Say Floating Bridge ^^^
Review and Synthesis of Available
Information to Estimate Human Impacts
to Dissolved Oxygen in Hood Canal
DEPARTMENT OF
ECOLOGY
State of Washington
I
111
March 2013
Ecology Publication No. 13-03-016
EPA Publication No. 910-R-13-002
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Publication and Contact Information
This report is available on the Department of Ecology's website at
https://fortress.wa.gov/ecy/publications/SummaryPages/1303016.html
The Department of Ecology's Activity Tracker Code for this study is 11-046.
For more information contact:
Publications Coordinator
Environmental Assessment Program
P.O. Box 47600, Olympia, WA 98504-7600
Phone: (360)407-6764
Washington State Department of Ecology - www.ecy.wa.gov/
o Headquarters, Olympia (360) 407-6000
o Northwest Regional Office, Bellevue (425) 649-7000
o Southwest Regional Office, Olympia (360)407-6300
o Central Regional Office, Yakima (509) 575-2490
o Eastern Regional Office, Spokane (509) 329-3400
Cover figure: Map of Hood Canal (in Puget Sound in western Washington State).
Any use of product or firm names in this publication is for descriptive purposes only and
does not imply endorsement by the author or the Department of Ecology.
If you need this document in a format for the visually impaired, call 360-407-6764.
Persons with hearing loss can call 711 for Washington Relay Service.
Persons with a speech disability can call 877-833-6341.
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Review and Synthesis of Available
Information to Estimate Human Impacts
to Dissolved Oxygen in Hood Canal
by
Ben Cope
Office of Environmental Assessment
Environmental Protection Agency, Region 10
Seattle, Washington
Mindy Roberts
Environmental Assessment Program
Washington State Department of Ecology
Olympia, Washington
Water Resource Inventory Area (WRIA) and 8-digit Hydrologic Unit Code (HUC) numbers
for the study area:
• WRIAs: 14, 15, 16, 17
• HUCs: 17110017, 17110018, 17110019
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This page is purposely left blank
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Table of Contents
Page
List of Figures and Tables 5
Abstract 7
Acknowledgements 8
Introduction 9
Conceptual Framework 13
Oxygen and Nutrients 14
Circulation 21
Fish Kills 24
Review Scope and Approach 25
Question 1: How Much Nitrogen Do Humans and Other Sources in the Watershed
Contribute to Hood Canal? 27
Measured Forms of Nitrogen 28
Alderbrook Resort Wastewater Treatment Plant 28
Total Tributary Loadings (Natural plus Human) 29
Human Contributions within Tributary Loadings 31
Overview of Methods to Estimate Loadings from Groundwater and Shoreline
On-site Sewage Systems 33
Atmospheric Deposition (including Rainfall) 47
Summary of Watershed Loadings 47
Question 2: How Much Nitrogen Do Humans Contribute to the Surface Layer of
Hood Canal Compared to Marine Sources of Nitrogen? 51
Estimates based on Observations 51
Estimates based on Aggregated Models 52
Estimates based on ROMS Water Quality Model 57
Summary of Aggregated Model and ROMS Model Estimates of Marine Nitrogen ...58
Synthesis of Marine Flux Estimates and Limitations 58
Question 3: What Is the Impact of Human Nitrogen Contributions on Dissolved
Oxygen in Hood Canal? 63
Background- Sediment Core Study 63
Trends in Dissolved Oxygen Measurement Data 65
Oxygen Depletion Estimates Based on Aggregated Models 67
Oxygen Depletion Estimates Based on ROMS Model Predictions 76
Synthesis of Human Dissolved Oxygen Impact Estimates 80
Analysis of Episodic Fish Kill Events 80
Uncertainty 87
Overview 87
Aggregated and Three-Dimensional Marine Models 87
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Uncertainty in Nitrogen Loading Estimates 89
Factors Not Explored in Available Documents 90
Hood Canal Science Review Summary 92
Recommendations for Future Technical Work 95
References 97
Glossary, Acronyms, and Abbreviations 105
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Figures
List of Figures and Tables
Page
Figure 1: Hood Canal watershed, place names, and naming conventions used in this
document 12
Figure 2: Conceptual model of processes related to dissolved oxygen 13
Figure 3: Examples of vertical dissolved oxygen profiles at three locations in Hood
Canal in August 2004 15
Figure 4: Continuous water quality data from the Twanoh ORCA buoy in Lynch
Cove 16
