Shallow Groundwater Movement in the Skagit River Delta ...Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington Scientific Investigations Report 2009–5208

Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

Scientific Investigations Report 2009–5208

Prepared in cooperation with the Skagit County Public Works Department, Washington State Department of Ecology, and Skagit County Public Utility District No. 1

U.S. Department of the InteriorU.S. Geological Survey

Cover: Photograph of the Skagit River delat area, Skagit County, Washington. Image ©2009 Digital Globe ©2007Google™.


By Mark E. Savoca, Kenneth H. Johnson, and Elisabeth T. Fasser

Prepared in cooperation with the Skagit County Public Works Department, Washington State Department of Ecology, and Skagit County Public Utility District No. 1


U.S. Department of the InteriorU.S. Geological Survey

U.S. Department of the InteriorKEN SALAZAR, Secretary

U.S. Geological SurveySuzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:Savoca, M.E., Johnson, K.H., and Fasser, E.T., 2009, Shallow groundwater movement in the Skagit River Delta Area, Skagit County, Washington: U.S. Geological Survey Scientific Investigations Report 2009–5208, 22 p.

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Contents

Abstract ...........................................................................................................................................................1Introduction.....................................................................................................................................................1

Purpose and Scope .............................................................................................................................2Description of Study Area ...................................................................................................................2Geologic Setting ....................................................................................................................................2Well-Numbering System ......................................................................................................................4

Methods of Investigation ..............................................................................................................................4Well Inventory and Water-Level Measurements ............................................................................4River Stage .............................................................................................................................................6Groundwater-Level and Flow-Direction Maps ................................................................................7Ocean Tides ...........................................................................................................................................8

Hydrogeology..................................................................................................................................................8Groundwater Movement.............................................................................................................................10

Groundwater-Flow Directions ..........................................................................................................10Groundwater-Level Fluctuations ......................................................................................................17

Summary and Conclusions .........................................................................................................................20Acknowledgments .......................................................................................................................................20Selected References ...................................................................................................................................21

Figures Figure 1. Map showing location of the Skagit River Delta study area, Skagit County,

Washington ……………………………………………………………………… 3 Figure 2. Diagram showing well numbering system used in Washington ………………… 4 Figure 3. Map showing location of wells and U.S. Geological Survey streamflow-gaging

station, Skagit River Delta area, Washington …………………………………… 5 Figure 4. Simplified surficial geologic map of the Skagit River Delta area, Washington …… 9 Figure 5. Map showing water-level altitude and inferred groundwater-flow directions

in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, August 2007 ………………………………………………………… 11

Figure 6. Map showing water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, November 2007 ……………………………………………………… 12

Figure 7. Map showing water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, February 2008 ……………………………………………………… 13

Figure 8. Map showing water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, May 2008 …………………………………………………………… 14

Figure 9. Map showing regression analysis derived annual water-level altitude and calculated groundwater-flow direction in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, 2007 and 2008 …………………… 16

Figure 10. Hydrographs of water levels in well 34N/03E-28M02 and precipitation at Anacortes, Washington, October 2007 through May 2008 ………………………… 17

iv

Figure 11. Map showing wells that likely are influenced by stage on the Skagit River, Washington ……………………………………………………………………… 18

Figure 12. Hydrographs of water levels in well 34N/03E-23B01 and stream stage at U.S. Geological Survey streamflow-gaging station 12200500, Skagit River near Mount Vernon, Washington, October 2007 through May 2008 ……………… 19

Figure 13. Hydrographs of water levels in well 34N/03E-19F01 and nearby tidal fluctuations near La Conner, Washington, October 2007 ………………………… 19

Figures—Continued

Tables Table 1. Hydrogeologic characteristics of simplified geologic units used in this study

and correlation with previous unit designations ………………………………… 10 Table 2. Fitted regression coefficients and calculated hydrogeologic parameters for

wells in the Skagit River Delta area, Washington, 2007 and 2008 ………………… 15

Conversion Factors and Datums

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)acre 0.004047 square kilometer (km2)section (640 acres or 1 mi2) 259.0 square hectometer (hm2)square foot (ft2) 0.0929 square meter (m2)square mile (mi2) 259.0 hectare (ha)square mile (mi2) 2.590 square kilometer (km2)

Hydraulic gradient

foot per foot (ft/ft) 1 meter per meter (m/m)foot per mile (ft/mi) 0.1984 meter per kilometer (m/km)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88), referred to in this report as “sea level.”

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above or below sea level.


By Mark E. Savoca, Kenneth H. Johnson, and Elisabeth T. Fasser

AbstractShallow groundwater movement in an area between

the lower Skagit River and Puget Sound was characterized by the U.S. Geological Survey to assist Skagit County and the Washington State Department of Ecology with the identification of areas where water withdrawals from existing and new wells could adversely affect streamflow in the Skagit River. The shallow groundwater system consists of alluvial, lahar runout, and recessional outwash deposits composed of sand, gravel, and cobbles, with minor lenses of silt and clay. Upland areas are underlain by glacial till and outwash deposits that show evidence of terrestrial and shallow marine depositional environments. Bedrock exposures are limited to a few upland outcrops in the southwestern part of the study area, and consist of metamorphic, sedimentary, and igneous rocks.

Water levels were measured in 47 wells on a quarterly basis (August 2007, November 2007, February 2008, and May 2008). Measurements from 34 wells completed in the shallow groundwater system were used to construct groundwater-level and flow-direction maps and perform a linear-regression analysis to estimate the overall, time averaged shallow groundwater-flow direction and gradient. Groundwater flow in the shallow groundwater system generally moves in a southwestward direction away from the Skagit River and toward the Swinomish Channel and Skagit Bay. Local groundwater flow towards the river was inferred during February 2008 in areas west and southwest of Mount Vernon. Water-level altitudes varied seasonally, however, and generally ranged from less than 3 feet (August 2007) in the west to about 15 feet (May 2008) in the east. The time-averaged, shallow groundwater-flow direction derived from regression analysis, 8.5° south of west, was similar to flow directions depicted on the quarterly water-level maps.

Seasonal changes in groundwater levels in most wells in the Skagit River Delta follow a typical pattern for shallow wells in western Washington. Water levels rise from October through March, when precipitation is high, and decline from April through September, when precipitation is lower. Groundwater levels in wells along the eastern margin of the study area also are likely influenced by stage on the Skagit River. Water levels in these wells remained elevated through

April, and did not seem to begin to decline until the end of May in response to declining river stage. Groundwater levels in a well equipped with a continuous water-level recorder exhibited periodic fluctuations that are characteristic of ocean tides. This well is less than 1 mile east of the tidally influenced Swinomish Channel, and exhibited water-level fluctuations that correspond closely to predicted tidal extremes obtained from a tide gage near La Conner, Washington.

IntroductionIn Washington State, the availability of water for out-

of-stream uses must be determined before water can be appropriated. This determination is most often made as part of an application for a water right; however, certain uses are exempted from the water rights permitting system. To prevent water withdrawals from affecting other out-of-stream and instream uses, Washington State may reserve a specific quantity of water in a stream basin for out-of-stream uses as part of the regulation establishing minimum instream flows (the Instream-Flow Rule). The reservation allows for new groundwater withdrawals in basins where all available water is appropriate. Once the total of new withdrawals equals the quantity specified in the reservation, subsequent new uses would have to find an alternative source of water, obtain an existing water right, or provide compensating mitigation for affected streamflow.

