“, the corresponding annual amount cannot be lower than 10000 ton year-’, leading to an input of lo8 Bq year-’ of 137Csdischarged into the lagoon. 6. ACKNOWLEDGEMENTS
We wish to thank Prof. F. Princivalle, from the Mineralogy Institute of Trieste University for the clay mineral analyses and interpretation, and colleagues from the “Laboratorio di Biologia Marina” of Aurisina (Trieste) that kindly collaborated during the field work. The authors are also grateful to Dr. G. Mattassi and his group from “U.S.L. no. 8 Bassa Friulana” for the fruitful cooperation, and to the “Consorzio per la Bonifica e lo Sviluppo Agricolo della Bassa Friulana” for providing hydraulic and climate data. This work was partially supported by Italian MURST 60% “AmbientiUmidi” Research Project. 7. REFERENCES 1. 2.
3.
ENEA-DISP. Incidente di Chernobyl - Conseguenze Radioecologiche in Italia. Relazione a1 27/05/86, DOC.DISP (86) 1. ENEA-DISP. Incidente di Chernobyl - Conseguenze Radioecologiche in Italia. Relazione a1 30/11/86, DOCDISP (86) 14. Ventura, G.,M. Belli, U. Sansone, P. Nicolai, P. Spezzano, C. Papucci, M. Marinaro and G. Mattassi, 1988. Radioecological research in northern Adriatic lagoons: activities and first results. Proc. of the International Conference on Environmental Radioactivity in the Mediterranean Area, Barcelona, 10-13 May 1988, pp. 467-478.
108 4. Baldi, R. Delfanti, V. Fiore, C. Galli, 0. Lavarello, C. Papucci, A. Pentassuglia, G.
5. 6. 7.
8. 9. 10. 11. 12.
13.
14. 15. 16.
Ventura, F. De Guamni and G. Mattassi, 1989. ENEA, Sicurezza e Protezione, no. 21 Sept.-Dec. 1989, pp. 89-99. Feruglio, 1925. La zona delle risorgive del basso Friuli fra Tagliamento e Torre. Parte 11. Studio fisico-chimico e agronomico. Ann. Staz. Chim. Agr. Sper. di Udine, Sene 111, Vol. I, Udine, pp. 345476. Mosetti, 1983. Sintesi sull'idrologia del Friuli Venezia Giulia Quad. Ente Tutela Pesca, Udine, p. 296. Ufficio Idrografico Magistrato alle Acque, 1960-1971. Annali Idrologici MM.LL.PP., Servizio Idrologico, Venezia. Dorigo, 1965. La laguna di Grado e le sue foci. Ricerche e rilievi idrografici. Ufficio Idrografko del Magistrato alle Acque, Pubbl. no. 155, Venezia, p. 231. Finocchiaro, F., 1987. I1 particellato nella zona costiera: provenienza e dispersione. Universith di Trieste, Istituto di Geologia e Paleontologia, Tesi di Dottorato di Ricerca in Scienze Ambientali (Scienza del Mare), (not published). Definizione dei caratteri idraulico-sedimentologicidelle Lagune di Marano e Grado e dei fondali marini antistanti. Contratto di ricerca tra ENEA-DISP Roma e Istituto di Geologia e Paleontologia di Trieste, Linea B, 1993 (not published). Marocco, R., 1989. Evoluzione quatemaria della laguna di Marano (Friuli Venezia Giulia). I1 Quaternario, Napoli, 2: 125-137. Brambati, G.P. Fanzutti and F. Finocchiaro, 1990. Effetti della risospensione indotta da vento sulle concentrazioni e dimensioni del particellato nel bacino di Lignano (Laguna di Marano Adriatic0 settentrionale). Atti 8 Congr, Trieste. AIOL, pp. 191-212. Schell, A.L. Sanchez, T.H. Sibley and J.R. Clayton Jr., 1979. Distribution coefficients for radionuclides in aquatic environments. 111. Adsorption and desorption Studies of lo6Ru, 137Cs,241Am, and 237Puin maline and freshwater systems. Annual report: August 19784uly 1979. NUREGKR-0803.76 pp. Jannasch, B.D. Honeymann, L.S. Balistieri and J.W. Murray, 1988. Kinetics of trace elements uptake by marine particles. Geochim. Cosmochim. Acta, 52: 567577. Stefanini, 1976. Composizione delle acque fluviali del Friuli-Venezia Giulia durante la fase di magra e di piena dei corsi d'acqua. Istituto di Ricerca sulle acque 28 (5). Benes, P., M. Cernik and P. Lam Ramos, 1992. Factors affecting interaction of radiocaesium with freshwater solids. J. Radiochem. Nucl. Chem., 159 (2):201-218.
