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Scientific Investigations Report 2008–5059

Scientific Investigations Report 2008–5059

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Reliable information describing the configuration of the water table in the Portland area is needed to resolve a variety of water-resource issues including evaluation of aquifer susceptibility to contamination, effects from stormwater injection systems such as UIC systems (underground injection control systems), construction, well drilling, and monitoring, and as a baseline to identify changes in water levels resulting from natural or human-induced causes. This report presents maps of estimated depth to water and water-table elevation for the Portland metropolitan area along with estimates of the relative uncertainty and an estimate of seasonal water-table fluctuations to help answer these needs.

The method of analysis used to determine the configuration of the water table in the Portland area relied on two types of readily available information: (1) water-level data from shallow wells, and (2) surface-water features that are representative of the water table. The largest source of available data on ground-water levels in wells in the Portland area are reports filed by well constructors at the time of new well installation. However, analysis of this extensive dataset by comparison with water levels measured by the U.S. Geological Survey (USGS) in the same or nearby wells indicated a poor agreement for wells less than 300 ft deep. Further examination revealed that water-level measurements at the time of well construction frequently appear to be unrepresentative of static water-level conditions. The measurements reported in well-construction records generally were shallower than the measurements by the USGS, though many measurements were substantially deeper than USGS measurements. The magnitudes of differences in depth to water from the two data sources ranged from -119 to 156 ft with a mean of the absolute value of the differences of 36 ft. One possible cause for the differences is that the water levels in many wells at the time of construction were not at equilibrium when measured. Shallower than expected water levels could be the result of the temporary incorporation of drilling fluids or well completion activities and deeper than expected water levels could be a consequence of measurements that were taken while a well was undergoing recovery following heavy pumping during well completion in aquifers with low permeabilities.

Owing to the large disparities, water-level measurements reported on well-construction records were deemed unsuitable for use in determining the water-table position for this study without further verification. The analysis of the water-table configuration primarily relied on water levels measured by the USGS as part of the current study or used in previous USGS ground-water studies conducted in the Portland area. The median water level was used for wells with multiple measurements collected over a period of time. Supplemental water-level data were acquired by identifying candidate wells and conducting a field effort to locate and measure 20 wells in areas of sparse data. The resulting dataset of wells used from previous USGS studies and wells located and measured for this study consisted of 582 wells. Due to the scarcity of well data in some areas, surface-water features such as major rivers, streams, lakes, wetlands, and springs representative of where the water table is at land surface were used to augment the analysis. Water-table elevation for these features was set equal to land-surface elevation, which was obtained from 2- and 10-meter lateral resolution digital elevation models.

Ground-water and surface-water data were combined for use in interpolation of the water-table configuration. Two conventions typically are used to define the water-table position: (1) depth to the water table below land surface and (2) elevation of the water table above a datum. The two conventions are related and can be equated if the land-surface elevation is known. However, the interpolation of the depth-to-water and water-table elevation datasets can produce substantially different results and may represent the end members of a spectrum of possible interpolations, depending on the conditions controlling the water-table position, such as the geometry of the land surface, rate and location of ground-water recharge and discharge, aquifer properties, and extent, thickness, and shape of the aquifer and adjacent confining units. Depth-to-water and water-table elevation datasets were interpolated independently for the current study and then combined to create a single representation of the water-table configuration. The two interpolations were used to create a combined map of depth to water by taking the mean of the values at each spatial position after transforming the interpolated water-table elevation values to depth to water. A map of the combined water-table elevation was developed by subtracting the combined depth-to-water map from land-surface elevation at each spatial position in the grid. Kriging, a type of spatial moving average, was the method of interpolation used for the study. Parameters for the kriging analysis were determined through the use of semivariograms developed individually for the depth-to-water and water-table elevation datasets.

