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mangrove system using environmental tracers in Babitonga Bay (Santa Catarina, Brazil)
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Continental Shelf Research 28 (2008) 682–695 www.elsevier.com/locate/csr
Hydrological mixing and geochemical processes characterization in an estuarine/mangrove system using environmental tracers in Babitonga Bay (Santa Catarina, Brazil) Virgı´ nia Barros Gracea,b, Josep Mas-Plac, Therezinha Oliveira Novaisb, Elisa Sacchid, Gian Maria Zuppia, a
Dipartimento di Scienze Ambientali, Universita` Ca’ Foscari, Dorsoduro 2137, 30123 Venezia, Italy b UnivilleUniversity of Joinville Region, Santa Catarina, Brazil c Departament de Cie`ncies Ambientals, and Centre de Geologia i Cartografia Ambiental (Geocamb), Universitat de Girona, 17071 Girona, Spain d Dipartimento di Scienze della Terra, Universita` di Pavia, Via Ferrata 1, 27100 Pavia, Italy Received 5 July 2007; received in revised form 28 November 2007; accepted 17 December 2007 Available online 31 December 2007
Abstract The hydrologic complex of Babitonga Bay (Brazil) forms a vast environmental complex where agriculture, shellfish farming, and industries coexist with a unique natural area of Atlantic rain forest and mangrove systems. The origin of different continental hydrological components, the environmental transition between saline and freshwaters, and the influence of the seasonality on Babitonga Bay waters are evaluated using isotopes and chemistry. End-member mixing analysis is used to explore hydrological processes in the bay. We show that a mixing of waters from different origins takes place in the bay modifying its chemical characteristics. Furthermore, biogeochemical processes related to well-developed mangrove systems are responsible for an efficient bromide uptake, which limit its use as a tracer as commonly used in non-biologically active environments. Seasonal behaviours are also distinguished from our datasets. The rainy season (April) provides a homogenization of the hydrological processes that is not seen after the dry season (October), when larger spatial differences appear and when the effects of biological processes on the bay hydrochemistry are more dynamic, or can be better recognized. Moreover, Cl/Br and stable isotopes of water molecule allow a neat definition of the hydrological and biogeochemical processes that control chemical composition in coastal and transition areas. r 2007 Elsevier Ltd. All rights reserved. Keywords: Babitonga Bay; Estuarine; Mangrove; Halogens; Stable isotopes; Mixing processes
1. Introduction Estuarine and transitional areas constitute natural reactors in which heterogeneous processes may affect the ecological equilibrium due to local changes in the sedimentation and in the biogeochemical processes and in the salinity conditions (Wolanski et al., 2007). Besides the natural sources, human inputs significantly contribute to the water chemistry. These include large volumes of poorly Corresponding author. Tel.: +39 0 412348666; fax: +39 0 412348584.
E-mail addresses: [email protected] (V. Barros Grace), [email protected] (J. Mas-Pla), [email protected] (T. Oliveira Novais), [email protected] (E. Sacchi), [email protected] (G.M. Zuppi). 0278-4343/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2007.12.006
defined human wastes that are sometimes discharged directly or after treatment into rivers and bays. Besides the bulk effluents, anthropogenic sources also supply agricultural chemicals and off-products of industrial processes. Solid loads, anthropic pollutants, and variable water salinity control the environmental conditions of these areas. The distinction between the effects of natural and human sources in a bay’s chemical processes is not always simple. Babitonga Bay (BB), one of the main estuarine formations with mangrove in the south of Brazil, is an excellent location to monitor and understand such processes. In this paper, we present a multi-parameter approach, combined with classical hydrochemical and isotopic
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methods, to depict those biogeochemical processes. All of them are investigated according to the hydrogeological setting of the BB and its inland watershed which define the main controls on surface and groundwater, and finally upon the estuarine water quality. Our aim is to seek for a process-based model that describes the hydrochemical and nutrient functioning of bay and related lowland permeable catchments detecting, also, the effect of organic matter (OM) (natural and anthropical) in the bay. Environmental tracers as halogens and stable isotopes are used in this research as they describe the movement of water through hydrological systems. Nevertheless, biogeochemical processes, considered as a whole in a bay environment, may even modify this behaviour and produce a change of their content. For instance, chloride and bromide are generally used as tracers in complex environment, because their low chemical reactivity. As a consequence, the Cl/Br ratio is a conservative indicator for the origin and movement of natural waters (Fontes and Matray, 1993; Edmunds, 1996; Davis et al., 1998; Vengosh and Pankranov, 1998). The ratio in freshwaters is primarily controlled by the initial ratio in precipitation, which in coastal regions must have a value similar to that of seawater (Fontes and Matray, 1993), although water–rock interactions or human pressures may affect its original value. Halogens in rainfall result initially from physical processes that entrain atmospheric aerosols and control their size (Junge, 1972). Near coastal areas, the Cl/Br ratio also reflects the influence of ocean spray on local precipitation (Kaufmann et al., 1984; Long et al., 1993; Davis et al., 1998; Alcala´ and Custodio, 2004). In the study area, this ratio remains relatively inert during transport above the Atlantic Forest and Serra do Mar, despite other atmospheric processes like continentality, elevation, and precipitation amount (Winchester and Duce, 1967; Brimblecombe, 1986; Goni et al., 2001). Studies in continental and urban areas demonstrate, on the contrary, a lower Cl/Br ratio in precipitation (Slanina et al., 1982; Vengosh and Pankranov, 1998). This relatively low Cl/Br ratio was attributed to a contribution of bromine (Vengosh and Pankranov, 1998; Davis et al., 2001), showing the effects of human pressures on natural processes. However, significant assimilation of bromide by green plants and peat deposits is presently well documented (Lundstrom and Olin, 1986; Vengosh and Pankranov, 1998; Whitmer et al., 2000; Hammer et al., 2005). Bromide is assimilated like adsorbable organic bromine (AOBr), following seasonal cycles (Putschew et al., 2003). Actually in wetlands, lakes, and lagoon, in late summer during periods of rapid plant growth, bromide concentrations assimilated in OM are higher than those during the rest of the year (Whitmer et al., 2000; Carpenter et al., 2003; Hammer et al., 2005). The non-conservative behaviour of bromide is caused by the strong preference for Br over Cl by the plants and by photosynthetic bacteria, or by better chemical interactions between organic matrixes (Blodau and Moore, 2002).
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Furthermore, stable isotopes of the water molecule (d18O and dD) are also appropriate tracers to depict surface and groundwater flow lines and recharge areas (Clark and Fritz, 1997; Kendall and McDonnell, 1998). Both of them, halogens and stable isotopes are studied using end-member mixing analysis to describe the influence of different water inputs into the saline BB water, as well as the occurrence of biogeochemical processes in this particular environment. 2. Site environment description The estuarine system of BB covers an area of 1400 km2, including the last great south hemisphere’s mangrove ecosystem (130 km2; Fig. 1). The complex is located northeast of Joinville, the most industrialized and urbanized city of Santa Catarina State. The bay’s drainage basin (261020 –261280 S and 481280 –481500 W) forms a vast environment, where agriculture and shellfish farming as well as a broad spectrum of industries, like textile, ‘‘metalmechanical’’, foundries, etc., coexist with a unique natural area of Atlantic Forest. The BB is connected to the Atlantic Ocean via an opening of 1850 m and presents a salt-wedge circulation system. A significant tide oscillation brings an appreciable water renewal to whole ecosystem. Tides are mixed with semidiurnal dominance and diurnal inequalities (Truccolo and Schettini, 1999). Non-linear effects result in the amplification of the astronomical constituents towards the interior of the bay, presenting a hypersynchronous behaviour. The mean tidal amplitudes in BB range from 0.14 m during neap tides to 1.53 m during spring tides (FEMAR, 2000). The region is characterized by a humid subtropical climate with a rainfall average of 2000 mm/year. It presents two differentiated seasons: summer (from November to April) and winter (from May to October). During summer, weather is characterized by high temperature and humidity, with intense precipitation. During winter, the influence of polar air masses brings a decline of temperature and precipitation in the region. Geomorphological analysis reveals large differences in lithology and structure, including the occurrence of welldeveloped Quaternary formations. The basin and range topography of the region generates pronounced altimetric contrasts. The whole Babitonga basin is characterized by four geological domains: granites and a complex Paleozoic tectonic basement; molasses; associated vulcanites; and sedimentary deposits from the Quaternary. It is divided in four hydrographic watersheds: Cachoeira, Palmital, Cubata˜o, and Parati. The Cachoeira River basin has an area of 84.82 km2, completely located in the urban area of Joinville, and it runs out to Saguac- u Lagoon. There is no accurate information about the Cachoeira River discharge, roughly estimated as 3–5 m3/s. Water quality is very low due to pollution from domestic and industrials effluents. Only 16.40% of the domestic sewages receive partial water treatment. Nevertheless, the largest chemical pollution of
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Fig. 1. Geographical sketch of the Babitonga Bay area, and location of the sampling points within the bay and its continental surrounding areas.