97
The role of a spring river as a source of 137Csin a lagoon environment: the case of the Stella river (Marano lagoon, Northern Adriatic Sea) M. Bellia, E. Colizzab,G.P. Fanzuttib,F. Finocchiarob,R. Melisb, R. Pianiband U. Sansonea ' W P A , Via Vitaliano Brancati, 48-00144Roma, Italy bZstituto di Geologia e Paleontologia, Universita di Trieste, via Edoardo Weiss, Comprensorio di S. Giouanni, 34127 Trieste, Italy
ABSTRACT The results from some cruises made in order to determine the influence of the Stella river as a source of 13'Cs within the Lignano basin (Marano Lagoon, Northern Adriatic Sea) are presented. The suspended solid transport of this spring river is previously estimated. Furthermore, particle size and composition features of the Stella river particulate suspended matter are used to discuss the 137Csin situ Kd measured in suspended matter. The pattern of salinity of water bodies in the lagoon, nearby the Stella mouth, confirm the distribution of radiocaesium in the bottom sediments.
1. INTRODUCTION
After a radioactive fall-out on a drainage basin, soils act as temporary repository of deposited radionuclides. Radiocaesium can be then slowly removed by erosion and transported from the catchment area towards water bodies. Following the Chernobyl accident, the Friuli-Venezia Giulia region, located in north-easternmost Italy, was subjected to heavier rainfall than other Italian regions. On average, it received the highest radioactive deposition [1,21.The concentration values are particularly high on the mountain areas and then decrease towards the coast. Since 1986, concentrations of 137Csin sediment, water and suspended matter were monitored in the Marano and Grado lagoons, and in the Adriatic Sea between Trieste and the Tagliamento river delta fan [3,41, because these environments would be considered the final reception areas of radiocaesium transported from inland. This monitoring activity allowed the identification of two different areas in the lagoons: the inner part, into which spring rivers flow,
98
characterized by fine sediments and high caesium concentration; the outer part near the lagoon inlet, where marine water exchanges prevail, is characterized by sandy sediments and low caesium contamination. The high caesium concentration at the Stella mouth seems to be in contrast with the regimen of the river, which is considered a spring river. In this paper the authors provide an assessment of the radiocaesium associated with the solid transport of the Stella river, reaching the western part of the Marano lagoon, and the dispersion processes of river suspended load into the lagoon. 2. STUDY AREA
In the Friuli Plain, all the rivers with a mountain drainage basin lose their water discharge after a few kilometres due to the gravelly deposits of the high plain. Below the spring-line, in the low plain characterized by sandy and silty deposits, rivers receive waters from the ground-water table. Only during main floods, particularly in spring and autumn, do waters flow all along the riverbed, from mountain area to the sea. The Stella river (47km length) is the most important river flowing into the Lignano basin, the westernmost basin of the Marano lagoon (Fig. 1).Its source is formed by a large number of springs located some kilometres south-west of Codroipo (Udine), south of the spring-line. After a few kilometres, three main trunks can be recognized: Taglio, Stella and Torsa. Afterwards they join in a single channel. The Stella river slightly erodes the silty-clayed soils of the plain, producing a wide and flat depression, recognizable as far as the lagoon border [51. The influent trunk of the Stella river, named Corno stream, lies in the gravelly and permeable high plain; it becomes normally dry a few kilometres south of the Tagliamento morainic amphitheatre. During very important floods its waters can also flow into the Taglio river, and therefore in the Stella [6]. Sometimes it receives also the overflow water of Ledra-Tagliamento canal, diverted from the Tagliamento river for agricultural needs. From 1926 to 1950,at Casali Sacile station, an average annual discharge of 33.6 m3 s-I, was calculated [61.A similar value (32.6m3s-') was provided at the station of Ariis during the 1966-1974 period 171.The monthly average discharge of Stella is quite regular through the year: the minimum value seldom falls under 25 m3 s-l, while the maximum value is about three times the average discharge. By examining and comparing the daily discharge with the rainfall data in two barycentric stations (Fig. 21, it can be noted that the former increases for one or two days aRer heavy rains (over 25 mm), owing to a rapid drainage. In conclusion, two kinds of meteorological events are superimposed on the regularity of the spring water discharge: a short and heavy rainfall on the low plain increases the river discharge for one or two days. Moreover, when the rain is more intense and long lasting in the mountain area, the water of Corno stream and Ledra-Tagliamento canal can cross the permeable high plain and add to the Stella river waters.
