Vertical separation

In Ontario the required vertical separation between a practice and the water table or bedrock is frequently cited as 1 meter. 
This comes from the 2003 Stormwater Management Planning and Design Manual.
"The depth to bedrock should be greater than or equal to 1 metre below the bottom of the perforated pipe storage media to ensure adequate drainage/hydraulic potential"https://www.ontario.ca/document/stormwater-management-planning-and-design-manual/stormwater-management-plan-and-swmp-design
"If a pervious pipe system is implemented in an area where the seasonally high water table is higher than the obvert of the pipe, the pipe will drain the groundwater table. In this scenario, depending on the native soil characteristics and whether the trench or pipe is wrapped in geotextile fabric, soil can be transported into the pipe system undermining the pipe foundation and leading to structural failure. Pervious pipe systems should not be implemented in areas where the seasonal high groundwater level is within 1 metre of the bottom of the storm sewer backfill to ensure this does not happen."
Both of these considerations are equally relevant to the underdrain of any structural BMP. 
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Whilst this is a great rule of thumb, like all aspects of LID, this 1 meter figure might require amendment on a site-by-site basis. 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.
Factors to consider include: 
*risks due to short periods of groundwater mounding and potentially unobserved seasonal fluctuations. 
*the potential for functional impacts associated with reduced percolation rates,
*retaining an unsaturated zone beneath the practice maintains the physical and biochemical water quality treatment benefits provided within the vadose zone.
 
===Other references of interest===
The CSA standard W200, Design of bioretention systems, requires a minimum vertical separation of 0.6 m between the base of bioretention and the water tableCSA Group. (n.d.). W200-18 Design of bioretention systems. Retrieved from https://store.csagroup.org/ccrz__ProductDetails?sku=2704497 
The Ontario Building Code requires 
Province of Ontario. “O. Reg. 332/12: BUILDING CODE,” 2018. https://www.ontario.ca/laws/regulation/120332.
Karathanasis, A D, T G Mueller, B Boone, and Y L Thompson. “Nutrient Removal from Septic Effluents as Affected by Soil Thickness and Texture.” Journal of Water and Health 4, no. 2 (June 2006): 177–95. http://www.ncbi.nlm.nih.gov/pubmed/16813011.
Stall, Christopher, Aziz Amoozegar, David Lindbo, Alexandria Graves, and Diana Rashash. “Transport of E. Coli in a Sandy Soil as Impacted by Depth to Water Table.” Journal of Environmental Health 76, no. 6: 92–100. Accessed October 23, 2018. http://www.ncbi.nlm.nih.gov/pubmed/24645419.
Rhode Island (largely located on glacial till with loamy sands) clarified their position on vertical separation in 2016 in relation to LID including:
*reduction to 0.6 m when infiltrating water from residential roofs and driveways, 
*flexibility when siting projects on brownfield (contaminated) land, 
*limited flexbility in retrofit scenarios, and
*accounting for bioretention media as part of the overall separation depth from surface water to groundwater. 
RI Department of Environmental Management. “Section 5.5.1 and 5.5.4 RISDISM Guidance -- Filtering Systems: Separation to Seasonal High Groundwater Table,” 2016.
Ontario. “F-6-1 Procedures to Govern Separation of Sewers and Watermains | Ontario.Ca,” 2016. https://www.ontario.ca/page/f-6-1-procedures-govern-separation-sewers-and-watermains.
Rathfelder, K., and M. Wei. “Underground Stormwater Infiltration: Best Practice for Protection of Groundwater Resources in British Columbia,” 2014. http://www.env.gov.bc.ca/wsd/plan_protect_sustain/groundwater/library.html in turn citing Washington St.ate Department of Ecology. “Guidance for UIC Wells That Manage Stormwater,” 2006.
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