Cachoeira River is attributed to untreated industrial wastes. The Palmital River basin drains an area of 357.60 km2 and its estuary receives contributions of several streams, all of them under tidal influence. The Palmital River is almost completely surrounded by mangrove forests. The Cubata˜o River, running for more than 75 km, constitutes the largest watershed of the whole hydrologic complex of BB (484 km2). Its river head is located at the Serra do Mar (1300 m a.s.l.). Mean discharge at the Cubata˜o River mouth is 17.70 m3/s, but it varies strongly as a function of rainfall distribution and intensity on the watershed. A large percentage of the Joinville domestic and industrial supply is derived from the Cubata˜o River. In addition, Cubata˜o waters are also collected for agricultural purposes. The Parati River basin has an area of 72.20 km2 partly preserved and partly occupied by agriculture. Groundwater occurrence is related to three main aquifer formations: the weathered crystalline rocks, the fractured granulite complex, and the Quaternary sedimentary formations (Baggio, 1997). Groundwater movement is generally towards BB. According to Baggio (1997), the hydraulic gradient is of 104, generally controlled by tidal oscillation, and the underground discharge rate from the crystalline aquifer to surface waters is up to 15 m3/h. 3. Sampling and analytical techniques Hydrochemical and isotopic data were obtained from 10 sampling points in BB for surface water (numbered from 1
to 10, sampled at two different depths—labelled S for surface and F for deep samples) and fit in the sampling network (Fig. 1) established in the past (CENTRAN, 2004) to cover the BB environment. Further information have been achieved for the Cubata˜o and Cachoeira River basins from two wells in the alluvial formation and two on the granite basement, for Cubata˜o: Rud (alluvial) and Hub (granite basement); and for Cachoeira: NHJ (alluvial) and TC (granite basement). Moreover, two freshwater springs located in one of the main islands within the bay (Sub1, Sub2) and two surface water points (Cub, Cach) have been sampled (Fig. 1). Analyses (Table 1) include electrical conductivity (EC), pH, major ions, trace elements and heavy metals (not discussed here), and stable isotopes of the water molecule. Electrical conductivity, pH, and redox potential were measured in situ, as well as, in most cases alkalinity by HCl titration. Samples were subsequently filtered at 0.45 mm and collected in pre-cleaned bottles. A sample aliquot was acidified to 1% HNO3 for the analysis of cations and metals. Field analyses were repeated for control in the laboratory. Chemical analyses were performed by the Dipartimento di Scienze Ambientali, University of Venice. Anions were determined by ion chromatography, whilst cations and trace elements were determined by atomic absorption, ion chromatography, and inductively coupled plasma source-atomic emission spectroscopy (ICPS-AES). All reported values have an ionic balance within 7.5%. Samples for stable isotope analysis were collected according to the procedures described by Clark and Fritz (1997). Hydrogen isotope
T
pH
0.22 3.52 6.21 20.30 14.60 24.30 33.60 36.30 36.40 39.60 34.70 34.50 30.30 32.60 27.80 28.70 26.70 26.70 26.50 28.80 0.09 0.05 0.16 0.80 0.07 0.14 1.06
Eh
160 186 195 201 188 197 176 166 173 166 167 168 171 165 163 85 163 8 9 111 114 35
Alkalinity
SO4
Cl
Br
F
Ca
Mg
Na
20.3 27.3 25.1 49.2 46.7 85.6 101.3
21.5 108.5 194.5 852.5 609.5 1092.0 2030.5 2245.0 2175.0 2445.0 1711.5 1237.5 1727.5 1755.5 1464.5 1529.0 1451.5 1462.0 1440.0 1569.5
131.5 964.5 1913.0 7931.5 5277.5 9823.5 15520.0 14530.0 16660.0 17460.0 14420.0 15030.0 12810.0 14090.0 11090.0 11620.0 10380.0 10480.0 10360.0 11640.0 5.8 6.0 6.8 52.0 13.0 13.0 191.8
0.0 3.2 4.7 15.5 12.0 20.5 31.5 27.5 31.5 34.5 31.5 22.5 26.5 23.0 21.0 22.0 23.0 18.0 18.5 25.0
29.0 10.0 8.0 5.5 2.5 3.0 8.5
4.7 29.7 51.4 201.0 133.8 255.5 442.0 456.5 475.8 515.0 429.5 429.5 366.5 403.2 325.2 334.5 293.0 273.2 287.2 336.0 6.9 2.9 15.8 68.9 1.1 21.1 2.0
3.8 82.5 159.0 426.0 301.5 535.5 772.5 754.5 844.5 1218.0 736.5 771.0 739.5 763.5 601.5 648.0 580.5 591.0 532.5 927.0 6.0 2.6 9.6 35.4 0.8 0.9 1.0
252 1020 2460 2440 5920 4220 7320 9820 10440 11400 9060 10180 8860 9340 8080 7580 7360 7240 6960 7740 6.6 16.3 12.28 89 11.14 9.3 248
124.8 133.3 119.6 116.3 114.1 95.4 99.0 92.3 101.6 102.2 103.8 40.3 12.0 88.5 347.4 12.0 58.7 95.2
2.7 9.5 4.0 20.8
0.5
4.0 2.5 3.5 1.0 7.5 8.5 7.0 1.5 7.5 3.5 5.5 9.0 2.0 4.3 1.4 2.5 5.5 2.5 3.3
K
3.3 43.3 168.2 109.4 199.6 305.2 325.6 336.4 378.4 303.6 313.2 265.2 327.2 264.4 250.8 220.4 249.2 230.0 243.2 1.4 1.6 1.5 25.7 2.3 2.2 2.4
dD
d18O
19.88
4.42
12.43 14.17 9.15 1.86 3.21 4.00 0.12 0.63 0.38
3.11
4.80
21.41 19.88 11.51 19.45 19.79
4.04 2.33 3.08 1.69 0.51 0.16 0.27 0.59 0.28 0.27 0.72 0.45 1.01 1.01 1.10 1.05 1.23 0.94 4.58 4.65 4.23 3.03 4.06 3.53 4.11
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April survey (24 April 2004) 1S 22.8 8.90 1F 22.8 8.40 2S 23.0 8.16 2F 24.0 7.72 3S 24.4 7.50 3F 24.7 7.70 4S 25.5 7.03 4F 25.0 7.50 5S 24.9 7.26 5F 24.9 7.45 6S 25.5 7.40 6F 25.4 7.54 7S 25.1 7.67 7F 25.4 7.60 8S 25.3 7.69 8F 25.2 7.68 9S 25.3 7.68 9F 24.8 7.65 10S 25.4 7.70 10F 25.8 7.71 Rud 23.1 8.87 Cub 21.4 8.70 Hub 22.1 8.90 NHJ 23.3 8.15 SUB1 22.9 9.07 SUB2 23.9 7.70 TC 23.5 8.06
EC
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V. B. Grace et al. / Continental Shelf Research 28 (2008) 682–695
Table 1 Hydrochemical data for April and October surveys at Babitonga Bay
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Table 1 (continued ) Point
Point
T
pH
T
pH
Eh
Alkalinity
SO4
Cl
Br
F
Ca
Mg
Na
K
d18O
dD
Eh
Alkalinity
SO4
Cl
Br
F
Ca
Mg
Na
K
dD
d18O
DO
3.30 11.74 21.57 23.78 30.94 31.04 47.19 48.10 49.85 50.21 49.77 47.96 46.03 46.13 42.46 42.44 41.44 41.21 34.58 37.33 47.33 42.92 0.16 0.11 0.24 0.62 0.11 0.17 0.49
71 59 141 129 77 73 45 63 123 111 129 122 129 136 133 40 40 56 35 42 85 81 104 62 12 8 43 33 4
36.6 48.8 73.2 85.4 85.4 97.6 109.8 122.0 109.8 122.0 109.8 97.6 97.6 109.8 109.8 146.3 85.4 97.6 122.0 109.8 134.1 134.1 73.2 97.6 73.2 274.4 24.4 36.6 85.4
419.0 568.9 1197.2 1375.3 1625.7 1644.0 2459.8 2526.6 2656.2 2718.5 2564.2 2623.0 2387.0 2395.0 2158.5 2169.4 2134.2 2054.6 1573.2 1865.1 2579.5 2484.5 7.3 7.0 8.2 28.5 8.0
944.1 1350.9 6506.4 8758.0 10883.3 10946.4 18374.5 17736.2 19314.4 19763.3 18651.6 18816.4 17259.2 17231.2 17259.2 15183.0 14362.4 12079.2 10820.0 13562.7 18325.4 17946.6 14.1 11.8 18.1 16.3 37.2 38.7 43.2
0.3 3.1 16.8 18.6 20.5 20.7 52.0 52.0 56.0 56.2 58.7 54.5 45.5 43.2 37.1 35.1 32.3 32.8 18.9 28.1 54.5 56.5 0.0 0.0 0.0 0.0 0.1 0.6 0.1
14.8 94.0 108.8 108.8 113.8 116.8 128.6 136.0 133.1 132.1 128.6 135.6 125.7 133.1 123.2 126.2 123.7 119.7 113.8 119.2 130.1 127.1 2.8 2.4 3.9 2.2 2.0 2.1 0.0
35.9 85.8 176.7 213.4 292.9 237.3 392.6 422.9 414.1 416.6 407.8 404.0 316.9 383.8 329.5 295.4 314.4 309.3 253.8 277.7 435.6 491.1 6.8 3.1 17.3 69.5 1.3 9.4 18.5
60.8 159.3 331.0 485.4 568.1 666.9 1372.1 1332.6 1519.1 1293.1 1511.7 1517.8 853.4 1257.2 888.0 900.3 895.4 913.9 548.3 863.3 1074.5 1283.2 3.8 1.2 8.3 17.4 0.9 1.0 4.0
576 1338 4204 5303 6081 5981 9825 10170 10646 10467 10507 10760 8105 9486 8453 7878 8140 7896 6127 7217 9997 9887 7.2 4.8 12.1 13.7 10.2 10.7 43.6
60.1 124.9 205.3 220.2 476.3 215.2 364.9 461.4 416.9 437.9 387.2 372.3 346.4 418.1 345.1 277.1 310.5 314.2 290.7 277.1 440.4 430.5 1.0 1.0 3.2 10.3 1.4 2.4 7.5
15.75 14.70 8.19 5.26 1.66 1.75 1.84 1.82 3.11 3.32 3.05 3.27 2.72 2.60 1.90 1.98 2.33 0.78 4.58 1.18 3.11 3.61 20.21 19.52 19.01 9.87 17.57 13.71 13.94
3.61 3.73 1.75 1.58 0.99 0.95 0.10 0.13 0.34 0.47 0.34 0.45 0.35 0.21 0.01 0.06 0.10 0.19 1.19 0.62 0.25 0.41 5.61 5.87 6.14 2.82 3.69 2.98 3.36
4.26 5.86 5.02 4.62 4.84 3.77 5.85 5.16 6.09 5.67 5.55 5.77 5.29 4.98 5.66 5.08 5.78 4.7 3.64 3.91 4.64 4.89 4.25 5.48 2.07 2.33 3.99 2.7 0.19
34.9
Temperature (T) is given in 1C, electrical conductivity (EC) in mS/cm, redox potential (Eh) is in mV, alkalinity (as HCO3), dissolved oxygen (DO), and other major elements/compounds are in mg/L, isotopes are in % of their respective standards of reference.
