NJDEP requires that the impacts of groundwater mounding on proposed infiltration basins be analyzed as a condition of approvals.

The prescribed method for conducting this analysis is using the Hantush Spreadsheet, which can be downloaded from here. In the associated design manual, it is indicated that the 'hi(0)' value (Initial Thickness of the Saturated Zone) cannot be altered from its default value of 10 feet without additional geotechnical data. In my experience with utilizing the spreadsheet, I recognized early on that there is A LOT of benefit to my mounding analysis results if I can supplement my report with the necessary geotechnical data to warrant increasing the Saturated Zone's thickness, so I almost always required getting that information.

Lately, I've been contemplating why increasing this thickness has any effect on the calculation at all. The basis for the spreadsheet stems from USGS Publication 2010-5102. On page 22, the foundational calculations are provided for the spreadsheet, which implicitly references the head of the beneath the basin in relation to the base of the aquifer. Based on the manner by which the calculations are structured, it seems to make sense that increasing the Saturated Zone's thickness would have a beneficial effect.

However, I'm struggling with this conceptually. Given that water is incompressible, shouldn't the thickness of the Saturated Zone be irrelevant to calculating the height of a groundwater mound? I'm aware of, but not particularly familiar with groundwater flow nets; are there concepts therein which would suggest that an increased aquifer thickness would facilitate a reduced mound under an infiltration feature?



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