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The Saco da Mangueira bay, located on the banks of the municipality of Rio Grande - RS, has suffered over the years, processes of pollution and contamination associated with its significant role in fishing, industrial, leisure activities, among others. Based on the above, this work aims to evaluate the water quality of the Saco da Mangueira bay, and for this to build a bottom sediment sampler in order to allow the extraction of interstitial water to evaluate its quality together with the of surface and bottom, through its classification and framing based on CONAMA Resolution No. 357 of 2005, in addition to the eutrophic classification by the Trophic State Index, as a form of aid in the planning and management of the water resources of this ecosystem. A sampling campaign was carried out on February 13, 2020, in which water samples were collected at 6 locations distributed in the study region, under which physical, chemical and microbiological parameters were evaluated. The results obtained showed that the waters of Saco da Mangueira are in Class 2 environmental condition of CONAMA Resolution 357/05, except for coliforms in some places. The Trophic State Index indicated that the bay has a low to intermediate amount of nutrients, classified from mesotrophic to oligotrophic. Thus, it is concluded that the bay can suffer degradation associated with different sources of human inputs, regardless of its framework, which reinforces the need for continuous monitoring of the quality of these waters and an integrated environmental management.

O canal São Gonçalo se caracteriza por ser um importante elo de ligação econômica e estratégica entre Brasil e Uruguai, bem como, é peça chave na gestão da irrigação na Planície Lagunar do estado do Rio Grande do Sul. Devido a esses motivos, objetivou-se fazer uma caracterização morfométrica dos sedimentos de fundo do canal São Gonçalo, buscando encontrar um padrão de comportamento e distribuição granulométrica na região de estudo. De tal forma, realizaram-se seis campanhas de coletas de sedimentos de fundo espacializadas de forma sazonal, possuindo cada uma dessas, 8 pontos distribuídos espacialmente. Obtiveram-se a partir das coletas, as distribuições granulométricas referentes a cada uma das 48 amostras coletadas. Esses dados permitiram a construção de um triângulo textural para o sedimento de fundo da região, curvas de distribuição granulométrica dos pontos, e análise estatística das granulometrias. O conjunto de resultados obtidos não permitiu a obtenção de um padrão de distribuição granulométrica do local, devido à grande variabilidade sazonal e espacial dos resultados.

The chemical speciation analysis of chromium in one of the most important South American Estuary was performed for the first time. Samples were collected in Patos Lagoon Estuary (Brazil) and were analysed by adsorptive cathodic stripping voltammetry, with the following analytical figures of merit: limit of detection, 0.1 nmol L(-1); precision RSD = 3%, n = 7; linearity, from limit of quantitation up to 20 nmol L(-1); and accuracy of 99.8%, expressed as recovery. No labile chromium forms were identified in samples, beside industries and a city were near the study area. It is pointed out a reverse correlation between total and non-active chromium and salinity, which could be explained by biogeochemical processes.

The southern coastal region of Brazil is highly productive and many of its oceanographic processes are still poorly understood due to a great number of forces, such as the continental input from the Plata River and Patos Lagoon, several distinct oceanic water masses, and a complex contribution from groundwater. The shelf near Albardão is highly rich in dissolved nutrients, particularly in the coastal zone. The narrow sandy barrier that separates the ocean from Mangueira Lagoon has led us to wonder about the possibility of groundwater, and if it might play a role in the productivity of the surrounding area. In order to test this hypothesis, nutrient data from Mirim and Mangueira Lagoons and from Albardão were collected during a rainy period. The present work shows the need to include groundwater transport from Mangueira Lagoon to the coastal zone as one of the main sources for the productivity in that region. However, for such phenomenon to be quantified and better understood, it becomes necessary to use natural isotopes as tracers of the underground input.A região sul do Brasil é uma área altamente produtiva, cujos processos oceanográficos ainda são pouco compreendidos devido ao grande número de forçantes, tais como aportes continentais do Rio da Prata, Lagoa dos Patos, diferentes massas de água oceânicas e a complexa contribuição da água subterrânea. A plataforma da região do Albardão é extremamente rica em nutrientes dissolvidos, especialmente na zona costeira. O estreito cordão arenoso que separa a região do Albardão da Lagoa Mangueira levou-nos a questionar a possibilidade do transporte de água subterrânea e se este poderia contribuir de forma expressiva na produtividade dessa região. Para testar essa hipótese, associamos dados de nutrientes das Lagoas Mirim e Mangueira a dados inéditos da plataforma da região do Albardão, todos em período de alta precipitação. Este trabalho demonstra a necessidade de incluir o transporte de água subterrânea da Lagoa Mangueira para a região costeira, como uma das principais fontes para o aumento da produtividade da região. Entretanto, para quantificar e compreender melhor este fenômeno faz-se necessário o uso de isótopos naturais como traçadores desses aportes subterrâneos.