Figure 5: All measured vertical profiles of oxygen, chlorophyll, and nitrate for July
2008 at the Twanoh ORCA buoy in Lynch Cove 17
Figure 6: Longitudinal pattern of minimum dissolved oxygen concentrations in Hood
Canal (near-bottom depth) 18
Figure 7: Average dissolved oxygen concentrations in Central Hood Canal 19
Figure 8: Seasonal variation in dissolved oxygen at depth at Hoodsport (near the
Great Bend) and Twanoh (Lynch Cove) 20
Figure 9: Simplified two-layer estuarine circulation 22
Figure 10: Idealized Puget Sound circulation 22
Figure 11: Observed seasonal variation of salinity with depth at Hoodsport 23
Figure 12: Factors contributing to episodic fish kills 24
Figure 13: General nitrogen sources to Hood Canal 27
Figure 14: Forms of nitrogen and relationships among variables 28
Figure 15: USGS estimates of monthly DIN loads to Hood Canal 30
Figure 16: Monthly TDN in tributaries to the main arm of Hood Canal and Lynch
Cove 31
Figure 17: Mason County shoreline sampling areas and results of targeted sampling
for nutrients in discharges with elevated fecal coliform concentrations 37
Figure 18: DIN concentration in freshwater seep samples along Mason County
shoreline areas of Hood Canal 38
Figure 19: Watershed areas defined in terms of buffer distances, unsampled area
delineations, and tributary catchments 45
Figure 20: Summary of watershed nitrogen sources (metric tons/month) 50
Figure 21: Conceptual model used to define layers 54
Figure 22: Hypothetical Lynch Cove dissolved oxygen profiles for a 3 layer flux
analysis 56
Figure 23: Range of marine nitrogen vertical flux estimates for Lynch Cove 61
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Figure 24: Sediment core hypoxia indicator and Pacific Decadal Oscillation indicator
for the period 1600-2005 64
Figure 25: Statistically significant trends (seasonal Kendall) in dissolved oxygen at
five locations in Puget Sound and the Strait of Juan de Fuca 66
Figure 26: Lynch Cove impacts based on Monte Carlo analysis 74
Figure 27: Sensitivity of impact estimates to marine flux term 75
Figure 28: Comparison of predicted and measured dissolved oxygen conditions at
Hoodsport for 2006 77
Figure 29: Water quality model prediction of surface oxygen during the 2006 fish kill..81
Tables
Table 1: Source documents on Hood Canal reviewed under this synthesis 11
Table 2: Tributary loads to Hood Canal and Lynch Cove by source 32
Table 3: Available estimates for flow, nitrogen concentration, and nitrogen loading
for groundwater discharges to marine waters 39
Table 4: Best estimates of range of seep DIN concentrations, groundwater flow, and
shoreline groundwater loadings for Lynch Cove 40
Table 5: Estimated shoreline OSS nitrogen contributions based on per capita
calculations 44
Table 6: Assumptions used in per capita estimation of summer shoreline OSS loading
to Lynch Cove 44
Table 7: Watershed nitrogen loads to Hood Canal and Lynch Cove in available
studies 48
Table 8: Potential range of watershed nitrogen loads to Lynch Cove in summer
(June through September) based on review and synthesis of available
studies 49
Table 9: Estimates for nitrogen loads to the full euphotic zone and surface layer of
Hood Canal and Lynch Cove 60
Table 10: Potential range of marine nitrogen loading to surface water in Lynch Cove
and relative contribution of human loadings. Synthesis of available
information for 2005-2006 summer conditions 62
Table 11: Estimates of OSS impacts on dissolved oxygen in Lynch Cove in summer. ...71
Table 12: Assumed distributions for the uncertainty analysis 73
Table 13: ROMS model predictions for average dissolved oxygen impact in Lynch
Cove at depths greater than 10 meters 79
Table 14: Comparison of predicted human impact on average summer1 dissolved
oxygen concentration in Lynch Cove below the euphotic zone2 80
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Abstract
The U.S. Environmental Protection Agency and the Washington State Department of Ecology
reviewed available science regarding human impacts to dissolved oxygen in Hood Canal at the
request of the Hood Canal Coordinating Council. We conclude that human nitrogen loadings are
not contributing substantially to low dissolved oxygen in Central Hood Canal, including the
Hoodsport region where episodic fish kills have occurred. Episodic fish kills in this area are
caused by a cascade of natural circulation and weather events.
