Final Environmental Impact Report - Whittier Bridge/I-95 ...

Whittier Bridge/I-95 Improvement Project FEIR
Chapter 1.0: Changes Since the Draft Environmental Impact Report and Resolution of Additional Issues
forested wetland adjacent to the existing marsh by planting young saplings and shrubs, and seeding
to establish a dense herbaceous layer.
This wetland replication will replace the lost acreage at Wetlands I and H at an approximately 1:1
ratio and will replace the lost principle functions of Floodflow Alteration, Sediment/Toxicant Retention
and Nutrient Removal. Functions gained by the wetland replication will include flood storage,
sediment/toxicant retention, nutrient transformation/retention, and wildlife habitat. The functional
quality for wildlife habitat of the replication wetland will be limited by the disturbed nature of the
existing wetland caused by the dominance by invasive species (common reed and purple
loosestrife) and the developed upland surrounding most of the area.
Hydrology
The hydrology of Wetland H relies on a mix of groundwater, surface flow from Wetland G and runoff
from the adjacent uplands, including stormwater flows from I-95. Observations of the soil profiles at
Soil Pits 3 and 4 in the existing wetland were used to estimate the hydrologic regime in the proposed
mitigation site. Soil Pit 3 appears to have saturation to the surface for extended periods of time and
routine saturation within the upper foot of the soil profile. Soil Pit 4, located at the toe of slope,
indicated seasonal high water occurs at a depth of approximately 8 inches, with permanent
saturation indicated by a gleyed matrix beginning at 14 inches depth. These water levels suggest
that grades set to the toe of slope (approximately 28 feet elevation) will provide a hydrologic regime
suitable for supporting a forested wetland, with seasonal high water within 1 foot of the soil surface.
Three long-term monitoring wells/piezometers will be installed in March 2012 in the mitigation site as
shown on Figure 1-11 following the US Army Corps of Engineers monitoring well installation
protocol. Water level data will be collected weekly beginning in early April through the end of June,
and biweekly through the end of October 2012. These data will be used to corroborate water level
estimates implied by Soil Pits 3 and 4 in the adjacent wetland. Any necessary minor adjustments to
the grading plan as a result of the long-term water level monitoring will be made in final design by
the design/build contractor.
Grading and Soils
The proposed grading plan is shown on Figure 1-11 and a profile of the mitigation area is shown on
Figure 1-12. The toe of slope will be pulled back approximately 50 feet to elevation 28. Excavated
material will be removed from the site and reused within the Whittier/I-95 project as suitable. Any soil
stockpiling will occur within designated work areas for the project, and will be properly contained to
prevent sedimentation into surrounding areas. The butternut tree shown on the grading plan to the
east of the replication area will be left undisturbed, with minimal grading disturbance to its rooting
zone to provide partial shade for the planted woody species.
Approximately 1 foot of topsoil will be placed over subgrades in the wetland. The existing loamy
sand subsoils appear to be suitable in texture to support the wetland topsoil, although this should be
confirmed during the boring to install the monitoring well in the existing upland. The wetland topsoil
will consist of a fertile, friable sandy loam similar to the soils in nearby forested wetlands, with a
minimum 20% organic matter content by dry weight. The upland topsoil will be a sandy loam placed
to a minimum depth of 6 inches, with a minimum 3% organic matter content by weight. The existing
upland topsoil should be inspected for suitability for stockpiling and re-use after completion of the
wetland replication.
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