99
Fig. 1. Sketch map of Friuli-Venezia Giulia plain and Marano and Grado lagoons.
The morphological evidence of the Stella river influence in the Lignano basin (surface: 50 km'; mean depth: 0.8 m) is represented by its delta, which was artificially cut and connected with a lagoon channel (Cialisia channel). The Lignano basin is characterized by wide tidal flats and by marshes of small extension. Its inlet is 500 m wide and about 11m deep; water flow through the inlet is up to 40-50 million m3 water at every tidal hemi-cycle [81. The tide is semidiurnal, with 65 cm mean tidal range; spring tides can reach range of 105 cm. The tidal stream velocity decreases &om inlet (up to 100 cm s-') to the inner part of the Cialisia channel. Here, during syzygy tides, a mean velocity of 25-30 cm s-' was measured.
100
‘O
t
I
d
7-
.-
dli
40
35
30
e
C
I
30 -
--
mrainfall discharge
Fig. 2. Relationships between daily discharge of the Stella river at Ariis station and rainfall at Talmassons station during 1973 (data from Magistrato alle Acque, Venice).
3. MATERIALS AND METHODS
In order to provide a complete set of environmental data to characterize such a complex environment, several field activities were carried out from 1991 to 1992. The physico-chemical characteristics of water and suspended matter were determined using samples collected near the surface and the bottom during seasonal surveys. Samples for 13%sdetermination were collected in May and October 1991 and November 1992. To assess the fluxes of substances from river to the lagoon environment, Total Suspended Matter (TSM) concentrations were measured by filtering 1 dm3 of water, sampled by means of a Niskin bottle, on a Whatman GF/F fibreglass filter (0.8 Fm pore size, 47 mm filter diameter) [9].The carbonate fraction was determined as difference in dry weight of TSM before and after treatment with HCl 1N. Elemental particulate organic carbon (POC) and nitrogen (PON) analyses were performed by ignition
101
in pure oxygen atmosphere, using helium as the carrier gas in a Perkin Elmer 2400 CHN Elemental Analyzer. Grain-size analyses of the suspended matter were performed on water sub-samples using a Coulter Multisizer 11, with an orifice tube of 140 pm. The TSM mineralogical composition was performed on a Siemens D 500 diffractometer, using CuKa radiation; scanning interval ranged between 2"and 35" of 20, pitch 0.1",2 seconds of computation per pitch. Semi-quantitative computation of the mineralogical phases were done using the diffractogram height of peaks. Samples of water and suspended matter for radiocaesium determination were carried out at the same depth (about 1 m from water surface) using two devices capable of filtering large amounts of water (more than 1000 dm3)using cartridge filters of 0.45 pm porosity. Each system was equipped with resin columns (ammonium hexo-cyano-cobaltferrate, NCFN) to fix radiocaesium dissolved in water. To determine the efficiency of the resins, two resins columns (diameter of 20 mm and height of 160 and 80 mm respectively) connected in series were used. During sampling, conductivity, pH and temperature were measured. 137Csconcentrations were determined in samples by gamma-spectrometry using high purity Germanium detectors (HPGe). 4.RESULTS 4 .1 . Concentration of total suspended matter in the Stella river
The solid flow of the Stella river has been always considered scarce owing to its spring characteristics [61.For this reason time series of solid measurement are lacking. Conversely, during researches on the suspended matter in the Lignano basin, it was observed that TSM concentrations at the Stella mouth could not be considered negligible, when comparing with those of the lagoon [lo].Figure 3 reports the frequency distribution of TSM concentrations in the Stella river, using all the available data (about 55 determinations at channel and mouth) collected duringvarious surveys from 1985 t o 1993 [9,10,121.The most frequent concentrations range between 5 and 10 mg dm-3; whereas concentrations between 10 and 30 mg dm-3 are observed in more than one third of cases. Concentrations higher than 60 mg dm3, measured only during flood tide, can be considered exceptional. 4.2. Characteristics of the Stella river total suspended matter
Some chemical and mineralogical analyses on TSM, referred to seasonal surveys (October 1991, February, April and July 1992) pointed out a prevailing clastic fraction over the organic one (Table 1). In fact, the mean percentage of POC is 6.9 _+ 3.1%of TSM weight and are equal or lower to these in the lagoon, The average of Stella river TSM mineralogical composition, results as follows
102
<5
5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 >60
mg/l Fig, 3.Frequency distribution of Total Suspended Matter (TSM) concentrations in the Stella river.
