Duwamish Industrial Area Hydrogeologic Pathways Project

Transcription

1 Duwamish Industrial Area Hydrogeologic Pathways Project Duwamish Basin Groundwater Pathways Conceptual Model Report Prepared for : City of Seattle Office of Economic Development and King Country Office of Budget and Strategic Planning Prepared by: Derek Booth, Ph.D., University of Washington Lori Herman, CGWP, Hart Crowser, Inc. with assistance from: Floyd & Snider Inc., Black & Veatch, Gambrell Urban, Inc., and the City of Seattle GIS Department Produced by: Hart Crowser, Inc. April 1998

2 Duwamish Basin Groundwater Pathways Conceptual Model Report Duwamish Basin Groundwater Pathways Conceptual Model Report Duwamish Basin Groundwater Pathways Conceptual Model Report Duwamish Basin Groundwater Pathways Conceptual Model Report Duwamish Basin Groundwater Pathways Conceptual Model Report

3 ACKNOWLEDGMENTS This report was written as a component of the Duwamish Industrial Area Hydrogeologic Pathways Project. The project is jointly sponsored by the City of Seattle Office of Economic Development and King County Office of Budget and Strategic Planning. This project comprises a key element of the Duwamish Coalition's strategy and partnerships to promote the environmentally responsible redevelopment of brownfield properties in the Duwamish Industrial Corridor. We would like to acknowledge the invaluable contributions of every member of the project team. Their intellectual contributions have helped make this unique project a success and help to make Seattle and King County leaders in national brownfields efforts, as recognized recently by the Office of Vice President Al Gore and the Environmental Protection Agency. Thomas Boydell, Principal, with the Seneca Consulting Group, has guided the project from its initial conception through implementation on behalf of the City of Seattle and King County governments. Ben Wolters has represented the City of Seattle Office of Economic Development. Michael Alvine and Lucy Auster have managed the project for King County. Tom, Ben, Mike and Lucy have all had an unwavering focus on beneficial outcomes throughout the process. Through a competitive process, the City of Seattle and King County selected the firm of Floyd & Snider Inc. as the lead consultant for the Hydrogeologic Pathways Project. The environmental consulting team led by Floyd & Snider Inc. also included environmental and technical systems experts from Hart Crowser, Inc., Black & Veatch, and Gambrell-Urban Inc. Through an intergovernmental memorandum of agreement, the City and County also secured assistance with the project from the University of Washington's Center for Urban Water Resources Management. The combination of the consultant and university resources provided the synergy of ideas and technical skills essential to this project. Lori Herman, CGWP, a principal at Hart Crowser, Inc., and Derek Booth, PhD, who is Director of the Center for Urban Water Resources Management and Professor of Geology in the University of Washington Department of Civil Engineering, are the primary authors collaborating on the production of this report. Dr. Booth has brought a wealth of knowledge and experience to the Regional Framework section of the report, which identifies the geologic history of the Duwamish Area, and regional stratigraphy. Dr. Booth is a widely recognized expert on the geologic history and structural geology of the Puget Sound area. Dr. Booth and Ms. Herman worked jointly with the data to develop the geologic cross sections and produce the written text. Dr. Booth is the author of the geologic map used in this report. Duwamish Pathways Project

4 Ms. Herman is responsible for development of the hydrogeological components of this report, including the conceptual groundwater model. A widely recognized expert in regional hydrogeology, Ms. Herman has a great deal of experience leading hydrogeologic work for both water supply and environmental remediation projects within the Duwamish study area, and has been a leader in several of the related Duwamish Coalition efforts. This work builds on Ms. Herman's previous work for the Duwamish Coalition entitled Criteria and Approach for Area-Wide Determination of Surface Water Protection as Most Beneficial Use of Groundwater, sponsored by the City of Seattle and issued in January In addition, Dr. Booth's team has worked on development of a numerical model, which is being developed in order to more quantitatively simulate groundwater flow conditions and explore some of the consequences of the conceptual model, in support of the work provided by Ms. Herman. The data collection and review process was collectively designed by the authors, other members of the Floyd & Snider Inc. team, our clients, Dan Cargill of the Washington State Department of Ecology, and Bernie Zavala and Howard Orleans of the U.S. Environmental Protection Agency Region 10, to ensure that the study would meet agency expectations. Data was collected and summarized by Black & Veatch, who developed the electronic database and associated queries. Figures were developed by Carrie Dovzak, working with Gambrell-Urban, Inc. The City of Seattle's Geographic Information Systems Division, with leadership from Vicki Evans, has done an outstanding job of finalizing and producing many of the figures for the report. Kate Snider of Floyd & Snider Inc. managed the overall project for the consultant team and its collaborating partners and facilitated the definition of project strategy and process. Duwamish Pathways Project

