Groundwater Mounding Beneath Stormwater Infiltration Basins: Lessons learned
from simulations and introduction of a public-domain tool for calculating
groundwater mounds
Infiltration of stormwater is increasingly encouraged or even required by local, state, and federal
regulations. An unintended consequence of infiltrating stormwater in a focused location (for example,
an infiltration basin) can be groundwater mounding which leads to flooding of nearby basements or
other structures. Groundwater mounding, if not anticipated in the design, can intersect the bottom of
an infiltration basin, causing drainage to be slower than indicated by calculations that only consider flow
in the unsaturated zone.
The U.S. Geological Survey, in cooperation with the New Jersey Department of Environmental
Protection undertook a study that simulated groundwater mounding beneath basins using a number of
values of different input variables. These variables include the hydraulic conductivity of the aquifer
(saturated zone), aquifer thickness, specific yield, basin shape, basin depth, and basin area. Although all
of the variables can substantially influence groundwater mounding, hydraulic conductivity stands out as
the most important variable to know yet is the most difficult to measure and the most likely to vary over
orders of magnitude.
Until recently practitioners needing to estimate the height and extent of groundwater mounding had
limited freely-available tools for making rigorous calculations. The USGS developed a spreadsheet based
on analytical solution of Hantush (1967) which allows the user to enter the (horizontal) hydraulic
conductivity, initial saturated thickness, and specific yield of the aquifer, length and width of the basin,
and rate and duration of recharge. The spreadsheet then calculates the height of groundwater
mounding at user-specified distances from the center of the basin. Although limited by the assumptions
of the analytical equation and accuracy of the user input, the output is, nonetheless, a rigorous
calculation that can be used for planning purposes.