We hypothesize that biogeochemical processes in a subterranean estuary in the northeastern Gulf of Mexico significantly alter nutrient concentrations that are discharged into the ocean via submarine groundwater discharge (SGD). This subterranean estuary is a site of steep gradients over small spatial scales, so selecting the appropriate groundwater endmember for flux calculations is difficult. Two-endmember mixing models typically applied to investigate biogeochemical processes in surface estuaries are not useful in this case because of aquifer heterogeneity and complex solid-fluid interactions. Groundwater residence times, redox potential, source strength, and aquifer matrix appear to be the most important drivers for dissolved organic carbon (DOC) and nutrient concentrations in this pristine coastal plain aquifer. The distribution of dissolved species was consistent with a sequence of reactions in a narrow (,2 m) surface layer in which nitrate is initially exhausted (likely because of denitrification), organic nitrogen is remineralized releasing ammonium, and some DOC remains. On the basis of a 222 Rn-derived non-steady state box model, total (terrestrial + marine) SGD rates were estimated to oscillate around 11 cm d 21 . The SGD is composed almost entirely of recirculated seawater (accounting for ,95% of total SGD fluxes). Conservative estimates of groundwater-derived nitrogen inputs into the coastal ocean were 8.2 6 1.2 mmol m 22 d 21 , consisting of ammonium (58%), organic nitrogen (28%), and nitrate (14%). We suggest that these mainly represent recycled rather than new nutrient additions. When extrapolated to the Florida Gulf coast, the nitrogen and DOC fluxes were comparable to the main regional rivers, supporting the proposition that SGD plays a major role in coastal biogeochemistry.

Acrylic fibers impregnated with MnO 2 (Mn-fiber) have become a valuable tool for concentrating dissolved radium for oceanographic applications. With four naturally-occurring radium isotopes ( Rn. We present a proven, air-tight cartridge design that allows one to use these measurement techniques. We then review the procedures for three radon-based nondestructive measurement techniques for 226 Ra on Mn-fiber (via RAD7, RaDeCC, and Rn emanation line systems) and perform an intercomparison among them, using the standard technique of γ-spectrometry as a reference. We find that all methods statistically agree in terms of measured activity. The Rn emanation line and the RaDeCC systems (both based on Lucas cell counting) provide the lowest measurement uncertainties and minimum detectable activities (MDAs) for a given counting time. The RAD7 technique, on the other hand, offers the advantage of being an automated system, thus requiring minimal user interaction. The standard γ-spectrometry technique, while more time-consuming and sample destructive, has the advantage of providing a simultaneous measurement for 228 Ra.

We review the usefulness, limitations, significance, and coastal management implications of radionuclide measurements in Brazilian coastal environments. We focus on the use of radionuclides as tracers of sedimentary processes and submarine groundwater discharge (SGD). We also discuss artificial radionuclide contamination and high natural radioactivity areas. The interpretation of 14C-, 137Cs-, and 210Pb-derived sedimentation rates has provided evidence that inappropriate soil use by urban and agricultural activities has intensified erosion processes on land, which is reflected in depositional environments, such as coastal lagoons, estuaries and mangroves. Of the processes discussed in this paper, SGD is the one that requires the most scientific effort in the short-term. There have been only two case studies using 222Rn and radium isotopes as groundwater tracers in Brazil. These investigations showed that SGD can be a major source of nutrients and other dissolved species to the coastal ocean. Baseline 137Cs, 90Sr, 239+240Pu, and 238Pu concentrations in seawater from the whole Brazilian coastal zone are very low. Therefore, in spite of contamination problems in many ecosystems in the northern hemisphere, artificial radionuclide pollution appears to be negligible along the Brazilian coast. Phosphate fertilizer industries and petroleum processing facilities are the main economic activities producing Technologically Enhanced Naturally Occurring Radioactive Materials (TENORM). Even though a few attempts have been made to assess the radiological effects of these activities, their potential threats indicate a need for the radiological control of their wastes. This review showed that the number of studies within the field of environmental radioactivity is still small in Brazil and much more research is needed to understand processes of high interest for environmental managers. In the near future, it is likely that such studies in Brazil will move from descriptive, environmental quality-based assessments to approaches that attempt to quantify chemical, physical, and biological processes in the environment.
Este artigo revisa a utilidade, limitações, importância e implicações para o gerenciamento costeiro das medições de radionuclídeos em ambientes costeiros do Brasil. O artigo enfoca o uso de radioisótopos como traçadores de processos sedimentares e descarga de água subterrânea na zona costeira. Também, é apresentada uma discussão sobre contaminação por radionuclídeos e áreas de alta radioatividade natural. A interpretação de taxas de sedimentação baseadas em 14C, 137Cs e 210Pb demonstra que o uso inadequado do solo por atividades agrícolas e urbanas intensificou os processos de erosão no continente, o que é refletido em ambientes deposicionais, como lagoas costeiras, estuários e manguezais. Existem apenas dois estudos de caso sobre o uso de isótopos como traçadores de água subterrânea no Brasil demonstrando que águas subterrâneas podem ser uma importante fonte de nutrientes e outras substâncias dissolvidas para o ocean...

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To better predict how ocean acidification will affect coral reefs, it is important to understand how biogeochemical cycles on reefs alter carbonate chemistry over various temporal and spatial scales. This study quantifies the contribution of fresh groundwater discharge (as traced by radon) and shallow porewater exchange (as quantified from advective chamber incubations) to total alkalinity (TA) dynamics on a fringing coral reef lagoon along the southern Pacific island of Rarotonga over a tidal and diel cycle. Benthic alkalinity fluxes were affected by the advective circulation of water through permeable sediments, with net daily flux rates of carbonate alkalinity ranging from −1.55 to 7.76 mmol m⁻² d⁻¹, depending on the advection rate. Submarine groundwater discharge (SGD) was a source of TA to the lagoon, with the highest flux rates measured at low tide, and an average daily TA flux of 1080 mmol m⁻² d⁻¹. Both sources of TA were important on a reef wide basis, although SGD acted solely as a delivery mechanism of TA to the lagoon, while porewater advection was either a sink or source of TA dependant on the time of day. On a daily basis, groundwater can contribute approximately 70% to 80% of the TA taken up by corals within the lagoon. This study describes overlooked sources of TA to coral reef ecosystems that can potentially alter water-column carbonate chemistry. We suggest that porewater and groundwater fluxes of TA should be taken into account in ocean acidification models in order to properly address changing carbonate chemistry within coral reef ecosystems