5 DUWAMISH BASIN GROUNDWATER PATHWAYS CONCEPTUAL MODEL REPORT 1.0 INTRODUCTION The Duwamish Basin Groundwater Pathways Study began as part of an area-wide interest to understand the groundwater flow pathways in the Duwamish Valley. The project area (Figure 1) includes the Duwamish Valley from the origin of the Duwamish River at the confluence of the Green and Black Rivers in Tukwila, to the river mouth at Elliott Bay. The river is dredged through most of its lower half and in this area is called the Duwamish Waterway. The study area falls within the jurisdictions of the City of Seattle, the City of Tukwila, and unincorporated King County. The Duwamish Valley area is a heavily industrial area, south of downtown Seattle, which includes approximately 5,000 acres of land designated for industrial activity. Typical uses include manufacturing, warehousing, heavy and light industrial activities, commercial uses, and container shipping and support activities. The area constitutes nearly 80 percent of the City of Seattle s industrial-zoned land and is a very important economic center for both the City and the region as a whole. For example, both Boeing aircraft production facilities and the primary shipping terminals for the Port of Seattle are located here. The legacy of long-term industrial activity includes soil, groundwater, surface water, and sediment contamination. Over the past several decades, concern over the contaminant impacts to the long-term health of the area in general, and impacts to the Duwamish River and Puget Sound in particular, have combined with regulatory actions to improve operations and encourage cleanup. Spurred by growth management mandates, current efforts are now focused on promoting reuse and redevelopment of this area for industrial purposes, which also encourages cleanup and pollution prevention. A consistent area-wide understanding of groundwater conditions facilitates long-term water quality protection and brownfields redevelopment. This project has focused on summarizing groundwater flow patterns in the Duwamish Valley, so that ultimately a better understanding of the contaminant risk to human health and the environment can be made. Duwamish Pathways Project Page 1

6 1.1 The Duwamish Industrial Area Hydrogeologic Pathways Project The Duwamish Industrial Area Hydrogeologic Pathways Project was conceived by the Duwamish Coalition committee - Preserving and Reclaiming Industrial Lands. The project is jointly funded by the City of Seattle and King County Offices of Economic Development. The City and County are working with an environmental consulting team led by Floyd & Snider, Inc., as well as the University of Washington s Center for Urban Water Resources Management. Both the Department of Ecology and the EPA Region 10 Brownfield Office have been active participants in the scoping and execution of the project. The goal of the Duwamish Hydrogeologic Pathways Project is to facilitate the redevelopment of brownfields in the Duwamish Corridor by improving the quality and pace of cleanup-related decision making in the area. This goal will be met in part by establishing a conceptual framework for the area-wide hydrogeologic and contaminant transport setting, and in part by developing a mathematical model in support of the conceptual hydrogeologic framework. These documents will be subsequently evaluated to determine the advisability of designating the highest beneficial use of shallow groundwater in the area to be its discharge to surface water, not its use as a drinking water source. 1.2 Data Sources Another product of the project will be a publicly accessible database, with GIS display capabilities for groundwater flow, geology, land use, and land use history information. The database will include references to publicly available environmental reports, and links to publicly available site lists, which are currently available electronically from Ecology and EPA. The GIS/database front end is being developed by the City of Seattle GIS office, based on database and mapping products produced by the Consultant and University team. The scope of work for data collection was determined jointly by the Project Team, the Department of Ecology, and the EPA. Key members from these parties worked closely together to identify optimal use of the project budget to collect a representative set of data from which to develop the conceptual model. It was agreed that all publicly available information could not be collected within the constraints of the available budget. Rather, criteria was developed by all parties to define what subset of data collection would be acceptable for conceptual model development. These criteria were Duwamish Pathways Project Page 2

7 summarized in a March 21, 1997 Memorandum by Lori Herman regarding Data Collection Criteria that identified the data sources and the types of data to prioritize for collection. Black & Veatch lead the data collection and database efforts for the consultant team. The principal data sources included regional geologic and hydrogeologic studies, major geotechnical studies, and contaminated site investigations (especially MTCA Remedial Investigations and RCRA Corrective Action Investigations) as these typically contained the greatest quantity of hydrogeologic information with desired quality. The Department of Ecology (Ecology), the U.S. Environmental Protection Agency (EPA), and the U.S. Geological Survey (USGS) were the primary sources for data. Team members also contributed data based on their previous work in the project area. The principal data used in preparing this report included: All wells log files on record with the Department of Ecology including the Resource Protection Well Reports, the Water Well Reports, and the old card files on water wells; Selected Ecology and EPA site investigations based on a review of contaminated databases for USTs, LUSTs, CERCLA, MTCA and RCRA; the most comprehensive studies were identified and copied with the assistance of Dan Cargill at Ecology and Howard Orlean at EPA; Regional water resource reports especially those prepared by the USGS (Woodward et al., 1995 and Leisch et al., 1963) and for the South King County Groundwater Management Plan (EES et al., 1989); Regional geologic reports particularly Waldron (1962), Waldron and others (1962), and Mullineaux (1970); Highline well field reports prepared by Hart Crowser (1984, 1985, 1993); Metro studies prepared for the Renton Effluent Transfer System reports (Converse Consultants, 1985) and the Metro Duwamish groundwater Study (Sweet Edwards & Harper Owes, 1985); and Reports on the Duwamish River Estuary, King County, Washington, USGS Professional Paper 990; USGS Water-Supply Paper 1873-C; and Muckleshoot Indian Tribe, Duwamish Pathways Project Page 3

