Groundwater

The target separation between the base of an exfiltrating LID practice and the water table is 0.9 m.

This is to mitigate risks due to short periods of groundwater mounding and potentially unobserved seasonal fluctuations. In areas where a 0.9 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 requirement for additional investigation and/or documentation supporting a proposed design may be reduced in areas where ≥ 0.9 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. This tool uses Hantush's derivation (1967).

Download groundwater mounding calculator(.xlsm)

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

Reducing groundwater mounding with underdrains

The yellow box represents the recommended hydraulic conductivity of bioretention filter media

Mounding of groundwater can be mitigated by correctly sizing and spacing of the pipes within the underdrain structure. In most large infiltrating LID systems (e.g. Bioretention, permeable paving or dry ponds, lateral drains should be spaced between 5 - 6 m apart. But when groundwater mounding must be minimized, this distance will be reduced and should be recalculated.

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This recommendation is supported by an analysis of Hooghoudt's equation in relation to loamy or clayey native soils, where K_media>>K_soil, finds the first term of the numerator negligible, so that the original equation: $Drain\ spacing={\sqrt {\frac {8K_{soil}H\left(D_{i}-D_{d}\right)\left(D_{d}-D_{w}\right)+4K_{soil}\left(D_{d}-D_{w}\right)^{2}}{q}}}$ may be simplified to: $Drain\ spacing={\sqrt {\frac {4K_{media}\left(D_{d}-D_{w}\right)^{2}}{q}}}$

Where:

• K_media is expressed in m/day
• D_d is the depth to the drain pipe (m)
• D_w is the minimum acceptable depth to the water table during infiltration event
• q is the inflow volume expressed as a depth over the entire surface (m)

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,
• Flow-through planters,
• Green roofs, or
• Blue roofs.

Sources of groundwater data

• Ontario groundwater
• Provincial groundwater monitoring network map

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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.

A parameter that describes the capability of a medium to transmit water.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

A perforated pipe used to assist the draining of soils.

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.

Relating to the properties, distribution and effects of water on and below the earth’s surface, and in the atmosphere.