Levantamento dos níveis de radioatividade natural em águas do alto Vale do Ribeira à planície costeira do litoral sul do Estado de São Paulo / Assessment of natural radioctivity levels in waters from Higher Ribeira Valley until the southern Sao Paulo State coastal plain

AUTOR(ES)
FONTE

IBICT

DATA DE PUBLICAÇÃO

22/11/2010

RESUMO

The complex exchange of fluvial, subsurface and seawater within a coastal area directly affects global biogeochemical cycles and the application of isotopic tracers, mainly natural radionuclides from U and Th series, is a powerful tool to track sources and sinks of trace elements and nutrients to this systems. The unique Ra signature applied to quantify the contribution of such fluxes is acquired within the subterranean estuary, a mixing zone between fresh groundwater and seawater in coastal aquifers. In the U and Th natural radioactive decay series there are four radium isotopes: 223Ra (t1/2 = 11.4 d), 224Ra (t1/2 = 3.7 d), 228Ra (t1/2 = 5.7 y) and 226Ra (t1/2 = 1,600 y). Their wide range in half-lives corresponds well with the duration of many coastal processes. All these Ra isotopes derive from decay of Th parents, which are tightly bound to particles. Because Ra is mobilized in the marine environment, sediments provide a continuous source of Ra isotopes to estuarine waters at rates set by its decay constants. The Th isotope activities in the sediments and the distribution coefficient of Ra between the sediments and water determine the potential input of each Ra isotope to the water. Because 228Ra is regenerated much faster than 226Ra, estuaries with high 228Ra/ 226Ra activity ratios in the water must have a high degree of exchange with sediments on the sea bed or with groundwater draining nearby. This information is useful to elucidate the contribution of estuarine systems to the exchange of trace elements, N, P and C in the mixing zone. The combined source functions for Ra in a coastal area include riverine particulates/ dissolved input, oceanic dissolved concentrations, input from sediments and groundwater. The relative significance of each of these sources is usually a function of the site-specific hydrogeology and where the samples are taken relative to the salinity gradient (extent of freshwater/saltwater mixing). Thus, the Ra isotopes provide fundamental information on the interaction of sediments, groundwater and estuarine waters. In this project, the distribution of natural Ra isotopes was studied in surface, groundwater and estuarine water samples collected from dry and wet seasons (2009 2010) campaigns performed in Ribeira Valley, Southern São Paulo State. The inventory allowed the application of Ra isotopes as tracers of fluvial and groundwater discharges to the Cananéia-Iguape estuarine complex. The exchange of groundwater/ surface water in Ribeira do Iguape River basin and related fluxes of several constituents for the Cananéia-Iguape estuarine complex mass balance is still not very well known. The results obtained in this research work evidenced that there is a prevalence of 228Ra isotope in all the set of samples analyzed. However, the activity concentrations of Ra isotopes determined from Higher Ribeira Valley through the Southern Coastal Plain of São Paulo are representative of natural background levels, showing low or minimal human intervention. In the set of samples collected along Ribeira do Iguape River, Cananéia and Iguape outlets, the higher concentrations of Ra were observed in bottom waters, indicating the diffusion of 228Ra from sediments recently deposited as a potential source of the increased concentrations of this isotope when compared with others. The activity concentrations of the short-lived Ra isotopes were negligible, lower than the limit of the detection. Fluxes of Ra for Cananéia outlet are strongly influenced by tidal oscillations, which modulate the increase and decrease of Ra concentrations in response of the respective increase and decrease of waters salinity. In Iguape outlet and in hydrochemical stations performed along Ribeira do Iguape River it was observed a linear relationship between the amount of suspended matter and the increase of 228Ra activity concentration. When we evaluate qualitatively the differences in behavior of both long-lived Ra isotopes, the concentrations of 226Ra have not shown similar distribution to 228Ra. This demonstrates negligible contribution from advective porewaters and groundwater to the studied scenario. Dominant fluxes of trace-elements, radionuclides and nutrients have their main sources centered on fluvial, sediments and suspended matter compartiments.

ASSUNTO(S)

cananéia cananéia iguape ra-226 ra-226 ra-228 ra-228 sgd sgd iguape

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