Recent population growth along the Interstate 5 corridor near Mount Vernon, Washington, has led to increased water use, with many new domestic wells serving residents in the lower part of the Skagit River basin in areas not served by a regional public water system. Planning for future development in the lower basin, including the reservation of water for new domestic wells, requires identification of areas where withdrawals from existing and new wells could adversely affect streamflow in the Skagit River or its tributaries. Skagit County, as the land use authority for unincorporated areas without access to public water systems, requires a scientifically credible basis for implementing land use restrictions to protect instream resources.

2 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

In June 2006, the U.S. Geological Survey (USGS) in cooperation with Skagit County, the Washington State Department of Ecology, and Skagit County Public Utility District No. 1, began a project to characterize shallow groundwater movement in an area between the lower Skagit River and Puget Sound, referred to as the “Skagit River Delta” in this report (fig. 1), and to investigate the possible presence of a groundwater divide to assist in the identification of areas where wells could have an adverse affect on flow in the Skagit River, and therefore would be subject to regulation under the Skagit River Instream-Flow Rule.

Purpose and Scope

This report presents information used to characterize shallow groundwater movement in the part of the Skagit River Delta north of the North Fork distributary. The report includes seasonal water-level maps, a linear-regression analysis of water levels, and descriptions of groundwater-flow direction, gradient, and seasonal fluctuations. Groundwater level and Skagit River stage data were collected by the USGS quarterly from August 2007 through May 2008.

Description of Study Area

The study area covers about 42 mi2 along the lower Skagit River before it enters Skagit Bay in southwest Skagit County, Washington (fig. 1). The area is bounded by the Skagit River to the east and south; an approximate watershed divide between the Samish and Skagit Rivers to the north; and Padilla Bay, Swinomish Channel, and Skagit Bay (all parts of Puget Sound) to the west. The Skagit River occupies a large, relatively flat alluvial valley that primarily is underlain by fluvial and deltaic sand and gravel deposits associated with the present and ancient Skagit River, and locally preserved lahar-runout deposits originating from Glacier Peak (located outside the study area to the southeast). Local upland areas are underlain by glacial till and outwash deposits that show evidence of terrestrial and shallow marine depositional environments. Bedrock (consisting of a complex assemblage of metamorphic rocks, sedimentary units, and igneous rocks) underlies the alluvial valley and upland areas. Surficial exposures of bedrock are limited to a few upland outcrops in the southwestern part of the study area.

Land-surface altitude in the study area ranges from near sea level adjacent to the Swinomish Channel to about 130 ft in upland areas. The study area has a temperate marine climate with warm, dry summers and cool, wet winters. Temperatures

are moderated by the Pacific Ocean and Puget Sound, and these bodies of water provide an abundant supply of moisture for storms that typically approach the area from the west. Mean annual (1971–2000) precipitation at Mount Vernon is 32.7 in. (National Oceanic and Atmospheric Administration, 2007). Summers (June–August) typically are dry with a mean precipitation of 4.5 in. at Mount Vernon. Winters (December–February) are wetter than summers with a mean precipitation of 11.0 in. at Mount Vernon. The mean monthly temperature at Mount Vernon ranges from about 40°F in January to about 63°F in August (National Oceanic and Atmospheric Administration, 2007).

Geologic Setting

A brief summary of major geologic events in the study area is described below, and is based on the work of Hansen and Mackin (1949), Easterbrook (1969), Marcus (1981), Johnson (1982), Tabor (1994), Booth (1994), Dragovich and Grisamer (1998), Dragovich and others (2002), and Dragovich and DeOme (2006). The geology of the study area records a complex history of accretion along the continental margin, mountain building, deposition of terrestrial and marine sediments, igneous intrusion, and the repeated advance and retreat of continental glaciers. Bedrock in the study area consists of low-grade metamorphic rocks, formed during Late Jurassic or Early Cretaceous continental margin subduction, and sedimentary units deposited in alluvial fan, braided stream, and near shore shallow marine settings.

Continental glaciers advanced into Skagit County several times during the Pleistocene Epoch. This ice is part of the Cordilleran ice sheet, and is known as the Puget Lobe. The most recent period of glaciation, the Vashon Stade of the Fraser glaciation, began about 17,000 years ago when the continental ice sheet in Canada expanded and the Puget Lobe advanced southward into western Skagit County, and eventually covered the entire Puget Sound Basin before halting and retreating. During the Everson Interstade, beginning about 13,500 years ago, the climate warmed and the lobe wasted back allowing marine waters to enter the Puget Sound Basin, which had been depressed due to glacial isostatic loading. Marine inundation buoyed the retreating ice and produced marine and estuarine conditions in the study area. Postglacial filling of the Skagit River valley, which had been excavated by subglacial meltwater, was accomplished through Holocene fluvial, estuarine, and deltaic deposition and volcanic-lahar deposits originating from Glacier Peak.

Introduction 3

Figure 1. Location of the Skagit River Delta study area, Skagit County, Washington.

tac09-9722-0386_SkagitDelta_Fig 01

3 MILES21

3 KILOMETERS21

0

0

Base from U.S. Geological Survey digital data, 1983, 1:100,000Universal Transverse Mercator projection, Zone 10Horizontal Datum: North American Datum of 1927 (NAD 27)

T.35N.

T.34N.

T.33N.

48°21'

122°30'

25'

48°29' 122°20'25'

R. 02 E. R. 04 E.

I5

I5

SR20

SR20

SR538

SR536

Sou

th

F

ork

Skagit Bay

Padilla Bay

Ska

git

River

North Fork

Swinomish Channel

Burlington

MountVernon

La Conner

Skagit River Delta study area

WASHINGTON

Figure location

Skagit RiverDelta

Boundary of Skagit River Delta study area

EXPLANATION


Well-Numbering System

Wells in the study area were assigned a local number that identifies each well within the Public Land Survey rectangular grid system for Washington State (fig. 2). For example, for well 34N/03E-09L01, the number and letter preceding the slash (34N) indicates the township north of the Willamette Base Line. The number and letter between the slash and the hyphen (03E) indicates the range east of the Willamette Meridian. The number following the hyphen (09) indicates the section number within the township. The letter following the section number (L) indicates the 40-acre quarter-quarter tract within the section. The number following the quarter-quarter tract (01) is a sequence number used to distinguish individual wells in the same quarter-quarter tract. A “D” following the sequence number indicates a well that has been deepened.

Methods of InvestigationMethods used to compile and

analyze information to characterize shallow groundwater movement in the


Will

amet

te M

erid

ian

Willamette Base Line

W A S H I N G T O N

Study area

SECTION 09

34N/03E-09L01ABCD

E F G H

JKLM

N P Q R

R. 3 E.

T.

34

N.

123456

7 12

1318

19 24

2530

31 32 33 34 35 36

TOWNSHIP

Figure 2. Well numbering system used in Washington.study area include measurement of water levels in wells, estimates of Skagit River stage, construction of water-level maps, and determination of ocean tides.

Well Inventory and Water-Level Measurements

Characterization of the groundwater-flow system relied on the analysis of spatially distributed information about groundwater levels, and the physical and hydraulic properties of the geologic units detected during well construction. This information was obtained through the measurement of water levels in wells, and the evaluation of lithologic descriptions from well drillers’ logs. Lithologic information was used to identify wells completed in the shallow groundwater system. Well records were compiled from USGS and Washington State Department of Ecology databases to identify potential wells to be used in this study. Candidate wells were selected for field inventory based on the location and depth of the well, and

the availability of the drillers’ log. The goal of the inventory was to obtain an even distribution of wells throughout the study area. However, this was not possible for the entire study area because of a lack of wells in less populated areas. During the field inventory in August 2007, permission was obtained to measure water levels in 43 wells (fig. 3) on a quarterly basis (August 2007, November 2007, February 2008, and May 2008). Four additional wells were added to the quarterly monitoring network as they became available for measurement. One of these wells (34N/03E-35N01) was installed for this study using a rotary drill rig and constructed of 6 in. steel casing and 0.01 in. stainless steel screen. Four quarterly monitoring wells were instrumented with continuous water-level recorders (fig. 3).

