SPATIAL VARIATIONS OF ENVIRONMENTAL TRACER DISTRIBUTIONS IN WATER FROM
4. GEOCHEMICAL RESULTS AND INTERPRETATION
The sampling campaigns were made after the two extreme seasons in the region, i.e., after the austral summer (April 2004) and winter (October 2004).
On the diagram (Figure 1) three groups of water appear:
1. Continental water samples related to the Cubatão River basin (CWCR) are characterised by the lightest isotopic content. Their shift from the Global Meteoric Water Line (GMWL) points out an isotopic enrichment that can be attributed to fractionation processes of local rainfall. Deuterium excess is around 25–30‰. 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/re-precipitation cycles occur repeatedly along the storm track. In fact, the high d-excess values of surface and ground waters indicates that an isotopically fractionated evapotranspiration fl ux contributes to the atmospheric water balance over the region, similar to the steady state evapotranspiration model developed for the Amazon Basin [2, 3]. Furthermore, d-excess values may also refl ect a decrease in condensation temperature as indicated by the elevation of the Serra do Mar rising inland along this transect [4]. This interpretation also suggests that an altitude effect contributes to the control of the d-excess values of surface and ground waters.
2. Surface and ground water from the Cachoeira river (SGCR) (including the deep well TC from the April survey) as well as bay points 1S and 1F, and the island springs, within BB. Specifi cally, these fi rst two groups show a very similar isotopic content in the April survey, almost indistinguishable in the plot. Such similarity is attributed to the effective recharge of rainfall and surface water to the aquifers during the wet season, suggesting the harmonisation of hydrological and hydrogeological systems.
3. Bay water (BBW). Isotopic results are arranged in a line extending from the second group towards the ocean isotopic composition in both surveys, and they refl ect the variable infl uence of ocean water. Isotopic content of sampling point 5 is taken as the ocean reference. Mixing rates could be FIG. 1. Water stable isotope content in hydrologic complex of Babitonga Bay. Symbols represent: (•) April survey and (o) October survey.
inferred from such a distribution, although the infl uence of the fi rst and second groups would not be clearly differentiated.
Mixing processes between fresh and ocean water are better represented in Figure 2, showing the relationship between chloride and δ18O 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. For the April survey, the homogeneity in the values of both components in the continental waters (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 defi ned, based on the aforementioned fi rst and second groups , and two distinct mixing lines are drawn accordingly.
13C and 15N in BB cannot be reasonably modelled by merely varying the proportion of a marine and a general terrestrial end-member. Those isotopes and the C/N ratio indicate that, although most of the organic matter have 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 organic matter is present in several sampling sites of the Babitonga Bay. The mixing between continental system and the ocean is clearly described by the isotopic composition of POC and PON as a function of salinity (Fig. 3a and 3b). The more pristine continental FIG. 2. Mixing processes between fresh and ocean water in BB. (•) April survey and (o) October survey.
end-member, the River Cubatão had a δ13CPOC value of –26.2‰. On the other hand, the River Cachoeira had a higher δ13CPOC value (–23.9‰) typical of sewage-impacted rivers. The sampling site closest to the Atlantic Ocean (7 and 8), had δ13C values around –22 to –23‰, which is typical of ocean samples. Most of the other samples plotted between these two extremes with a tendency of increase the δ13CPOC as a function of salinity (Fig. 3a).
The δ15N values describe a rather signifi cant correlation with salinity than δ13C (Fig. 3b). Overall, the δ15N values of POM can be explained in terms of simple mixing between the terrestrial particulate matter (riverine or sewage derived) with generally lower δ15N values, and marine phytoplankton, which has mainly inherited the δ15N of nitrate from deeper oceanic waters. Larger δ15N values were obtained at stations 7 and 8 near the BB mouth. These results therefore suggest that δ15N of the oceanic water is around +10‰, slightly enriched in comparison to the latitudinal distribution pattern of δ15NPOM reported in the literature [5, 6].
The above mentioned observations are in agreement with the information obtained using chloride and bromide. Halides support the idea that an effective bromine uptake by biological processes occurs in the Babitonga Bay, which is noticeable at those points located near the coast, which are characterized by shallow depths and are protected from the main tidal infl uences. Such facts illustrate the limit of bromine application as tracer of biogeochemical processes in complex hydrological systems such as the estuarine/mangrove systems.
FIG. 3: (a) δ13C values of the particulate organic carbon (δ13CPOC) versus salinity in the BB; (b) δ15N values of the particulate organic nitrogen (δ15NPON) versus salinity in the BB.
5. CONCLUSIONS
The Babitonga Bay could be considered as an exceptional natural model in predicting the future of natural environments stressed by recent anthropic activities, and provide a spectacular opportunity to develop a sound understanding of the changes induced to lagoon systems. In fact, the use of end- member mixing analyses with distinct components has helped to differentiate the origin of water fl owing into the bay, the origin of particulate organic matter as well as to identify the non-conservative behaviour of bromine in such highly biologically active environments.
ACKNOWLEDGEMENTS
The authors thank Prof. R.L. Victoria for access to the analytical facilities at the Centro de Energia Nuclear na Agricultura (CENA), Universidade de São Paulo. This work was fi nancially supported by the Region of Veneto and by UNIVILLE. The authors wish to thank Claudio Tureck and Mariele Simm (University of UNIVILLE - Joinville) for the collection of samples and Digital Cartography Laboratory (UNIVILLE) and Enrico Allais (ISO 4, Turin) for oxygen and deuterium analyses.
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