Hydrogeological Study vs Geotechnical Report

When you need both

• Basements or garages near the water table. The geotechnical engineer designs the foundation and drainage; the hydrogeologist sets the design groundwater level and works out whether dewatering or permanent drainage is needed, and how much.
• Construction dewatering. Under water taking EASR regulation (O. Reg. 63/16), the water taking report for an EASR registration has to analyze soil settlement and land subsidence and their effect on nearby infrastructure. That analysis needs geotechnical soil properties as well as the predicted drawdown.
• Infiltration and LID. The hydrogeologist needs the seasonal high water table and infiltration rates; the geotechnical engineer needs to know the infiltration will not soften the subgrade or affect foundations.
• Slopes and valley lands. Groundwater seeping from a slope is often what makes it unstable, so slope stability work leans on hydrogeological data.

Sharing the boreholes

Combining the drilling is usually the cheapest approach, but only if it is planned for both reports before the rig arrives. Three things decide whether a geotechnical borehole can also serve the hydrogeology:

1. Depth. Geotechnical holes stop where the foundation design needs them. Hydrogeological wells often have to go deeper: Brampton's terms of reference ask for monitoring wells at least 2 m below the lowest elevation of the proposed structures.
2. The well itself. A water level noted in an open hole is not a monitoring well. Screens have to be set in the formations that matter, and Richmond Hill's hydrogeological study requirements ask for single-well response tests in the formations that dewatering will affect, so the wells must be built to be tested.
3. Who builds it. Under R.R.O. 1990, Reg. 903, a hole made to test groundwater or gather information about it, and not meant to supply drinking or farm water, is a "test hole", and a test hole counts as a well. A test hole or dewatering well no deeper than 3.0 m below the ground surface is exempt from the well rules (section 1.1). The exemption does not apply if it is built in a contaminated area or where conditions are likely to produce flowing wells, or if it goes all the way through a formation that is not an aquifer, meaning a layer that cannot transmit enough water to serve as a water supply. Any other test hole has to be built to the regulation's well construction rules, so the drilling contractor needs the right well licence. Even an exempt shallow hole must be filled in properly once it is no longer used.

Once installed, monitoring, sampling or testing equipment can generally be put in a test hole without further licensing, as long as it does not alter the well or test its yield. The practical sequence: the geotechnical engineer and the hydrogeologist agree the borehole locations and depths, the hydrogeologist specifies which holes become monitoring wells and where the screens go, and the logs are shared between both reports.

Mistakes that cost a second mobilization

• Boreholes stopped above the depth the hydrogeological terms of reference ask for.
• Holes backfilled after logging, with no monitoring wells installed, so water levels cannot be tracked over the seasons.
• Wells screened across several layers, which mixes water levels and makes response tests hard to interpret.
• Too few wells to show flow direction across the site.
• Geotechnical drilling done months before anyone thought about monitoring, so the seasonal record starts late.

If a planning application is likely to need a hydrogeological study, bring the hydrogeologist in when the geotechnical drilling is scoped. Our development hydrogeological study page covers what the study includes and what it costs.