Methods of Investigation 5


09L0109L02

19F01

19F02

27Q01

35N01

05B0105B02

01R0105R01

10A0110B0112A0109E01

08R01

16G0114K01

16J0115R01

16P0121A01 23D01

23B0122F01

23G0122H0122F0220M01

21K0124L01 24K0120J01

21M01

26D01

26K01

28M0128M02

26Q01 25N0135D01

35G0132K01

35R01

35R02

04C01

05H02

03J01

3 MILES21

3 KILOMETERS21

0

0


T.35N.

T.34N.

T.33N.

R. 02 E. R. 03 E. R. 04 E.

48°21'

122°30'

25'

48°29' 122°20'25'

I5

I5

SR20

SR20

SR538

SR536

12200500

Sou

th

F

ork

Skagit Bay

Padilla Bay

Swinomish Channel

Ska

git

River

North Fork

Burlington

MountVernon

La Conner

Water-level observation wellObservation well with continuous water-level recorder

Skagit River Delta study area

USGS streamflow-gaging station

Well centroid

EXPLANATION

12200500

05H0232K01

Figure 3. Location of wells and U.S. Geological Survey streamflow-gaging station, Skagit River Delta area, Washington.


The depth to groundwater below land surface was measured using a calibrated electric tape or graduated steel tape, both with accuracy to 0.01 ft. Latitude and longitude locations were determined for each well using a Global Positioning System receiver with a horizontal accuracy of one-tenth of a second (about 10 ft). Altitudes of the land surface at each well were interpolated from USGS 1:24,000–scale topographic maps, with an accuracy ranging from ±5 ft (low-relief areas) to ±10 ft (high-relief areas). Light Detection and Ranging (LiDAR) derived values for the altitude of land surface became available after completion of the field inventory, and were used to determine the altitude of land surface at each well for the computation of water-level altitudes. LiDAR data were collected and processed by a private vendor under contract with the USGS. Vertical accuracy was evaluated for internal consistency (repeatability), and conformance with independent ground-control points.

All water-level measurements were made by USGS personnel according to standardized techniques of the USGS (Drost, 2005). Well information collected during the field inventory and the quarterly monitoring-network measurement periods were entered into the USGS National Water Information System (NWIS) database and published in Fasser and Julich (2009).

River Stage

Estimates of Skagit River stage (the altitude of the water surface in the river) were used to constrain groundwater-level contours along the eastern margin of the study area, and to infer groundwater recharge from or groundwater discharge to the river. Daily values of river stage were obtained for USGS steamflow-gaging station 12200500 (Skagit River near Mount Vernon, Wash.; fig. 3) and a mean gage height was computed for each of the quarterly monitoring-network measurement periods. The initial measurement period (August 2007) required locating suitable wells and obtaining owner permission and took 8 days to complete. All subsequent quarterly measurements (November 2007, February 2008, and May 2008) were accomplished in 2 days. Values of river stage reported in vertical datum NGVD 29 were converted to the datum used for the monitoring network, NAVD 88, using the on-line program VERTCON (http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl) provided by the National Geodetic Survey.

River stage at the USGS steamflow-gaging station was extrapolated along the length of the Skagit River within the study area using several data coverages and Geographic Information System (GIS) procedures. Spatial data points were developed to represent the centerline of the

river based on available coverages of hydrography, aerial photography, USGS quadrangle topographic maps, and LiDAR imagery. These data were cross checked, and adjusted using ArcMapTM, version 9.3, by ESRI Inc., to produce a consistent rendering of river hydrography. The points for the centerline were converted to x and y coordinates (NAD 83 State Plane Washington North) in the software, and exported to Microsoft Excel®. The distances between centerline points were computed and summed to obtain a total distance from the mouth of the Skagit River. River channel distance was measured along the North Fork of the Skagit River, and extended out of the study area to open water in Skagit Bay, where water levels were expected to average to mean sea level. The total river mile distance from open water to the USGS gage was computed to be 17.5 mi. GIS points were then interpolated at one-half-river-mile intervals, and each point was assigned a river-stage value by proportionally adjusting the stage value at the USGS gage (for example, by a factor of (river mile)/17.5).

These estimates of river stage were considered during construction of groundwater-level maps and were compared to nearby groundwater levels measured in the delta to help infer the nature of the hydraulic connection between the Skagit River and adjacent groundwater system. In areas where estimates of river stage were higher than nearby groundwater levels, the potential for groundwater recharge from the river exists, and the orientation of groundwater-level contours were drawn to reflect surface-water recharge.

The linear interpolation described above might differ from the actual stream-stage values due to unaccounted for variations in river hydraulics such as changes in cross section, gradient, and the effects of distributary channels (such as the South Fork of the Skagit River), and tidal influence. To check the procedure, LiDAR altitudes were obtained with GIS at the points used for estimating river miles. Despite the fact that the laser is reflected and/or absorbed rather than directed back to the detector, it seems that sufficient LiDAR elevation points are picked up immediately along the banks of the river to give a good estimate of river stage along the river. Linear regression of the LiDAR data indicated a linear relation between river mile and LIDAR derived river stage of 0.99 ft/mi, corresponding to a river stage value of approximately 16.5 ft at the USGS gaging station. The computed mean gage height at the USGS gaging station for the quarterly measurement periods ranged from 16.46 to 25.63 ft. The regression analysis of LiDAR data gave a good fit to the linear interpolation of river stage with a root-mean-square error of 1.75 ft for the data points available, and supports the use of linear interpolation to estimate river stage.

http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl

http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl

Methods of Investigation 7

Groundwater-Level and Flow-Direction Maps

Of the 47 wells inventoried (fig. 3), measurements from 34 wells completed in alluvial deposits (shallow groundwater system) were used in the construction of groundwater-level and flow-direction maps, and linear-regression analysis. Groundwater level (altitude) was calculated by subtracting depth to water from the LiDAR derived ground-surface altitude at each well. Groundwater-level altitude contours and flow lines were generated manually for each of the quarterly measurement periods. Wells completed in glacial deposits or below confining clay layers within the alluvium were not included in the construction of groundwater-level and flow-direction maps or linear-regression analysis because water levels from these wells are likely influenced by hydrogeologic conditions that are distinctly different from those controlling water levels in the shallow groundwater system. Furthermore, upland areas consisting of relatively low permeability glacial deposits and bedrock were interpreted as probable shallow groundwater-flow barriers.

Water levels in well 22F02 were influenced by active pumping during all quarterly measurements, and water levels in well 26Q01 were significantly lower than surrounding wells leading to uncertainties regarding the accuracy of well-construction and lithologic information or the possibility of nearby pumping. Therefore, water-level data from these two wells were not used in the construction of groundwater-level and flow-direction maps or linear-regression analysis. Water levels in wells 08R01 and 20J01 were influenced by pumping during the August 2007 quarterly measurement and were not used for the construction of maps or regression analysis. Some wells only had partial water-level records (water levels were not available for all quarterly measurements) and were only used in the construction of maps and regression analysis for quarterly measurements in which data were available. Hydrographs of groundwater levels were generated for all inventoried wells and published in Fasser and Julich (2009).