TABLE 1 Main characteristics of suspended matter at the mouth of the Stella river October 1991 Parameters
Surf.
February 1992 April 1992
Bottom Surf.
Bottom Surf.
July 1992
Bottom Surf.
Bottom
2.95 10.13 5.04 7.30 6.17 36.89 Concentration (mgdm3) 9.13 43.8 48.0 47.4 30.5 40.5 Carbonate (%I 39.3 39.0 33.5 38.8 Organic carbon (mg dm-3) 506.2 2138.5 383.3 562.6 284.0 340.7 698.6 1667.1 5.6 Organic carbon (%I 5.5 4.9 13.0 5.6 4.7 11.3 4.5 11.6 13.1 7.3 8.9 9.8 6.7 7.7 9.5 C M ratio 7.2 8.7 8.6 12.2 Mean diameter (pn) 11.9 16.1 10.0 11.7 2.5 15.0 1.1 9.4 3.6 31.3 Salinity (%d 1.6 5.2 4.4 11.4 8.7 11.5 Modal diameter (pn) 13.6 21.8 11.4 14.2 22.6 25.6 23.9 33.5 5th percentile (pn) 29.3 38.3 21.7 28.5
(in percent): calcite (14 f 31, dolomite (50 f 91, quartz (18 f lo), feldspar (3 f l), illite (7f 41, kaolinite (2 It 21, chlorite (6 f 3).Dolomite is the most important mineral, followed by quartz and calcite. The sum of clay mineral percentages can reach 15%;illite and chlorite content are almost equal, kaolinite is lower. Montmorillonite or other expandable clay minerals were not detected.
103
~
-
.... .. ...... ... . . . ~
.... ....._
.........
i
.-.-..... ....
Diameter (yrn)
Fig. 4. Grain size frequency curve of the Stella river suspended matter (solid) compared with a lagoon basin one (dotted).Sampling of October 1991.
The grain size distribution of TSM shows a modal diameter ranging from 21.8 to 4.4 pm. The highest values relate to the measurements of October, during which a high water discharge was recorded; the lowest to April, in which the lowest tidal range was recorded. The mean diameter has a narrow range from 7.2 to 16.1pm. The diameter of the 5th (coarser)percentile does not exceed 40 pm. Figure 4 shows an example of grain size frequency curve of the Stella river particulate matter compared with one from the lagoon basin. 4.3. Water salinity
Salinity data (Fig. 5) show that brackish water (salinity <6%0) is confined to the northernmost part of the Cialisia channel, up to 1.5-2.0 km from the Stella mouth during ebb tide. During flood tide the area is further on restricted in a narrow strip along the lagoon border. 4.4. Radiocaesium content
Figure 6 reports the mean 137Csconcentrations measured in the superficial layer of bottom sediment samples collected in the Marano lagoon. The distribution of 137Csconcentrations range from 4 to 60 Bq kg-'. Table 2 indicates the mean 137Csconcentration measured in water and suspended matter of the Stella river. From May 1991 to November 1992 the 137Csconcentration in the Stella river water was quite uniform, ranging between 4.1 x 10"' and 12.0 x 10"' with a mean of 8 x lo4 Bq 1-'. A low variability it is also observed in the 137Cs concentrations measured in the suspended matter collected in the same period. Table 2 summarizes also the results of 13'Cs measurements in water and
104
Fig. 5. Surface salinity patterns in the Lignano basin near the Stella river mouth (ebb tide hemicycle):(a) October 8, 1991; (b) February 18, 1992; (c) April 28,1992; (d) July 30,1992.
suspended matter. I3'Cs concentrations in brackish water are from 2 to 4.5 times higher than those measured in fresh water. The concentrations of 137Cs in suspended matter are very close to that determined for samples taken in the Stella river.
105
Fig. 6. '37Cs in the superficial layer of Marano lagoon bottom sediments (Bq kg-'1.
In the same table the in situ Kdvalues are given for suspended matter from the Stella river and from the westernmost part of the Marano lagoon. The partitioning of 137Csbetween soluble and particulate phases is defined here as the ratio of 137Cssorbed in the particulate phase to the concentration of this radionuclide in solution.