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October survey (18 October 2004) 1S 22.9 6.77 1F 22.1 7.27 2S 22.5 7.35 2F 22.5 7.40 3S 22.7 7.54 3F 22.7 7.54 4S 22.9 8.01 4F 22.2 7.98 5S 22.3 8.05 5F 21.6 8.07 6S 22.2 8.07 6F 22.3 8.07 7S 22.5 7.96 7F 22.4 7.95 8S 23.8 7.74 8F 23.8 7.73 9S 25.0 7.64 9F 24.6 7.64 10S 23.6 7.45 10F 23.6 7.55 G1S 22.1 7.99 G1F 22.4 7.98 Rud 23.0 6.41 Cub 21.4 6.25 Hub 21.7 6.66 NHJ 22.1 7.23 Sub1 20.6 5.86 Sub2 21.6 6.13 Cachoeira 23.6 6.89
EC
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composition was measured by water reduction over metallic zinc (Coleman et al., 1982), whilst d18O was analysed by water–CO2 equilibration at 25 1C (Epstein and Mayeda, 1953); both results are expressed in V-SMOW (Gonfiantini, 1978; Gonfiantini et al., 1995). The analytical errors are 71% and 70.1%, respectively. All gases were analyzed on a Finnigan MAT 250 Mass Spectrometer at ISO4 s.s., Torino, Italy. 4. Results Chemical and isotopic data reflect how the BB waters are connected to, and affected by, seasonal, regional, and local hydrological processes. Water chemistry and isotopes are strongly influenced by both freshwater flow and water exchange between the continent and the ocean (Table 1). These data are evaluated to discuss the origin of different hydrological components and their mixing, the influence of seasonality, and the role of biological processes in the transition environment between saline and freshwaters. 4.1. Chemical data The spatial and temporal variation of the entire hydrology of BB is firstly reflected by conductivity data (Fig. 2). EC distribution shows the influence of the Atlantic Ocean on the bay waters. The EC lay in a range about 70% of that of seawater (represented by sampling point 5) in an extended part of the bay (points 4, 6, 7, 8, 9, and 10), and it progressively diminishes in its inner sectors (points 1, 2, and 3). EC of continental water in rivers is significantly lower than that of the bay by more than two orders of
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magnitude. Temporal variability is pointed out by the difference between April and October conductivity datasets. Conductivity values for the bay sampling points are consistently larger in October than those in April. Sampling locations in the estuarine area show seasonal increments from 27% to 55%, being larger in most of the points closer to the coastline. Contrarily, those points related to freshwater (wells, rivers, and springs) show a notable decrease in conductivity, and a minor seasonal difference. The difference between the wet (April) and the dry (October) season shows the dilution effect by continental water inputs within the bay. Similar observations can be achieved by pH that varies from 8.1 to 6.7 (being the range of continental waters from 5.6 to 7.3) as consequence of ocean water dilution. BB waters undergo to stratified reduced conditions, as indicated by Eh variations. In particular, at the river mouths, where seawater arrives particularly altered, the high load in continental OM gives place to a more reduced environment. The order of cation dominance is Na–Mg– Ca4K, while anion dominance is Cl4SO4–HCO3 (Table 1). Although the high load of OM and consequential reduction processes, chloride and sulphate can be used as tracers in the mixing processes that occur between continental and thus reduced water and ocean waters in the BB. Lowest values correspond to river mouth samples, whilst the highest to ocean ones, following the mixing ratio between continental and marine waters (Barros, 2005). The movement of the tides into and out of the BB and the associated effects created by the incursion of oxidized waters and posterior retraction control the redox changes
Fig. 2. Electrical conductivity distribution at sampling locations for April and October surveys.
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Fig. 3. Sulphate versus chloride relationship for April and October surveys.
in water bodies, and as consequence the presence of dissolved nutrients (Schettini et al., 1996; Barros, 2005). Fig. 3 shows a neat linear relationship between the two species in both surveys. April data, however, show a larger dispersion for chloride concentrations lower than 20.0 g/L (SO4 ¼ 0.13Cl32.4; with R2 ¼ 0.91 and n ¼ 18), whereas October data fit a better linearity (SO4 ¼ 0.12Cl+378.95; R2 ¼ 0.98 and n ¼ 18); which can be attributed to a heterogeneous behaviour of the bay hydrologic dynamics (local continental inflow variability, tides, etc.). Freshwater samples (Clo200 mg/L) lay on linear ratio, except those located in the Cachoeira River watershed (NHJ, Cach) that show a slight increase on sulphate as well (Table 1). The relationship between chloride and bromine at the Babitonga way is plotted in Fig. 4, and it allows observing seasonal and spatial effects at the bay area. A first observation refers to the overall depletion of bromine if compared to the ratio in oceanic waters with a mean dissolved chloride of 19.3 g/L and bromine 66 mg/L; that is, Cl/Br ¼ 293 (Fontes and Matray, 1993). Notice that for the ocean waters at the BB (sampling point 5), this average ratio is set to Cl/Br ¼ 517, and 348 for the April and October surveys, respectively; showing a deficit of bromine with respect to the reported mean value in oceans. The good fit to the local ocean ratio observed for the April data is not reproduced by the October results, where a depletion of bromine persists in some bay waters with respect to the local oceanic ratio of Cl/Br ¼ 348. In particular, large Cl/Br ratios were observed in those sampling locations closer to the shoreline (points 1, 2, 3, 8, 9, and 10), whereas those located in the central axis of the bay or close to its mouth (points 4, 5, 6, and 7) show a ratio similar to the local oceanic value. Remarkably, point 1S shows a very
Fig. 4. Bromine versus Chloride relationship for April and October surveys. Constant Cl/Br lines for mean ocean and local values are also plotted (see text for explanation).
large Cl/Br ratio of 4383 and 3405 for the April and October surveys (Fig. 5). Such distribution supports the idea that an effective bromine uptake by biological processes (Lundstrom and Olin, 1986; Vengosh and Pankranov, 1998; Whitmer et al., 2000; Hammer et al., 2005) occurs at the BB, which is noticeable at those points located near the coast, areas with low depths and gently preserved from the main tidal influences, and exceptionally at point 1S (Table 1). Human pollution is then discarded as a possible water contribution because local industrial and urban wastewaters possess a low Cl/Br ratio (Cl/Br ¼ 220). The comparison between seasonal and spatial influences upon the Cl/Br ratio is more clearly seen in Fig. 5. In this plot, the depletion in bromide at those points close to the shoreline, resulting in a higher Cl/Br ratio, stands out in comparison with the bay locations; reaching values even higher than those of the April survey. The variations of Cl/Br with chloride concentrations show high ratio values for bay waters with chlorides concentrations below 15.0 g/L, approximately (Table 1); that is, in those locations near to the bay shoreline. 4.2. Isotopic data The d18O and dD stable isotopic content is reported in Fig. 6, and it offers a complementary insight on the origin of water and the mixing processes in BB. Fig. 6 allows differentiating three distinct sample groups, especially in the October survey. Continental water samples from groundwater related to the Cubata˜o River basin (group 1; Cub, Rud, Hub) show the lightest isotopic content. Their departure from the
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Fig. 5. Distribution of the Cl/Br ratio at sampling locations for April and October surveys.
Fig. 6. Water stable isotope content (d18O and dD) for April and October surveys. Class groups based on the October survey dataset are described in the text.
global meteoric water line (GMWL) points out an enrichment in deuterium that can be attributed to fractionation processes of local rainfall, that is, re-evaporation; in particular, during the advection of water vapour from Atlantic Ocean to the Serra do Mar. Deuterium excess, defined by Dansgaard (1964) as d-excess ¼ dD8d18O, is around 25–30% for this group, being larger than other
continental waters of the area (10–15%; see next sample group 2) for the October dataset. The isotopic composition of local precipitation is thus primarily controlled by regional scale processes, like the trajectories of the water vapour transport over the continent and the average rainfall history of the air masses giving precipitation at a particular place (Epstein and Mayeda, 1953; Craig, 1961;
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Dansgaard, 1964; Craig and Gordon, 1965; Gonfiantini, 1965, 1998; Merlivat and Jouzel, 1979; Siegenthaler and Oeschger, 1980; Rozanski et al., 1992, 1993; Jouzel et al., 1997). The high d-excess values of surface and groundwaters suggest that an isotopically fractionated evapotranspiration flux contributes to the atmospheric water balance over the region, similar to the steady-state evapotranspiration model developed for the Amazon Basin (Gat and Matsui, 1991; Martinelli et al., 1996; Maurice-Bourgoin et al., 2003). Moreover, d-excess values may reflect also a decrease in condensation temperature as indicated by the elevation of the Serra do Mar rising inland along this transect. This suggests that also an altitude effect contributes to the control on the d-excess values of surface and groundwaters. The large transpiration fluxes from flooded areas and from the Mata Atlantica forest, coupled to moisture transported from the Ocean and the BB, contribute to the significant amount of rainfall (2000 mm/year) on Serra do Mar in this region. Thus, the increase in d-excess inland is most easily explained by the contribution of recycled moisture via evaporation and re-precipitation. Moreover the increase in d-excess suggests that evaporation/reprecipitation cycles occur repeatedly along the storm track. A second group (group 2 in Fig. 6) is constituted by surface and groundwater from the Cachoeira River (sampling points Cach, NHJ) including the deep well TC from the April survey, as well as bay points 1S and 1F both linked to Cubata˜o River contribution, and the island springs (sampling points Sub1, Sub2). They show an isotopic content that aligns parallel to the GMWL,
revealing the continental origin of the water, specifically from the basins surrounding the bay, and the almost absence of mixing with ocean water. Specifically, these first two groups show a very similar isotopic content in the April survey, and they lay very close in the plot. Such similarity is attributed to the effective recharge of rainfall and surface water to the aquifers during the wet season. Finally, a third group is arranged in a line from the second group towards the ocean isotopic composition in both surveys. Isotopic content of sampling point 5 is taken as the ocean reference. This last group is constituted by sampling points located in the bay area, and they reflect the variable influence of ocean water. 5. Hydrological mixing and biological processes Mixing processes between fresh and ocean water are better represented in Fig. 7, showing the relationship between chloride and d18O data, and therefore combining chemical and isotopic information. Mixing lines have been drawn assuming an average linear composition of both components of the mixing system members (Albare`de, 1995). For the April survey, the homogeneity in the values of both components in the continental pole (previous groups 1 and 2) does not allow a further distinction of their origin. In any case, data from the April survey convey a complete mixing process as actual data overlay the estimated mixing line. For the October dataset, two continental end-members are defined, based on the first and second groups already mentioned, and two distinct mixing lines (‘‘October 1’’ and
Fig. 7. d18O content versus chloride for April and October surveys, and possible mixing lines between identified end-members. Symbols within the mixing lines correspond to proportions of each end-member set at 0.1%, 1.0%, 10%, 20%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, 97.5%, 99%, and 99.9%.
