Hydrologic Studies in Oregon

Hydrogeology of the Unsaturated Zone and Ground-Water Flow System Assessment, Portland, Oregon

Project Chief: Daniel T. Snyder

Cooperators:

City of Portland, Bureau of Environmental Services
Clackamas County, Water Environment Services
City of Gresham
Multnomah County

BACKGROUND

The Portland metropolitan area within Oregon consists of about 1.5 million residents. About one third of these residents are within the city limits of Portland .  Activities such as the installation of underground injection control (UIC) systems (sumps), leaking underground storage tanks, point source and non-point source pollution threaten the ground-water resource.  Within the metropolitan area are a number of public water-supply systems that utilize ground water.  This includes the City of Portland’s Columbia South Shore wellfield which was planned as a backup water supply but which has become relied on as a supplemental source of water.  As a result, the need for reliable, quantitative knowledge of the unsaturated zone and characterization of the ground-water flow system has become of greater importance.

The City of Portland Bureau of Environmental Services (BES) has installed approximately 9,300 underground injection control (UIC) systems throughout the city.  Estimates of the number of UIC systems installed through private development are greater than 25,000. These systems, or sumps, are large perforated pipes sunk vertically into the ground that allow stormwater runoff to soak into the soil.  Use of the sumps helps to remove a large portion of stormwater from the combined sewer system, which reduces overflows to the Willamette River and the Columbia Slough.  BES is presently working with the Oregon Department of Environmental Quality (ODEQ) and the U.S. Environmental Protection Agency (EPA) on the permitting requirements for the sumps. 

Adequate hydrogeologic information about the unsaturated zone in the area of sump installation is recognized as an important aspect of the permitting process.  The thickness of the unsaturated zone (equivalent to depth to water) and the lithologic and hydraulic characteristics of the subsurface materials are factors that affect the ability of a sump to function properly and to filter and process particulate and dissolved substances that may be present in the water entering a sump.  This information can be used to help assess the potential for contamination of the ground-water resource via sumps.  In March, 2003 BES requested the assistance of the U.S. Geological Survey (USGS) in the characterization of the unsaturated zone thickness and lithology.

The request by BES illustrates the need for this type of information which has broad applications for government agencies, private businesses and organizations, and residents in the region.  The most frequent request for information pertaining to ground water received by the USGS Oregon District is “how deep is the water table?”  Previous studies help to answer these questions but only in an indirect way.  Existing maps show the elevation of the potentiometric surface by hydrogeologic unit rather than the thickness of the unsaturated zone.  No single map or resource is currently available that quickly and easily depicts the general unsaturated zone thickness for the Portland metropolitan area.  In addition, data for these studies were collected prior to 1990 and do not take advantage of additional information now available as a result of more recent investigations or data from wells that have been drilled since that time.

There is also a further need for information and analysis pertaining to the characterization of the ground-water flow system in the region.  The USGS developed a ground-water flow model for the Portland Basin (Morgan and McFarland, 1996) which has been used to describe a number of aspects regarding the ground-water flow system such as the location of recharge areas for aquifers, potential down gradient impacts of septic systems, and time of travel.  This information was published as part of a ground-water vulnerability study for the Clark County, Washington portion of the basin (Snyder and others, 1996).  Unfortunately, the analyses for the Oregon side of the Portland Basin were never fully developed or published.  This information would be of value to BES and other water-resource planners and managers in helping to protect and maintain ground-water resources.

STUDY AREA

The scope and extent of the study include most of the region covered by the USGS Portland Basin Ground-Water Study (McFarland and Morgan, 1997) with the greatest emphasis on Multnomah and northern Clackamas Counties in Oregon (figure 1).  The area encompassed by the Johnson Creek drainage basin will receive greater scrutiny due to initial analyses by BES indicating that the suitability of sumps in this area is largely uncertain.

OBJECTIVES

The primary goal of the project proposed here is to provide the hydrogeologic information and understanding necessary to support effective storm-runoff and resource-management decisions regarding the suitability of sumps in areas throughout the Portland metropolitan area. The secondary goal is to provide hydrogeologic information of broad interest and value to the community to improve the understanding of the unsaturated zone and aquifers in the Portland area, and help answer questions regarding the ground-water flow system.  To achieve these goals, we will:

• Develop an unsaturated zone thickness map which can be used by BES, ODEQ, or EPA to evaluate the appropriateness of using sumps based in certain areas.
• Provide information on the unsaturated zone lithology.  This information will be useful to BES, ODEQ, or EPA in the determination of the ability of the subsurface materials to process and filter water entering the sumps and to mitigate possible water-quality concerns.
• Perform a variety of analyses using the existing USGS regional ground-water flow model of the Portland Basin that will be useful in characterizing the ground-water flow system.  Analyses will include vertical gradient maps, contributing area maps for selected wellfields and surface-water features, water budgets for selected features, maps showing the time of travel or age of ground-water, and discussion of the significance of this information to UIC systems.