The kriging analysis also was used to evaluate the reliability or uncertainty associated with the values of the water-table position. Standard deviation maps were generated for depth-to-water and water-table elevation analyses. These maps are similar and were averaged to produce a single standard deviation map for the water-table position. The resulting standard deviation map was rescaled to determine a relative uncertainty. The values of the relative uncertainty range between 0 and 1 where 0 represents a low relative uncertainty and 1 represents a high relative uncertainty. These values can be used to determine the relative uncertainty associated with an estimate of the water-table position at any location on the map and also were used to limit the extent of the analysis where the level of relative uncertainty was deemed to be unsatisfactory.

Because the water table is not a stationary surface and continually fluctuates in response to changes in recharge to or discharge from the aquifer, an analysis was conducted to determine the range of seasonal fluctuations. The magnitude of seasonal water-table fluctuations depends on the resulting change in storage in the aquifer and the effective porosity of the aquifer. The analysis of seasonal water-table fluctuations was performed with the use of summary statistics of data from 127 shallow wells that had multiple water-level measurements distributed throughout the seasons. Results of the analysis revealed that the spatial distribution of seasonal fluctuations of the water table is largely a function of the effective porosity of the aquifer in the zone of fluctuation of the water table and can be categorized by hydrogeologic unit.

Depth to water in the study area ranges from 0 feet below the surface along major rivers and streams to a maximum estimated at more than 1,200 feet below land surface for parts of the mountains to the east. Depth to water exceeds 100 feet in the terrace deposits throughout much of northern and eastern Portland. Depth to water greater than 300 feet is limited to a few higher elevation areas and includes parts of the Tualatin Mountains, the foothills of the Cascade Range, and much of the Boring Hills. In addition to the low-lying areas along major rivers and streams, areas of notably shallow depths to water include the area consisting of former alluvial channels extending from the confluence of Johnson and Crystal Springs Creeks northward to the Willamette River and southward to the Clackamas River, much of the area adjacent to Johnson Creek, the area extending from Beggars Tick Marsh eastward to Holgate Lake at the western end of Powell Butte, the area around Fairview Creek, and the western end of Lake Oswego. These regions of shallow depths to water may represent areas of concern with regard to existing or planned stormwater injection systems.

Elevation of the water table in the study area ranges from 11 feet NAVD 88 along most areas of the Columbia and Willamette Rivers to more than 2,000 feet NAVD 88 on the western slopes of the Cascade Range. The water-table elevation is similar to land surface with the highest elevations in the Tualatin Mountains, Boring Hills, and foothills of the Cascade Range and the lowest elevations along major rivers and streams and in the terrace deposits in the area approximately bounded by the Columbia River to the north, the Willamette River to the west, and Mount Tabor and Rocky Butte to the southeast. Shallow ground-water flow directions generally are toward adjacent streams and rivers with overall flow toward the major ground-water discharge areas, consisting of the Columbia, Willamette, and Clackamas Rivers. A notable exception is in the middle and upper parts of the Johnson Creek drainage basin, where much of the shallow ground-water flows toward the Sandy, Columbia, or Willamette Rivers. Ground-water mounds are located at the top of hills or mountains such as the Tualatin Mountains, Mount Tabor, or the Boring Hills. The largest closed ground-water depression is adjacent to the northwestern side of Powell Butte.

Ground-water levels in the Portland area normally are highest during the spring following the winter period of high precipitation and low evapotranspiration. Water levels recede during the summer in response to diminished precipitation and high evapotranspiration and are lowest in the autumn. The range of seasonal water-table fluctuations observed in wells used for analysis ranged from 1 to 22 feet, with a mean of 7 feet. Small seasonal water-table fluctuations occur throughout the study area but are concentrated more heavily in the terraced areas between the Willamette and Columbia Rivers north and west of the Boring Hills, where the wells typically obtain water from the unconsolidated sedimentary aquifer. The largest seasonal changes generally occur in the Sandy, Boring, and Damascus areas with few exceptions.

The accuracy of the water-table configuration maps depends on various factors, including data limitations and errors, the method of interpolation, and assumptions made during the analysis. The water-table configuration maps generally are representative of the conditions in the study area; however, the actual position of the water table may differ from the estimated position at specific locations, and short-term, seasonal, and long-term variations in the differences can be expected.

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