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‘‘October 2’’, respectively) are drawn accordingly in Fig. 7. It can be observed that the line originated from the second group, that is, the one corresponding to continental waters from the Cachoeira basin (group 2), fits with the measured composition of the bay water samples, suggesting an efficient groundwater flow towards the bay originated in the surrounding aquifers that mixes with ocean water. A minor influence of continental waters is seen from the group related to the hydrographic network of the plain, as the Cubata˜o River. Maximum continental water contribution (both from surface and from aquifer) is estimated in a E20% in point 1, and it decreases towards the ocean. In particular, sampling locations 1S and 1F, located at the end of a large tidal channel with chloride concentrations near 1000 mg/L, lay away from the best mixing line (line ‘‘October 1’’ in Fig. 7), pointing to a slight contribution from the Cubata˜o River discharge. An additional mixing analysis can be performed using chloride and bromine as the two components of a system. Results are also plotted in Fig. 8, where ratio Cl/Br for the mixing line is estimated as the quotient between the chloride and bromide concentrations for a given mixing proportion; that is, Cl Cl1 x1 þ Cl2 x2 ¼ Br Br1 x1 þ Br2 x2 where Cli and Bri are the chloride and bromide concentrations of end-member i (i ¼ 1, 2), and xi is the proportion of each end-member being x1+x2 ¼ 1. Indeed, constant Cl/Br lines in Figs. 4 and 5 are based upon the erroneous assumption that the ratio will remain constant through the water mixing process. Despite the Cl/Br signature of rainfall in surface and groundwater, bay
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waters will reflect the effects of biogeochemical cycles as well as human pressures, and this will therefore produce a modification of the Cl/Br ratio. In this mixing analysis, the two chosen end-members are points 5S and 5F, for the ocean side, and freshwater spring samples of an island located in the middle of the bay (points Sub1 and Sub2) and the deep well Hub in the plain. This second end-member is considered as representative of the continental input mainly as groundwater as concluded from Fig. 7. In fact, deep crystalline aquifers are confined (Baggio, 1997), and thus groundwater is not in contact with mangrove or anthropogenic systems. Particularly, sampling points Sub1 and Sub2 represent the outcropping of groundwater in a central island in the bay. Such upwelling is attributed to the fault system that affects the geological structure of the area, and that acts as a preferential flow path that govern this upward flow. Such explanation satisfies their hydrochemical composition, characterized by a low chloride content, and their isotopic signature as well. For that reason, we make use of them as representative of the continental end-member. Mixing line for the April survey in Fig. 8 shows an acceptable fit with actual data, as expected from the relatively constancy of the Cl/Br ratio as already shown in Fig. 4. Nevertheless, the October survey data show a significant variation of this ratio that place actual data above the hypothetical mixing line. As the influence of the continental end-member (location Sub2) on the mixing line becomes significant at proportions above 90%, we shall assume that the bromine behaviour is mainly due to the non-conservative nature of this element in the mangrove environment suggesting biological uptake, and it is thus not related to the exactness of the component concentration defined at the continental pole.
Fig. 8. Value of the Cl/Br ratio versus chloride concentration for April and October surveys, and possible mixing lines between identified end-members.
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In that way, bromine uptake by biogeochemical processes can be quantified assuming the conservative behaviour of chloride. We shall use the actual concentration of chloride at each sampling location to estimate the mixing proportions of the two end-members, that is, Cl ¼ Cl1 x1 þ Cl2 ð1 x1 Þ then, ClCl1 x1 ¼ Cl 1 Cl2
x2 ¼ 1 x1 where Cl is the measured chloride concentration, and Cli and xi were described before. Once the mixing proportions (x1, x2) are known, we estimate the bromine concentration corresponding to a conservative mixing process (Br*), and subsequently its difference from actual bromine concentrations. Using location points 5 and Sub2 as the two endmembers, with chloride and bromine concentrations of 19.54 g/L and 56.1 mg/L, and 38.7 and 0.59 mg/L, respectively, the mixing proportions for the BB water samples and their deviation from the conservative bromine concentration for the October survey are calculated and presented in Table 2. Results show that relevant bromine uptake occurs in locations 1, 2, 3, 7, 8, 9, and 10. Deviation from the supposed conservative behaviour is considerably larger than possible sampling or analytical errors. Mean deviation factor for those locations is 32.175.1%
(excluding 1S), which is attributed to the assimilation of bromine by mangrove vegetation. In BB, OM has generally double origin: natural and anthropic (Barros, 2005). However, the special location of point 1S in a tidal channel, within a well-developed mangrove site, justifies its large deviation factor due to the accentuated bromine assimilation by green plants, depending of vegetation seasonal cycles (Putschew et al., 2003; Hammer et al., 2005). Organic uptake in BB samples is furthermore supported by the relation between SO4 and Br (Table 1 and Fig. 9) which slope is higher than that of classical seawater dilution. On the contrary, the SO4/Cl ratios (Table 1 and Fig. 3) are similar to that of seawater. Besides, in both cases, the lowest values correspond to river mouth samples, whilst the highest to ocean ones, following the mixing ratio between continental and marine waters (Barros, 2005). There is also a seasonal effect of bromide depletion with respect to sulphate in October in the bay waters, as described in Fig. 4. The movement of the tides into and out of the BB and the associated effects created by the incursion of oxidized waters and posterior retraction control the redox changes in water bodies and, consequently, the presence of dissolved nutrients (Barros, 2005). In fact, environmental stress like the lack of electron acceptors, such as dissolved nutrients, favours the formation of organic bromine compounds (Putschew et al., 2003). Moreover, concentrations of organically bound bromide in OM or in peat strongly depend on the degree
Table 2 Estimation of bromine uptake factor using mixing analysis Point
1S 1F 2S 2F 3S 3F 4S 4F 5S 5F 6S 6F 7S 7F 8S 8F 9S 9F 10S 10F G1S G1F
Measured concentrations
Mixing proportions
Cl (mg/L)
Br (mg/L)
x1
x2
Br* (mg/L)
BrBr*
Deviation factor
944.1 1350.9 6506.4 8758.0 10883.3 10946.4 18374.5 17736.2 19314.4 19763.3 18651.6 18816.4 17259.2 17231.2 17259.2 15183.0 14362.4 12079.2 10820.0 13562.7 18325.4 17946.6
0.28 3.09 16.78 18.58 20.49 20.71 51.98 52.01 55.96 56.25 58.73 54.52 45.47 43.19 37.12 35.09 32.29 32.81 18.91 28.08 54.53 56.48
0.954 0.933 0.668 0.553 0.444 0.441 0.060 0.092 0.012 0.012 0.045 0.037 0.117 0.118 0.117 0.223 0.265 0.383 0.447 0.306 0.062 0.082
0.046 0.067 0.332 0.447 0.556 0.559 0.940 0.908 0.988 1.012 0.955 0.963 0.883 0.882 0.883 0.777 0.735 0.617 0.553 0.694 0.938 0.918
3.17 4.33 19.00 25.41 31.46 31.64 52.79 50.97 55.46 56.74 53.57 54.04 49.61 49.53 49.61 43.70 41.36 34.87 31.28 39.09 52.65 51.57
2.89 1.24 2.23 6.83 10.97 10.93 0.81 1.04 0.50 0.49 5.15 0.48 4.14 6.34 12.49 8.61 9.07 2.05 12.37 11.01 1.88 4.92
1042.59 40.15 13.27 36.74 53.57 52.80 1.55 2.00 0.89 0.87 8.77 0.88 9.11 14.68 33.66 24.53 28.08 6.25 65.41 39.19 3.45 8.70
Deviation factor (%) ¼ 100(BrBr*)/Br.
Estimated bromine
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6. Conclusions The main conclusions are summarized as follows:
Fig. 9. Sulphate versus bromine relationship for the sampling points in April and October surveys.
of organic compound decomposition: bromide is primarily bound to OM molecules (Biester et al., 2006). The input of OM in BB is very substantial. Its main input is due to flooding of the major rivers and is, therefore, prevalent in the rainy season. The application of bromine as a tracer of biogeochemical processes in estuarine/mangrove systems underlines the anthropic activity in BB environment throughout the estuarine zone. This is particularly evident in the superficial portion of the water body and during the warmer season. In addition to the significant assimilation of bromide by green plants and peat deposits, in Cachoeira basin, which is completely an urban setting, industrial and domestic wastewaters bring relatively greater quantities of Br (0.50 ppm) described by a low Cl/Br ratio (220). Urban effluents are, thus, characterized by an excess of bromine and bromo-compounds used for the dyeing and finishing processes of the textile industries, flameproofing, biomass, and sanitizers in sewages of agri-food industries (Magazinovic et al., 2004; Reineke et al., 2006). These observations are in agreement with the information obtained with 13C in OM and 15N (Barros, 2005). BB cannot be reasonably modelled by merely varying the proportion of a marine and a general terrestrial endmember because of the described biogeochemical processes. Carbon and nitrogen isotopes and the C/N ratio have additionally indicated that, although most of the OM has a riverine source in sample sites near the coast (1, 2, and 3), and a marine source in other sites (5, 6, 7, and 8), a component of sewage OM is also present in several sampling sites of the BB.
(1) The participation of all the hydrologic cycle components (surface runoff, groundwater flow, and ocean seawater) in a estuarine mass balance can be verified in BB, showing seasonal differences as indicated by chemical and isotopic data. (2) Signal of the mixing between continental and ocean waters is observed in the whole bay until its mouth, as suggested by stable isotopes and Cl/Br ratio. Continental water proportions in bay water may reach up to 20% in the inner parts of BB. (3) Bromine acts as a non-conservative tracer as it is assimilated by mangrove green plants. Such reactive behaviour lowers bromine concentration by an average 3275% with respect to the expected Cl/Br ratio. (4) BB environment experiences the anthropic activity in its boundaries until the mouth, particularly at the superficial portion of the water body and during the warmer season. The input of organic matter in BB is very substantial. This can also be proved by the application of bromine as a tracer of biogeochemical processes in estuarine/mangrove systems. In synthesis, the inconsistency of bromine concentrations with those expected from linear mixing processes supports the hypothesis of a significant bromine uptake by organic matter, either anthropic or natural, at the BB. This fact illustrates the application of bromine as a tracer of biogeochemical processes in complex hydrological systems, such as the estuarine/mangrove environment, rich in organic matter. Thus, the use of end-member mixing analyses with distinct components has helped to differentiate the origin of water flows into the bay, as well as to identify the non-conservative behaviour of bromine in such highly biologically active environments. Finally, the landward movement of tides in BB acts as a pulse of electron acceptors in regions where redox conditions are reduced. The synergism between low tides and the great continental organic matter input can generate very polluted environments, with repercussions in the aquatic life and consequently in the life of the people that live nearby the bay. Acknowledgements The authors thank Dr Enrico Allais for access to the analytical facilities at the ISO4, Universita` di Torino. This work was financially supported by the Region of Veneto and by UNIVILLE. The authors wish to thank Claudio Tureck and Mariele Simm (University of UNIVILLEJoinville) for the collection of samples and Digital Cartography Laboratory (UNIVILLE). The constructive and insightful comments of Tarcisio Possamai, Ilaria
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Baneschi, Massimo Guidi, and Vittorio Lucchini helped to improve the manuscript.
