G.7
Infiltration of large volumes of water may result in a rise in the water table or development of a shallow water table on hydraulically‑restrictive materials that slow the downward percolation of water. If this mounding of water is excessive, the infiltration BMP may become less effective and/or adjacent structures or facilities may be impacted by the rising water table. In addition, if the infiltration BMP is adjacent to a slope, slope stability may be decreased.
Thresholds for triggering groundwater mounding and seepage analysis are summarized in Table II-5.4.
The mounding analysis shall evaluate the impact of the infiltration BMP on local groundwater flow direction and water table elevations and determine whether there would be any adverse effects caused by seepage zones on nearby building foundations, basements, roads, parking lots or sloping sites. If the results of the mounding analysis indicate that adverse conditions could occur, as determined by a licensed professional, the infiltration BMP shall not be built.
If infiltration on the site may result in shallow lateral flow (interflow), the conveyance and possible locations where that interflow may re‑emerge should be assessed by a licensed hydrogeologist.
For deep infiltration BMPs, the following shall also be evaluated:
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Extent of groundwater mounding under the design flow rate.
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Potential impacts from the groundwater mounding to:
- Deep infiltration BMP performance.
- Surrounding infrastructure, including, but not limited to, infiltration BMPs, drainage facilities, foundations, basements, utility corridors, or retaining walls.
- Offsite slope stability.
- Down-gradient existing contamination plumes.
Several analytical tools are available to evaluate potential groundwater mounding beneath infiltration BMPs. These include both analytical and numerical groundwater flow software. In general, public domain software programs shall be used (such those initially authored by the United States Geological Survey (USGS) or the Environmental Protection Agency).
The software program MODRET is considered a standard tool for evaluating infiltration BMPs, and is recommended in the Ecology Manual. Although MODRET is a proprietary computer program, it is readily available for purchase and is based on USGS software. However, MODRET is limited to evaluation of a single BMP at a time, and generally will not be suitable for evaluating clustered BMPs.
The preferred program for simulating groundwater mounding beneath infiltration BMPs is the USGS‑based program MODFLOW. MODFLOW can be used to simulate a wide range of aquifer conditions and geometries. The primary limitation with MODFLOW is that most versions of the program do not simulate the movement of water through the unsaturated zone, which would normally be expected to slow the downward movement of water and allow for lateral spreading of water before reaching the water table. Instead, infiltrating water is input directly to the water table. For a shallow water table or perching layer this limitation should not greatly influence the overall results of the mounding simulation and represents a more conservative approach to simulating mounding.
Licensed hydrogeologists with formal training and experience in developing groundwater flow models should conduct these analyses. It should also be noted that groundwater models do not provide specific answers but are tools to help understand the behavior of groundwater systems under a variety of conditions. The results of any model should be used in the context of the overall goal of the project and be applied as warranted by the risk tolerance of the owner.
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