Ecotoxicological risks of calcium nitrate exposure to freshwater tropical organisms: Laboratory and field experiments.

This study aimed to analyze laboratory and field data to assess the ecotoxicological risks of calcium nitrate exposure to freshwater tropical biota. Short-term laboratorial tests resulted in estimated EC50 values of 76.72 (67.32-86.12)mg N-NO3₋ L(-1) for C. silvestrii and 296.46 (277.16-315.76) mg N-NO3₋ L(-1) for C. xanthus. Long-term laboratorial tests generated IC25 values of 5.05 (4.35-5.75) and 28.73 (26.30-31.15) mg N-NO3₋ L(-1) for C. silvestrii and C. xanthus, respectively. The results from in situ mesocosm experiments performed in the Ibirité reservoir (a tropical eutrophic urban water body located in SE Brazil) indicated that C. silvestrii and C. xanthus were not under severe deleterious acute impact due to the treatment because the higher nitrate concentrations determined were 5.2 mg N-NO3₋ L(-1) (t=24 h; sediment-water interface) and 17.5 mg N-NO3₋ L(-1) (t=600 h; interstitial water). However, an abrupt decrease in the densities of Cyanophyceae members and other benthic taxa was observed. In summary, the present work contributes greatly to the toxicity data linked to two taxonomically distinct organisms that have never been screened for calcium nitrate sensitivity. Furthermore, considering the problem of the management and restoration of eutrophic environments, our study reports a comprehensive field assessment that allows the elucidation of the possible toxic impacts caused by the addition of calcium nitrate (a remediation technique) on aquatic and benthic organisms as well as the implications on the aquatic ecosystem as a whole, which may greatly allow expanding the current knowledgebase on the topic.

Calcium nitrate addition to control the internal load of phosphorus from sediments of a tropical eutrophic reservoir: microcosm experiments.

The main objective of this study was to perform laboratory experiments on calcium nitrate addition to sediments of a tropical eutrophic urban reservoir (Ibirité reservoir, SE Brazil) to immobilize the reactive soluble phosphorus (RSP) and to evaluate possible geochemical changes and toxic effects caused by this treatment. Reductions of 75 and 89% in the concentration of RSP were observed in the water column and interstitial water, respectively, after 145 days of nitrate addition. The nitrate application increased the rate of autotrophic denitrification, causing a consumption of 98% of the added nitrate and oxidation of 99% of the acid volatile sulfide. As a consequence, there were increases in the sulfate and iron (II) concentrations in the sediment interstitial water and water column, as well as changes in the copper speciation in the sediments. Toxicity tests initially indicated that the high concentrations of nitrate and nitrite in the sediment interstitial water (up to 2300 mg L(-1) and 260 mg L(-1), respectively) were the major cause of mortality of Ceriodaphnia silvestrii and Chironomus xanthus. However, at the end of the experiment, the sediment toxicity was completely removed and a reduction in the 48 h-EC50 of the water was also observed. Based on these results we can say that calcium nitrate treatment proved to be a valuable tool in remediation of eutrophic aquatic ecosystems leading to conditions that can support a great diversity of organisms after a restoration period.

Assessment of the acute toxicity of eutrophic sediments after the addition of calcium nitrate (Ibirité reservoir, Minas Gerais-SE Brazil): initial laboratory experiments / Toxicidade aguda de sedimentos eutrofizados (Represa Ibirité, Minas Gerais-SE, Brasil) após adição de nitrato de cálcio: experimentos iniciais de laboratório