Sediment cores indicate that oxygen levels in Central Hood Canal were lower before 1900 than
between 1900 to 2005, contrary to the pattern expected if humans are the dominant influence.
While measured oxygen levels indicate a decline in Central Hood Canal between the 1950s and
2000s, this pattern also occurs in the Strait of Juan de Fuca and elsewhere in Puget Sound.
Lynch Cove, where the relative influence of humans is highest, has increasing oxygen levels.
Historical oxygen levels are consistent with climate cycles.
Researchers concur that:
1. Marine water upwelling delivers most of the nitrogen to the surface waters.
2. Shoreline on-site sewage systems represent the dominant human source of nitrogen.
3. Human impacts are highest in Lynch Cove.
We could not determine conclusively whether human nitrogen loadings cause Lynch Cove
dissolved oxygen levels to violate water quality standards. While we estimate that humans
contribute 0.03 to 0.3 mg/L of oxygen depletion in Lynch Cove, the methodology used by
researchers was problematic and the wide range of marine nitrogen flux estimates led to high
uncertainty in the relative human contribution.
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Acknowledgements
The U.S. Environmental Protection Agency (EPA) and the Washington State Department of
Ecology (Ecology) appreciate the assistance we received from the following individuals and
organizations during the development of this document:
• Joel Baker, Puget Sound Institute
• Corrine Bassin, University of Washington
• Jill Brandenberger, Pacific Northwest National Laboratory
• Mike Brett, University of Washington
• Michael Cox, EPA
• Al Devol, University of Washington
• John Eliasson, Washington Department of Health
• Amy Georgeson, Mason County
• Keith Grellner, Kitsap County
• Julie Horowitz, Hood Canal Coordinating Council
• Andy James, Puget Sound Institute
• Mitsuhiro Kawase, University of Washington
• Christopher Krembs, Ecology
• Rochelle Labiosa, EPA
• John Mickett, University of Washington
• Jan Newton, University of Washington
• Tony Paulson, U.S. Geological Survey
• Greg Pelletier, Ecology
• Jeff Richey, University of Washington
• Brandon Sackmann, Ecology
• Rich Sheibley, U.S. Geological Survey
• Mark Warner, University of Washington
Independent Review Panel
• Alexandria Boehm, Stanford University
• Paul Harrison, University of British Columbia
• James O'Donnell, University of Connecticut
• Hans Paerl, University of North Carolina
• Harvey Seim, University of North Carolina
• Ivan Valiela, Marine Biological Laboratory, Woods Hole, Massachusetts
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Introduction
Hood Canal is a long, deep fjord-like waterbody in Puget Sound with relatively low human
development in the surrounding watershed. The tidal exchange between Hood Canal and
Admiralty Inlet is small relative to the overall depth and volume of the canal, and a sill at the
north end of Hood Canal restricts circulation. Because of these characteristics, low dissolved
oxygen (hypoxia) is a natural condition in the deep waters of Hood Canal.
Fish and other aquatic organisms can be acutely and chronically impacted by the depletion of
dissolved oxygen. Periodic fish kills in Hood Canal have been attributed to low dissolved
oxygen conditions. Because of the severity of the natural hypoxia, it is important that we
understand whether human impacts, even if relatively small, are exacerbating the impacts to fish
and other aquatic life. The Washington State water quality standards include provisions to
minimize human-caused impacts to dissolved oxygen in waters with naturally low dissolved
oxygen such as Hood Canal. The standards require that human impacts be restricted to an
impact of less than 0.2 mg/L (interpreted as a non-detectable change from natural conditions)
any place in the water column and at any time of year or day.
A key question for the U.S. Environmental Protection Agency (EPA), the Washington State
Department of Ecology (Ecology), and the public is "What is the human contribution to the low
dissolved oxygen in Hood Canal?" This is a difficult question to answer. An investigation of
this question involves analysis of watershed conditions, population and land use development,
inputs from septic systems, processes that affect pollutant transport, and flushing and
productivity in Hood Canal. This review centers around the introduction of human-caused
nitrogen loadings to the surface waters of Hood Canal. Nitrogen releases can alter the ecosystem
condition in a number of ways, but this assessment is narrowly focused on the effects of nitrogen
pollution on dissolved oxygen conditions. This report is organized around three sequential
questions:
1. How much nitrogen do humans and other sources in the watershed contribute to Hood Canal?
2. How much nitrogen do humans contribute to the surface layer of Hood Canal compared to
marine sources of nitrogen?