106
TABLE 2 '37Cs concentrations and in situ Kd values
Location
Sampling periods
Number samples
Water (Bq l-'x104)
Suspended matter
(Bqg - ' ~ l O - ~ )
in situ Kd (mlg-1x10-5)
Stella River
May 1991 octo. 1991 Nov. 1992 May 1991 Oct. 1991
2 2 4 2 2
9f3 9f2 6f4 20f2 28f4
8.2M.2 11.35of0.001 9fl 7f2 7fl
1.of0.3 1.3f0.3 1.8f0.8 0.4m.i 0.25f0.01
Marano Lagoon
5. DISCUSSION AND CONCLUSIONS
The sediment 137Csconcentrations in the inner part of the lagoon, in which the influence of the Stella river prevails, are significantly higher than those measured in sediments taken near the lagoon inlet-zone (Fig. 6). As already seen, the salinity, together with the compositional and grain-size characteristics of the Stella particulate matter confirm that the influence of the river is restricted to the northern part of the Cialisia channel (Fig. 5). The bottom of the inner part of the lagoon is therefore a reservoir of fine sediments [ l l l , and consequently a sink for 13%s, transported by the Stella river. The decrease of caesium values from the Stella mouth towards the lagoon inlet can be due to the dispersion processes due to tidal stream or wind-induced resuspension on bottom sediments [121. As can be seen from the data given in Table 2, the in situ & values are very variable and highly dependent on the composition of water. The & values for suspended matter collected in the Stella river (mean conductivity value = 0.6 mS cm-') range between 1.0 and 1.8 ml g-' x 10'. The values determined for suspended matter collected in brackish water, where the mean conductivity value is fifty times higher (30 mS cm-'), are about five times lower (from 0.25 to 0.4 ml g-' x lo5)than those determined in the Stella river. K ' and Na' are the major ions competing with radiocaesium in brackish environments and there is a considerable literature 113,141 reporting values of & lower in the marine than in the fresh water environments. The differences in behaviour could be simply explained considering that when different water types interact, changes in the major chemical composition of the resulting water can alter the availability of 13'Cs adsorption sites on particulate matter. The desorption processes of 13%s on particles of the Stella river are favoured by an ion exchange mechanism due to the higher presence of ions such as K ' in the lagoon environment. The & presented in this paper cannot be evaluated in terms of K' and Na' variability in fresh and salt water because these data were not
107
measured during our samplings. Only for the Stella river literature values are available [Elfor ; the sampling area of the river considered in this paper, K+ values range from 2 to 3 ppm and Na’ from 3 to 40 ppm. Finally the in situ Kdvalues reported in Table 2 are higher (about two order of magnitude) if compared with measurements carried out by other authors [16]in similar environments. Our values could be explained considering the presence of significant concentrations of clay minerals (about 15%) in particles transported by the Stella river. Besides the TSM concentration values normally measured in Stella water can be compared to those in lagoon water (5-10 mg dm3), the flux of particulate, mainly inorganic matter from the Stella river cannot be considered negligible. Firstly, because the liquid discharge of the river is steady in time and its influence can be registered inside a conservative environment as the lagoon. Secondly, because during heavy rains the river solid discharge widely increases, thus providing an enrichment of caesium content in suspended matter coming from the Tagliamento mountain catchment area through the Ledra and Corn0 hydraulic systems. Combing the TSM data with the average annual discharge of Stella river, it is possible to evaluate the amount ofTSM and 137Cstransported into the lagoon. Assuming a TSM mean concentration value of 10 mg dm“, the corresponding annual amount cannot be lower than 10000 ton year-’, leading to an input of lo8 Bq year-’ of 137Csdischarged into the lagoon. 6. ACKNOWLEDGEMENTS
We wish to thank Prof. F. Princivalle, from the Mineralogy Institute of Trieste University for the clay mineral analyses and interpretation, and colleagues from the “Laboratorio di Biologia Marina” of Aurisina (Trieste) that kindly collaborated during the field work. The authors are also grateful to Dr. G. Mattassi and his group from “U.S.L. no. 8 Bassa Friulana” for the fruitful cooperation, and to the “Consorzio per la Bonifica e lo Sviluppo Agricolo della Bassa Friulana” for providing hydraulic and climate data. This work was partially supported by Italian MURST 60% “AmbientiUmidi” Research Project. 7. REFERENCES 1. 2.
3.
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