The project has been designed to avoid and minimize negative effects on sensitive natural areas as much as possible and will mitigate impacts that cannot be avoided. Effects on natural resources are described below.
Geology and Soils
The project has the potential to increase erosion, result in long-term settlement of fill and has new bridges that could be impacted by earthquakes. Strategies to minimize erosion, including covering exposed slopes, installing drains, and/or limiting soil moving to dry weather conditions, would be implemented during construction of the project, and all new bridges and retaining walls would be designed to current standards for earthquake resilience.
Construction of the project could have a temporary effect on nearby water bodies, floodplains and surface/groundwater quality. Runoff from new pavement would be treated and infiltrated or similarly managed for quantity control. Increased stormwater treatment of runoff, where currently there is none, would provide some benefit to nearby surface water bodies and groundwater resources.
Six wetlands are located within the South Study Area. One of the wetlands, Bell Marsh, is a Category I and is located immediately south of Steilacoom-DuPont Road. The other five wetlands are rated Category III.
The project has been designed to avoid impacts to wetlands; however, total avoidance is not possible. The project would result in permanent fill to a portion of two Category III wetlands, a loss of 0.30 acres of wetland area, and loss of 1.59 acres of wetland buffer. Construction would temporarily impact an additional 0.08 acres of wetlands and 1.04 acres of wetland buffers. The project would compensate for wetland impacts using the Pierce County approved in-lieu fee program.
Fish, Wildlife, and Vegetation
The project would result in some temporary impacts to land cover such as tree removal and placement of fill. There may also be noise impacts that affect wildlife.
Long-term, the project would result in the conversion of existing vegetated land to new paved areas, stormwater treatment facilities, or other disturbed areas that provide a lower level of habitat functions (e.g., forest to lawn). As part of this conversion, the project would result in 4.94 acres of permanent impacts and 2.05 acres of temporary impacts to Oregon white oak habitat. Most of the impacts are focused on highly disturbed habitat directly adjacent to I-5 and interchange ramps. The project would also result in water quality improvement associated with added treatment for stormwater runoff that currently discharges to wetlands and streams without treatment. The project would minimize vegetation removal, especially native vegetation such as Oregon white oaks. Where oak removal is unavoidable, it would be mitigated by replanting the same species at the required replacement ratios. Other areas of vegetation removal would be restored in accordance with WSDOT’s Roadside Policy Manual (2015).
For More Information
To read more about the study approach, existing conditions, and effects on natural resources, please see Sections 3.6 through 3.9 of the Supplemental Environmental Assessment.