Integrating biological degradation potential into ecological risk assessment

Expanding agricultural activities and industrial operations have resulted in the accumulation of toxic chemicals in our environment, and have become a potential threat to human health as well as to the flora and fauna in our ecosystems. The assessment of contamination risk to our environment mainly relies on mathematical models that estimate the contaminant concentrations and the leaching rates into aquifers. The contaminant level is commonly determined by comparing the concentrations predicted by models against a threshold concentration, and the ecosystem is considered at risk if the predicted contaminant concentration is above a threshold value. Although environmental models can describe contaminant transport and adsorption processes relatively well, most of these models do not take into account the capability of microorganisms to degrade toxic contaminants into less toxic or non-toxic molecules. Many experimental and in-situ studies have shown that soil microbial activity can result in a relatively fast rate of contaminant degradation, therefore implying that the time required to “clean-up” a contaminated site may be substantially shorter in the presence of receptive microorganisms. Hence, ecological risk assessments not accounting for microbial load and specific catabolic processes can overstate the risks and lead to the making of policies and management strategies only partly suitable to specific contaminated ecosystems. In this study, we integrated the biodegradation potential ψB of a contaminant into ecological risk assessment index through the specific biomass affinity ϕ. We focused on atrazine (ATZ), one of the most extensively used herbicides in Australia, as the model contaminant using data from la Cecilia and Maggi (2017), and the Hazard Quotient as the modeled ecological risk index (Suter II, 2007). Model parameters were estimated from laboratory experiments, while simulations were extended to a 60-year time scale to analyze the Hazard Quotient of ATZ contamination in groundwater with the inclusion of ATZ biodegradation potential under scenarios of different ATZ application rates. These analyses demonstrated that ATZ contamination level can be overestimated if the biodegradation potential were not taken into account. The use of contaminant biodegradation potential in ecological risk assessments can improve information for optimum decision making in environment and resources management.