Portland Harbor RI/FS Final Remedial Investigation Report February 8, 2016
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Portland Harbor RI/FS Final Remedial Investigation Report February 8, 2016 3.0 ENVIRONMENTAL SETTING This section describes the current and historical physical characteristics and human uses of the Portland Harbor Superfund Site (Site). Physical characteristics of the Site include meteorology, regional geology and hydrogeology, surface water hydrology, the physical system (which includes bathymetry, sediment characteristics, and hydrodynamics and sediment transport), habitat, and surface features. Human characteristics of the Site that are discussed here include historical and current land and river use, the municipal sewer system, and human access and use. In addition to providing context to the RI sampling and analysis, the factors presented in this section are considered in the refinement of the study area-wide CSM, which is presented in Section 10. Section 3.1 focuses primarily on the physical setting within the study area (RM 1.9 to 11.8). However, the physical features of the Willamette River from Willamette Falls (RM 26) to the Columbia River (RM 0), as well as the upstream portion of Multnomah Channel, are discussed as needed to place the study area’s physical characteristics into a regional context. The Willamette River basin has a drainage area of 11,500 square miles and is bordered by foothills and mountains of the Cascade and Coast ranges up to 10,000 ft high to the south, east, and west (Trimble 1963). The main channel of the Willamette forms in the southern portion of the valley near Eugene, at the convergence of the Middle and Coast forks. It flows through the broad and fertile Willamette Valley region and at Oregon City flows over the Willamette Falls and passes through Portland before joining the Columbia River (Map 3.1-1). The Willamette flows predominantly from the south to the north and has a total length of about 309 miles. It is the 19th largest river in the contiguous United States in terms of discharge. The portion of the river from Willamette Falls to the Columbia River is considered the lower Willamette River (see Map 1.0-1). Multnomah Channel is a distributary channel of the lower Willamette River that begins at RM 3.1 and flows northwest approximately 21 miles to its confluence with the Columbia River. The upstream reaches of the Willamette River above Willamette Falls constitute a meandering and, in some cases, braided river channel. Upstream flooding is largely controlled by 13 major tributary reservoirs (Uhrich and Wentz 1999). In the lower Willamette River, especially in the vicinity of Portland Harbor, the channel banks have been stabilized in several areas by the placement of riprap, and construction of seawalls, bulkheads, etc. These measures have created a much more stable channel in the lower Willamette River. The portion of the river where the federal navigation channel is maintained at –40 ft CRD (see Section 3.1.4.1) defines Portland Harbor and extends upstream from the Columbia River (RM 0) to the Broadway Bridge (RM 11.7; Map 1.0-1). From 1973 3-1 Portland Harbor RI/FS Final Remedial Investigation Report February 8, 2016 through 2007, average annual mean flow in the Willamette River was approximately 33,800 cfs at the Morrison Bridge (near RM 12.8) in Portland.1 3.1 PHYSICAL ENVIRONMENT 3.1.1 Meteorology Located about 65 miles inland from the Pacific Ocean, the city of Portland and Portland Harbor are situated near the confluence of the Willamette and Columbia rivers. This area lies approximately 20 ft above sea level and is about midway between the Coast Range to the west and the Cascades Range to the east. The climate of Portland is usually described as temperate or oceanic, with mild, damp winters and relatively dry, warm summers. The Coast Range provides limited protection from Pacific Ocean storms while the steep slope of the Cascades Range impedes moisture-laden westerly winds, resulting in moderate rainfall in the area, especially during the winter months (NOAA 2010). Precipitation falls primarily as rain, with nearly 90 percent occurring between mid- October and mid-May. Rainfall varies across the metropolitan area, with the West Hills (located to the west of the study area) receiving nearly 60 inches of rain per year while the Portland International Airport (located to the east of the study area) only receives about 36 inches. Forest Park, which is located in the West Hills, drains to the study area. Measurable snow accumulations are rarely more than 2 inches, occurring most frequently at elevations over 500 ft (including the West Hills) or along Portland’s eastern boundary near the Columbia River Gorge at Troutdale (NOAA 2010). The city has experienced some major