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Todos os Santos Bay (BTS) is the 2nd largest bay in Brazil and an important resource for the people of the State of Bahia. We made measurements of radon and radium in selected areas of the bay to evaluate if these tracers could provide estimates of submarine groundwater discharge (SGD) and flushing times of the Paraguaçu Estuary and BTS. We found that there were a few areas along the eastern and northeastern shorelines that displayed relatively high radon and low salinities, indicating possible sites of enhanced SGD. A time-series mooring over a tidal cycle at Marina do Bonfim showed a systematic enrichment of the short-lived radium isotopes 223Ra and 224Ra during the falling tide. Assuming that the elevated radium isotopes were related to SGD and using measured radium activities from a shallow well at the site, we estimated groundwater seepage at about 70 m3/day per unit width of shoreline. Extrapolating to an estimated total shoreline length provided a first approximation of total (fresh + saline) SGD into BTS of 300 m3/s, about 3 times the average river discharge into the bay. Just applying the shoreline lengths from areas identified with high radon and reduced salinity results in a lower SGD estimate of 20 m3/s. Flushing times of the Paraguaçu Estuary were estimated at about 3-4 days based on changing radium isotope ratios from low to high salinities. The flushing time for the entire BTS was also attempted using the same approach and resulted in a surprisingly low value of only 6-8 days. Although physical oceanographic models have proposed flushing times on the order of months, a simple tidal prism calculation provided results in the range of 4-7 days, consistent with the radium approach. Based on these initial results, we recommend a strategy for refining both SGD and flushing time estimates.

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RESUMEN

Water flow pathways and water balance are fundamental components for understanding the dynamics of C in the soil/water interface of small basins. The objective of this study was to describe the seasonal variations and estimate the annual balance of dissolved organic carbon (DOC) by comparing two tropical microbasins (preserved forest-PF and cacao plantation-CP). Twenty-one weekly collections were conducted from September to December 2012 and from April to June 2013. The calculation of the partial balance considered precipitation (P) as inflow and the stream as outflow. The samples were filtered and analyzed using a TOC analyzer. Overall, the DOC was higher CP compared with FP. The behavior of both venues showed that rainy season caused an increase in concentrations in the overland flow (OF) and in the stream, and a decrease in the precipitation (P) and in the throughfall (T). In the CP, the outflow and the soil were chiefly responsible for the high DOC concentrations in the stream, when compared to the PF, which is the result of constant OM decomposition. Soil composition contributes to the control of DOC consumption in each type of soil. The balances were negative in both microbasins, although losses were higher in the AFS (agroforestry systems) when compared to the PF, especially during rainy seasons (-8.98 and -3.05 kg ha-1 year-1, CP and FP, respectively). Thus, the high annual loss of DOC in the CP of the microbasins during the rainy season indicates changes in ecosystem metabolism due to the vegetation cover and to the interactions with the soil.

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Sampling was conducted monthly during a transition period between the dry and rainy seasons in order to evaluate the effectiveness of a municipal sewage treatment plant (STP) in eutrophication control. STP effluent and fluvial input data were also estimated. In the dry period, high concentrations of nutrients, chlorophyll a (up to 360 µg L(-1)), and anoxia in bottom waters were observed in the upper portion of the estuary. Nitrate was scarce during the dry months, although high concentrations were observed at the river sources and the upper estuary. The N:P and Si:P molar ratios were usually below 16:1, and the Si:N ratio was higher than 1:1. The fluvial inputs were a greater source of nutrients to the estuary than the STP, but nutrient loading by these effluents were also important in contributing to the eutrophication of the upper estuarine zone, especially in the dry season when symptoms were more intense.

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