Ontario Power Generation's Response to the Joint Review

Attachment 1 to OPG letter, Albert Sweetnam to Dr. Stella Swanson, “Deep Geologic Repository Project for Low and Intermediate Level Waste – Submission of
Responses to the Final Sub-set of Package #4 Information Requests”, CD#: 00216-CORR-00531-00143.
IR# EIS Guidelines
Section
Information Request and Response
increased to accommodate potential impacts of climate change on extreme rainfall intensity over the life of the project.
There is sufficient space on the DGR project site for this potential expansion of the pond.
3. Stormwater Management Pond Design – Storm Events
The SWMP is being designed to retain runoff and control suspended solids concentration in effluent discharges. A
minimum retention time of 24 hours for runoff from the 6-hour, 25 mm rainfall event is one of several design criteria for
the SWMP. The SWMP is also being designed to satisfy water quality sizing criteria in MOE (2003). The pond is being
designed in accordance with guidelines in MOE (2003). It will be a wet pond facility consisting of:
� a permanent pool and extended detention storage for control of total suspended solids concentrations in
effluent discharges that satisfy MOE volumetric water quality criteria; and
� an active storage volume for control of post-development peak discharges to predevelopment values during
24-hour storm events with return periods ranging from 2 to 100 years.
Retention of the 6-hour 25 mm rainfall event is specified as one of several measures to be considered in the evaluation
of stormwater control alternatives on existing industrial sites when conducting a Storm Water Control Study in
accordance with the requirements of Sector Effluent Monitoring and Effluent Limits Regulations under the Ontario
Environmental Protection Act (MOEE 1994). Rainfall depths of 12.5 to 25 mm are generally considered to represent
the “first flush”, runoff that occurs at the beginning of a storm event, which has higher contaminant concentrations.
MOE (2003) note the use of these rainfall depths as water quality design criteria. However, they propose an alternative
approach to the volumetric sizing of stormwater facilities for water quality control in Ontario, which is based on
continuous simulation modelling of end-of-pipe stormwater management practices that assesses the variation in
pollutant removal with stormwater management practice type and the impervious level of the tributary catchment. MOE
developed volumetric criteria that reflect a long-term climatic record and consider the effect of storms in series, event
overflows and winter melt conditions, as well as the basic characteristics of different stormwater management practice
types.
4. Seepage Water Pumping
During construction, seepage water (i.e., primarily groundwater inflow into the shafts) and construction process water
will be collected in sumps underground then pumped to surface for discharge into the ditch leading to SWMP. It is
likely that water will be pumped continuously during underground construction (shaft sinking and lateral development)
but the pumping rate will vary depending on the types of construction activities being carried out.
During operations it is envisaged that the seepage water will be collected in the main sump and then periodically
pumped to surface for discharge into the ditch leading to SWMP (see response to Information Request LPSC-01-19)
(OPG 2012b). The frequency at which water will be pumped to surface will be largely dependent on rate of
groundwater inflow to shafts (expected to be small) and the final size of the operations phase sump.
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