snow and ice storms caused by cold air outflow from the gorge. A storm in 1893 resulted in approximately 60 inches of snow accumulation (NOAA 2010). Winds are from the north and west during the late spring and summer dry season and from the east and south during the fall and winter rainy season. Annual monthly wind speeds average 8.0 mph at the Portland airport (NOAA 2011). Average temperatures range from a low of 45°F (7°C) in the winter months to a high of the middle 90s (~35°C) in the late summer (NOAA 2000). The lowest temperature ever recorded in Portland was −3°F (−19°C), which occurred on February 2, 1950. The highest temperature ever recorded was 107°F (42°C), on July 30, 1965 and again on August 8 and 10, 1981 (NOAA 2011). 3.1.2 Geology 3.1.2.1 Geologic Setting The study area is located along the southwestern edge of a large geologic structure known as the Portland Basin. The Portland Basin is a bowl-like structure that is 40 miles long and 20 miles wide and bounded by folded and faulted uplands. These 1 Data obtained from the USGS Water Resources web site (http://waterdata.usgs.gov/or/nwis/sw). 3-2 Portland Harbor RI/FS Final Remedial Investigation Report February 8, 2016 northwest-trending structural zones are interpreted as dextral wrench faults that delineate the Portland pull-apart basin (Beeson et al. 1985; Yelen and Patton 1991). The Tualatin Mountains (Portland West Hills) form a ridge that runs parallel to the Willamette River to the west, from the Multnomah Channel to the City of Portland. The mountains define the western edge of the Portland Basin; groundwater and creeks and channels along the east face of the mountains flow downward to the Willamette River. The basin has been filled with up to 1,400 ft of alluvial and glacio-fluvial flood deposits since the middle Miocene (approximately 12 million years ago). These sediments overlie older (Eocene and Miocene) rocks including the Columbia River Basalt Group (CRBG), Waverly Heights basalt, and older marine sediments. The older rocks are exposed where uplifting has occurred (e.g., RM 7 west side in the Doane Lake area) on the margins of the basin, including adjacent to the study area. Because the study area is located at the edge of the basin, both the older rocks and overlying sediments are present near the surface and play a significant role in defining interactions between groundwater and the river. The geologic units found in the vicinity of the study area are illustrated in Figure 3.1-1 and briefly described below, from youngest to oldest (Beeson et al. 1991; Swanson et al. 1993): 3.1.2.1.1 Recent Anthropomorphic Fill Anthropomorphic fill blankets much of the lowland area next to the river and is predominantly dredged river sediment, including fine sand and silty sand. Hydraulic dredge fill was used to fill portions of the flood plain, such as Doane Lake, Guild’s Lake, Kittridge Lake, Mocks Bottom, Rivergate, and a number of sloughs and low-lying areas. The fill also was used to connect Swan Island to the east shore of the Willamette River and to elevate or extend the bank along significant lengths of both sides of the riverfront by filling behind artificial and natural silt and clay flood levee dike structures. Rocks, gravel, sand, and silt also were used to fill low-lying upland and bank areas. The thickness of this unit ranges from 0 to 20 or more feet. The permeability of this unit, where composed of clean dredge fill sand, is higher than the natural fine-grained alluvium. The presence of silt fill or a silty matrix in the sand fill generally reduces the permeability of the unit significantly. 3.1.2.1.2 Fine-grained Pleistocene Flood Deposits and Recent Alluvium (Undifferentiated) This unit includes fine-grained facies of the Pleistocene Flood Deposits, as well as recent alluvium deposited by the present Willamette River. This unit generally consists of silt, clay, silty sand, and fine-to-medium sand that borders and underlies the present floodplain of the river (Beeson et al. 1991). The lower portions of this unit and where it forms the large bluffs bordering the east side of the river likely consist of the fine- grained facies of the flood deposits, whereas the upper portions near the river are likely more recent alluvium. The upper fine-grained portion of the unit has likely been reworked and deposited by the present Willamette River. The sands of this unit may be 3-3 Portland Harbor RI/FS Final Remedial Investigation Report February 8, 2016 indistinguishable from overlying dredge fill in some places (Landau 2002a). The thickness of this unit ranges from 20 to over 100 ft. The permeability of the clay, silt, and silty sand of this unit is generally relatively low, whereas the portions of the unit consisting of clean sands may have a relatively higher permeability.