A linear-regression analysis of groundwater altitudes in the Skagit Delta area was done to estimate the overall, time-averaged shallow groundwater-flow direction and gradient. The data used in the regression analysis were similar to those used for construction of the groundwater-altitude maps and included: (1) x and y coordinates for the wells according to the NAD 83 State Plane projection in feet for Washington (North) and (2) quarterly groundwater altitudes from the 34 wells completed in alluvial deposits.

The regression analysis was calculated using Microsoft Excel® Data Analysis tool. Each set of quarterly water-level measurements was analyzed separately, and then the seasonal

regression coefficients were combined to obtain an overall (annual) flow regime description. The groundwater altitudes (z) were input as the dependent variable and the two columns of x and y coordinates of the wells were entered as the independent variables. In this case, linear regression was used to estimate coefficients a0, a1, and a2 that describe a planar surface for the potentiometric surface as:

est 0 1 2

est

0

1

2

z =a +a *x+a *y ,

wherez is estimated elevation of the potentiometric

surface at location (x,y);a is intercept constant;a is slope of the surface in the x direction; anda is slope of the surface in the y direction.

(1)

These coefficients were then used to estimate the common hydrogeologic parameters of the groundwater regime for that quarterly measurement:

Gradient = SQRT (a12 + a2

2) is theslope of the potentiometric surface.

Direction = ATAN2 (a1, a2) is theupstream/downstream direction of the surface.

Then, the relative water level was calculated as the elevation of the plane at the geographic center of all well locations (centroid; fig. 3):

Water level at the centroid = z cent = a0 + a1 * x cent + a2 * y cent

After measurements for all four sets of quarterly measurements were processed through the regression method, an estimate of direction and gradient of the annual groundwater system was calculated by averaging the regression coefficients from each of the quarterly measurements, for example:

a0, annual = (a0, August + a0, November + a0, February + a0, May) / 4

After averaging to get the annual coefficients, the annual gradient, direction, and water level at the centroid were calculated from the annual regression coefficients.


Ocean Tides

Predicted values of tidal extremes were used in an analysis of possible correlation between groundwater-level fluctuation at monitoring well 19F01 (fig. 3) and ocean tides. The tide data were obtained from the closest National Oceanic and Atmospheric Administration tide gage (National Oceanic and Atmospheric Administration, 2009) that had calibrated tidal coefficients (tide gage 9448558 “La Conner, Swinomish Slough, Wash.”). The resulting analysis (discussed later in this report) is only approximate, because of several factors: (1) the groundwater-level data from well 19F01 also were influenced by recharge or other factors; (2) the tidal record used in the analysis is only of predicted astronomical highs and lows, not actual measured tidal levels, that may have been affected by wind and barometric pressure; (3) the location for the predicted tide is at La Conner, more than 2 mi to the south, not at the point in Swinomish Channel closest to the well, and different tidal components may travel differently along the narrow and shallow channel; and (4) high tides seem to have less influence on the well than do low tides (estimated from the correlation coefficients), although this may be caused by tidal attenuation along the Swinomish Channel.

HydrogeologyA simplified surficial geologic map (fig. 4) for the

Skagit River Delta area was constructed based on previous mapping by Schuster (2000). Geologic units were grouped into one of five new unit designations (table 1) based on similarities in lithologic and hydrogeologic characteristics. The hydrogeologic characteristics attributed to these units are similar to those defined for similar units by other investigations in areas adjacent to this study (Thomas and others, 1997; Dragovich and Grisamer, 1998; GeoEngineers, 2003). Descriptions of the units used in this study are given below.

The alluvial and recessional outwash aquifer (Qago) is present throughout the low lying areas of the Skagit River Delta and represents: (1) active or abandoned channel and overbank deposits associated with the present and ancient Skagit River, (2) lahar-runout deposits, originating from Glacier Peak, and (3) terrestrial and marine recessional and deltaic glacial-outwash deposits. This unit comprises the shallow groundwater system in the study area, and consists of sand, gravel, and cobbles, with minor lenses of silt and clay. Groundwater in this aquifer is unconfined where it is not fully saturated or exposed at land surface, however, confined conditions are likely where it is fully saturated and overlain by confining layers of clay.

The till confining unit (Qgt) is present within local upland areas and shows evidence of terrestrial and shallow marine depositional environments. This low-permeability unit is composed of glacial diamicton and consists of various proportions of clay, silt, sand, gravel, cobbles, and boulders, with locally occurring sand and gravel lenses capable of providing water for domestic use.

Exposures of advance outwash aquifer (Qga) deposits are limited to a few areas along the base of the glacial upland, and the presence of this unit is inferred, at least in places, beneath the till confining unit within upland areas. The unit consists primarily of sand and gravel with minor amounts of silt and scattered layers of pebble-cobble gravel and local silt and clay interbeds. Groundwater in this aquifer is potentially confined by the overlying till confining unit, however, unconfined conditions may occur locally where it is not fully saturated or exposed at land surface.

Generally, glacial deposits are believed to be largely absent beneath low lying areas of the Skagit River Delta to a depth of approximately 300 ft below sea level, likely due to removal by southward flowing subglacial meltwater, prior to subaerial exposure of the glacier bed during ice recession (Booth, 1994; Dragovich and others, 1994). This interpretation is supported by the absence of glacial deposits in wells from low lying areas of the delta, and similar interpretations from previous investigations (Dragovich and Grisamer, 1998; GeoEngineers, 2003). Glacial deposits in upland areas within the delta likely represent erosion remnants of a previously more continuous distribution of glacial units.

The sedimentary bedrock aquifer (OEc) crops out within and adjacent to uplands in the southern part of the study area; a lack of outcrop and well-log information makes it difficult to determine the extent of this unit in the subsurface. The unit consists primarily of pebble and cobble conglomerate and medium- to coarse-grained sandstone, with fine-grained intervals of mudstone, siltstone, coal, and shale. Groundwater in the bedrock aquifer is unconfined where it crops out; however, confined conditions are likely where it is fully saturated and overlain by glacial till and glaciolacustrine units. Fine-grained intervals within the sedimentary bedrock aquifer also may produce locally confined conditions.

The igneous and metamorphic confining unit (KJ) crops out within and adjacent to uplands in the southwestern part of the study area; a lack of outcrop and well-log information makes it difficult to determine the extent of this unit in the subsurface. This low-permeability unit is composed of igneous and metamorphic rocks and consists of a complex assemblage of volcaniclastic deposits, and low-grade metasediments, and metavolcanics. This unit is considered to be nonwater bearing except in localized areas of fracturing.

Hydrogeology 9

Figure 4. Skagit River Delta area, Washington.


3 MILES21

3 KILOMETERS21

0

0


T.35N.

T.34N.

T.33N.

R. 02 E. R. 03 E. R. 04 E.

48°21'

122°30'

25'

48°29' 122°20'25'

Sou

th

F

ork

Skagit Bay

Padilla Bay

Swinomish Channel

Ska

git

River

North Fork

Alluvial and Recessional Outwash (Aquifer)SIMPLIFIED GEOLOGIC UNITS

EXPLANATION

Advance Outwash (Aquifer)Sedimentary Bedrock (Aquifer)

Till (Confining Unit)

Igneous and Metamorphic Rocks (Confining Unit)

KJKJ KJ

KJ

KJ

KJ

Qago

Qago

Qago

Qago

Qago

Qago

Qago

Qago

Qago

Qago

Qago

Qago

Qago

Qgt

Qgt

Qgt

Qgt

Qgt

Qgt

Qgt

Qgt

Qgt

Qgt

Qgt

QgtQgt

Qgt

Qgt

Qga

Qga

Qga

OEcOEc

OEcOEc

OEc

OEc

OEc

OEc

KJ

KJ

KJ

KJ

KJ

Qago

Qgt

Qga

OEc

KJ

Water

Burlington

MountVernon

La Conner


Groundwater MovementThe direction of horizontal groundwater movement

can be inferred from maps of water-level altitude contours. Groundwater flow generally is from areas of recharge to areas of discharge, in the direction of decreasing water-level altitudes and perpendicular to the water-level altitude contours. Water-level altitude maps and regression analysis were used to determine shallow groundwater-flow directions and gradients in the Skagit River Delta area.

