Groundwater

Target groundwater separation

In many areas of Ontario the target separation between the base of an infiltrating LID practice and the seasonally water table or bedrock surface elevation is 1.0 m. This is to mitigate risks due to short periods of groundwater mounding and potentially unobserved seasonal fluctuations. Seasonally high water table elevation should be determined through continuous water level monitoring of wells in the vicinity of the proposed infiltration facilities during spring months (March to June), when water levels in shallow aquifers are typically at their peak. In areas where a 1.0 m separation cannot be provided, or where conditions dictate that an even greater separation may be warranted, additional discussion and/or analysis specific to the physical characteristics of the site and the proposed design should be completed. The design practitioner is advised to consult with approval agencies to understand their requirements and/or expectations prior to undertaking work, and to complete an appropriate level of analysis to support their conclusion. The detail of investigation and/or documentation supporting a proposed design may be reduced in areas where ≥ 1.0 m separation is anticipated.

Retention of an unsaturated zone beneath the practice :

• Minimizes the potential for functional impacts associated with reduced percolation rates,
• Maintains the physical and biochemical water quality treatment benefits provided within the vadose zone.

Groundwater mounding

Groundwater mounding describes the localised raising of the water table beneath infiltration practices. It may be of concern if it affects nearby structures including building foundations. When you wish to model the extent of groundwater mounding beneath an infiltration facility, use this Tool that incorporates Hantush's derivation (1967).

Download the Hantush groundwater mounding tool

Note that this is a minor adaptation (metric units and formatting) from the original tool, written and hosted by the USGS.

Preventing groundwater interaction

Many LID systems rely upon reuse, or evaporation and transpiration instead of infiltration to the ground. If the site cannot support any infiltration, consider

• Rainwater harvesting,
• Stormwater planters,
• Green roofs, or
• Blue roofs.

Oak Ridges Moraine Groundwater Program

ORMGP Public mapping portal

Depth to groundwater raster and contours from the ORMGP

The Oak Ridges Moraine Groundwater Program works collaboratively with government agencies and consultants to better understand and manage the water resources of the Oak Ridges Moraine physiographic region, which stretches from the Credit and Nottawasaga River watersheds in the west, to the Trent River in the east and reaches from the shores of Lake Ontario northwards to Lake Simcoe and the Kawartha Lakes watersheds. The program delivers a multi-agency collaborative approach to collecting, analyzing and disseminating geological and hydrogeological knowledge as a basis for effective decision making and stewardship of water resources in this part of Ontario. Visit the program website to learn more about program elements and publications. Through the public mapping portal portion of the program website users have the ability to generate maps of estimated water table elevation and depth to water table for sites captured by the regional groundwater model and locate existing well and borehole records. This provides LID designers with a powerful tool for identifying opportunities for, and constraints to implementating stormwater infiltration facilities with regard to groundwater levels.

Other sources of groundwater data

• Ontario groundwater
• Provincial groundwater monitoring network map

Planning

Hydrogeology

The Conservation Authorities Geoscience Group has prepared a guidance document which attempts to standardize the hydrogeological study requirements for development applications made to Conservation Authorities. It is a helpful guide for all low impact development applications which include infiltration. It is provided with the caveat that not all sections or content are necessary or appropriate for every case. Pre-consultation with your local conservation authority and municipality is strongly recommended in all cases.

Hydrogeological Assessment Submissions Conservation Authority Guidelines for Development Applications

Beginning an infiltration BMP design?

See Infiltration testing and Design infiltration rates pages

See also

• Site considerations

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Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

Layer of rock or soil that holds or transmits water.

Abiotic transfer of water vapour to the atmosphere.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.