2151002Princeton Hydro West Essex Stormwater Comments to Planning Board 20250310

Stormwater Management Review for Municipal Planning Board Approval West Highlands, Inc. Block No. 179, Lot No.32, West Orange Township, Essex County, New Jersey March 2025 Princeton Hydro, LLC: 2151.002 3 If the work was done at other times, the depth to SHWT can be obtained from the NRCS Web Soil Survey. Mottling is defined as contrasting or “blotchy” color patterns within a dominant soil color. It is forms as the SHWT rises into oxygenated soils, changing the conditions in the soils from aerobic to anoxic resulting in a gray “blotches”, see Figure 1 below. The coloring of the soil is caused when the water table rises, createing anaerobic (oxygen-depleted) conditions. Bacteria that thrive in anaerobic conditions use the oxygen in iron and manganese oxides, changing the color of the minerals. When the water table drops, the dissolved minerals are deposited on the soil particles. The deposited minerals appear as coatings on the soil particles in colors such as yellow, red, orange, brown, blue, or black. Areas that have been saturated for long periods can lose all of their iron and manganese oxides, turning gray. The mottling will show the highest elevation that SHWT has occurred historically. This process takes years to occur and is considered more accurate than the water elevation because it is not impacted by recent weather conditions. Figure 1: Example of Mottling in Soil While reviewing the multiple subsurface investigation reports for this project, the soil profiles never make any mention of mottling. That is very unusual, as stated above the BMP Manual states that mottling should be used to established using mottling with the observed water level. Importantly, soil logs must also be specific in their identification of various color throughout the profile, as is consistent with the Field Indicators of Hydric Soils in the United States referenced by the NJDEP BMP Manual. Color identification is key in correctly identifying mottling by recording both the number of different colors observed, as well as the value and chroma of the observed colors. Figure 2 provides a comparison of a sample soil profile which uses the Munsell method per the Field Indicators of Hydric Soils (top) and the soil profiles for this project (bottom). It appears that the SHWT was only established using the elevation of the water found in the test pit or boring and no effort was made

ydric Soils (top) and the soil profiles for this project (bottom). It appears that the SHWT was only established using the elevation of the water found in the test pit or boring and no effort was made to document mottling. As noted in the NJDEP BMP Manual, there are instances where mottling can be absent from a soil profile, but it is highly unusual that mottling was not found in any of the soil test pits or borings provided by the applicant. Mottling is a natural and common process, thus it is fair to assume mottling should have been observed in some of the soil profiles. The concern is that the groundwater level found in a test pit can fluctuate on a daily basis based on the recent weather. Using the mottling to determine the SHWT is recommended because it estimates the highest (most conservative) elevation that does not vary with time. If one only uses the observed groundwater elevation, it is safe to assume that the SHWT elevation is probably underestimated. It is recommended that the professionals who documented these test pits should provide the Planning Board with testimony to explain why mottling was never documented and discuss the how their reports could be underestimating the elevation of SHWT.