Groundwater-Flow Directions

Groundwater flow in the alluvial and recessional outwash aquifer generally moves in a southwestward direction away from the Skagit River and towards the Swinomish Channel and Skagit Bay (figs. 5–8). Local groundwater flow toward the river was inferred during February 2008 in an area west of Mount Vernon near well 24K01, and in an area southwest of Mount Vernon near wells 35R01 and 35R02. Water-level altitude contours in these areas indicate a south to south eastward groundwater-flow direction (fig. 7). Contours in these areas do not indicate groundwater flow to the river during the other quarterly measurement periods, and flow to the river in these areas likely is seasonal or transient in nature. Water-level altitudes in well 35R02 also were greater than nearby estimates of river stage during November 2007 and May 2008 (figs. 6 and 8; however, water-level altitude

contours near this well indicate a westward groundwater-flow direction away from the river. Water-level altitudes varied seasonally, however, generally ranged from 1 to 2 ft (August 2007, fig. 7) in the west to about 15 ft (May 2008, fig. 8) in the east. Concave and convex bends in the water-level altitude contours indicate the presence of convergent and divergent groundwater-flow patterns on a local scale. These flow patterns likely reflect variations in hydraulic conditions within the aquifer resulting from spatial variations in aquifer material properties (clay, silt, and sand). Local groundwater withdrawals and agricultural drainage systems also may influence the groundwater-flow patterns.

The fitted regression coefficients and calculated groundwater gradient, flow direction, and water level at the centroid are presented in table 2. An estimate of goodness of fit (statistical parameter R2) for the regression also is given for each seasonal estimate. The high R2 values indicate that the planar representations of the groundwater potentiometric surfaces are in good agreement with the quarterly water-level measurements. The large negative a0 (intercept) coefficients are due to the origin of the State Plane System being far to the southwest and well away from the project area.

Contours of equal values of z (time-averaged groundwater altitude for the four quarterly measurements) were calculated algebraically from the annual a0, a1, and a2 coefficients (fig. 9). Estimates of the overall (time averaged) groundwater-flow direction (8.5° south of west) and gradient (2.67 ft/mi) also were computed (table 2).

Table 1. Hydrogeologic characteristics of simplified geologic units used in this study and correlation with previous unit designations.

[Geologic units used in this report are simplified and are from Schuster (2000). Symbol: –, not applicable]

Period EpochHydrogeologic

characteristic of unitsSimplified

geologic unitsGeologic units

Qua

tern

ary

Holocene toPleistocene

Alluvial and Recessional Outwash Aquifer

Qago Qac, Qas, Qf, Qvlk, Qgom

Plei

stoc

ene

Till Confining Unit Qgt Qp, Qls, Qgdme, Qgt

Advance Outwash Aquifer Qga Qga, Qgas

Tertiary Oligocen to Eocene Sedimentary Bedrock Aquifer OEc OEcb, Ecc

Cretaceousto

Jurassic

– Igneous and Metamorphic Confining Unit

KJ KJvcf, KJmsg, KJmvg

Groundwater Movement 11

Figure 5. Water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, August 2007.


3 MILES21

3 KILOMETERS21

0

0


Well Characteristic

Well used to construct water-level map

Well with short local number and water-level altitude

Wells not used to construct water-level map

EXPLANATION

Shallow groundwater system wells

Well completed in glacial depositsWell completed below confining clayWell influenced by pumping

5.64Estimated river stage and altitude

Inferred groundwater- flow direction

Water-level altitude

August 2007All altitude measurements

are in feet above NAVD 88

T.35N.

T.34N.

T.33N.

R. 02 E. R. 03 E. R. 04 E.

48°21'

122°30'

25'

48°29' 122°20'25'

I5

I5

SR20

SR20

SR538

SR536

Sou

th

F

ork

Skagit Bay

Padilla Bay

Ska

git

River

North Fork

Swinomish Channel

09L01

09L02

19F01

19F02

27Q01

35N01

05B0105B02

01R0105R01

10A0110B01 12A0109E01

08R01

16G0114K01

16J0115R01

16P01 21A0123D01

23B01

22F01

23G0122H0122F02

20M01

21K01 24L0124K01

20J01

21M01

26D01

26K0128M01

28M02

26Q01

25N0135D01

35G01

32K01 35R0135R02

35R02

04C01

05H02

03J01

Burlington

MountVernon

La Conner

3

3

31 2 4

1110

987

65

31 2 4

11

10

98765

4.7

12.7

9.88

9.41

8.94

8.47

7.99

7.52

7.05

6.58

6.113.29

4.23 5.643.76 5.17

15.5215.05

14.58

14.11

13.64

13.17

12.23

11.29 11.7610.82

10.35

16.4615.99

1.00

5.9

5.10

0.66

2.43

9.288.14

8.31

2.81

4.69

5.65

3.35 4.978.63

9.598.64

4.731.47

3.3710.302.62

5.55

8.99

1.12

5.88

9.477.04

7.25

7.827.95

7.95

0.65

17.1815.28

-3.93 14.96-5.13

11.15

10.66

10.82-4.53

10.04

13.42

-11.42



T.35N.

T.34N.

T.33N.

48°21'

122°30'

25'

48°29' 122°20'25'

09L0109L02

19F01

19F02

27Q01

35N01

05B01

05B02

01R0105R01

10A0110B01 12A01

09E01

08R01

16G01

14K0116J01

15R0116P01

21A0123D01

23B01

22F01

23G0122H01

22F02

20M0121K01

24L01 24K0120J01

21M01

26D01

26K0128M01

28M02 26Q01 25N01

35D0135G01

32K01 35R0135R02

04C01

05H02

03J01

I5

I5

SR20

SR20

SR538

SR536

Sou

th

F

ork

Skagit Bay

Padilla Bay

Ska

git

River

North Fork

Swinomish Channel

Burlington

MountVernon

La Conner

3

2

4

12

11

10987

65

3

2

4

1211

10

9

8

7

6

5

6.4

12.8

9.89

9.31

8.738.15

7.574.07

5.24 5.82 6.98

6.98

4.66

15.71

15.13

13.97 14.5513.39

12.22

11.64

11.06

10.48

19.2118.62

20.3718.04

17.46

16.88

16.306.00

9.305.60

9.90

5.532.04

1.44

2.97

8.98

3.32

3.46

5.28

4.045.49

8.976.55

8.782.47

4.073.41

3.48

6.23

9.22

2.55 5.89 9.71

7.82

8.39

10.09

4.99

10.64

10.31

15.96

11.42

11.27

12.83 12.12

10.3411.38

13.03

-9.03

3 MILES21

3 KILOMETERS21

0

0


Well Characteristic




EXPLANATION



Estimated river stage and altitude



November 2007All altitude measurements


R. 02 E. R. 03 E. R. 04 E.

35R027.95

3

19.79

Figure 6. Water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, November 2007.