8 Figure 2 presents a generalized map showing the coverage of the well data and study area reports compiled in the database. Tables B-3 and B-4, Appendix B, present a bibliography provided by Black and Veatch from the project database. The database references provided in Tables B-3 and B-4 are indicated in the text by Reference ID # and are included in a project library. Additional data sources and references used to evaluate the groundwater flow conditions in the Duwamish Basin are given in the List of References at the end of this report. 1.2 Scope of the Conceptual Model Development This conceptual model report follows a preliminary groundwater conceptual model developed for the City of Seattle Office of Economic Development. The previous work (Hart Crowser, 1997) was part of an effort aimed at identifying the criteria for designation of surface water as the most beneficial use of shallow groundwater in the Duwamish Corridor. One of the critical criteria for consideration of a surface water designation for the groundwater in the area includes understanding the hydrogeologic conditions which define the groundwater system. These conditions include: The geologic history and framework; Aquifer and aquitard occurrence; Recharge and discharge factors; Groundwater flow patterns; and Water quality. This report focuses on further developing the conceptual model of groundwater flow within the Duwamish Basin, by providing a more detailed characterization of the geologic and hydrogeologic conditions, based on more comprehensive data collection and data analyses. The scope of work for development of this model study included: Compilation and review of data collected from Ecology, EPA, Metro, King County, USGS, and Consultant Team Files, including assistance with development of the database and mapping displays; Selection of the well and boring data for inclusion in the database, and criteria for the GIS mapping coverage for the area including topography, geology, groundwater elevations, and other supporting mapping for presentation of the conceptual model; Duwamish Pathways Project Page 4

9 Review and analysis of site investigation reports for data supporting understanding of the stratigraphy and groundwater flow at individual sites; Development of hydrogeologic cross sections, groundwater elevation contour maps, and illustrative groundwater-flow graphs and maps for construction of the conceptual model. In addition to these data, we have used the collective efforts of regional experts on the geology and groundwater flow, and the information in previously published reports, to develop as complete a picture as the available data and scope of work allow. We believe this study provides the best picture of the Duwamish Groundwater Pathways as is currently known. Future work should be able to build on this effort, and where appropriate, suggestions for future data collection efforts are identified. However, we do not anticipate that any substantive augmentation of the existing data will materially change the elements of the conceptual model. Numerical modeling of this groundwater flow by the University of Washington, in progress as part of the subsequent phase of this project, will be using the hydrogeologic framework developed here to generate more quantitative results. 2.0 REGIONAL FRAMEWORK The following sections provide the regional framework within which the Duwamish Valley groundwater flow system is defined. The framework begins with an overview of the geologic history. The geologic history defines the nature and occurrence of the principal geologic units, which are provided a nomenclature in this report section. Description of the occurrence of the geologic units within the study area is then presented with reference to the available data and regional cross sections, drawn to present our best interpretation of the Duwamish region stratigraphy. Finally, we bring the regional geologic discussion into the perspective of Duwamish Valley groundwater by describing the regional recharge and discharge conditions that define the boundary conditions to regional groundwater flow into the Duwamish Valley. 2.1 Geologic History of Duwamish Area The geology of the Duwamish Valley reflects many of the processes that have shaped the Puget Lowland as a whole. The valley is a relic arm of Puget Sound, which was carved by the overriding ice sheet that last advanced into this area from British Columbia about 15,000 years ago. Through the scouring action of flowing ice and sub-glacial water, many Duwamish Pathways Project Page 5

10 hundreds of feet of sediment have been flushed out of what are now the multiple channels of the Sound, and subsequently inundated with sea water once the ice retreated and global sea level rose to its present-day level. At the time of ice retreat, the Duwamish arm of Puget Sound extended many miles farther south than it does today. Up until about 5700 years ago its shoreline reached to the city of Auburn, about 20 miles upstream of the present mouth of the waterway at Elliott Bay. At that time, a tremendous mudflow, the Osceola Mudflow, descended from the flanks of Mount Rainier along the valley of the White River, building a voluminous fan of sediment into the marine waters at Auburn and progressing down-valley as a submarine flow at least as far north as Kent (Dragovich and others, 1994). Over the subsequent centuries, the mudflow sediments were re-eroded from higher up the White River and re-deposited farther downstream. During this time flow from the White River apparently trended north, down the present Green River valley, and so this sediment rapidly advanced the shoreline up the Duwamish arm of Puget Sound, filling the bed upward with fine silt while the coarser sand and gravel advanced laterally at the water s edge. This sediment completely buried the pre-5700-year-old form of the valley; only a few bedrock knobs, high enough for their tops to remain above the level of this infilling deposit, remain exposed at the modern ground surface. After a long but indeterminate time of probably several thousand years, the advancing deposits of the ancestral White River reached Elliott Bay. Here, further advance of the sediment wedge dramatically slowed because of the great depth of the bay. Upstream, the Duwamish arm was now land, probably completely filled by floodplain deposits as the combined flow of the White and Green rivers continued to transport sediment from Mount Rainier and the nearby Cascade Range down to Puget Sound. As the river episodically flooded and migrated back and forth across the floodplain, the sediments already there were reworked by the river and locally augmented by additional riverine and floodplain deposits. About 1,100 years agos, 20 feet of uplift occurred on the south side of the Seattle Fault (Bucknam et al., 1992), whose east-west trace passes just north of the study area in Elliott Bay. The immediate consequences of that uplift would have been expressed in a local shallowing of the lower Duwamish River and by uplift of the then-active floodplain relative to sea level. Re-entrenchment of the river would have subsequently occurred, but some of those pre-1,100-year floodplain deposits probably still remain preserved at the modern ground surface, particularly just west of Kellogg Duwamish Pathways Project Page 6

