Difference between revisions of "Groundwater"

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


This is to mitigate risks due to short periods of groundwater monitoring 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.  
==Target groundwater separation==
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.
In many areas of Ontario the target separation between the base of an exfiltrating LID practice and the water table is 1.0 m.
This is to mitigate risks due to short periods of groundwater mounding and potentially unobserved seasonal fluctuations. 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 requirement for additional 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 :
Retention of an unsaturated zone beneath the practice :
*Minimizes the potential for functional impacts associated with reduced percolation rates,
*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.
*Maintains the physical and biochemical water quality treatment benefits provided within the vadose zone.
 
 
==Groundwater mounding==
==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.
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).   
When you wish to model the extent of groundwater mounding beneath an infiltration facility, use this Tool that incorporates Hantush's derivation (1967).   


{{Clickable button|[[Media:Hantush.xlsm|Download groundwater mounding calculator(.xlsm)]]}}
{{Clickable button|[[Media:Hantush.xlsm|Download the Hantush groundwater mounding tool]]}}


Note that this is a minor adaptation (metric units and formatting) from the original tool, written and [https://pubs.usgs.gov/sir/2010/5102/ hosted by the USGS].
Note that this is a minor adaptation (metric units and formatting) from the original tool, written and [https://pubs.usgs.gov/sir/2010/5102/ hosted by the USGS].


===Reducing groundwater mounding with underdrains===
===Preventing groundwater interaction===
[[File:Drain spacing.jpg|thumb|The yellow box represents the recommended hydraulic conductivity of bioretention filter media]]
Many LID systems rely upon reuse, or evaporation and transpiration instead of infiltration to the ground. If the site cannot support any infiltration, consider
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.
*[[Rainwater harvesting]],  
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*[[Stormwater planters]],
This recommendation is supported by an analysis of Hooghoudt's equation <ref>H.P.Ritzema, 1994, Subsurface flow to drains. Chapter 8 in: H.P.Ritzema (ed.), Drainage Principles and Applications, Publ. 16, pp. 236-304, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. ISBN 90-70754-33-9</ref><ref>W.H. van der Molen en J.Wesseling, 1991. A solution in closed form and a series solution to replace the tables for the thickness of the equivalent layer in Hooghoudt's drain spacing equation. Agricultural Water Management 19, pp.1-16</ref><ref>van Beers, W.F.J. 1976, COMPUTING DRAIN SPACINGS: A generalized method with special reference to sensitivity analysis
*[[Green roofs]], or  
and geo-hydrological investigations, International Institute for Land Reclamation and Improvement (ILRI) Wageningen, The Netherlands</ref> in relation to loamy or clayey native soils, where ''K<sub>media</sub>''>>''K<sub>soil</sub>'', finds the first term of the numerator negligible, so that the original equation:
*[[Blue roofs]].
<math>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}}</math>
may be simplified to:
<math>Drain\ spacing=\sqrt{\frac{4K_{media}\left(D_{d}-D_{w}\right)^{2}}{q}}</math>
{{Plainlist|1=Where:
*''K<sub>media</sub> is expressed in m/day
*''D<sub>d</sub>'' is the depth to the drain pipe (m)
*''D<sub>w</sub>'' 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===
==Oak Ridges Moraine Groundwater Program==
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]].
[[File:ORMGP.JPG|thumb|Depth to groundwater raster and contours from the ORMGP]]
https://oakridgeswater.ca/
 
The Oak Ridges Moraine focused program 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 beyond Lake Simcoe and the Kawartha Lakes. Within the publicly accessible maps section there is a depth to water table map layer. 
 
===Other sources of groundwater data===
*[http://ontariogroundwater.com/ Ontario groundwater]
*[https://www.ontario.ca/environment-and-energy/map-provincial-groundwater-monitoring-network Provincial groundwater monitoring network map]
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Latest revision as of 13:45, 10 August 2020

Target groundwater separation[edit]

In many areas of Ontario the target separation between the base of an exfiltrating LID practice and the water table is 1.0 m. This is to mitigate risks due to short periods of groundwater mounding and potentially unobserved seasonal fluctuations. 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 requirement for additional 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[edit]

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[edit]

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

Oak Ridges Moraine Groundwater Program[edit]

Depth to groundwater raster and contours from the ORMGP

https://oakridgeswater.ca/

The Oak Ridges Moraine focused program 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 beyond Lake Simcoe and the Kawartha Lakes. Within the publicly accessible maps section there is a depth to water table map layer.

Other sources of groundwater data[edit]

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.

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.

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