RELEVANCE AND BENEFITS

The general objective of this study-to provide information and understanding to support resource-management decisions-is directly relevant to the Strategic Plan of the USGS (U.S. Geological Survey, 2000), as well as the Strategic Directions (U.S. Geological Survey, 1999) and the Cooperative Program Priorities (U.S. Geological Survey, 2002) of the Water Resources Discipline.  This work will provide useful, relevant, and broadly transferable information to local resource managers, ground-water consultants, and property owners assisting them in making effective resource-management decisions to comply with regulations and to meet local water-resources protection and development goals.

The specific objectives of describing the hydrogeologic setting in areas of UIC systems for use by BES is also anticipated to be of benefit to nearby  communities in the Portland Basin.  These communities may also be required to determine this type of information with regard to UIC systems.  The knowledge gained from this study should help to provide much of the information that will be needed.  In addition to their use in UIC system permitting, the results of the study will provide valuable information to much of the Portland Basin for such activities as drilling of domestic or other types of supply wells, determining the risk posed by leaking underground storage tanks, and identifying areas where land-use activities could threaten ground-water resources.

Throughout this study the project team work closely with BES, EPA, and ODEQ to solicit their input regarding the proposed approach and methods. The involvement of these agencies in this study will serve to increase their understanding and acceptance of the results and ensure that the products developed meet the needs of BES.

APPROACH

Considerable expertise and information has been developed during previous investigations of the Portland Basin by the USGS and other agencies and the present study is designed to utilize these resources.  To meet the objectives of the study, existing hydrogeologic information as well as simulation tools such as the USGS Ground-Water Flow Model of the Portland Basin will be used to determine the unsaturated zone thickness, the lithology of the unsaturated zone, and to perform a variety of analyses for characterizing the ground-water flow system.

Because of the urgent need for information, the study will be conducted in two phases.  Phase I will be conducted during the latter part of the 2003 Federal Fiscal Year and the early part of the 2004 Federal Fiscal Year and will concentrate on the data needs required immediately by BES which include a preliminary unsaturated zone thickness map.  Phase II will take place in the 2004 Federal Fiscal Year and additional cooperators to share the study costs will be sought (by BES and USGS) among Municipal, County, and Regional agencies that are anticipated to have a need for the acquisition and analysis of the type of hydrogeologic information proposed.  A meeting with these other agencies will be planned to solicit their participation and input.  BES and USGS are committed to seeking funding for completion of the project in its entirety.

Unsaturated Zone Thickness

Unsaturated zone thickness, or depth to water, is a primary factor in determining the suitability of a location for sump installation.  A preliminary unsaturated zone thickness map will be developed early in Phase I using readily available data.  The principal purpose for this map will be for BES to quickly identify the potential status of existing sumps for planning and design purposes.  The unsaturated zone thickness map will be revised during later portions of Phase I using data from a variety of sources.  This map will take longer to create, but will have less uncertainty associated with the estimates of unsaturated zone thickness.  Information on land-surface elevation provided by BES, Metro, and the USGS will be used to convert some of the source data from elevations of the ground-water surface above sea level to values representing depth to water (unsaturated zone thickness).  The completed unsaturated zone thickness map will be used in conjunction with the land-surface elevation maps to create a map depicting the elevation of the water table which may be of value for some analyses.

Preliminary Map

Three primary data sources will be used to create the initial unsaturated zone thickness map for the region.  The best source of information on ground-water levels in the Portland Basin is the set of water-level maps from the USGS regional ground-water study of the Portland Basin (McFarland and Morgan, 1997).  These maps were developed from more than 1,500 wells where the well locations were accurately established, the source aquifer determined, and the water levels measured.  Additional sources of information will be required for areas where the low density of wells in the surficial aquifer results in a high uncertainty of the unsaturated zone thickness.  The simulated ground-water levels determined from the surficial layer of the USGS ground-water flow model developed for the above study (Morgan and McFarland, 1996) will be used to supplement water-level information.  An additional data set of wells with locations that are only known approximately and that were not included in the USGS study will be used to augment these data sets.  Finally, supplemental water-level information will be utilized as needed and as time permits from sources such as the Johnson Creek Basin Hydrologic Monitoring Study and the incorporation of the locations of rivers, lakes, wetlands, and springs.