11 2.2 Regional Stratigraphy Island, perched above the level of modern river flooding and floodplain deposition. The most recent phase of valley s geologic history includes the filling of tideflats and floodplains, and the subsequent dredging of a straightened channel of the meandering Duwamish River. Completed between 1913 and 1917 by the Duwamish Waterway District, the new channel was 4-1/2 miles in length south from the East and West Waterways. As a result of the waterway development, 12-1/2 miles of old river bed were abandoned. The excavated waterway material was used to fill the old channel areas and the lowlands above flood levels. Subsequent filling for land development purposes has resulted in a surficial layer of fill over most of the lower Duwamish Valley. The heterogeneous nature of fill materials can locally affect both infiltration characteristics and groundwater flow where the water table is below these fill materials. Because much of the thicker fills that occur below the water table are dredged from the Duwamish River, the silts and sands are difficult to distinguish from the native alluvium and have the same general hydrogeologic properties. Each of the stages in the geologic history of the study area is reflected by deposits and landforms within the valley and adjacent uplands. The regional stratigraphic units that make up the upland and valley deposits are described sequentially, from oldest to youngest, in the following section. There are essentially three principal geologic assemblages within the Duwamish Basin that help to define the hydrogeologic system: Bedrock, which occurs as a basement material and bounds the area aquifers; A sequence of glacial and non-glacial sediments within the uplands which flank and define the valley, and establish groundwater flow boundaries to the valley aquifer system; and Valley alluvial deposits that fill in the Duwamish trough and contain the aquifers and groundwater of most interest to this study. The following sections describe the characteristics of these assemblages in the regional setting and define the nomenclature used to distinguish geologic units. Duwamish Pathways Project Page 7

12 2.2.1 Geologic Units Tertiary Bedrock Bedrock is exposed in the eastern and southern parts of the study area, most extensively on the east side of the Duwamish River (Waldron and others, 1962; Waldron, 1962). North of these exposures, the bedrock surface descends from several hundred to over a thousand feet below the ground surface. Where exposed, bedrock consists of marine and continental sedimentary rocks and isolated igneous intrusions, all deposited during the Tertiary period (between about 40 and 10 million years in age in this area). More extensive exposures farther east suggest thicknesses of these geologic units in excess of 3,000 feet. The Duwamish area bedrock expresses part of a much larger, regional, bedrock rib that thrusts westward from the Cascade Range and extends, recognizably but somewhat discontinuously, across the entire Puget Lowland (Yount and Gower, 1991; Frizzell and others, 1984). To the east it is locally known as the Newcastle Hills, forming the high ridges of Cougar, Tiger, and Rattlesnake mountains. To the west it is less well exposed, but it does crop out on the south end of Bainbridge Island and again on Gold Mountain. To the south, the bedrock surface descends gradually as much as 2,000 feet below the surface; but to the north, the bedrock descends down one of the steepest subsurface escarpments recognized in North America, attaining a depth of over 3,600 feet over a horizontal distance of only a mile or two. This escarpment marks the location of the Seattle Fault which produced the source of a major earthquake about 1,100 years ago and which has likely been active for many hundreds of thousands of years. A southerly, presumably older and now inactive strand of this fault passes directly through the study area at about the latitude of Boeing Field, separating younger rocks to the north from older rocks, now uplifted, to the south. Structurally, the bedrock in the area straddles the axis of the Newcastle anticline of Weaver (1916), a ridge of folded bedrock that projects northwest some 30 miles from the Cascade range front (Mullineaux, 1970; Booth and Minard, 1992) towards the Olympic Mountains. These rocks are exposed at the surface in the east-central and southern part of the study area, where they overlook the younger glacial and postglacial deposits that fill the valley or mantle the hillsides. Although the bedrock strata would have been originally deposited horizontally, most of them now dip to either the northwest (in the north half of the study area) or southwest (south half of the study area) by 10 O to more than 40 O. Duwamish Pathways Project Page 8