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Continental Shelf Research 28 (2008) 682–695 www.elsevier.com/locate/csr
Hydrological mixing and geochemical processes characterization in an estuarine/mangrove system using environmental tracers in Babitonga Bay (Santa Catarina, Brazil) Virgı´ nia Barros Gracea,b, Josep Mas-Plac, Therezinha Oliveira Novaisb, Elisa Sacchid, Gian Maria Zuppia, a
Dipartimento di Scienze Ambientali, Universita` Ca’ Foscari, Dorsoduro 2137, 30123 Venezia, Italy b UnivilleUniversity of Joinville Region, Santa Catarina, Brazil c Departament de Cie`ncies Ambientals, and Centre de Geologia i Cartografia Ambiental (Geocamb), Universitat de Girona, 17071 Girona, Spain d Dipartimento di Scienze della Terra, Universita` di Pavia, Via Ferrata 1, 27100 Pavia, Italy Received 5 July 2007; received in revised form 28 November 2007; accepted 17 December 2007 Available online 31 December 2007
Abstract The hydrologic complex of Babitonga Bay (Brazil) forms a vast environmental complex where agriculture, shellfish farming, and industries coexist with a unique natural area of Atlantic rain forest and mangrove systems. The origin of different continental hydrological components, the environmental transition between saline and freshwaters, and the influence of the seasonality on Babitonga Bay waters are evaluated using isotopes and chemistry. End-member mixing analysis is used to explore hydrological processes in the bay. We show that a mixing of waters from different origins takes place in the bay modifying its chemical characteristics. Furthermore, biogeochemical processes related to well-developed mangrove systems are responsible for an efficient bromide uptake, which limit its use as a tracer as commonly used in non-biologically active environments. Seasonal behaviours are also distinguished from our datasets. The rainy season (April) provides a homogenization of the hydrological processes that is not seen after the dry season (October), when larger spatial differences appear and when the effects of biological processes on the bay hydrochemistry are more dynamic, or can be better recognized. Moreover, Cl/Br and stable isotopes of water molecule allow a neat definition of the hydrological and biogeochemical processes that control chemical composition in coastal and transition areas. r 2007 Elsevier Ltd. All rights reserved. Keywords: Babitonga Bay; Estuarine; Mangrove; Halogens; Stable isotopes; Mixing processes
1. Introduction Estuarine and transitional areas constitute natural reactors in which heterogeneous processes may affect the ecological equilibrium due to local changes in the sedimentation and in the biogeochemical processes and in the salinity conditions (Wolanski et al., 2007). Besides the natural sources, human inputs significantly contribute to the water chemistry. These include large volumes of poorly Corresponding author. Tel.: +39 0 412348666; fax: +39 0 412348584.
E-mail addresses: [email protected] (V. Barros Grace), [email protected] (J. Mas-Pla), [email protected] (T. Oliveira Novais), [email protected] (E. Sacchi), [email protected] (G.M. Zuppi). 0278-4343/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2007.12.006
defined human wastes that are sometimes discharged directly or after treatment into rivers and bays. Besides the bulk effluents, anthropogenic sources also supply agricultural chemicals and off-products of industrial processes. Solid loads, anthropic pollutants, and variable water salinity control the environmental conditions of these areas. The distinction between the effects of natural and human sources in a bay’s chemical processes is not always simple. Babitonga Bay (BB), one of the main estuarine formations with mangrove in the south of Brazil, is an excellent location to monitor and understand such processes. In this paper, we present a multi-parameter approach, combined with classical hydrochemical and isotopic
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methods, to depict those biogeochemical processes. All of them are investigated according to the hydrogeological setting of the BB and its inland watershed which define the main controls on surface and groundwater, and finally upon the estuarine water quality. Our aim is to seek for a process-based model that describes the hydrochemical and nutrient functioning of bay and related lowland permeable catchments detecting, also, the effect of organic matter (OM) (natural and anthropical) in the bay. Environmental tracers as halogens and stable isotopes are used in this research as they describe the movement of water through hydrological systems. Nevertheless, biogeochemical processes, considered as a whole in a bay environment, may even modify this behaviour and produce a change of their content. For instance, chloride and bromide are generally used as tracers in complex environment, because their low chemical reactivity. As a consequence, the Cl/Br ratio is a conservative indicator for the origin and movement of natural waters (Fontes and Matray, 1993; Edmunds, 1996; Davis et al., 1998; Vengosh and Pankranov, 1998). The ratio in freshwaters is primarily controlled by the initial ratio in precipitation, which in coastal regions must have a value similar to that of seawater (Fontes and Matray, 1993), although water–rock interactions or human pressures may affect its original value. Halogens in rainfall result initially from physical processes that entrain atmospheric aerosols and control their size (Junge, 1972). Near coastal areas, the Cl/Br ratio also reflects the influence of ocean spray on local precipitation (Kaufmann et al., 1984; Long et al., 1993; Davis et al., 1998; Alcala´ and Custodio, 2004). In the study area, this ratio remains relatively inert during transport above the Atlantic Forest and Serra do Mar, despite other atmospheric processes like continentality, elevation, and precipitation amount (Winchester and Duce, 1967; Brimblecombe, 1986; Goni et al., 2001). Studies in continental and urban areas demonstrate, on the contrary, a lower Cl/Br ratio in precipitation (Slanina et al., 1982; Vengosh and Pankranov, 1998). This relatively low Cl/Br ratio was attributed to a contribution of bromine (Vengosh and Pankranov, 1998; Davis et al., 2001), showing the effects of human pressures on natural processes. However, significant assimilation of bromide by green plants and peat deposits is presently well documented (Lundstrom and Olin, 1986; Vengosh and Pankranov, 1998; Whitmer et al., 2000; Hammer et al., 2005). Bromide is assimilated like adsorbable organic bromine (AOBr), following seasonal cycles (Putschew et al., 2003). Actually in wetlands, lakes, and lagoon, in late summer during periods of rapid plant growth, bromide concentrations assimilated in OM are higher than those during the rest of the year (Whitmer et al., 2000; Carpenter et al., 2003; Hammer et al., 2005). The non-conservative behaviour of bromide is caused by the strong preference for Br over Cl by the plants and by photosynthetic bacteria, or by better chemical interactions between organic matrixes (Blodau and Moore, 2002).
683
Furthermore, stable isotopes of the water molecule (d18O and dD) are also appropriate tracers to depict surface and groundwater flow lines and recharge areas (Clark and Fritz, 1997; Kendall and McDonnell, 1998). Both of them, halogens and stable isotopes are studied using end-member mixing analysis to describe the influence of different water inputs into the saline BB water, as well as the occurrence of biogeochemical processes in this particular environment. 2. Site environment description The estuarine system of BB covers an area of 1400 km2, including the last great south hemisphere’s mangrove ecosystem (130 km2; Fig. 1). The complex is located northeast of Joinville, the most industrialized and urbanized city of Santa Catarina State. The bay’s drainage basin (261020 –261280 S and 481280 –481500 W) forms a vast environment, where agriculture and shellfish farming as well as a broad spectrum of industries, like textile, ‘‘metalmechanical’’, foundries, etc., coexist with a unique natural area of Atlantic Forest. The BB is connected to the Atlantic Ocean via an opening of 1850 m and presents a salt-wedge circulation system. A significant tide oscillation brings an appreciable water renewal to whole ecosystem. Tides are mixed with semidiurnal dominance and diurnal inequalities (Truccolo and Schettini, 1999). Non-linear effects result in the amplification of the astronomical constituents towards the interior of the bay, presenting a hypersynchronous behaviour. The mean tidal amplitudes in BB range from 0.14 m during neap tides to 1.53 m during spring tides (FEMAR, 2000). The region is characterized by a humid subtropical climate with a rainfall average of 2000 mm/year. It presents two differentiated seasons: summer (from November to April) and winter (from May to October). During summer, weather is characterized by high temperature and humidity, with intense precipitation. During winter, the influence of polar air masses brings a decline of temperature and precipitation in the region. Geomorphological analysis reveals large differences in lithology and structure, including the occurrence of welldeveloped Quaternary formations. The basin and range topography of the region generates pronounced altimetric contrasts. The whole Babitonga basin is characterized by four geological domains: granites and a complex Paleozoic tectonic basement; molasses; associated vulcanites; and sedimentary deposits from the Quaternary. It is divided in four hydrographic watersheds: Cachoeira, Palmital, Cubata˜o, and Parati. The Cachoeira River basin has an area of 84.82 km2, completely located in the urban area of Joinville, and it runs out to Saguac- u Lagoon. There is no accurate information about the Cachoeira River discharge, roughly estimated as 3–5 m3/s. Water quality is very low due to pollution from domestic and industrials effluents. Only 16.40% of the domestic sewages receive partial water treatment. Nevertheless, the largest chemical pollution of
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Fig. 1. Geographical sketch of the Babitonga Bay area, and location of the sampling points within the bay and its continental surrounding areas.
