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<poem>Where possible, it is recommended that [[infiltration]] BMPs maintain a 1.0 m vertical separation above the seasonally high groundwater table. {{:Depth to GW}}
Retention of this unsaturated zone:
*Minimizes the potential for functional impacts associated with reduced percolation rates,
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.
==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).
{{Clickable button|[[Media:Hantush.xlsm|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===

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