13 The oldest, and structurally lowest rocks here were originally given the name Puget Group. This term was applied by White (1888) and later by Willis (1898) to thick, continental and brackish-water, coal-bearing arkosic sandstone and shale that earlier were described by Willis (1886) as the Coal measures of the Puget Sound basin. As now used in this region, the Puget Group includes not only arkosic rocks but also interstratified continental volcanic sedimentary rocks. Two main units are recognized and were named by Waldron (1962) in the Duwamish area: a lower assemblage of volcanic sedimentary rocks, the Tukwila Formation; and an upper unit of arkosic sedimentary rocks, the Renton Formation. Overlying the Puget Group, are the youngest rocks of this area, the Blakely Formation named by Weaver (1916). Characteristics of these bedrock units are summarized below. Tpt---The Tukwila Formation is named for the town of Tukwila, where beds in the lower part of the Puget Group crop out on the southeast flanks of the valley wall. The formation at these type sections consists of a lower volcanic sedimentary rock unit that constitutes most of the formation, a middle arkosic unit about 260 feet thick that is lithologically similar to arkosic rocks of the overlying Renton Formation, and an upper volcanic sedimentary rock unit that is less than 500 feet thick. Tpr---The overlying Renton Formation, named for the city of Renton, is exposed on the southernmost valley flanks of the study area. The formation includes the Renton coal measures, which were extensively mined in Renton in the early part of the century. Most of the formation consists of fine- to medium-grained arkosic and feldspathic sandstone, and lesser amounts of siltstone, sandy shale, coal, and carbonaceous shale. At most places the sandstone is only weakly cemented. Tb---The Blakely Formation includes marine sandstone, some conglomerate, and minor amounts of siltstone. These beds were deposited in a shallow marine to coastal environment and have a distinctive collection of marine fossils. They are found in the northeast part of the study area on the north-dipping limb of the Newcastle anticline, having long since eroded away everywhere farther south. Ti---Locally, irregular masses of andesite and basalt appear to have intruded the Puget Group and now crop out at several localities adjacent to the Duwamish Valley and near Tukwila. The rocks generally are massive with columnar fracturing in only a few places. Duwamish Pathways Project Page 9

14 Groundwater flow in the bedrock is not expected to be significant relative to the glacial and alluvial sediments. The sedimentary rocks are generally not important sources of groundwater, because the cementation and fine-grained nature of the sandstones preclude rapid movement of subsurface water, and the intrusive rocks are generally massive and even less able to store and transmit water. Groundwater in these deposits has probably best been studied in the Newcastle area (Hart Crowser, 1981, 1985). In the Newcastle area, the Blakely Formation and coal seams within the Renton Formation have been tapped for water supply, although yields are generally low and finding suitable water can take several drilling attempts because of the discontinuous nature of the fracture zones which supply water. In the project area, the bedrock is considered a poor water producer, particularly in the deposits mapped along the valley walls. Bedrock data collected during subsurface investigations along the Metro sewer line indicated impervious and relatively impervious conditions within the Tukwila and Renton Formations encountered in the upper Duwamish Basin (Reference ID# 127 and 128) Upland Glacial and Nonglacial Sequence The topography of the Duwamish area is dominated by an extensive upland plain, mantled by a rolling surface of glacial till deposited during the last occupation of the Puget Lowland by the Vashon stade of the Fraser glaciation (Armstrong and others, 1965) about 15,000 years ago. Beneath the Vashon deposits is a complex sequence of older unconsolidated sediments that extends far below sea level across most of the study area. Our understanding of the older glacial and nonglacial sequence in the upland areas, and the Vashon deposits in the project area, is discussed in the following sections Older Glacial and Interglacial Deposits The older sediments are exposed at the modern ground surface where recent erosion has sliced through the glacial till, notably by the Duwamish River itself, and also by wave action along Puget Sound to the west. These older sediments are very compact, having been weighted down by one or more episodes of glaciation subsequent to their deposition. Many are also cemented by oxidation, a consequence of the many tens or hundreds of thousands of years of weathering that they have experienced. Duwamish Pathways Project Page 10

15 This older sequence of deposits is very complex, recording the interfingering of two unrelated, but overlapping, geologic phenomena glacial advances from British Columbia and volcanic eruptions from Mount Rainier superimposed on the less dramatic, but ongoing, westward transport of sediment out of the Cascade Range by rivers. Glacial deposits can generally be discriminated from non-glacial deposits on the basis of rock and mineral assemblages that are related to different source areas. These criteria are particularly useful in surface exposures; unfortunately, they are less easily applied where the primary data sources are drill cores. Some of the older efforts to name and correlate these deeper Quaternary deposits have been shown to be incorrect, but adequate substitutes have yet to be developed and widely accepted. A discussion of the stratigraphic uncertainties in identifying Quaternary deposits is given in Appendix A. This report follows the convention established by most recent groundwater studies in the region by avoiding formal stratigraphic names for all deposits, save those of the most recent glacial advance (the Vashon ) and the interglacial period that immediately preceded it (the Olympia ). The terminology used here is based on work completed for the South King County Ground Water Management Plan and recently published by the USGS (Woodward et al., 1995). The pre-olympia deposits are defined as follows: Q(A)c Coarse-grained deposits, presumably immediately underlying Olympia age deposits (correlation is speculative); Q(B)f Fine-grained deposits, immediately underlying the Q(A)c unit; speculative correlation would imply an age of at least 100,000 years old; Q(B)c Coarse-grained deposits, immediately underlying unit Q(B)f; Q(C)f Older undifferentiated, unconsolidated fine-grained deposits; Qto Older (pre-vashon) glacial till, undifferentiated and uncorrelated with glacial advances named elsewhere in the Puget Lowland Deposits of the Vashon Ice Advance In contrast to the uncertainties about relatively old and generally deeply buried Quaternary sediments, deposits at or near the constructional land surface generally can be assigned readily to the youngest of the regionally recognized glacial advances, the Vashon stade of the Fraser glaciation. During this time an ice sheet progressively advanced along the axis of the Duwamish Pathways Project Page 11