09L0109L02

19F01

19F02

27Q01

35N01

05B01

05B02

01R0105R01

10A0110B01 12A01

09E01

08R01

16G01

14K0116J01

15R0116P01

21A01 23D01

23B0122F01

23G0122H01

22F0220M01

21K01

24L01 24K01

20J01 21M01

26D01

26K0128M01

28M0226Q01

25N0135D01

35G01

32K01 35R01

35R02

04C01

05H02

03J01

T.35N.

T.34N.

T.33N.

48°21'

122°30'

25'

48°29' 122°20'25'

I5

I5

SR20

SR20

SR538

SR536

Sou

th

F

ork

Skagit Bay

Padilla Bay

Ska

git

River

North Fork

Swinomish Channel

8.99

8.19

5.29

3.40

2.82

1.44

3.44

7.58

6.21

11.96

4.21

4.766.84

7.96

6.69 8.31

12.103.04

5.42

13.30

4.24 4.42

6.45

3.948.1310.50

9.31

9.44

9.44

5.24

3.75

11.25

17.3210.22

10.36

13.57

11.01

13.03

11.22

12.02

11.88

10.66

10.26

13.42

3 4

1312111098765

14

3

413

12

11

1098

76

5

14

13.10

12.10 12.60

9.58

9.07

8.57

8.06

7.567.06

6.553.53

4.54 5.04 6.054.03 5.54

15.62

15.12

14.62

14.11

13.61

11.59

11.09

10.58

10.08

16.6317.1416.13

17.64

Burlington

MountVernon

La Conner

3 MILES21

3 KILOMETERS21

0

0


Well Characteristic




EXPLANATION



Estimated river stage and altitude3



February 2008All altitude measurements


R. 02 E. R. 03 E. R. 04 E.

35R02

6.05

Figure 7. Water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, February 2008.



09L0109L02

19F01

19F02

27Q01

35N01

05B0105B02

01R0105R01

10A0110B01 12A01

09E01

08R01

16G01

14K0116J01

15R01

16P01 21A01

23D01

23B01

22F01

23G0122H01

22F0220M01 21K01

24L01

24K0120J01

21M01

26D01

26K0128M01

28M02 26Q0125N01

35D01

35G01

32K01

35R0135R02

04C01

05H02

03J01

T.35N.

T.34N.

T.33N.

48°21'

122°30'

25'

48°29' 122°20'25'

I5

I5

SR20

SR20

SR538

SR536

Sou

th

F

ork

Skagit Bay

Padilla Bay

Ska

git

River

North Fork

Burlington

MountVernon

La Conner

3

20.5

5.13

6.59 7.32 8.79

8.79

5.86 8.06

24.1723.43

21.24

19.77

17.57

16.84

15.38

14.65

13.91

13.18

12.45

11.72

10.98

10.25

9.52

25.6322.70

21.97

19.04

18.31

16.11

24.90

3

4

13121110

987

65

14

15

2

3

4

1312

11109

87

6

5

1415

2 6.6

7.1

3.2

8.652.48

1.12

2.82

6.67

9.766.389.79

3.75

4.36

6.48

7.78

4.86 6.43

8.07

5.013.84

4.03

2.83

7.73

9.88

5.51

6.29

11.6311.68

14.33

11.27

13.53

12.1611.04

14.24

15.15

11.47

11.83

10.71

12.4514.17

13.51

18.04

Swinomish Channel

3 MILES21

3 KILOMETERS21

0

0


Well Characteristic




EXPLANATION



Estimated river stage and altitude



May 2008All altitude measurements


R. 02 E. R. 03 E. R. 04 E.

35R0214.17

Figure 8. Water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, May 2008.


The overall groundwater-flow direction derived from the regression analysis is similar to those depicted on the water-level maps; shallow groundwater flows slightly south of west from the mainstem of the Skagit River out to the Swinomish Channel and Skagit Bay. Regression analysis of the four quarterly measurements generally agrees with the annual analysis, with only slight variations in gradient or direction (table 2).

Time-averaged water levels in individual wells are always higher or lower than the fitted planar surface (fig. 9). Where measured water levels in a subarea are higher on average than the fitted plane, the potentiometric surface is diverging from the uniform planar flow direction; conversely where water levels in a subarea are lower than the plane the flow is converging. Differences in measured water levels and the plane indicate a pattern in which groundwater converges from the river meander, diverges into two preferential channel areas, converges to flow between the till outcrop uplands, and finally diverges to flow separately toward Skagit and Padilla Bays. Taking into account this final divergence, the sea level (z = 0) contour would coincide fairly well with sea level in the bays and the Swinomish Channel.

To compare the overall slope of the ground surface in the area with that of the groundwater surface, a similar regression calculation was performed on ground-surface elevation at the

quarterly monitoring well locations used in the construction of the water-level maps. Ground surface slopes were determined to be slightly steeper (3.82 ft/mi) and slightly more to the southwest (W 24.0° S) than the groundwater potentiometric surface.

The potential for vertical groundwater movement within the alluvial and recessional outwash aquifer was evaluated in areas where a comparison of water-level altitudes could be made in closely spaced wells completed above and below clay layers within the aquifer (figs. 5–8). Wells completed below clay layers are widely spaced and along the margins of the study area, however, comparisons were possible at three locations (19F01/19F02, 09L01/09L02, and 10A01/10B01). Water-level altitude differences at the first two well-pair locations indicate the potential for year-round downward flow, with higher altitudes in wells completed above clay layers for all quarterly measurements. A more complex relation was observed at the last well-pair location in which higher altitudes in wells completed above clay layers were observed during August 2007 and May 2008 measurements indicating the potential for downward flow, and lower altitudes in wells completed above clay layers during the November 2007 and February 2008 measurements indicating the potential for upward groundwater flow.

Table 2. Fitted regression coefficients and calculated hydrogeologic parameters for wells in the Skagit River Delta area, Washington, 2007 and 2008.

[Abbreviations: ft/ft, foot per foot; ft/mi, foot per mile; NAVD 88, North American Vertical Datum of 1988]

Measurement August 2007 November 2007 February 2008 May 2008 Annual

Fitted coefficients, from Microsoft Excel®

a0 (Intercept) -628.10 -589.49 -659.44 -768.90 -661.48a1 (x variable 1) 0.000456 0.000462 0.000488 0.000597 0.000501

a2 (x variable 2) 0.000116 0.000030 0.000104 0.000051 0.000075

Calculated hydrogeologic parameters

Gradient (ft/ft) 0.00047 0.00046 0.00050 0.00060 0.00051Gradient (ft/mi) 2.49 2.44 2.63 3.16 2.67

Direction W 14.3° S W 3.7° S W 12.1° S W 4.9° S W 8.5° S

Water level at centroid (feet above NAVD 88)

5.71 6.40 7.52 7.54 6.79

Goodness of fit, from Microsoft Excel®

R2 0.896 0.879 0.848 0.874



0

2

4

6

8

10

12

14

4

W8.5°S

2.67 feet per mile

09L0109L02

19F01

19F02

27Q01

35N01

05B01

05B02

01R0105R01

10A0110B01 12A01

09E01

08R01

16G01

14K0116J01

15R0116P01

21A01 23D01 23B0122F01

23G0122H0122F02

20M01

21K01

24L01 24K0120J01

21M01

26D01

26K0128M01

28M02

26Q0125N01

35D01

35G01

32K01 35R01

35R0204C01

05H02

03J01

5.89

6.90

7.462.44

1.17

2.92

7.09

6.43

10.7

9.319.83

4.55

3.76

3.85

5.82

6.854.38

9.97

7.21

9.922.33

4.46

12.43.83

3.64

6.33

2.41

2.71 8.81

7.82

9.98

5.215.02

10.45

17.1817.11

14.96

12.62

12.12

10.94 12.32

10.39

10.41

10.36

10.7413.34

Wells with partial recordWells with full record

3 MILES21

3 KILOMETERS21

0

0

T.35N.

