A probabilistic assessment of the impairment status of Areas of Concern in the Laurentian Great Lakes: How far are we from delisting the Hamilton Harbour, Lake Ontario, Canada?

摘要

Environmental policy analysis aims to identify the important drivers of degradation, elucidate sources of controversy, and put the necessary risk assessment tools in place to prepare for the unexpected. It is thus critical to develop risk assessment frameworks that ensure both continuity in the decision-making process, but also iterative adjustments to accommodate the non-stationarity of open environmental systems or the intrinsic uncertainty reflecting our incomplete knowledge. In this context, we reappraise the idea of probabilistic water quality criteria as a pragmatic way to accommodate the frequently experienced situation when the prevailing conditions may not always be favourable, even if the central tendency of the system is on par with what is defined as an acceptable ecosystem state. Our case study is the Hamilton Harbour, a large embayment located at the western end of Lake Ontario, with a long history of environmental pollution problems. Based on a probabilistic framework, our basic objective is to provide a comprehensive assessment of the current state of eutrophication and fish contamination in the system. Notwithstanding the significant improvements brought about by the remedial measures in the area, our analysis shows that the system continues to experience excessively high exceedance frequencies of the water quality goals, as well as of the consumption advisory PCB benchmarks of eight sport fish species examined. Specifically, 95.6% of the measured TP concentrations exceeded the proposed threshold level of 20 ?g L-1, while 43.9% of the measured values exceeded the proposed chlorophyll a targeted value of 10 ?g L-1. Likewise, the recent THg exceedance frequencies of the advisory benchmark were consistently very low, except from freshwater drum, but we still found high frequency violations of the advisory PCB benchmark across all fish species examined. These results clearly suggest that the Hamilton Harbour is still a long way off from being delisted as an Area of Concern. While our modelling analysis does not negate -in principle- the optimism that the goal of restoring environmental health of the system is achievable, it cautions that there are many ?ecological unknowns? that may determine the degree and timing of restoration. The present framework encapsulates one of the founding concepts of the Hamilton Harbour remediation, whereby management strategies and quality goals heavily rely on continuous research and monitoring and dynamically adapt to the contemporary changes of the ecological conditions.