16 Lowland from the north (Clague, 1981), reaching its maximum extent about 15,000 yr B.P. and covering the Puget Lowland to a maximum depth of about 5,000 feet (Booth, 1987). Deposits of the Vashon stade have a variety of textural characteristics and topographic expression, owing to the rapidly changing depositional environments caused by the advance and retreat of the ice sheet as described below. Qtb The Transitional Beds were formed as the ice first advanced, and blocked northward lowland drainage out the Strait of Juan de Fuca, which now connects Puget Sound with the Pacific Ocean. In the impounded lakes that formed in the course of establishing southerly drainage out of the Puget Lowland, laminated silt and clay were deposited. This material is mapped farther east and north as either Lawton Clay (Qvl) (e.g., Waldron and others, 1962) or more recently as Transitional beds (Qtb; e.g., Minard and Booth, 1988; Yount and others, 1993), because the unit may include deposits of pre-vashon lowland lakes as well as those formed in the subsequent ice-dammed environment. This unit is a regionally significant aquitard, allowing very little movement of groundwater. Qva---The Vashon Advance Outwash marks the initiation of the subsequent Vashon stade when coarser outwash (Qva) was deposited by streams derived from the advancing ice sheet. Coarse advance outwash at least a few tens of feet thick (and locally as much as 300 feet) underlies the broad uplands in the central part of the quadrangle. This deposit inundated the pre-vashon topography of the lowland and resulted in a south-sloping surface at about 500 feet elevation in this area (Booth, 1994). Much of the total thickness of the bluffs along Puget Sound, from the City of Des Moines north to Duwamish Head, consists of advance outwash. Most exposures consist of light-gray to very light brown sand or sand and pebble to cobble gravel. The outwash here is chiefly fine to medium sand, but it includes some pebble gravel and also some laminated silts. It ranges from poorly to well sorted, and it is typically moderately to well bedded. This deposit is the primary shallow aquifer in the upland areas throughout the region. Qvt---Lodgment till (Qvt) was deposited by the melt-out of debris at the base of the glacier, as ice covered the region. This heterogeneous, compact sediment blankets the area to depths of, at most, several feet; the ground surface underlain by this deposit is locally fluted with elongated hills displaying a weak but uniform north-south orientation across the uplands here. Where present at the surface, till provides a low- Duwamish Pathways Project Page 12

17 permeability cover to underlying aquifers, reducing recharge but also offering protection from surface contaminants. Elsewhere, it is overlain by younger and more permeable sediment, but the till is still commonly present at depth and so slows groundwater migration and recharge. The layer is nearly continuous across the tops of the upland plateaus of the region but has eroded away on their flanks. It is an important but imperfect barrier to groundwater infiltration into the underlying advance outwash, greatly slowing the migration of any near-surface contaminants, but it is neither completely impervious nor everywhere present. The matrix of the till differs from place to place in relative proportions of clay, silt, and sand, but mostly it is a gravelly silty to very silty sand. In many places, the till is sub-stratified and contains lenses of sand, gravel, and silt. Rock fragments, scattered throughout the till, are mostly pebble to cobble size; boulders more than 3 feet in diameter are uncommon, and only a few more than 5 feet in diameter have been seen. The unoxidized till is light gray and compact; the oxidized till is light yellowish gray and is generally loose. Although a weak brown soil is developed on the till, oxidation rarely extends more than a few feet below the surface. Qvr---Coursing between the hills of this upland surface, and in places lapping up onto their flanks, is the recessional outwash, channel deposits of the long-vanished rivers that issued from the snout of the retreating ice sheet as it withdrew to the north. Recession of the ice sheet was accompanied by both outwash deposits and ice-dammed lakes, analogous to those formed during the ice advance. Water from the melting ice sheet and the Cascade Range drained southward and westward, spilling over divides that were later abandoned as the ice pullback exposed lower routes farther north. By the time that the Duwamish uplands were uncovered by the retreating ice, proglacial drainage was well established across most of the eastern and southern lowland. Water was impounded in a lake that inundated much of the central and southern Puget Lowland, because over one thousand feet of glacial ice still filled the Strait of Juan de Fuca. This water body, Glacial Lake Russell, drained out through the Black Hills into the Chehalis River many tens of miles to the south (Thorson, 1980). Streams draining from the ice sheet into that lake have left channels and more localized patches of gravel and sand that form unconfined, perched aquifers above the till. Where fine-grained deposits of the glacial lake bottom are present, little near-surface groundwater movement can occur. Duwamish Pathways Project Page 13