This study evaluated the acute toxicity of sediment in a eutrophic reservoir after remediation with a calcium nitrate solution to retain phosphorus. The study involved microcosms of surface sediments and water from the sediment-water interface in the Ibirité reservoir. This reservoir, located in the vicinity of metropolitan Belo Horizonte (Minas Gerais, SE Brazil), is a water body that receives treated effluents from an oil refinery (REGAP-Petrobras), as well as high loads of untreated urban effluents from the city of Ibirité and surrounding areas and industrial effluents from a major industrial park. Incubation times of the treatment experiments were: t = 0, t = 5, t = 10, t = 25, t = 50, t = 85 and t = 135 days. One control microcosm and three treated microcosms were analysed in each time interval. Acute toxicity of water samples was assessed with Ceriodaphnia silvestrii Daday, 1902 and that of bulk sediment samples with Chironomus xanthus Rempel, 1939. Toxicity tests were carried out concomitantly with chemical analyses of dissolved inorganic nitrogen species (ammonia, nitrate and nitrite), sulfate and metals in the water samples of the microcosms. Acid volatile sulfides (AVS), simultaneously extracted metal (SEM) and potentially bioavailable metal were analyzed in bulk sediment samples. Neither of the tested organisms showed toxicity in the control microcosm samples. The water column of the treated microcosm showed toxicity to C. silvestrii, starting at t = 10 days, while the sediment pore water toxicity started at t = 0 day. However, toxicity was found to decline from t = 85 days to t = 135 days. Sediments showed toxicity to C. xanthus during the entire experiment, except at the longest incubation time (t = 135 days). The overall results indicate that nitrate, which reached concentrations exceeding 1,200 mg N-NO3- L-¹ in the sediment pore water of the treated microcosms, was most probably responsible for the toxicity of the samples. Although the calcium nitrate technology proved effective in retaining phosphorus, promoting sediment oxidation via denitrification, from the ecotoxicological standpoint and under the experimental conditions of this study, the application of nitrate for remediation of the sediments in the Ibirité reservoir did not prove effective up to a period of 135 days of incubation. However, we presume that after longer periods of incubation, treated sediments may recover their ability to sustain a benthic community. More advanced experiments are planned involving longer incubation times, thus extending the denitrification process, which may lead to a higher phosphorus retention capacity and to more complete abatement of sediment toxicity.

O presente trabalho visou à avaliação da toxicidade aguda da aplicação de solução de nitrato de cálcio, como procedimento para remediação dos sedimentos de um reservatório eutrofizado com vistas à retenção de fósforo. O estudo foi realizado através de microcosmos com sedimento e amostras de interface sedimento-água da Represa Ibirité. A represa, situada na região metropolitana de Belo Horizonte (Minas Gerais, Brasil), é corpo receptor de efluentes tratados de refinaria de petróleo (REGAP-Petrobras), de altas cargas de esgoto sanitário não tratado da cidade de Ibirité e áreas vizinhas, e de efluentes industriais de importante parque industrial. Os tempos de incubação dos experimentos foram: t = 0, t = 5; t = 10; t = 25; t = 50; t = 85; e t = 135 dias. Em cada um deles, foram analisadas amostras de um microcosmo-controle e três microcosmos-tratamento. A Ceriodaphnia silvestrii Daday, 1902 foi o organismo utilizado para avaliação da toxicidade aguda das amostras de água, enquanto que o Chironomus xanthus Rempel, 1939 foi empregado para a avaliação do sedimento integral. Paralelamente aos testes de toxicidade, foram realizadas análises químicas da série nitrogenada (amônia, nitrato e nitrito), sulfato, e metais nas amostras de água. Nos sedimentos foram analisados os sulfetos volatilizáveis por acidificação (SVA), metais extraídos simultaneamente (MES) e metais potencialmente biodisponíveis. Tanto as amostras de água como dos sedimentos totais dos microcosmos-controle não se mostraram tóxicos aos organismos testados. As amostras de água de interface sedimento-água dos micorocosmos-tratamento foram tóxicas para a C. silvestrii desde o tempo t = 10 dias e as amostras de águas intersticiais, desde o período t = 0. No entanto, foi notada uma diminuição da toxicidade do tempo t = 85 para t = 135 dias. Para o organismo C. xanthus, os sedimentos em tratamento se mostraram tóxicos durante todo o experimento, exceto no tempo t = 135 dias. Os resultados mostraram que o nitrato, com concentração superior a 1.200 mg N-NO3- L-¹ nas amostras de água intersticial dos sedimentos dos microcosmos-tratamentos, é a causa mais provável da toxicidade das amostras. Embora a tecnologia de adição de nitrato tenha se mostrado eficaz na retenção de fósforo nos sedimentos, pois causa a oxidação dos sedimentos através do incremento substancialmente da taxa de desnitrificaçao, do ponto de vista ecotoxicológico e para as condições experimentais deste trabalho, a aplicação do nitrato como forma de intervenção para remediação dos sedimentos da Represa Ibirité não se mostrou adequada até o período de 135 dias. Após este período, presume-se que os sedimentos tratados possam recuperar a capacidade de sustentar uma comunidade bentônica. Experimentos mais avançados foram planejados, visando um tempo de incubação mais estendido, que, por conseguinte, pode levar a uma maior capacidade de retenção de P e ao abatimento total da toxicidade dos sedimentos e da água, devido a um maior avanço do processo da desnitrificação.