Summary We investigated the distribution of naturally occurring geochemical tracers ( 222 Rn, 223 Ra, 224 Ra, 226 Ra, CH 4 , δ 18 O, and δ 2 H) in the water column and adjacent groundwater of Mangueira Lagoon as proxies of groundwater discharge. Mangueira Lagoon is a large (90 km long), shallow (∼4–5 m deep), fresh, and non-tidal coastal lagoon in southern Brazil surrounded by extensively irrigated rice plantations and numerous irrigation canals. We hypothesized that the annual, intense irrigation for rice agriculture creates extreme conditions that seasonally change groundwater discharge patterns in the adjacent lagoon. We further supposed that dredging of irrigation canals alters groundwater fluxes. While the activities of 222 Rn in shallow groundwater were 2–3 orders of magnitude higher than in surface water, CH 4 and radium isotopes were only ∼1 order of magnitude higher. Therefore, 222 Rn appears to be the preferred groundwater tracer in this system. Radon concentrations and conductivities were dramatically higher near the pump house of rice irrigation canals, consistent with a groundwater source. Modeling of radon inventories accounting for total inputs (groundwater advection, diffusion from sediments, and decay of 226 Ra) and losses (atmospheric evasion, horizontal mixing and decay) indicated that groundwater advection rates in the irrigation canals (∼25 cm/d) are over 2 orders of magnitude higher than along the shoreline (∼0.1 cm/d). Nearly 75% of the total area of the canals is found in the southern half of the lagoon, where groundwater inputs seem to be higher as also indicated by methane and stable isotope trends. In spite of the relatively small area of the canals, we estimate that they contribute nearly 70% of the total (∼57,000 m 3 /d) groundwater input into the entire Mangueira Lagoon. We suggest that the dredging of these canals cut through aquitards which previously restricted upward advection from the underlying permeable strata. The irrigation channels may therefore represent an important but previously overlooked source of nutrients and other dissolved chemicals derived from agricultural practices into the lagoon.

This paper characterizes major ion distributions and investigates whether groundwater exerts a major control on the chemical functioning of Mangueira Lagoon, a large (90 km long), shallow (∼4–5 m deep), and fresh coastal lagoon in southern Brazil. Water volumes equivalent to ∼80% of the total annual input are used in the summer for irrigating nearby rice plantations, the most important regional economic activity. While Na+ and Cl− are the major ions in local groundwater, Na+ and HCO 3 − are the most enriched ions in lagoon water. The ion concentrations measured in Mangueira Lagoon were homogeneous, except for a few samples affected by rainwater and groundwater inputs. A shore-normal transect starting at the pump house of a rice irrigation canal indicated strong groundwater input at this canal. In spite of the small volume contribution (∼2% of precipitation), groundwater discharge accounts for 50–70% of major ion inputs into the lagoon, with ∼70% of the groundwater inputs being anthropogenically derived (e.g., from the rice irrigation canals). This may have serious implications for the management of the coastal water resources from Mangueira Lagoon and other similar areas as groundwater associated with agricultural systems may be contaminated by fertilizers and pesticides. The results imply that groundwater should not be neglected in dissolved species’ budgets even when its volume contribution is small.

This study assesses the importance of groundwater discharge to dissolved nutrient levels in Mangueira Lagoon. A transect of an irrigation canal in the margin of Lagoon demonstrated a strong geochemical gradient due to high groundwater inputs in this area. Using 222Rn as a quantitative groundwater tracer, we observed that the flux of dissolved inorganic nitrogen (DIN), silicate and phosphate (1178 and 1977; 26190 and 35652; 167 and 188 mol d-1 for winter and summer, respectively) can continually supply/ sustain primary production. The irrigation canals act as an artificial underground tributary and represent a new source of nutrients to coastal lagoons.