T.34N.

T.33N.

48°21'

122°30'

25'

48°29' 122°20'25'

R. 03 E.R. 03 E.R. 02 E. R. 03 E. R. 04 E.

I5

I5

SR20

SR20

SR538

SR536

North Fork

Sou

th

F

ork

Skagit Bay

Padilla Bay

Ska

git

River

Swinomish Channel

Burlington

MountVernon

La Conner

Shallow groundwater system wells used in regression analysis


Calculated groundwater- flow direction

Well CharacteristicDerived water-level altitude, in feet above NAVD 88

Estimated Skagit River stage showing river mile and annual average water-level altitude, in feet above NAVD88

EXPLANATION35R0210.74


9

8

7

654

16

15

14

13

12

11

10

9.5

8.5

7.5

6.5

5.54.5

3.5

17.51716.5

15.5

14.5

13.5

12.511.5

10.5

10

4.01

4.585.15 5.72

6.29

6.87

7.44

8.018.58

9.15

9.73

10.30

10.87

11.44

12.02

12.59

13.16

13.7314.30

14.88

15.45

16.02

16.5917.16

17.74

18.3118.88

19.45 20.03

11.44

Figure 9. Regression analysis derived annual water-level altitude and calculated groundwater-flow direction in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, 2007 and 2008.


Groundwater-Level Fluctuations

Groundwater levels fluctuate over time, both seasonally and long term, in response to changing rates of recharge to and discharge from the groundwater system. When recharge exceeds discharge, the amount of water stored in an aquifer increases and water levels rise; when discharge exceeds recharge, groundwater storage decreases and water levels decline. Groundwater levels also may respond to changes in nearby stream stage. When stream stage exceeds nearby groundwater levels, streamflow may recharge the aquifer, causing a rise in groundwater levels; when groundwater levels exceed nearby stream stage, discharge from the aquifer to the stream may occur, resulting in a decline in groundwater levels.

Seasonal changes in groundwater levels in many of the wells in the Skagit River Delta follow a typical pattern for shallow wells in western Washington. Water levels rise in autumn and winter, when precipitation is high, and decline during spring and summer, when precipitation is low (fig. 10). Groundwater levels in wells along the eastern margin of the study area also are likely influenced by stage of the Skagit River (fig. 11). Water levels in these wells remained elevated through April, likely due to groundwater recharge from the river, and did not seem to begin to decline until the end of May in response to declining river stage (fig. 12).

Results of the regression analysis of quarterly groundwater levels, indicated by the calculated water level at the centroid (table 2), are in general agreement with the typical

pattern for shallow wells in western Washington. Regression analysis also indicates the groundwater gradient was steepest in May, likely due to elevated groundwater levels associated with Skagit River stage, and that by August the water levels along the river declined in response to declining river stage, and the gradient was reduced (table 2). The groundwater gradient increases through the winter probably in response to groundwater recharge from precipitation.

Groundwater levels also may respond to changes in nearby ocean tides. Cyclical water-level fluctuations in a well equipped with a continuous water level recorder exhibited a periodicity that is characteristic of ocean tides (for example, component wave lengths of about 24 hours, and 12 hours and 25 minutes). The well (34N/03E-19F01) is north of the town of La Conner and less than a mile east of the Swinomish Channel, a tidally influenced surface-water body that connects Skagit Bay to the south to Padilla Bay to the north (fig. 3). Water-level fluctuations in the well correspond closely to predicted tidal extremes obtained from the National Oceanic and Atmospheric Administration tide gage near La Conner (fig. 13). Through a trial-and-error correlation analysis, it was noted that water levels in the well showed similar relative high and low points, with a delay of approximately 28 hours after the tidal high or low, and an attenuation factor of about 3 percent. The peak-to-peak fluctuation in the well was about 0.3 ft compared to the tidal fluctuation of about 10 ft at the La Conner gage.

Figure 10. Water levels in well 34N/03E-28M02 and precipitation at Anacortes, Washington, October 2007 through May 2008.


Anacortes Weather StationWell 34N/03E-28M02

0

1.0

2.0

3.0

4.0

5.0

6.0

GROU

NDW

ATER

LEV

EL, I

N F

EET

ABOV

E N

AVD

88

0.5

0

1.0

1.5

2.0

2.5

3.0

3.5

4.0

PREC

IPIT

ATIO

N, I

N IN

CHES

APRMAR

2008FEBJANDECNOV

2007OCT MAY



T.35N.

T.34N.

T.33N.

48°21'

122°30'

25'

48°29' 122°20'25'

I5

I5

SR20

SR20

SR538

SR536

Sou

th

F

ork

Skagit Bay

Padilla Bay

Ska

git

River

North Fork

Swinomish Channel

Burlington

MountVernon

La Conner

3 MILES21

3 KILOMETERS21

0

0


Well outside the area of likely influence

EXPLANATION

Shallow groundwater system wellsWell completed in glacial depositsWell completed below confining clayWell influenced by pumping

R. 02 E. R. 03 E. R. 04 E.

Well with short local number likely influenced by stage on Skagit River

35R02

35R01

25N01

26D01

24K0123G01

23B0123D01

14K01

35N01

14K01

Figure 11. Wells that likely are influenced by stage on the Skagit River, Washington.


Figure 12. Water levels in well 34N/03E-23B01 and stream stage at U.S. Geological Survey streamflow-gaging station 12200500, Skagit River near Mount Vernon, Washington, October 2007 through May 2008.

Figure 13. Water levels in well 34N/03E-19F01 and nearby tidal fluctuations near La Conner, Washington, October 2007.


Gaging Station No 12200500Well 34N/03E-23B01

10.0

10.4

10.8

11.2

11.6

12.0

12.4

12.8

13.2

13.6

14.0GR

OUN

DWAT

ER L

EVEL

, IN

FEE

T AB

OVE

NAV

D 88

10

12

14

16

18

20

22

24

26

28

30

STRE

AM G

AGE

HEIG

HT, I

N F

EET

ABOV

E N

AVD

88

APRMAR

2008FEBJANDECNOV

2007OCT MAY

tac09-9722-0328 Skagit Delta_Fig 13

PRED

ICTE

D HI

GH A

ND

LOW

TID

AL R

ANGE

,IN

FEE

T AB

OVE

NAV

D 88

TRAN

SDUC

ER R

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D, IN

FEE

T(M

EAN

SEA

LEV

EL)

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

-10

-8

-6

-4

-2

0

2

4

6

262524OCTOBER 2007

232221 27

Tide PredictionWell 34N/03E-19F01


Summary and ConclusionsRecent population growth along the Interstate 5 corridor

near Mount Vernon, Washington, has led to increased domestic water use, with many new wells serving residents in the lower part of the Skagit River basin. Planning for future development in the lower basin, including the reservation of water for new domestic wells requires identification of areas where withdrawals from existing and new wells could adversely impact streamflow in the Skagit River or its tributaries. A study to characterize shallow groundwater movement in an area between the lower Skagit River and Puget Sound was conducted by the U.S. Geological Survey to assist Skagit County and the Washington Department of Ecology with the identification of areas where withdrawals from existing and new wells could have adverse impact on streamflow in the Skagit River.

