Does the treatment of contaminated groundwater with a nanoscaled zero valent iron-based remediation agent increase or decrease the environmental risk?

In this post, Mirco Weil reports on their recently published paper “Environmental risk or benefit? Comprehensive risk assessment of groundwater treated with nano Fe0-based Carbo-Iron®.”

Considering the dependence of urban settlements and agricultural economy on groundwater, any contamination of it is clearly undesirable. Among the common sources for contamination are tank leakages and accidental spills of chlorinated hydrocarbons. In the EU, a total of 132 groundwater bodies are heavily contaminated with the chlorinated alkenes tetrachloroethene and trichloroethene. The extensive use of groundwater highlights the need for groundwater remediation. For the treatment of groundwater contaminated with chlorinated hydrocarbons, research focused on nano-scaled zero-valent iron (nFe0) in recent years. These particles are placed in high concentrations into the groundwater-bearing aquifer to create a permeable reactive barrier.

Treatment of contaminated groundwater with nFe0-based Carbo-Iron® and the approach to assess the unknown environmental risk or benefit of this remediation (graphic by M. Weil)

While a risk-cost-benefit analysis is performed prior to the application of the remedial agent, this analysis does only rarely consider the potential risk of the remediation method for the local environment. We believe that the assessment of the environmental risk before the remediation is necessary, since any remedial agent may have negative impacts on groundwater ecosystems at the local or regional scale. These ecosystems provide important services, including the purification of water by microbiological biodegradation of chemical substances, the elimination of pathogens and the metabolization of dissolved organic matter. Thereby they improve the water quality and keep aquifer pore spaces open.

Our recent study gathers data required for the environmental risk assessment of the nFe0-based remedial agent Carbo-Iron®. We complemented the available data for effects assessment by studies with the crustacea Daphnia magna, the algae Scenedesmus vacuolatus, larvae of the insect species Chironomus riparius and nitrifying soil microorganisms. With these data, we determined the most sensitive endpoint and calculated a predicted no effect concentration (PNEC) of 0.1 mg/L. The data on predicted environmental concentrations (PEC) of the Carbo-Iron® were taken from a recent field study and compared with the PNEC to derive risk quotients. The PEC of Carbo-Iron® exceeds the PNEC by a factor of 6500 directly in the permeable barrier. With increasing distance to this zone, the risk quotient decreases and reaches a value of 5 approx. 10 m from the barrier. Beyond this zone, the risk quotient is assumed to be below 1, since migration of Carbo-Iron® is limited.

The use of the data from the risk assessment to determine the suitability of Carbo-Iron® as a remedial agent is restricted, since previous environmental risk to Carbo-Iron® application is not considered. Therefore, a Triad-based approach was developed, integrating the assessment of chemical, physico-chemical and ecotoxicological risks in the groundwater before and after treatment with Carbo-Iron®. Here, we determined some negative impacts of Carbo-Iron®, but the beneficial influence of the remedial agent predominated.

The paper was authored by Mirco Weil, Katrin Mackenzie, Kaarina Foit, Dana Kühnel, Wibke Busch, Mirco Bundschuh, Ralf Schulz and Karen Duis, and published in Science of the Total Environment.