An estimate of the spatial distribution of the uncertainty of the unsaturated-zone thickness estimates will be made for each primary data source.  The largest factor influencing the uncertainty is the proximity of a known value of the unsaturated-zone thickness to a point of interest on the map.  A map showing the distance to the nearest data value will be created for each primary data source.  Uncertainty maps for data sources with locations that are only known approximately will have a constant value added to the uncertainty equal to the median of the precision of the locations of the data values. These maps can be combined to help identify where additional data is necessary to reduce the uncertainty.  In addition, there are seasonal fluctuations of ground-water levels that will be considered.  Water levels typically rise as a result of recharge from precipitation during late fall through early spring.  Water levels declines become more noticeable in the last spring through early fall as a result of losses to streams, springs, evapotranspiration, and pumpage.  A map will be developed depicting the range of seasonal fluctuation of the ground-water levels.  A final set of maps showing the minimum and maximum range of water levels will then be developed that incorporate the range of seasonal fluctuations.  These final maps will include both unsaturated zone thickness and the elevation of the ground-water table.

Revised Map

Sources for additional data on unsaturated zone thickness will be identified, assessed, and incorporated into a revised map.  Sources of data will be prioritized based on criteria such as their usefulness in refining ground-water levels in areas of greatest concern with regard to UIC systems, areas where there is greater uncertainty in the preliminary map, and areas of high population density.  This revised map, based on all available information, will be published as a USGS Open-File Report at the conclusion of Phase I.  Sources of data that are anticipated to be used include but are not limited to the following:

• Ground-water level observations made during recent or ongoing USGS investigations (such as the Johnson Creek Basin Monitoring Study, Willamette Basin Ground-Water Study, Willamette National Water-Quality Assessment Program, and BES Ground-Water Quality Study).
• Ground-water well databases such as the USGS National Water Information System, the Oregon Department of Water Resources (OWRD) Groundwater Resource Information Distribution, and the OWRD State Observation Well network.
• The locations and elevations of surface-water features such as rivers, streams, lakes, wetlands, and springs after assessing their usefulness in representing nearby ground-water levels.
• Ground-water investigations by other agencies or by ground-water consultants including modeling and monitoring of the Columbia South Shore wellfield.

Unsaturated Zone Lithology

During Phase II the surficial lithology will be obtained primarily from 1:24,000-scale mapping of surficial geology by the Oregon Department of Geology and Mineral Industries (DOGAMI) which covers most of the Portland Basin (Beeson and others, 1989; Beeson and others, 1991; Madin, 1990, Madin, 1994).  The information will include a description of the physical and mineralogical characteristics of the surficial geologic units and their origin.  These maps will be compiled and aggregated with the resulting composite map included in the final report.

Additionally, during Phase II the subsurface lithology and material characteristics of the unsaturated zone between the bottom of a typical sump (approximately 30 feet) to some depth below the sump (using criteria established by BES, other cooperators, EPA, and/or ODEQ) will be determined for use by BES.  The lithology will be described as a textural characterization based on classification from the dominant bulk texture within the depth interval, a composite or average bulk texture for the interval, or some other statistical determination of texture as decided in consultation with BES, other cooperators, EPA and/or ODEQ.  Sources of information that will be used include the database of well lithology developed for the USGS Portland Basin Ground-Water Study and the DOGAMI borehole geology database used for the surficial geology mapping.  Both sources will be supplemented with information from driller’s logs contained in OWRD’s Groundwater Resource Information Distribution database as needed.  The lithology of the unsaturated zone from 0 to 100 feet will also be determined for more general applications using the same data sources and a map produced for the characteristics of the materials.  The textural classification is important in understanding how water will move through the unsaturated zone from a sump to the water table.

Ground-Water Flow System Assessment

In Phase II the USGS regional ground-water flow model of the Portland Basin will be used to create a number of analyses and scenarios that will be useful in assessing the ground-water flow system.  The analyses will be performed for two scenarios representing two possible sets of conditions.  The first will represent the ground-water flow system using the original calibration conditions for the ground-water model.  This assumed the City of Portland ’s Columbia South Shore wellfield was used only minimally.  The second scenario will assume full utilization of the Columbia South Shore wellfield.  Vertical gradient maps will be created showing where the movement of ground-water is likely to be upwards or downwards.  Particle tracking will be performed to create contributing area maps for the wellfield, in addition to major rivers, lakes, and springs.  A water budget will be provided for each of these features.  Particle tracking will also be used to develop a map depicting the age of ground-water or the time of travel as ground-water moves from its point of entry (recharge) to its point of discharge.  Selected portions of these analyses and their significance with regard to UIC systems will be presented in the final report.

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