3. What is the impact of human nitrogen contributions on dissolved oxygen in Hood Canal?
The first question provides the foundation for estimating the potential human impacts to
dissolved oxygen. The second question provides a gross estimate of the relative significance of
human contributions and natural conditions on phytoplankton growth and dissolved oxygen
depletion. The third question directly addresses concerns about low dissolved oxygen and fish
kill events. It is also the most complicated to answer.
The purpose of this document is to review available scientific information and provide the best
estimates of the current human impact to dissolved oxygen levels in Hood Canal. Several
regulatory options described in Baldi and Eaton (2012) require an assessment as to whether
human contributions are meeting, nearing, or violating state water quality standards. We
consider uncertainty a fundamental part of the assessment, but we also recognize the need to
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move forward with management actions with the available information. The goals of this
document are to (1) accurately describe the extensive research to date in a condensed document,
and (2) provide "best estimates" of human impacts to dissolved oxygen based on all available
information and our best professional judgment. In some instances, we derive these estimates
from methods and datasets that have not been integrated to date.
This review was initiated by a request from the Hood Canal Coordinating Council to support the
development of the Aquatic Rehabilitation Action Plan (Brewer, 2011). We reviewed the
information available through January 2013, which included journal articles, published technical
reports, and draft chapters available through the Hood Canal Dissolved Oxygen Program
(HCDOP) web site (http://hoodcanal.washington.edu/news-docs/publications.jsp). We received
additional information from the lead authors of the referenced studies who reviewed various
drafts of this report. We held multiple meetings with researchers in 2011 and 2012 to discuss
findings and areas of agreement and disagreement.
Some of the studies reviewed in this report are papers that have been published in peer-reviewed
journals or technical reports, while others had not been peer reviewed prior to the development
of this report. This has led to a parallel development of this report and some of the underlying
studies. In particular, several chapters of the HCDOP report were released for the first time
during the EPA/Ecology review, and other chapters were revised during the development of this
report.
Recognizing the importance, complexity, and interdisciplinary aspects of the scientific questions
under review, EPA and Ecology requested that the Puget Sound Institute oversee an additional
round of review from an independent panel of experts in early 2012. The panel review was
guided by specific charge questions that focused on differences in methodology or interpretation
among the researchers. The panel identified a number of important issues in the analyses to date
(PSI, 2012). The independent panel report was shared with all researchers, who provided
feedback. We emphasize that peer review generally leads to better scientific products, but it
does not always lead to consensus. Some of the findings in this report are subjects of ongoing
debate among the researchers.
After the independent review, EPA and Ecology revised the draft report and released it for public
comment. We reviewed the comments and made revisions to this report. There was no
information in the comments that changed the conclusions of the draft report, but a number of
important suggestions and clarifications were provided in the comments that improved the final
report.
We begin this summary of available science with a conceptual framework of the important
system components, including a description of the problem. In turn we address each of the three
questions described above, based on the best available published information listed in Table 1
below. Finally, we address uncertainty, identify factors not explicitly addressed in currently
available information, summarize broader findings, and recommend topics for future technical
work that would reduce the uncertainty in available information.
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Table 1: Source documents on Hood Canal reviewed under this synthesis.
See References section for complete citations.
Study Topic
Analytical Method
Information Reviewed
Question 1:
Human Nitrogen
Contributions
Water Quality
Monitoring
Statistical Loading
Models
Embrey and Inkpen (1998).
Simonds, B., Sheibley, R., Rosenberry, D., Reich, C., and
Paulson, A. (2008).
Georgeson, A., Mathews, W., and Orth, P. (2008).
Sheibley and Paulson (2011).
James, A. (201 la).
Paulson, A, Konrad, C., Frans, L., Noble, M., Kendall, C.,
Josberger, E., Huffman, R., and Olsen, T. (2006).
Steinberg, P., Brett, M., Bechtold, I, Richey, J., Porensky L., and
Osborne, S. (2010).
Richey, J., Brett, M., Steinberg, P., Bechtold, J., Porensky L.,
Osborne, S., Constans, M., Hannafious, D., and Sheibley, R. (2010).
Question 2:
Human vs Marine
Nitrogen
Contributions
Current Meter Data
Analysis
Estuarine Aggregated
Models
Paulson, A, Konrad, C., Frans, L., Noble, M., Kendall, C.,
Josberger, E., Huffman, R., and Olsen, T. (2006).
