Evaluation of the potential toxicity (acute and chronic) of sediments from abandoned uranium mine ponds

Background, Aim and Scope. The superficial aquatic system of the Cunha Baixa uranium mine area is comprised by the flooded mine pit (M), which receives the acidic mine effluent resultant from in situ leaching of pore ore, a pond where this effluent is neutralised (T), and a potential reference pond (Ref). As part of the first tiers of an ecological risk assessment that is being performed in this area, the aim of this work was to evaluate the potential sediment toxicity of these ponds. Methodology. To perform this work, elutriates were produced from sediments collected at the three ponds in two distinct seasons (Spring and Winter). Acute and chronic toxicity of elutriates was evaluated by standard assay protocols, using Daphnia spp. as test species (D. magna - standard species and D. longispina native species). Results. In opposition to what could be previewed based on total metal concentrations, results showed acute toxicity only in M site (low pH, high metal levels) in spring for both species with EC50=94.7% and 96.3% (D. longispina and D. magna respectively). A stimulatory effect (in growth and fecundity) was observed in the chronic assays, for almost all of the tested concentrations of the three elutriates tested - except for the highest concentrations of the M elutriate. Some differences were observed in the responses of Daphnia, both between test species and seasons. Differences between the two sampling periods were also found for pH. Discussion. Although some toxicity was observed in M, overall no toxicity was found for sediments of the aquatic system, corroborating previous results from our team (including whole-sediment tests). Differences in pH may explain the observed toxicity, acting both as a stressor and mobilizing contaminants. Stimulatory phenomena, typical when dealing with natural samples, worked as confounding factors. Several explanations should be considered for these stimulatory effects, however the role of radionuclides (not measured in this work) cannot be ignored. Conclusions. Supported by the results gathered in this study and in previous evaluations already performed, it is possible to state that in the present situation sediments plays a secondary role in the toxicity of the Cunha Baixa uranium mine, probably working more as a barrier than as a source of contaminants to the water column. However, future reclamation works in this area should carefully consider the remobilisation of sediments, especially from pond M. Recommendations and Perspectives. Radiochemical contamination is expected to be higher in field situations where radiation emitter isotopes are present in all the sediment compartment and overlying water, emitting radiations in all the directions. Thus whole-sediment and elutriate laboratory bioassays are not representative of long-term field exposures to radiation. Based on this supposition, these laboratory responses should be validated by field surveys of the benthic and planktonic freshwater communities of these ponds and freshwater receiving resources, at the higher tiers of the local ecological risk assessment.