Cachoeira River is attributed to untreated industrial wastes. The Palmital River basin drains an area of 357.60 km2 and its estuary receives contributions of several streams, all of them under tidal influence. The Palmital River is almost completely surrounded by mangrove forests. The Cubata˜o River, running for more than 75 km, constitutes the largest watershed of the whole hydrologic complex of BB (484 km2). Its river head is located at the Serra do Mar (1300 m a.s.l.). Mean discharge at the Cubata˜o River mouth is 17.70 m3/s, but it varies strongly as a function of rainfall distribution and intensity on the watershed. A large percentage of the Joinville domestic and industrial supply is derived from the Cubata˜o River. In addition, Cubata˜o waters are also collected for agricultural purposes. The Parati River basin has an area of 72.20 km2 partly preserved and partly occupied by agriculture. Groundwater occurrence is related to three main aquifer formations: the weathered crystalline rocks, the fractured granulite complex, and the Quaternary sedimentary formations (Baggio, 1997). Groundwater movement is generally towards BB. According to Baggio (1997), the hydraulic gradient is of 104, generally controlled by tidal oscillation, and the underground discharge rate from the crystalline aquifer to surface waters is up to 15 m3/h. 3. Sampling and analytical techniques Hydrochemical and isotopic data were obtained from 10 sampling points in BB for surface water (numbered from 1
to 10, sampled at two different depths—labelled S for surface and F for deep samples) and fit in the sampling network (Fig. 1) established in the past (CENTRAN, 2004) to cover the BB environment. Further information have been achieved for the Cubata˜o and Cachoeira River basins from two wells in the alluvial formation and two on the granite basement, for Cubata˜o: Rud (alluvial) and Hub (granite basement); and for Cachoeira: NHJ (alluvial) and TC (granite basement). Moreover, two freshwater springs located in one of the main islands within the bay (Sub1, Sub2) and two surface water points (Cub, Cach) have been sampled (Fig. 1). Analyses (Table 1) include electrical conductivity (EC), pH, major ions, trace elements and heavy metals (not discussed here), and stable isotopes of the water molecule. Electrical conductivity, pH, and redox potential were measured in situ, as well as, in most cases alkalinity by HCl titration. Samples were subsequently filtered at 0.45 mm and collected in pre-cleaned bottles. A sample aliquot was acidified to 1% HNO3 for the analysis of cations and metals. Field analyses were repeated for control in the laboratory. Chemical analyses were performed by the Dipartimento di Scienze Ambientali, University of Venice. Anions were determined by ion chromatography, whilst cations and trace elements were determined by atomic absorption, ion chromatography, and inductively coupled plasma source-atomic emission spectroscopy (ICPS-AES). All reported values have an ionic balance within 7.5%. Samples for stable isotope analysis were collected according to the procedures described by Clark and Fritz (1997). Hydrogen isotope
T
pH
0.22 3.52 6.21 20.30 14.60 24.30 33.60 36.30 36.40 39.60 34.70 34.50 30.30 32.60 27.80 28.70 26.70 26.70 26.50 28.80 0.09 0.05 0.16 0.80 0.07 0.14 1.06
Eh
160 186 195 201 188 197 176 166 173 166 167 168 171 165 163 85 163 8 9 111 114 35
Alkalinity
SO4
Cl
Br
F
Ca
Mg
Na
20.3 27.3 25.1 49.2 46.7 85.6 101.3
21.5 108.5 194.5 852.5 609.5 1092.0 2030.5 2245.0 2175.0 2445.0 1711.5 1237.5 1727.5 1755.5 1464.5 1529.0 1451.5 1462.0 1440.0 1569.5
131.5 964.5 1913.0 7931.5 5277.5 9823.5 15520.0 14530.0 16660.0 17460.0 14420.0 15030.0 12810.0 14090.0 11090.0 11620.0 10380.0 10480.0 10360.0 11640.0 5.8 6.0 6.8 52.0 13.0 13.0 191.8
0.0 3.2 4.7 15.5 12.0 20.5 31.5 27.5 31.5 34.5 31.5 22.5 26.5 23.0 21.0 22.0 23.0 18.0 18.5 25.0
29.0 10.0 8.0 5.5 2.5 3.0 8.5
4.7 29.7 51.4 201.0 133.8 255.5 442.0 456.5 475.8 515.0 429.5 429.5 366.5 403.2 325.2 334.5 293.0 273.2 287.2 336.0 6.9 2.9 15.8 68.9 1.1 21.1 2.0
3.8 82.5 159.0 426.0 301.5 535.5 772.5 754.5 844.5 1218.0 736.5 771.0 739.5 763.5 601.5 648.0 580.5 591.0 532.5 927.0 6.0 2.6 9.6 35.4 0.8 0.9 1.0
252 1020 2460 2440 5920 4220 7320 9820 10440 11400 9060 10180 8860 9340 8080 7580 7360 7240 6960 7740 6.6 16.3 12.28 89 11.14 9.3 248
124.8 133.3 119.6 116.3 114.1 95.4 99.0 92.3 101.6 102.2 103.8 40.3 12.0 88.5 347.4 12.0 58.7 95.2
2.7 9.5 4.0 20.8
0.5
4.0 2.5 3.5 1.0 7.5 8.5 7.0 1.5 7.5 3.5 5.5 9.0 2.0 4.3 1.4 2.5 5.5 2.5 3.3
K
3.3 43.3 168.2 109.4 199.6 305.2 325.6 336.4 378.4 303.6 313.2 265.2 327.2 264.4 250.8 220.4 249.2 230.0 243.2 1.4 1.6 1.5 25.7 2.3 2.2 2.4
dD
d18O
19.88
4.42
12.43 14.17 9.15 1.86 3.21 4.00 0.12 0.63 0.38
3.11
4.80
21.41 19.88 11.51 19.45 19.79
4.04 2.33 3.08 1.69 0.51 0.16 0.27 0.59 0.28 0.27 0.72 0.45 1.01 1.01 1.10 1.05 1.23 0.94 4.58 4.65 4.23 3.03 4.06 3.53 4.11
685
April survey (24 April 2004) 1S 22.8 8.90 1F 22.8 8.40 2S 23.0 8.16 2F 24.0 7.72 3S 24.4 7.50 3F 24.7 7.70 4S 25.5 7.03 4F 25.0 7.50 5S 24.9 7.26 5F 24.9 7.45 6S 25.5 7.40 6F 25.4 7.54 7S 25.1 7.67 7F 25.4 7.60 8S 25.3 7.69 8F 25.2 7.68 9S 25.3 7.68 9F 24.8 7.65 10S 25.4 7.70 10F 25.8 7.71 Rud 23.1 8.87 Cub 21.4 8.70 Hub 22.1 8.90 NHJ 23.3 8.15 SUB1 22.9 9.07 SUB2 23.9 7.70 TC 23.5 8.06
EC
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Table 1 Hydrochemical data for April and October surveys at Babitonga Bay
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Table 1 (continued ) Point
Point
T
pH
T
pH
Eh
Alkalinity
SO4
Cl
Br
F
Ca
Mg
Na
K
d18O
dD
Eh
Alkalinity
SO4
Cl
Br
F
Ca
Mg
Na
K
dD
d18O
DO
3.30 11.74 21.57 23.78 30.94 31.04 47.19 48.10 49.85 50.21 49.77 47.96 46.03 46.13 42.46 42.44 41.44 41.21 34.58 37.33 47.33 42.92 0.16 0.11 0.24 0.62 0.11 0.17 0.49
71 59 141 129 77 73 45 63 123 111 129 122 129 136 133 40 40 56 35 42 85 81 104 62 12 8 43 33 4
36.6 48.8 73.2 85.4 85.4 97.6 109.8 122.0 109.8 122.0 109.8 97.6 97.6 109.8 109.8 146.3 85.4 97.6 122.0 109.8 134.1 134.1 73.2 97.6 73.2 274.4 24.4 36.6 85.4
419.0 568.9 1197.2 1375.3 1625.7 1644.0 2459.8 2526.6 2656.2 2718.5 2564.2 2623.0 2387.0 2395.0 2158.5 2169.4 2134.2 2054.6 1573.2 1865.1 2579.5 2484.5 7.3 7.0 8.2 28.5 8.0
944.1 1350.9 6506.4 8758.0 10883.3 10946.4 18374.5 17736.2 19314.4 19763.3 18651.6 18816.4 17259.2 17231.2 17259.2 15183.0 14362.4 12079.2 10820.0 13562.7 18325.4 17946.6 14.1 11.8 18.1 16.3 37.2 38.7 43.2
0.3 3.1 16.8 18.6 20.5 20.7 52.0 52.0 56.0 56.2 58.7 54.5 45.5 43.2 37.1 35.1 32.3 32.8 18.9 28.1 54.5 56.5 0.0 0.0 0.0 0.0 0.1 0.6 0.1
14.8 94.0 108.8 108.8 113.8 116.8 128.6 136.0 133.1 132.1 128.6 135.6 125.7 133.1 123.2 126.2 123.7 119.7 113.8 119.2 130.1 127.1 2.8 2.4 3.9 2.2 2.0 2.1 0.0
35.9 85.8 176.7 213.4 292.9 237.3 392.6 422.9 414.1 416.6 407.8 404.0 316.9 383.8 329.5 295.4 314.4 309.3 253.8 277.7 435.6 491.1 6.8 3.1 17.3 69.5 1.3 9.4 18.5
60.8 159.3 331.0 485.4 568.1 666.9 1372.1 1332.6 1519.1 1293.1 1511.7 1517.8 853.4 1257.2 888.0 900.3 895.4 913.9 548.3 863.3 1074.5 1283.2 3.8 1.2 8.3 17.4 0.9 1.0 4.0
576 1338 4204 5303 6081 5981 9825 10170 10646 10467 10507 10760 8105 9486 8453 7878 8140 7896 6127 7217 9997 9887 7.2 4.8 12.1 13.7 10.2 10.7 43.6
60.1 124.9 205.3 220.2 476.3 215.2 364.9 461.4 416.9 437.9 387.2 372.3 346.4 418.1 345.1 277.1 310.5 314.2 290.7 277.1 440.4 430.5 1.0 1.0 3.2 10.3 1.4 2.4 7.5
15.75 14.70 8.19 5.26 1.66 1.75 1.84 1.82 3.11 3.32 3.05 3.27 2.72 2.60 1.90 1.98 2.33 0.78 4.58 1.18 3.11 3.61 20.21 19.52 19.01 9.87 17.57 13.71 13.94
3.61 3.73 1.75 1.58 0.99 0.95 0.10 0.13 0.34 0.47 0.34 0.45 0.35 0.21 0.01 0.06 0.10 0.19 1.19 0.62 0.25 0.41 5.61 5.87 6.14 2.82 3.69 2.98 3.36
4.26 5.86 5.02 4.62 4.84 3.77 5.85 5.16 6.09 5.67 5.55 5.77 5.29 4.98 5.66 5.08 5.78 4.7 3.64 3.91 4.64 4.89 4.25 5.48 2.07 2.33 3.99 2.7 0.19
34.9
Temperature (T) is given in 1C, electrical conductivity (EC) in mS/cm, redox potential (Eh) is in mV, alkalinity (as HCO3), dissolved oxygen (DO), and other major elements/compounds are in mg/L, isotopes are in % of their respective standards of reference.
