Risk-Based Environmental Management Plan for Construction Dewatering

A significant requirement of large scale construction dewatering projects is the development of an Environmental Management Plan (EMP), designed to protect the health of the natural environment throughout the course of the project.  EMPs have traditionally relied on developing monitoring and mitigation plans for meeting compliance criteria, based on available background conditions.  Frequently, insufficient background data are available to reliably determine targets for impacts of dewatering on groundwater levels or streamflow.

The 16^th Avenue Trunk Sewer Project is part of a massive Regional Municipality of York (population 950,000) municipal sewer system improvement initiative to meet the long-term needs of the region.  The project involved tunnelling a 6.5 mile long gravity-fed sewer tunnel through overburden material, including a significant regional aquifer.  Construction of the tunnel involved the use of a Tunnel Boring Machine in conjunction with dewatering of the regional aquifer.

An original EMP was designed to meet the compliance requirements of several regulatory agencies at the federal, provincial, and municipal levels of government.  However, conflicting agency requirements made the EMP onerous with respect to compliance; and as the project neared completion, the requirements of the EMP could no longer be met.

Charged with creating a new EMP that was sustainable while maintaining the health of the natural environment, we engaged the various regulatory agencies to develop a risk-based adaptive EMP, the first of its kind for a construction project in Ontario.  The EMP relied on regular monitoring of shallow and deep groundwater levels and gradients, aquatic habitat, fisheries and benthic organisms, streamflow, and groundwater and surface water quality sampling to determine the need for implementing mitigation measures.  This paper presents the process and lessons learned of engaging and educating regulatory agencies to solve conflicting requirements en route to designing and implementing a sustainable adaptive environmental management plan.