We performed an Environmental Risk Assessment (ERA) to evaluate the impact on non-humanbiota from liquid radioactive effluents discharged by the Belgian Nuclear Power Plants(NPPs) of Doel and Tihange. A deterministic risk assessment for aquatic and terrestrialecosystems was performed using the ERICA tool and applying the ERICA screening value of 10µGy.h-1. The ERA was performed for the radioactive discharge limits and forthe actual releases (maxima and averages over the last 10 years, 1999-2008). All ERICAreference organisms were considered and depending on the assessment situation, additionalreference organisms were included in the analysis. It can be concluded that the currentdischarge limits for the Belgian NPPs do not result in significant risks to the aquaticand terrestrial environment and that the actual discharges, which are a fraction of theliquid discharge limits, are unlikely to harm the environment.

The toxicity and oxidative stress responses of 19-day old Arabidopsis seedlings induced by U (66 μM) and Cd (20 μM) alone or in a binary mixture set-up (equitoxic mixture) are studied in function of time. After 48h a significant decrease in root and shoot growth and a simultaneous increase in anthocyanin production was evident in all treated plants.

Production of O−2 or H2O2 was visualized by staining freshly harvested leaves with nitrobluetetrazolium or diaminobezidine, respectively. With this method production of O−2 was only significantly measurable after 168 h treatment which coincides with a significant decrease in biomass production and probably also plant cell death. For Cd treated plants a significant increase in H2O2 production was measurable from 24h onwards. In contrast, a similar H2O2 production could not be measured in U or U + Cd treated plants. Both water and lipophilic soluble antioxidants significantly increased in U treated plants after 48 h. These high antioxidant levels might detoxify potential H2O2 produced in the U treated plants. In contrast for Cd treated plants only after 168h a significant increase in water soluble antioxidants was measured whereas no difference in the lipophilic fraction was visible.

This study aimed at investigating biological effects in Arabidopsis thalianaleaves and roots irradiated for 72 h with 3.5 Gy or 30 Gy of gamma radiation, and to unravel oxidative stress related responses to achieve a better understanding of the importance of the cellular redox balance as a modulator in gamma radiation stress. A. thaliana performs like a rather radioresistant plant species as no alterations on growth and only minor alterations in the nutrient profile were observed. Gamma irradiation did not seem to induce an NADPH mediated oxidative burst and lipid peroxidation appeared to be directly induced by ionizing radiation rather than mediated through LOX activity. As ionizing radiation can cause indirect damage via water radiolysis, H2O2 is hypothesized to be an important reactive oxygen species under radiation stress. Although most H2O2-scavenging enzymes remained unchanged, important alterations were observed for CAT1, CAT2 and CAT3expression.

The reactive oxygen species (ROS)-signaling pathway is very important in heavy metal toxicity. Induction of the antioxidative defense mechanism, comprising ROS-scavenging enzymes and metabolites, in plants after environmental uranium contamination has been insufficiently studied in the past. This study aimed to analyze oxidative stress related responses in Arabidopsis thaliana after uranium exposure. Seventeen-day-old seedlings were exposed to 0, 0.1, 1, 10 and 100 μM uranium for 3 days. After exposure to 100 μM uranium, a decrease in fresh weight for leaves and roots was observed, leaves colored anthocyanous and roots were stunted and yellow. To reveal the importance of oxidative stress in uranium toxicity, alterations in ROS-scavenging enzymes were studied at protein and transcriptional level. Superoxide dismutase (SOD) capacities increased in leaves and roots after exposure to 100 μM uranium but no differences were observed for catalase (CAT) capacities. Transcript levels of different SODs located at various cellular compartments were affected depending on the place of action. Gene expression of CAT in leaves and roots was also affected after uranium exposure. Results indicate that oxidative stress plays an important role in uranium toxicity but suggest that plant responses differ for leaves and roots.

The biological responses induced by low-dose chronic gamma exposure of hydroponically grown Arabidopsis thaliana, irradiated during a full life cycle (seed to seed) were investigated. Applied dose rates were 2300, 375 and 85 µGray h-1. Plants (roots and shoots) were harvested after 24 day (inflorescence emergence), at 34 days (∼50% of flowers open) and at 54 days (silice ripening). Gamma exposure significantly reduced root weight compared to the control but no clear effect of dose rate level on root dry weight was observed. Leaf weight was significantly reduced at the highest irradiation level, only after 54 days exposure. ED-10 was estimated at 10 µGy h-1. Seed germination was not affected by gamma irradiation. For several of the stress enzymes studied enzyme capacity was generally stimulated at the low and intermediate gamma irradiation level compared to the control and highest irradiation level. No pattern was observed in concentration or reduction state of the non-enzymatic antioxidants, ascorbate and glutathione. Lipid peroxidation products in leaves were present highest at full flowering and decreased with exposure level at this growth stage. At the other two growth stages, lipid peroxidation products were unaffected by gamma treatment.

The measurement of diffusive gradients in thin films (DGT) has been proposed as a surrogate for metal uptake by plants. A small-scale experiment was performed to test the predictive capacity of the DGT method with respect to uranium and heavy metal availability and uptake by ryegrass. Correlation analysis was performed to compare the results obtained with the DGT device with more conventional bioavailability indices: concentration in pore water or in selective extractants. Six soils with different histories of uranium contamination and with distinct soil characteristics were screened. For uranium, the bioavailability indices evaluated were highly correlated, indicating that uranium pools assessed were at least partially comparable. For heavy metals, bioavailability indices were hardly correlated. Uranium concentration in the pore water was a better predictor for uranium uptake than amount recovered following selective extraction. The concentration measured with DGT (CDGT) was also highly correlated with plant uptake. However, the significance level was sensitive to the value of the diffusion coefficient (pH dependent or not) used to calculate CDGT. For the heavy metals screened, only in the case of Cd was uptake predicted by CDGT.

A greenhouse experiment was set up to evaluate the potential of enhanced phytoextraction to clean up U and heavy metal contaminated soils. One soil had a naturally high U concentration; the other soil was impacted by the radium extraction industry. Enhancement of solubility and uptake by plants (ryegrass and Indian mustard) was monitored after addition of 5 chemical amendments: citric acid, ammonium citrate-citric acid mixture, oxalic acid, EDDS and NTA. Solubilisation and uptake were highly influenced by the amendment applied and soil-plant combinations. For U, citric acid, the ammonium citrate-citric acid mixture or EDDS were most effective in increasing U uptake. EDDS was most effective in increasing Cu in mustard and ryegrass and Pb in ryegrass shoots. For other metals, increase in uptake was limited to at most a factor 5. Percentages annually removed with biomass ranged from 0.0002% to 1.52%., and were lowest for U, Cr and Pb and highest for Cd. A targeted 10% reduction in soil contaminant would require 7 years for Cd, 35 and 52 years for Cu and Zn, 203 and 384 years for U and Pb and 9433 years for Cr. Phytoextraction is hence not a feasible technique to decontaminate historically contaminated soils.