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October survey (18 October 2004) 1S 22.9 6.77 1F 22.1 7.27 2S 22.5 7.35 2F 22.5 7.40 3S 22.7 7.54 3F 22.7 7.54 4S 22.9 8.01 4F 22.2 7.98 5S 22.3 8.05 5F 21.6 8.07 6S 22.2 8.07 6F 22.3 8.07 7S 22.5 7.96 7F 22.4 7.95 8S 23.8 7.74 8F 23.8 7.73 9S 25.0 7.64 9F 24.6 7.64 10S 23.6 7.45 10F 23.6 7.55 G1S 22.1 7.99 G1F 22.4 7.98 Rud 23.0 6.41 Cub 21.4 6.25 Hub 21.7 6.66 NHJ 22.1 7.23 Sub1 20.6 5.86 Sub2 21.6 6.13 Cachoeira 23.6 6.89
EC
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composition was measured by water reduction over metallic zinc (Coleman et al., 1982), whilst d18O was analysed by water–CO2 equilibration at 25 1C (Epstein and Mayeda, 1953); both results are expressed in V-SMOW (Gonfiantini, 1978; Gonfiantini et al., 1995). The analytical errors are 71% and 70.1%, respectively. All gases were analyzed on a Finnigan MAT 250 Mass Spectrometer at ISO4 s.s., Torino, Italy. 4. Results Chemical and isotopic data reflect how the BB waters are connected to, and affected by, seasonal, regional, and local hydrological processes. Water chemistry and isotopes are strongly influenced by both freshwater flow and water exchange between the continent and the ocean (Table 1). These data are evaluated to discuss the origin of different hydrological components and their mixing, the influence of seasonality, and the role of biological processes in the transition environment between saline and freshwaters. 4.1. Chemical data The spatial and temporal variation of the entire hydrology of BB is firstly reflected by conductivity data (Fig. 2). EC distribution shows the influence of the Atlantic Ocean on the bay waters. The EC lay in a range about 70% of that of seawater (represented by sampling point 5) in an extended part of the bay (points 4, 6, 7, 8, 9, and 10), and it progressively diminishes in its inner sectors (points 1, 2, and 3). EC of continental water in rivers is significantly lower than that of the bay by more than two orders of
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magnitude. Temporal variability is pointed out by the difference between April and October conductivity datasets. Conductivity values for the bay sampling points are consistently larger in October than those in April. Sampling locations in the estuarine area show seasonal increments from 27% to 55%, being larger in most of the points closer to the coastline. Contrarily, those points related to freshwater (wells, rivers, and springs) show a notable decrease in conductivity, and a minor seasonal difference. The difference between the wet (April) and the dry (October) season shows the dilution effect by continental water inputs within the bay. Similar observations can be achieved by pH that varies from 8.1 to 6.7 (being the range of continental waters from 5.6 to 7.3) as consequence of ocean water dilution. BB waters undergo to stratified reduced conditions, as indicated by Eh variations. In particular, at the river mouths, where seawater arrives particularly altered, the high load in continental OM gives place to a more reduced environment. The order of cation dominance is Na–Mg– Ca4K, while anion dominance is Cl4SO4–HCO3 (Table 1). Although the high load of OM and consequential reduction processes, chloride and sulphate can be used as tracers in the mixing processes that occur between continental and thus reduced water and ocean waters in the BB. Lowest values correspond to river mouth samples, whilst the highest to ocean ones, following the mixing ratio between continental and marine waters (Barros, 2005). The movement of the tides into and out of the BB and the associated effects created by the incursion of oxidized waters and posterior retraction control the redox changes
Fig. 2. Electrical conductivity distribution at sampling locations for April and October surveys.
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Fig. 3. Sulphate versus chloride relationship for April and October surveys.
in water bodies, and as consequence the presence of dissolved nutrients (Schettini et al., 1996; Barros, 2005). Fig. 3 shows a neat linear relationship between the two species in both surveys. April data, however, show a larger dispersion for chloride concentrations lower than 20.0 g/L (SO4 ¼ 0.13Cl32.4; with R2 ¼ 0.91 and n ¼ 18), whereas October data fit a better linearity (SO4 ¼ 0.12Cl+378.95; R2 ¼ 0.98 and n ¼ 18); which can be attributed to a heterogeneous behaviour of the bay hydrologic dynamics (local continental inflow variability, tides, etc.). Freshwater samples (Clo200 mg/L) lay on linear ratio, except those located in the Cachoeira River watershed (NHJ, Cach) that show a slight increase on sulphate as well (Table 1). The relationship between chloride and bromine at the Babitonga way is plotted in Fig. 4, and it allows observing seasonal and spatial effects at the bay area. A first observation refers to the overall depletion of bromine if compared to the ratio in oceanic waters with a mean dissolved chloride of 19.3 g/L and bromine 66 mg/L; that is, Cl/Br ¼ 293 (Fontes and Matray, 1993). Notice that for the ocean waters at the BB (sampling point 5), this average ratio is set to Cl/Br ¼ 517, and 348 for the April and October surveys, respectively; showing a deficit of bromine with respect to the reported mean value in oceans. The good fit to the local ocean ratio observed for the April data is not reproduced by the October results, where a depletion of bromine persists in some bay waters with respect to the local oceanic ratio of Cl/Br ¼ 348. In particular, large Cl/Br ratios were observed in those sampling locations closer to the shoreline (points 1, 2, 3, 8, 9, and 10), whereas those located in the central axis of the bay or close to its mouth (points 4, 5, 6, and 7) show a ratio similar to the local oceanic value. Remarkably, point 1S shows a very
Fig. 4. Bromine versus Chloride relationship for April and October surveys. Constant Cl/Br lines for mean ocean and local values are also plotted (see text for explanation).
large Cl/Br ratio of 4383 and 3405 for the April and October surveys (Fig. 5). Such distribution supports the idea that an effective bromine uptake by biological processes (Lundstrom and Olin, 1986; Vengosh and Pankranov, 1998; Whitmer et al., 2000; Hammer et al., 2005) occurs at the BB, which is noticeable at those points located near the coast, areas with low depths and gently preserved from the main tidal influences, and exceptionally at point 1S (Table 1). Human pollution is then discarded as a possible water contribution because local industrial and urban wastewaters possess a low Cl/Br ratio (Cl/Br ¼ 220). The comparison between seasonal and spatial influences upon the Cl/Br ratio is more clearly seen in Fig. 5. In this plot, the depletion in bromide at those points close to the shoreline, resulting in a higher Cl/Br ratio, stands out in comparison with the bay locations; reaching values even higher than those of the April survey. The variations of Cl/Br with chloride concentrations show high ratio values for bay waters with chlorides concentrations below 15.0 g/L, approximately (Table 1); that is, in those locations near to the bay shoreline. 4.2. Isotopic data The d18O and dD stable isotopic content is reported in Fig. 6, and it offers a complementary insight on the origin of water and the mixing processes in BB. Fig. 6 allows differentiating three distinct sample groups, especially in the October survey. Continental water samples from groundwater related to the Cubata˜o River basin (group 1; Cub, Rud, Hub) show the lightest isotopic content. Their departure from the
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Fig. 5. Distribution of the Cl/Br ratio at sampling locations for April and October surveys.
Fig. 6. Water stable isotope content (d18O and dD) for April and October surveys. Class groups based on the October survey dataset are described in the text.
global meteoric water line (GMWL) points out an enrichment in deuterium that can be attributed to fractionation processes of local rainfall, that is, re-evaporation; in particular, during the advection of water vapour from Atlantic Ocean to the Serra do Mar. Deuterium excess, defined by Dansgaard (1964) as d-excess ¼ dD8d18O, is around 25–30% for this group, being larger than other
continental waters of the area (10–15%; see next sample group 2) for the October dataset. The isotopic composition of local precipitation is thus primarily controlled by regional scale processes, like the trajectories of the water vapour transport over the continent and the average rainfall history of the air masses giving precipitation at a particular place (Epstein and Mayeda, 1953; Craig, 1961;
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Dansgaard, 1964; Craig and Gordon, 1965; Gonfiantini, 1965, 1998; Merlivat and Jouzel, 1979; Siegenthaler and Oeschger, 1980; Rozanski et al., 1992, 1993; Jouzel et al., 1997). The high d-excess values of surface and groundwaters suggest that an isotopically fractionated evapotranspiration flux contributes to the atmospheric water balance over the region, similar to the steady-state evapotranspiration model developed for the Amazon Basin (Gat and Matsui, 1991; Martinelli et al., 1996; Maurice-Bourgoin et al., 2003). Moreover, d-excess values may reflect also a decrease in condensation temperature as indicated by the elevation of the Serra do Mar rising inland along this transect. This suggests that also an altitude effect contributes to the control on the d-excess values of surface and groundwaters. The large transpiration fluxes from flooded areas and from the Mata Atlantica forest, coupled to moisture transported from the Ocean and the BB, contribute to the significant amount of rainfall (2000 mm/year) on Serra do Mar in this region. Thus, the increase in d-excess inland is most easily explained by the contribution of recycled moisture via evaporation and re-precipitation. Moreover the increase in d-excess suggests that evaporation/reprecipitation cycles occur repeatedly along the storm track. A second group (group 2 in Fig. 6) is constituted by surface and groundwater from the Cachoeira River (sampling points Cach, NHJ) including the deep well TC from the April survey, as well as bay points 1S and 1F both linked to Cubata˜o River contribution, and the island springs (sampling points Sub1, Sub2). They show an isotopic content that aligns parallel to the GMWL,
revealing the continental origin of the water, specifically from the basins surrounding the bay, and the almost absence of mixing with ocean water. Specifically, these first two groups show a very similar isotopic content in the April survey, and they lay very close in the plot. Such similarity is attributed to the effective recharge of rainfall and surface water to the aquifers during the wet season. Finally, a third group is arranged in a line from the second group towards the ocean isotopic composition in both surveys. Isotopic content of sampling point 5 is taken as the ocean reference. This last group is constituted by sampling points located in the bay area, and they reflect the variable influence of ocean water. 5. Hydrological mixing and biological processes Mixing processes between fresh and ocean water are better represented in Fig. 7, showing the relationship between chloride and d18O data, and therefore combining chemical and isotopic information. Mixing lines have been drawn assuming an average linear composition of both components of the mixing system members (Albare`de, 1995). For the April survey, the homogeneity in the values of both components in the continental pole (previous groups 1 and 2) does not allow a further distinction of their origin. In any case, data from the April survey convey a complete mixing process as actual data overlay the estimated mixing line. For the October dataset, two continental end-members are defined, based on the first and second groups already mentioned, and two distinct mixing lines (‘‘October 1’’ and
Fig. 7. d18O content versus chloride for April and October surveys, and possible mixing lines between identified end-members. Symbols within the mixing lines correspond to proportions of each end-member set at 0.1%, 1.0%, 10%, 20%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, 97.5%, 99%, and 99.9%.
