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Soil & Groundwater - East Harbour Station

← Return to Early Works: East Harbour Station Draft Report

Key Findings

  • Soil within the study area consists primarily of clay, silt, sand and gravel.
  • The following source water features are present in the study area: an Intake Protection Zone*, a regional Highly Vulnerable Aquifer** and an Event Based Area***.
  • The area is serviced by municipal water supply from Lake Ontario.
  • Without mitigation, there is potential for impacts to groundwater (e.g., temporary drawdown of the water table) and soil (e.g., ground movement and settlement as a result of excavation and/or dewatering). The completion of hydrogeological and geotechnical investigations, a construction dewatering assessment and groundwater and soil management plans as planning progresses, and implementation of appropriate mitigation measures during early works activities are anticipated to reduce potential impacts (see Potential Effects & Mitigation Measures).

*Areas of land and water that contribute source water to a surface water drinking system intake.
**An aquifer underground layer of permeable soil and/or rock) susceptible to contamination due to its location near the ground surface or the characteristics of surrounding soils.
***Areas in a watershed where a spill could pollute the drinking water supply.

Small scale excavation support system.

Small scale excavation support system.
https://www.naxsa.org/mpage/2020-06-excavator

Potential Effects & Mitigation Measures

Potential Effects:

Groundwater

  • Subsidence/settlement of structures in the Zone of Influence* (ZOI) due to construction dewatering if dewatering is required.
  • Encounter previously contaminated groundwater during early works activities (e.g., dewatering).
  • Construction dewatering may impact groundwater-dependent natural features (i.e., Don River) as a result of decreases in groundwater discharge.

Soil

  • Potential displacement of soils as a result of early works activities may result in ground movement and settlement.
  • Early works activities (e.g., excavation) could expose and/or result in the spreading of contaminated materials.

*The Zone of Influence refers to the area where groundwater levels will be lowered due to dewatering.

Mitigation Measures:

Groundwater

  • As project planning progresses, determine water taking quantities, quality, and resultant dewatering ZOI through a site-specific hydrogeological investigation, construction dewatering assessment and groundwater management plan as required.
  • If anticipated, potential subsidence/settlement impacts to existing structures can be mitigated with measures such as the completion of pre-construction inspections of structures in the dewatering ZOI and implementation of a detailed settlement monitoring program, complete with settlement triggers that result in changes to the dewatering program if surpassed.
  • Adhere to source water protection best management practices where necessary, including the development of a salt management plan and a plan for handling and storage of chemicals.
  • Develop remedial action plans, risk assessment and risk mitigation plans for encountering contaminated groundwater.

Soil

  • Complete detailed soil investigations, as project planning progresses.
  • Employ excavation support systems if required and/or implement appropriate ground treatment such as jet grouting to reduce the risk of ground loss during early works activities.
  • Develop management plan(s) for handling, management and disposal of excavated material as required.
  • Develop and implement remedial action plans, risk assessment and risk mitigation plans for encountering contamination.

← Return to Early Works: East Harbour Station Draft Report