Aplicação da técnica de difusão em filmes finos por gradiente de concentração (DGT) em poço de monitoramento de água subterrânea
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Data
2021-05-11
Autores
Santos, Cristiane Aily
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Universidade Estadual Paulista (Unesp)
Resumo
A amostragem passiva é entendida como o procedimento para coleta de amostra de água em determinado intervalo do aquífero sem realização de purga no poço. A técnica Diffusion Gradients in Thin films – DGT – utiliza-se de um amostrador passivo simples que permite avaliar in situ a concentração de metais e outros elementos nos sistemas aquáticos, bem como a especiação desses elementos em diferentes tipos de amostras. A DGT tem sido aplicada com sucesso em águas com intensa agitação, tais como rios, córregos, alguns lagos e mares. No entanto, para sistemas onde há baixa vazão (águas não turbulentas), observa-se a influência de uma camada limite difusiva (Diffusive Boundary Layer - DBL) que se forma no dispositivo e que, devido a sua espessura, afeta as concentrações medidas com o uso da DGT. Por este motivo, a DGT tem sido pouco aplicada em sistemas não turbulentos, como em poços de monitoramento de água subterrânea. O presente trabalho visou determinar a espessura da DBL e verificar a aplicabilidade da DGT em poços de monitoramento de água subterrânea. Para isso foram realizados testes laboratoriais e aplicada a DGT em um poço instalado para monitoramento em um Posto Revendedor de Combustíveis. Dentre as vantagens de se utilizar a DGT para medidas das concentrações de metais em água tem-se que esta tem uma excelente sensibilidade devido à pré-concentração, possibilitando que baixíssimos limites de detecção sejam alcançados. É ideal para monitoramento ambiental, uma vez que é possível se obter a concentração média em relação ao tempo, não gera volume de água de purga, e não altera a dinâmica de fluxo subterrâneo natural, além de simplificar as operações durante a amostragem. Os resultados dos experimentos realizados em laboratório simulando um poço de monitoramento com baixo fluxo mostraram que a DBL está presente no sistema e que deve ser considerada na correção das concentrações obtidas pela técnica DGT. Uma DBL única, calculada a partir do coeficiente angular da reta para cada elemento, é proposta para ser utilizada nos cálculos das concentrações. As amostras pontuais coletadas in situ com bailer antes e após a retirada dos dispositivos mostraram grande variação nas concentrações. As concentrações calculadas para os dispositivos com uma DBL média foram maiores do que as das amostras pontuais ou foram consideravelmente menores. Dentre as interferências ocorridas in situ pode-se considerar a baixa força iônica na água local, a presença de coloides impedindo a difusão através do filtro, a complexa especiação dos elementos e, provavelmente, a questão relacionada à adequação do coeficiente de difusão clássico em ambiente de água subterrânea. A proposta de calcular uma única DBL a partir dos coeficientes angulares mostrou-se aplicável. A DGT aplicada in situ, até o momento pode ser considerada como uma boa técnica de screening para a água subterrânea.
Passive sampling is understood as a procedure for collecting water samples from a given aquifer interval without purging the existing water in the well. The Diffusion Gradients in Thin films - DGT device is a simple passive sampler that allows to evaluate in situ the concentration of metals, semi-metals and other elements in aquatic systems, as well as the speciation of these elements in different types of samples. DGT has been successfully applied since 1994 in waters with intense agitation such as rivers, streams and agitated surfaces of some lakes and seas. However, for low flow systems (non-turbulent waters) the influence of a diffusive boundary layer (DBL) formed in the device was observed and that, due to its thickness, affects the concentrations measured with the use of DGT. For this reason, DGT has not yet been applied to these non-turbulent systems such as groundwater monitoring wells. The present work aims to evaluate the applicability of DGT in groundwater monitoring wells. For this purpose laboratory tests were performed and performing tests in wells installed to monitor an area contaminated by metals are intended. Among the advantages of using DGT to measure metal concentrations in water is the excellent sensitivity due to its pre-concentration property, and very low detection limits can be achieved. It is also ideal for performing environmental monitoring, since it is possible to obtain the average concentration over time, it does not generate purge water volume, nor changes the natural underground flow dynamics and simplifies operations during sampling. The results of experiments carried out in a laboratory that simulated a monitoring well with low flow showed that DBL is present in this system and that it should be considered in the correction of the concentrations obtained by DGT. A single DBL calculated from the slope of the line for each element is proposed to be used in calculations of concentrations. The grab samples collected with bailer before and after the removal of the devices showed an expressive variation in the concentrations. The concentrations calculated for devices with an average DBL were higher than the concentration of the grab sample or were much lower. Among the interferences that have occurred in situ, we can consider the low ionic strength in the local water, the presence of colloids preventing diffusion through the filter, the complex speciation of the elements and probably an issue related to the adequacy of the classic diffusion coefficient in groundwater environment. The proposal to calculate a single DBL from the angular coefficients proved to be applicable. DGT applied in situ, until now, can be considered as a good screening technique for groundwater.
Passive sampling is understood as a procedure for collecting water samples from a given aquifer interval without purging the existing water in the well. The Diffusion Gradients in Thin films - DGT device is a simple passive sampler that allows to evaluate in situ the concentration of metals, semi-metals and other elements in aquatic systems, as well as the speciation of these elements in different types of samples. DGT has been successfully applied since 1994 in waters with intense agitation such as rivers, streams and agitated surfaces of some lakes and seas. However, for low flow systems (non-turbulent waters) the influence of a diffusive boundary layer (DBL) formed in the device was observed and that, due to its thickness, affects the concentrations measured with the use of DGT. For this reason, DGT has not yet been applied to these non-turbulent systems such as groundwater monitoring wells. The present work aims to evaluate the applicability of DGT in groundwater monitoring wells. For this purpose laboratory tests were performed and performing tests in wells installed to monitor an area contaminated by metals are intended. Among the advantages of using DGT to measure metal concentrations in water is the excellent sensitivity due to its pre-concentration property, and very low detection limits can be achieved. It is also ideal for performing environmental monitoring, since it is possible to obtain the average concentration over time, it does not generate purge water volume, nor changes the natural underground flow dynamics and simplifies operations during sampling. The results of experiments carried out in a laboratory that simulated a monitoring well with low flow showed that DBL is present in this system and that it should be considered in the correction of the concentrations obtained by DGT. A single DBL calculated from the slope of the line for each element is proposed to be used in calculations of concentrations. The grab samples collected with bailer before and after the removal of the devices showed an expressive variation in the concentrations. The concentrations calculated for devices with an average DBL were higher than the concentration of the grab sample or were much lower. Among the interferences that have occurred in situ, we can consider the low ionic strength in the local water, the presence of colloids preventing diffusion through the filter, the complex speciation of the elements and probably an issue related to the adequacy of the classic diffusion coefficient in groundwater environment. The proposal to calculate a single DBL from the angular coefficients proved to be applicable. DGT applied in situ, until now, can be considered as a good screening technique for groundwater.
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Palavras-chave
DGT, DBL, Água subterrânea, Poço de monitoramento, Metais, Metals, Monitoring well, Ground water