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‘‘October 2’’, respectively) are drawn accordingly in Fig. 7. It can be observed that the line originated from the second group, that is, the one corresponding to continental waters from the Cachoeira basin (group 2), fits with the measured composition of the bay water samples, suggesting an efficient groundwater flow towards the bay originated in the surrounding aquifers that mixes with ocean water. A minor influence of continental waters is seen from the group related to the hydrographic network of the plain, as the Cubata˜o River. Maximum continental water contribution (both from surface and from aquifer) is estimated in a E20% in point 1, and it decreases towards the ocean. In particular, sampling locations 1S and 1F, located at the end of a large tidal channel with chloride concentrations near 1000 mg/L, lay away from the best mixing line (line ‘‘October 1’’ in Fig. 7), pointing to a slight contribution from the Cubata˜o River discharge. An additional mixing analysis can be performed using chloride and bromine as the two components of a system. Results are also plotted in Fig. 8, where ratio Cl/Br for the mixing line is estimated as the quotient between the chloride and bromide concentrations for a given mixing proportion; that is, Cl Cl1 x1 þ Cl2 x2 ¼ Br Br1 x1 þ Br2 x2 where Cli and Bri are the chloride and bromide concentrations of end-member i (i ¼ 1, 2), and xi is the proportion of each end-member being x1+x2 ¼ 1. Indeed, constant Cl/Br lines in Figs. 4 and 5 are based upon the erroneous assumption that the ratio will remain constant through the water mixing process. Despite the Cl/Br signature of rainfall in surface and groundwater, bay
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waters will reflect the effects of biogeochemical cycles as well as human pressures, and this will therefore produce a modification of the Cl/Br ratio. In this mixing analysis, the two chosen end-members are points 5S and 5F, for the ocean side, and freshwater spring samples of an island located in the middle of the bay (points Sub1 and Sub2) and the deep well Hub in the plain. This second end-member is considered as representative of the continental input mainly as groundwater as concluded from Fig. 7. In fact, deep crystalline aquifers are confined (Baggio, 1997), and thus groundwater is not in contact with mangrove or anthropogenic systems. Particularly, sampling points Sub1 and Sub2 represent the outcropping of groundwater in a central island in the bay. Such upwelling is attributed to the fault system that affects the geological structure of the area, and that acts as a preferential flow path that govern this upward flow. Such explanation satisfies their hydrochemical composition, characterized by a low chloride content, and their isotopic signature as well. For that reason, we make use of them as representative of the continental end-member. Mixing line for the April survey in Fig. 8 shows an acceptable fit with actual data, as expected from the relatively constancy of the Cl/Br ratio as already shown in Fig. 4. Nevertheless, the October survey data show a significant variation of this ratio that place actual data above the hypothetical mixing line. As the influence of the continental end-member (location Sub2) on the mixing line becomes significant at proportions above 90%, we shall assume that the bromine behaviour is mainly due to the non-conservative nature of this element in the mangrove environment suggesting biological uptake, and it is thus not related to the exactness of the component concentration defined at the continental pole.
Fig. 8. Value of the Cl/Br ratio versus chloride concentration for April and October surveys, and possible mixing lines between identified end-members.
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In that way, bromine uptake by biogeochemical processes can be quantified assuming the conservative behaviour of chloride. We shall use the actual concentration of chloride at each sampling location to estimate the mixing proportions of the two end-members, that is, Cl ¼ Cl1 x1 þ Cl2 ð1 x1 Þ then, ClCl1 x1 ¼ Cl 1 Cl2
x2 ¼ 1 x1 where Cl is the measured chloride concentration, and Cli and xi were described before. Once the mixing proportions (x1, x2) are known, we estimate the bromine concentration corresponding to a conservative mixing process (Br*), and subsequently its difference from actual bromine concentrations. Using location points 5 and Sub2 as the two endmembers, with chloride and bromine concentrations of 19.54 g/L and 56.1 mg/L, and 38.7 and 0.59 mg/L, respectively, the mixing proportions for the BB water samples and their deviation from the conservative bromine concentration for the October survey are calculated and presented in Table 2. Results show that relevant bromine uptake occurs in locations 1, 2, 3, 7, 8, 9, and 10. Deviation from the supposed conservative behaviour is considerably larger than possible sampling or analytical errors. Mean deviation factor for those locations is 32.175.1%
(excluding 1S), which is attributed to the assimilation of bromine by mangrove vegetation. In BB, OM has generally double origin: natural and anthropic (Barros, 2005). However, the special location of point 1S in a tidal channel, within a well-developed mangrove site, justifies its large deviation factor due to the accentuated bromine assimilation by green plants, depending of vegetation seasonal cycles (Putschew et al., 2003; Hammer et al., 2005). Organic uptake in BB samples is furthermore supported by the relation between SO4 and Br (Table 1 and Fig. 9) which slope is higher than that of classical seawater dilution. On the contrary, the SO4/Cl ratios (Table 1 and Fig. 3) are similar to that of seawater. Besides, in both cases, the lowest values correspond to river mouth samples, whilst the highest to ocean ones, following the mixing ratio between continental and marine waters (Barros, 2005). There is also a seasonal effect of bromide depletion with respect to sulphate in October in the bay waters, as described in Fig. 4. The movement of the tides into and out of the BB and the associated effects created by the incursion of oxidized waters and posterior retraction control the redox changes in water bodies and, consequently, the presence of dissolved nutrients (Barros, 2005). In fact, environmental stress like the lack of electron acceptors, such as dissolved nutrients, favours the formation of organic bromine compounds (Putschew et al., 2003). Moreover, concentrations of organically bound bromide in OM or in peat strongly depend on the degree
Table 2 Estimation of bromine uptake factor using mixing analysis Point
1S 1F 2S 2F 3S 3F 4S 4F 5S 5F 6S 6F 7S 7F 8S 8F 9S 9F 10S 10F G1S G1F
Measured concentrations
Mixing proportions
Cl (mg/L)
Br (mg/L)
x1
x2
Br* (mg/L)
BrBr*
Deviation factor
944.1 1350.9 6506.4 8758.0 10883.3 10946.4 18374.5 17736.2 19314.4 19763.3 18651.6 18816.4 17259.2 17231.2 17259.2 15183.0 14362.4 12079.2 10820.0 13562.7 18325.4 17946.6
0.28 3.09 16.78 18.58 20.49 20.71 51.98 52.01 55.96 56.25 58.73 54.52 45.47 43.19 37.12 35.09 32.29 32.81 18.91 28.08 54.53 56.48
0.954 0.933 0.668 0.553 0.444 0.441 0.060 0.092 0.012 0.012 0.045 0.037 0.117 0.118 0.117 0.223 0.265 0.383 0.447 0.306 0.062 0.082
0.046 0.067 0.332 0.447 0.556 0.559 0.940 0.908 0.988 1.012 0.955 0.963 0.883 0.882 0.883 0.777 0.735 0.617 0.553 0.694 0.938 0.918
3.17 4.33 19.00 25.41 31.46 31.64 52.79 50.97 55.46 56.74 53.57 54.04 49.61 49.53 49.61 43.70 41.36 34.87 31.28 39.09 52.65 51.57
2.89 1.24 2.23 6.83 10.97 10.93 0.81 1.04 0.50 0.49 5.15 0.48 4.14 6.34 12.49 8.61 9.07 2.05 12.37 11.01 1.88 4.92
1042.59 40.15 13.27 36.74 53.57 52.80 1.55 2.00 0.89 0.87 8.77 0.88 9.11 14.68 33.66 24.53 28.08 6.25 65.41 39.19 3.45 8.70
Deviation factor (%) ¼ 100(BrBr*)/Br.
Estimated bromine
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6. Conclusions The main conclusions are summarized as follows:
Fig. 9. Sulphate versus bromine relationship for the sampling points in April and October surveys.
of organic compound decomposition: bromide is primarily bound to OM molecules (Biester et al., 2006). The input of OM in BB is very substantial. Its main input is due to flooding of the major rivers and is, therefore, prevalent in the rainy season. The application of bromine as a tracer of biogeochemical processes in estuarine/mangrove systems underlines the anthropic activity in BB environment throughout the estuarine zone. This is particularly evident in the superficial portion of the water body and during the warmer season. In addition to the significant assimilation of bromide by green plants and peat deposits, in Cachoeira basin, which is completely an urban setting, industrial and domestic wastewaters bring relatively greater quantities of Br (0.50 ppm) described by a low Cl/Br ratio (220). Urban effluents are, thus, characterized by an excess of bromine and bromo-compounds used for the dyeing and finishing processes of the textile industries, flameproofing, biomass, and sanitizers in sewages of agri-food industries (Magazinovic et al., 2004; Reineke et al., 2006). These observations are in agreement with the information obtained with 13C in OM and 15N (Barros, 2005). BB cannot be reasonably modelled by merely varying the proportion of a marine and a general terrestrial endmember because of the described biogeochemical processes. Carbon and nitrogen isotopes and the C/N ratio have additionally indicated that, although most of the OM has a riverine source in sample sites near the coast (1, 2, and 3), and a marine source in other sites (5, 6, 7, and 8), a component of sewage OM is also present in several sampling sites of the BB.
(1) The participation of all the hydrologic cycle components (surface runoff, groundwater flow, and ocean seawater) in a estuarine mass balance can be verified in BB, showing seasonal differences as indicated by chemical and isotopic data. (2) Signal of the mixing between continental and ocean waters is observed in the whole bay until its mouth, as suggested by stable isotopes and Cl/Br ratio. Continental water proportions in bay water may reach up to 20% in the inner parts of BB. (3) Bromine acts as a non-conservative tracer as it is assimilated by mangrove green plants. Such reactive behaviour lowers bromine concentration by an average 3275% with respect to the expected Cl/Br ratio. (4) BB environment experiences the anthropic activity in its boundaries until the mouth, particularly at the superficial portion of the water body and during the warmer season. The input of organic matter in BB is very substantial. This can also be proved by the application of bromine as a tracer of biogeochemical processes in estuarine/mangrove systems. In synthesis, the inconsistency of bromine concentrations with those expected from linear mixing processes supports the hypothesis of a significant bromine uptake by organic matter, either anthropic or natural, at the BB. This fact illustrates the application of bromine as a tracer of biogeochemical processes in complex hydrological systems, such as the estuarine/mangrove environment, rich in organic matter. Thus, the use of end-member mixing analyses with distinct components has helped to differentiate the origin of water flows into the bay, as well as to identify the non-conservative behaviour of bromine in such highly biologically active environments. Finally, the landward movement of tides in BB acts as a pulse of electron acceptors in regions where redox conditions are reduced. The synergism between low tides and the great continental organic matter input can generate very polluted environments, with repercussions in the aquatic life and consequently in the life of the people that live nearby the bay. Acknowledgements The authors thank Dr Enrico Allais for access to the analytical facilities at the ISO4, Universita` di Torino. This work was financially supported by the Region of Veneto and by UNIVILLE. The authors wish to thank Claudio Tureck and Mariele Simm (University of UNIVILLEJoinville) for the collection of samples and Digital Cartography Laboratory (UNIVILLE). The constructive and insightful comments of Tarcisio Possamai, Ilaria
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Baneschi, Massimo Guidi, and Vittorio Lucchini helped to improve the manuscript.
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