Land-surface altitude in the study area ranges from near sea level adjacent to the Swinomish Channel to about 130 ft in upland areas. The shallow groundwater system consists of alluvial, lahar runout, and recessional outwash deposits composed of sand, gravel, and cobbles, with minor lenses of silt and clay. The aquifer is unconfined where it is not fully saturated or exposed at land surface, however, confined conditions are likely where it is fully saturated and overlain by confining layers of clay. Upland areas are underlain by glacial till and outwash deposits that show evidence of terrestrial and shallow marine depositional environments. Bedrock exposures are limited to a few upland outcrops in the southwestern part of the study area, and consist of metamorphic, sedimentary, and igneous rocks.

Water levels were measured in 47 wells on a quarterly basis (August 2007, November 2007, February 2008, and May 2008), and measurements from 34 wells completed in the shallow groundwater system were used in the construction of groundwater-level maps and linear-regression analysis. Estimates of Skagit River stage were used to constrain groundwater-level contours along the eastern margin of the study area. Groundwater flow in the shallow groundwater system generally moves in a southwestward direction away from the Skagit River and towards the Swinomish Channel and Skagit Bay. Local groundwater flow towards the river was inferred during February 2008 in areas west and southwest of Mount Vernon. Water levels varied seasonally, however,

generally ranged from less than 3 ft (August 2007) in the west to about 15 ft (May 2008) in the east. Concave and convex bends in the shape of water-level altitude contours indicate the presence of convergent and divergent groundwater flow patterns on a local scale, and likely reflect variations in hydraulic conditions within the aquifer resulting from spatial variations in aquifer material properties (clay, silt, and sand). The time-averaged shallow groundwater-flow direction derived from regression analysis (8.5° south of west) was similar to flow directions depicted on the quarterly water-level maps, with a gradient of 2.67 ft/mi,

Seasonal changes in groundwater levels in most of the wells in the Skagit River Delta follow a typical pattern for shallow wells in western Washington. Water levels rise in the fall and winter, when precipitation is high, and decline during the spring and summer, when precipitation is lower. Groundwater levels in wells along the eastern margin of the study area also are likely influenced by stage on the Skagit River. Water levels in these wells remained elevated through April, and did not begin to decline until the end of May, in response to declining river stage. Groundwater levels in a well equipped with a continuous water-level recorder exhibited periodic fluctuations that are characteristic of ocean tides (for example, component wave lengths of about 24 hours, and 12 hours and 25 minutes). The well (34N/03E-19F01) is less than a mile east of the Swinomish Channel (tidally influenced), and exhibited water-level fluctuations that correspond closely to predicted tidal extremes obtained from a tide gage near La Conner.

AcknowledgmentsThe authors thank the many well owners, farmers, and

irrigators for their cooperation in providing access to wells. The authors also thank Skagit County and the Washington State Department of Ecology for providing assistance in the compilation of well information, and Aquatech Well Drilling & Pumps, Inc., for constructing a monitoring well at no cost to the project.

Selected References 21

Selected References

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Dragovich, J.D., and DeOme, A.J., 2006, Geologic map of the McMurray 7.5-minute quadrangle, Skagit and Snohomish Counties, Washington: Washington Division of Geology and Earth Resources Geologic Map GM-61, 18 p., 1 sheet, scale 1:24,000.

Dragovich, J.D., Gilbertson, L.A., Norman, D.K., Anderson, Garth, and Petro, G.T., 2002, Geologic map of the Utsalady and Conway 7.5-minute quadrangles, Skagit, Snohomish, and Island Counties, Washington: Washington Division of Geology and Earth Resources Open-File Report 2002-5, 34 p., 2 sheets, scale 1:24,000.

Dragovich, J.D., and Grisamer, C.L., 1998, Quaternary stratigraphy, cross sections, and general geohydrologic potential of the Bow and Alger 7.5-minute quadrangles, western Skagit County, Washington: Washington Division of Geology and Earth Resources Open File Report 98-8, 30 p., 6 plates.

Dragovich, J.D., Pringle, P.T., and Walsh, T.J., 1994, Extent and geometry of the Mid-Holocene Osceola mudflow in the Puget Lowland—Implications for Holocene sedimentation and paleogeography: Washington State Department of Natural Resources, Washington Geology, v. 22, no. 3, p. 3-26.

Drost, B.W., 2005, Quality-assurance plan for ground-water activities: U.S. Geological Survey Open-File Report 2005-1126, 27 p., available at http://pubs.usgs.gov/of/2005/1126/

Easterbrook, D.J., 1969, Pleistocene chronology of the Puget Lowland and San Juan Islands, Washington: Geological Society of America Bulletin, v. 80, no. 11, p. 2273-2286.

Fasser, E.T., and Julich, R.J., 2009, Hydrographs showing ground-water level changes for selected wells in the lower Skagit River basin, Washington: U.S. Geological Survey Data Series 441, available at http://pubs.usgs.gov/ds/441/

GeoEngineers, 2003, Lower and upper Skagit watershed plan Samish River sub-basin, ground water hydrology evaluation: File no. 7291-001-00-1180, 175 p.

Hansen, H.P., and Mackin, J.H., 1949, A pre-Wisconsin forest succession in the Puget Lowland, Washington: American Journal of Science, v. 247, no. 12, p. 833-855.

Johnson, S.Y., 1982, Stratigraphy, sedimentology, and tectonic setting of the Eocene Chuckanut Formation, northwest Washington: Seattle, University of Washington, Ph.D. thesis, 221 p., 4 plates.

Marcus, K.L., 1981, The rocks of Bulson Creek: Eocene-Oligocene sedimentation and tectonics in the Lake McMurray area, Washington: Bellington, Western Washington University, M.S. thesis, 84 p., 1 plate.

National Oceanic and Atmospheric Administration, 2007, Climatological data, annual summary, Washington, 2007: Asheville, North Carolina, National Climatic Data Center, v. 111, no. 13, 30 p.

National Oceanic and Atmospheric Administration, 2009, NOAA tides and currents: National Oceanic and Atmospheric Administration database, accessed May 21, 2009, at http://tidesandcurrents.noaa.gov/tides08/tab2wc1b.html

Schuster, J.E., 2000, Digital 1:100,000-scale geology of Washington State: Washington Division of Geology and Earth Resources Open File Report 2005-3, digital coverage, accessed August 26, 2009, at http://www.dnr.wa.gov/ResearchScience/Topics/GeologyPublicationsLibrary/Pages/pub_ofr05-3.aspx

Tabor, R.W., 1994, Late Mesozoic and possible Early Tertiary accretion in western Washington State–The Helena-Haystack mélange and the Darrington-Devils Mountain fault zone: Geological Society of America Bulletin v. 106, no. 2, p. 217-232, 1 plate.

Thomas, B.E., Wilkinson, J.M., and Embry, S.S., 1997, The ground-water system and ground-water quality in western Snohomish County, Washington: U.S. Geological Survey Water-Resources Investigations Report 96 4312, 218 p., 9 plates.

http://pubs.usgs.gov/of/2005/1126/

http://pubs.usgs.gov/of/2005/1126/

http://pubs.usgs.gov/ds/441/

http://tidesandcurrents.noaa.gov/tides08/tab2wc1b.html

http://tidesandcurrents.noaa.gov/tides08/tab2wc1b.html

http://www.dnr.wa.gov/ResearchScience/Topics/GeologyPublicationsLibrary/Pages/pub_ofr05-3.aspx




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Shallow Groundwater Movement in the Skagit River Delta ...Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington Scientific Investigations Report 2009–5208