G.7.2 Analysis Procedures

The initial step for any groundwater modeling analysis is the development of a conceptual model of the groundwater system. The conceptual model should describe the anticipated groundwater flow system including the data requirements described above, direction and rate of groundwater flow, potential model boundaries, and approach for simulating infiltration. The conceptual model provides the basis for constructing the computer model.

Because of the limited available data necessary for model inputs, a parametric analysis shall be performed whereby model inputs, especially aquifer parameters, are varied over a range of values to evaluate the potential impact on the mounding results. The range values shall be based on known variability in the parameter and experience with similar soils in the area by the licensed professional developing the model.

The following ranges of aquifer parameters shall be used in the parametric analysis:

• Hydraulic conductivity: one order of magnitude (e.g., + and ‑ a power of 10) for each receptor soil.

• Aquifer thickness: plus or minus 50 percent of the known values.

• Specific yield: minimum range of 0.05 to 0.2.

If known field conditions warrant, increase the above ranges as necessary.

In general, multiple infiltration scenarios will need to be simulated to evaluate potential mounding below the infiltration BMPs. For example, both short‑term peak storm events and long‑term seasonal precipitation should be evaluated. Additional scenarios may include a series of short‑term high precipitation events. Although the actual events that need to be simulated will depend on subsurface conditions, number and types of infiltration BMPs, and potential risk factors, as a minimum the following scenario is required:

• A typical wet season (November through April) based on average monthly precipitation followed by a single‑event rainfall modeling of the back‑loaded long‑duration storm for the 100‑year recurrence interval, using data from the closest rain gage.

The licensed hydrogeologist performing the mounding analysis should use professional judgment and experience to potentially modify the above scenario or add additional scenarios on a project specific basis, as needed.

As additional soil and groundwater information is collected during construction, testing, and operation of the infiltration BMP, the mounding analysis should be revised and refined to incorporate any new information. If groundwater monitoring indicates results inconsistent with the findings of the mounding analysis, in the opinion of a licensed hydrogeologist, the model should be re‑evaluated. The re‑evaluation should include simulation of the precipitation events prior to the observed groundwater monitoring data.