18 Valley Alluvial Deposits The near-surface sediments of the Duwamish Valley are set within the trough of the Duwamish estuary, carved by glacial ice and subsequently infilled by river sediment. The lower boundary of that trough is reached sporadically by deep borings, particularly in the south half of the study area. The pattern of those data indicate that the trough lies roughly two hundred feet below the modern ground surface along the axis of the valley, generally shallowing to the south and also towards the east and west valley walls. The boundary of these deposits is marked either by bedrock (where present to the south) or by very dense sediment that has been glacially overridden. Above this boundary, the geologic history of the area suggests that a sequence of estuarine deposits, typically fine sands and silts with shells, should be found and should progress up into a more complexly interbedded river-dominated sequence of sand, silt, and gravel marking the advance of the sedimentary wedge fed by the Osceola Mudflow and later deposits. The mudflow itself would have been very thin and subaqueaous along the Duwamish Valley when first deposited, so the chances of intersecting (and recognizing) that layer by drill cores is probably low. The upper part of the river-deposited sediment should, and does, show the classic signs of continued slow overbank deposition by fine sand and silt, colonization by marshland plants, and occasional erosion and refilling by coarser sediment associated with the main channel of the migrating Duwamish River. A more specific discussion of the valley alluvial deposits, the primary focus of the groundwater pathways project, follows this regional framework section Stratigraphic Sequence in the Study Area Although a sequential listing of geologic deposits implies a regular, orderly layering of these materials, actual conditions are much more complex. Individual deposits vary in thickness across the study area; some are present in certain areas but not in others; upper and lower contacts are not everywhere horizontal; and some geologic units gradually, but significantly, change their hydrologic characteristics from one end of the river to the other. Because of these complications, a geologic map (Plate 2) and three regional cross sections (Plate 3) are provided to illustrate the spatial relationships between these materials. Plate 2 is excerpted from a compilation of King County geology (Booth & Sacket, 1997). Plate 3 presents three regional cross sections of the upland-valley stratigraphy through the north, central, and southern Duwamish Basin. The basis for the interpretation and Duwamish Pathways Project Page 14

19 significance to the groundwater flow in the basin is discussed by cross section and region in the following sections Northern Region (Cross Section X-X ) Except within the Duwamish Valley itself, data are sparse across the northern uplands of the study area. The most complete information is provided by borings for a once-proposed Metro sewer-line tunnel that extended from the west end of the West Seattle Bridge directly west to the shore of Puget Sound. Because that line was to extend only just below sea level, borings for that project extend at most to elevation -35 feet (Reference ID #4). The next most complete additional data are available from the Washington State Department of Transportation investigations for the West Seattle Bridge (Reference ID #125), which provide boring data that locally extend below elevation -200 feet. Elsewhere in this part of the study area, two wells reach the underlying Tertiary bedrock: one (Well 18-19), just north of cross section X-X at a depth of 1,030 feet; and the other (Well 19-12), several thousand feet south of the cross section, at a depth of 319 feet. Aside from these bedrock contacts and the noted occurrence of generally fine-grained deposits, the logs for these two deep wells are very crude and generalized. As shown by cross section X-X, the modern valley lies as much as a thousand feet or more above the floor of a bedrock trough, probably formed originally at least several million years ago. The trend of this bedrock trough is crudely northward and approximates the trend of the modern Duwamish Valley. Passing under and north of Harbor Island into Elliott Bay, it descends ever-more abruptly out to the projected modern location of the Seattle Fault. Farther south, it continues to shallow until exposures of rock are seen sporadically at the modern ground surface near Boeing Field (see Plate 2). The post-glacial alluvium of the modern Duwamish River appears to fill an inset, glacial-age valley that is at least 170 feet deep in the center of the modern Duwamish Valley. The western part of that now-infilled glacial valley is walled and floored by silt and clay of the early Vashon transitional beds, equivalent to the deposits commonly called Lawton Clay and recognized throughout the north Seattle area. There are no data in the eastern part of the glacial valley, but near-sea-level exposures along the eastern wall of the Duwamish Valley suggest that the same silty deposits are present here as well. Borings for the West Seattle Freeway, Salmon Bay Steel well, and the foot boring for the Elliott Bay Mill Co. (Reference ID s #42, 70, and 125) Duwamish Pathways Project Page 15

20 variously encounter silt, clay, and glacial till below the post-glacial alluvium. These limited deep data suggest that the deep deposits are primarily finegrained with occasional coarse-grained layers ranging in lateral extent from perhaps a few thousand feet to possibly the width of the valley and individual layers thicknesses varying from a few feet to several hundred feet. The sedimentary structure beneath the western upland plateau is rather well defined to sea level based on multiple borings for the proposed Metro tunnel project (Reference ID #4) and interpreted by David McCormack (written communication, 1997). Vashon advance outwash (Qva) composes the entire thickness of the hillside above 150 to 200 feet elevation, but for a thin discontinuous covering of till (Qvt). Beneath the advance outwash, silt and clay of the transitional beds (Qtb) are everywhere recognized, but this unit thickens appreciably to the east and is likely to fully contain the postglacial alluvium of the Duwamish Valley, down to at least elevation -200 feet. Closer to Puget Sound, coarser deposits of the penultimate interglacial period (the Olympia, unit Qob) are found and have been unequivocally identified by radiocarbon dating of wood from several of the boreholes. Beneath the Olympia beds, and presumably extending well below sea level, is a second fine-grained unit (Q(B)f), stratigraphically separate from the transitional beds but similar in hydrologic behavior. Discriminating the two fine-grained units is important for the present application only if intervening coarse-grained sediment (unit Q(A)c), which could act as a lateral conduit for groundwater flow, is present between them. This does appear to be the case under much of the western uplands, at elevations below 150 feet. Farther east, however, coarser sediments appear to be entirely truncated by the fine deposits of the transitional beds everywhere adjacent to the modern Duwamish Valley Central Region (Cross Section Y-Y ) Data to constrain a regional cross section in the central part of the study area are very sparse and are compiled only from deep borings at the Highline well field (the Beverly Park test well [06-02]), nearly one mile to the southeast, and regional compilations (Woodward et al., 1995). This situation contrasts markedly with the level of hydrogeologic understanding within the Duwamish Valley itself, where the most complete information across all of the region is available (see Section 3.1.2). In the central region, the west uplands display a vertical sequence of alternating coarse-and fine-grained layers down to a level of several hundred Duwamish Pathways Project Page 16

