Interaction between cadmium and mercury accumulation by Daphnia magna Straus (Crustacea, Branchiopoda)

The objective of this study was to evaluate the effects and interactions of various factors on the simultaneous accumulation of cadmium and mercury contaminants. Two chemical forms of mercury (HgCl2 and CH3HgCl) were used in conjunction with inorganic cadmium (as CdCl2). The effects of direct and trophic modes of accumulation were investigated. Daphnids (Daphnia magna) were exposed for 5 days to cadmium (CdCl2) and mercury (HgCl2 or CH3HgCl) in different combinations and concentrations. Exposure was provided either directly via water, or indirectly by providing contaminated algae (Chlorella vulgaris) as a food source. As it is often the case for aquatic organisms, methylmercury was found to be the form of mercury most efficiently accumulated by D. magna. The way of exposure to mercury was also a determinant, with the results depending on the chemical form used. Therefore, even though the preferential way of inorganic mercury accumulation was through the water, the preferential way for D. magna contaminated with methylmercury was through contaminated algae. However, accumulation of Cadmium was not significantly affected by the exposure way. Both cadmium and mercury were able to inhibit each other accumulation. Even though HgCl2 was normally the mercury form that most efficiently inhibited cadmium accumulation, inhibition of CH3HgCl was more efficient in the presence of cadmium.El objetivo de este estudio fue evaluar los efectos e interacciones de varios factores sobre la acumulación simultánea de cadmio y mercurio. Se utilizaron dos formas químicas de mercurio (HgCl2 y CH3HgCl) junto con cadmio inorgánico (como CdCl2), y se investigaron los efectos de la vía de contaminación (directa y trófica). Se expusieron ejemplares de daphnias (Daphnia magna) durante 5 días a diferentes concentraciones de cadmio (CdCl2) y mercurio (HgCl2 y CH3HgCl) en varias combinaciones. La contaminación se efectuó directamente a través del agua, o indirectamente a través de algas previamente contaminadas (Chlorella vulgaris). Como suele observarse en organismos acuáticos, el metilmercurio fue la forma de mercurio más eficientemente acumulada por D. magna. La vía de contaminación por mercurio fue también determinante con resultados dependientes de la forma química usada. Por lo tanto, aunque el modo preferente de acumulación de mercurio inorgánico fue a través del agua, en D. magna contaminada con metilmercurio el modo preferente fue a través de algas contaminadas. Sin embargo, la acumulación de cadmio no se vio significativamente afectada por la vía de contaminación. Tanto el cadmio como el mercurio fueron capaces de inhibir el proceso de acumulación del otro metal. Aunque el HgCl2 fue normalmente la forma de mercurio que más eficientemente inhibía la acumulación de cadmio, la inhibición de CH3HgCl fue más eficientemente en presencia de cadmio

Long-term TNT and DNT contamination: 1-D modeling of natural attenuation in the vadose zone: case study, Portugal

The vadose zone of a trinitrotoluene (TNT) and dinitrotoluene (DNT) contaminated site was investigated to assess the mobility of those explosives under natural conditions. Located in the left margin of the River Tejo Basin, Portugal, the site is located on unconsolidated sediments. Wastewaters associated with the 50-year explosives production were disposed in excavated ponds, from where water would infiltrate and pollute the unsaturated and saturated parts of the local aquifers. Two boreholes were drilled to 9 m depth in such a former waste pond to investigate the contaminant's fate in the vadose zone. Sediment samples were taken every 1-2 m for analysis of the polynitroaromatics (p-NACs) and organic volatile compounds, pH, organic carbon content, cation exchange capacity and grain size analysis. The main contaminant was TNT representing >70 % of the total p-NACs concentration that peaked approximately 7 mg/kg in one borehole, even if the median in both boreholes was of similar to 1 mg/kg. DNT was 4-30 % of the total p-NACs and nitrotoluene (NT), up to 5 %. No other (volatile) organic compound was detected. The predominance of TNT as the main contaminant implies that any natural mass reduction has been inefficient to clean the site. Several 1-D model simulations of p-NACs cleaning of the vadose zone under natural conditions indicated that the most probable scenario of combined advection and partitioning will only remove TNT after 10's of years, whereas DNT and NT will hardly be removed. Such low concentrations and long times for the p-NACs removal, suggest that by now those compounds have been washed-out to a level below standard limits