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Ecotoxicological tests with liquid fractions of the sediment (interstitial water and elutriates) has been widely used in environmental assessments. The tests with interstitial water differ from tests with total sediment as they assess ecotoxicity in a liquid sample that occupies the spaces between the solid particles of the sediment. This study was intended to assess ecotoxicity in the Lacustrine Complex formed by seven natural lagoons of different dimensions within the property of a steel mill located in the State of Espírito Santo.

The region under study is subject to an inflow of domestic effluents and has not been rated yet regarding its use, pursuant to CONAMA Resolution 357/2005. The surface water and sediment samples, from which interstitial water was extracted, were collected from 11 locations. The number of locations for assessment was predefined based on the size of each lagoon. The study period covered the Autumn and Winter of 2013. Material was collected in accordance with the procedures established in the Standard ABNT NBR 15469 (2007). The surface water samples were collected with a Van Dorn sampler at 50 cm from the surface and, at the same locations, the sediment samples was with a Petersen-type bottom sampler. The material dredged from each location was stored temporarily in a stainless steel box, homogenized and, after resting for roughly five minutes, a sample of interstitial water was removed with the aid of a sterile syringe. This study used the organismsDaphnia similisandCeriodaphia dubia, and the tests were held in accordance with the Standards ABNT-NBR 12713(2009) and ABNT-NBR 13373(2010), respectively.

The results for D. similis evidenced acute effects in 2/11 (18%) of surface water samples collected during the Autumn and 1/11 (9%) during the Winter. However, regarding interstitial water samples, 7/11 (64%) evidenced effects in the Autumn and 8/11 (73%) in the Winter. As for C.dubia, the surface water samples evidenced chronic effects only in 3/11 (28%) locations in the Autumn and 10/11 (91%) in the Winter, while for the interstitial water matrix (Autumn and Winter) only 1/11 (9%) of the locations did not evidence toxic effects. Ecotoxicity of the interstitial water samples (sediment) was greater than in samples collected from lagoon surfaces. Usually, sediments are deposits for most of the persistent pollutants disposed in the water environment (MELO and ABESSA, 2002) and, then, may be converted into more enriched matrices when compared to the water column (FURLEY, 2006).

Therefore, the comparison between samples evidenced greater ecotoxicity when they were submitted to chronic assessments with C.dubia than in acute assessments with D.similis. It was also possible to assess that interstitial water samples taken from sediment evidenced ecotoxicity greater than in the water samples from lagoon surfaces.