Steinberg, P., Brett, M., Bechtold, J., Richey, J., Porensky L., and
Osborne, S. (2010).
Devol, A., Newton, J., Bassin, C., Banas, N., Kawase, M., Ruef, W,
Bahng, B., and Warner, M. (201 la).
Questions:
Human Impacts on
Dissolved Oxygen
Sediment Core Analysis
Water Quality
Monitoring
Aggregated Models
Water Quality Model of
Hood Canal
Brandenberger, J.M., E.A. Crecelius, P. Louchouarn, S.R. Cooper,
K. McDougall, E. Leopold, and G. Liu. (2008).
Brandenberger, J.M., P. Louchouarn, and E.A. Crecelius (2011).
Bassin, C.J., Mickett, J.B., Newton, J.A., and Warner, M.J. (2011).
Warner, M. (201 la).
Mickett, J., Alford, M., Newton, J., and Devol, A. (2011).
Devol, A., Newton, J., Bassin, C., Banas, N., Kawase, M., Ruef, W,
Bahng, B., and Warner, M. (2011).
Brett (20 lOa).
Kawase and Bahng (2010).
Kawase and Bahng (2012).
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Studies compiled for this assessment have used different naming conventions for the sub-regions
within Hood Canal. In this review, "Hood Canal" means the entire waterbody and/or watershed
south of the sill near Poulsbo on Figure 1. South of the sill, we distinguish between the main
arm of the Canal as "Central Hood Canal" (Poulsbo to Potlatch), a transition area called the
"Great Bend" (Potlatch to Sisters Point), and "Lynch Cove" (East of Sisters Point). Many
documents refer to the area east of Sisters Point as Lower Hood Canal and consider Lynch Cove
only the far eastern extent of that area.
Hood Canal
River Basin
Figure 1: Hood Canal watershed, place names, and naming conventions used in this document.
Source: Adapted from graphic on HCDOP Website (www. hoodcanal. Washington, edit).
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Conceptual Framework
All of the researchers studying Hood Canal hypoxia developed similar conceptual models of the
system and key attributes related to oxygen. While developed for studies of hypoxia in the Gulf
of Mexico, Figure 2 captures the critical physical, chemical, and biological processes that govern
dissolved oxygen in Hood Canal as well.
nutrients (N, P, Si)
sediments &
organic carbo
healthy ber.ttiie community
(worms, sn.
Euphotic Zone
Organic matter decomposes &
consumes oxygen
Marine nitrogen
mixed into surface
layer
Sediment oxygen demand
and nutrient flux
Figure 2: Conceptual model of processes related to dissolved oxygen.
Source: Adapted from Downing JA et al. Gulf of Mexico hypoxia: land and sea interactions.
Task force report no. 134. Ames, IA: Council for Agricultural Science and Technology, 1999.
Two black double-arrows added to the figure represent important processes in Hood Canal. The
top arrow highlights the estuarine mixing of nitrogen-rich water from deeper waters into the
surface layer, a natural mixing process that provides nutrients for phytoplankton in the surface
layer. The second arrow highlights the processes occurring at the sediment-water interface,
where enriched sediments exert an oxygen demand on the overlying water and release dissolved
nutrients under low oxygen conditions. A full understanding of human impacts on oxygen must
consider the effects of these processes.
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Oxygen and Nutrients
Dissolved oxygen conditions in Hood Canal have been monitored extensively by the Washington
State Department of Ecology and the University of Washington over the past several decades.
The Hood Canal Dissolved Oxygen Program (HCDOP) began an expanded monitoring effort in
2005. Roberts et al. (2005) described the initial 2005 monitoring program components. Funded
by the U.S. Navy and led by the University of Washington, HCDOP significantly increased the
resources focused on conditions in Hood Canal. HCDOP included several monitoring and
modeling programs:
• Marine water quality monitoring (4 ORCA buoys, cruises)
• Flow and water quality in freshwater streams and rivers
• Biotic impacts of low dissolved oxygen
• Phytoplankton productivity sampling and analysis
• Watershed models and other analyses
• Marine models and other analyses
In addition to these activities funded through HCDOP, several other research efforts addressed
conditions in Hood Canal, including:
• Sediment cores (Pacific Northwest National Laboratory, Texas A&M University, University
of Washington, USGS, and Bryn Athens College)
• Terrestrial, marine, and groundwater nitrogen loads to Hood Canal (USGS)
• Groundwater monitoring within the Hood Canal watershed (Mason County, Kitsap County,
and USGS)
The resulting body of information for the 2005-2009 intensive-monitoring period provides
insight into the spatial and temporal variation in oxygen concentrations, along with current
nitrogen loadings to Hood Canal.