21 feet below sea level. Bedrock is not encountered. The contacts between these layers are shown as (locally) horizontal, simply as an expression of anticipated relationships because no multiple wells are available to constrain this orientation. As also seen farther north (cross section X-X ), glacially overridden silt and clay of units Qtb and Q(B)f predominate for at least several hundred feet below about elevation +200 feet. A thin sea-level deposit of coarse sediment, mapped in this section as Q(A)c, separates finegrained units at the Beverly Park well but its lateral extent is unknown. It may or may not correlate with the mapped and dated Olympia-age deposits that appear to occupy the same stratigraphic position farther north. Unlike areas both north and south, the west slopes of the Duwamish Valley wall here are mantled in Vashon recessional outwash (unit Qvr). This deposit will be of roughly similar permeability to the Vashon advance outwash (unit Qva) but very much more permeable than the adjacent Transitional beds (Qtb). It thus provides a subsurface pathway for water to move into the valley alluvial sediments without first emerging as hillside seeps above the Transitional beds. In contrast to the west uplands, the east uplands here are underlain almost exclusively by sedimentary bedrock of the Blakely Formation. The rock descends both to the west and to the north. Locally it is capped by Vashon till, but nowhere is it anticipated to be a significant contributor of groundwater into the Duwamish Valley itself Southern Region (Cross Section Z-Z ) The data are plentiful for the stratigraphy of the west upland and valley area in the southern region. The studies conducted for west upland in this area include the 1983 Metro proposed South 146th Street Seahurst alignment for the Renton Effluent Transfer System (Reference ID # 139); extensive groundwater investigation, testing, and modeling completed for the City of Seattle Highline wellfield development data (Reference ID # 87, 107, and 119); and compilation of regional data for the Des Moines upland for the South King County Groundwater Management Plan (Woodward et al., 1995). In the valley, there were also several studies which provided valuable data for stratigraphic evaluation including the Metro RETS (Reference ID # 127 and 128) and deep test well data for the City of Tukwila s Foster Links golf course. Although data is very limited on the eastern flank of the valley, the massive bedrock outcrops provide a boundary to the valley aquifers and limit the significance of any groundwater inflow from this area. Duwamish Pathways Project Page 17

22 The southern region is marked by a narrow infilled valley of sediments bounded on both sides by bedrock (cross section Z-Z ). The valley sediments range from a few tens of feet to about 200 feet. The common outcrops of bedrock in this area and the detailed investigations completed in the west upland and in the valley provide valuable data on the regional stratigraphy. Vashon till mantles most of the west upland area, averaging between 20 and 50 feet in thickness. Beneath the till, the advance outwash comprises the first important water-bearing unit of the west upland. These outwash deposits are primarily comprised of sand and, where saturated, mark the shallow-most regional aquifer of the west upland. The aquifer is generally unconfined. Groundwater is typically encountered between elevations of 290 and 305 feet, and the bottom of the aquifer is typically found between 280 and 200 feet in elevation. The Highline Aquifer tapped for water supply by the City of Seattle occurs within an older glacial unit (Q(A)c) indicated by thick sands and gravels with a Cascade provenance. Referred to as the Intermediate Aquifer in the Highline studies, it occurs at an elevation ranging between 0 and 200 feet above mean sea level. Locally, in the Highline neighborhood, it is mostly horizontal and laterally extensive to the north and south. It is generally bounded to the east and west by fine-grained material. The aquifer is most permeable in the vicinity of the Riverton Heights supply well (21-13) and becomes less permeable radially outward from this area. The approximate extent of the Highline aquifer is outlined on Plate 1 and Figure 2 (Area 7). The aquifer area is represented as the transmissivity contour of roughly 20,000 gallons/day/foot for the Q(A)c aquifer as defined by Hart Crowser (1985). A deeper aquifer has been explored in the Highline Study Area to depths of 700 feet or more in at least four locations: the Riverton Heights water supply well area (Well 21-13), the Glacier High area (Well 16-03), boring B-6 drilled for the Metro Seahurst Study (Reference ID# 139), and the Beverly Park test well (Well 06-02). The aquifer zone generally occurred within a sand and gravel unit (Q(B)c) under the current terminology) in the -100 to -150 foot elevation range. This deep aquifer zone appears to be discontinuous; whereas over 100 feet of sand and gravel was observed at the Glacier High site, the aquifer is absent at the Riverton Heights site, and it appears to be less than 40 feet thick at the Beverly Park site. Pumping tests indicated the deep aquifer zone to be hydraulically connected to the intermediate aquifer zone. Duwamish Pathways Project Page 18