O30 Princeton Hydro Report Dated 9 3 25

West Essex Highlands Review West Orange, NJ Permit No. 722-04-0001.1 LUP230001 April 2025 must empty in 72 hours. This is a commonly used practice that has been previously accepted by the NJDEP. However, the applicant accounted for infiltration in the runoff routing calculations. That means that the model assumes some of the runoff is infiltrated, effectively removed from the simulation, and does not travel through the stormwater piping network. When one uses infiltration in their runoff routing calculations, one must provide groundwater mounding calculations for the water quality and 100-year storm. During a rain event, the groundwater will mound below BMPs that infiltrate. Therefore, the designed must provide these calculations to confirm the mounding does not encroach on the storage volume of the BMP. If the calculations indicate that the groundwater mound will intersect the basin bottom, the system will not infiltrate at the rate implied by the calculations and measured hydraulic conductivity. While the applicant did provide calculations for both scenarios, the groundwater mounding analysis for the 100-year event is critically flawed. Upon our further review, we believe that when properly completed the groundwater mounding analysis will indicate groundwater mound conflicts for all of the following BMPs: e BMP 3 © BMP 4 ° BMP1O Princeton Hydro will rely on PP3 as an example of the analysis that was performed on each of the BMPs to determine the mounding issue under the 100-year storm scenario. Princeton Hydro observed that the duration of infiltration in the groundwater mounding analysis for the water quality and the 100-year storm, were nearly identical for both storms (0.21 hours for water quality and 0.2 for one hundred year). The analysis for the water quality storm can be found on page 821 of 946 in the Drainage Report and the analysis for the 100-year storm on page 831 of 946 in the drainage report. The duration of these two different elements of a storm event should be very different. The water quality storm is only 1.25 inches where the 100-year storm is 8.33 inches. That is almost 7 times more water in the 100-year storm, due to the sheer volume difference the time should also be substantially different. The recharge rate is also different for both storms, Princeton Hydro does not know why the recharge rate would have different values for the

difference the time should also be substantially different. The recharge rate is also different for both storms, Princeton Hydro does not know why the recharge rate would have different values for the different storms and maintains that it should be the same for any storm. As noted previously, the design proposes to replace the existing soil from the bottom of the BMP down to the SHWT with a sandy soil that has a higher hydraulic conductivity to allow the runoff to infiltrate quicker through the BMP storage area. However, the native soil at the SHWT still has a very low hydraulic conductivity so the “infiltrated runoff" from the BMP will back up in the replacement soil since the soils at and below the SHWT infiltrate at a much slower rate. Typically, the stormwater report would provide calculations demonstrating that there was enough storage in the replacement soil to hold the “infiltrated runoff" while it infiltrates below the SHWT at a much slower rate. The calculations would show that the “infiltrated” runoff would be held in the replacement soil and not mound into the storage volume of the BMP. Groundwater mounding into the storage volume would directly impact the runoff peak flow rate calculations and the groundwater recharge calculations by reducing the amount of stormwater volume that could be managed by the BMP. Princeton Hydro, LLC Page | 13