We present several figures to illustrate the types of monitoring information and some of the
spatial and temporal patterns of dissolved oxygen in Hood Canal (Figures 3 through 8).
Dissolved oxygen levels deeper than 10 meters below the surface in Hood Canal and Lynch
Cove are perilously low for fish in the late summer and fall, with a sharp difference between the
surface mixed layer and the very low oxygen levels in the lower layer (Figure 3). Concentrations
below 1 mg/L can be lethal to some species (EPA, 2000), and Newton et al. (201 Ic) describes
potential chronic and acute effects of low dissolved oxygen on Hood Canal biota. Oxygen levels
fluctuate seasonally throughout the water column (Figures 4 and 5). The magnitude of low
dissolved oxygen levels varies spatially (Figure 6), and the volume-weighted concentrations
exhibit large interannual variability (Figures 7 and 8).
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DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER
i 6
II
—*— Lynch Cove-L19
--*--The Great Bend-Li3
-CD— Sisters Point-L14
Figure 3: Examples of vertical dissolved oxygen profiles at three locations in Hood Canal in
August 2004.
Source: USGS (Paulson et al, 2006).
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f „ -w^r
t
Temperature ( C)
Jan Rib
Salinity (psu)
Nitrate (umol)
11
It
Mjy
Aug Sap Oa Mm OK
Zl
I,
Figure 4: Continuous water quality data from the Twanoh ORCA buoy in Lynch Cove.
Source: Devol et al. (201 Ib).
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-10
-15
-20
-25
•30
0 200 400 0
Oxygen (umol/kg)
50
100 0
Chlorophyll (mg/md)
20 40
Nitrate (umol)
Figure 5: All measured vertical profiles of oxygen, chlorophyll, and nitrate for July 2008 at the
Twanoh ORCA buoy in Lynch Cove.
Source: Newton et al. (2011 a).
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Measured and Interpolated O2
August 2006 Oxygen
123'icncrw
123 'C<0"VV
122'5Q'0"W
024 8 Kjiornolori
Figure 6: Longitudinal pattern of minimum dissolved oxygen concentrations in Hood Canal
(near-bottom depth).
Source: Newton et al. (201 Ib).
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Average Dissolved Oxygen - Depth>20 m
B.QOQ
7.000
2.000
90 120 ISO 180 210 240
Day of Year
270 300 330 360
+ 19S2-3 —*—1954 • 195S * 19S6 * 19S7 » 1958 1959 • 1960 • 1961
« 1962 * 1963 * 196S —•-1966 • I99fl * 1999 • 2000 • 2001 2002
2003 • 2004 ES 2005 * 2006 a 2007 X 200B a 2009 A 2010 —*— 2011
Figure 7: Average dissolved oxygen concentrations in Central Hood Canal.
Aggregate of six stations, depth > 20 meters. Source: Warner (2011 a).
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Twaii
Hoodsport
Delta
Figure 8: Seasonal variation in dissolved oxygen at depth at Hoodsport (near the Great Bend)
and Twanoh (Lynch Cove).
Source: Brett (pers. comm., 20lie).
Low levels of oxygen result from the complex interaction of physical, chemical, and biological
processes. The physical characteristics of Hood Canal (length, depth, and sill-restricted
circulation) contribute to naturally low oxygen levels at depth. Human activities that release
nutrients into the Canal can further deplete oxygen concentrations. The relative nutrient loading
associated with humans, compared with the loading from natural sources, will define the
magnitude of human impact on oxygen concentrations.
Oxygen depletion occurs as a result of organic matter decomposition, and phytoplankton
represent a major source of organic matter. Phytoplankton, like other plants, require sunlight and
nutrients to grow. These nutrients include both nitrogen and phosphorus. Phytoplankton require
more nitrogen than phosphorus as indicated by the Redfield ratio for phytoplankton composition
(carbon:nitrogen:phosphorus = 106:16:1). In most rivers and lakes, the availability of
phosphorus in the photic zone controls the growth of phytoplankton. Because of the relative
abundance of phosphorus in marine environments, phytoplankton in estuarine and marine waters
are generally growth-limited by the supply of nitrogen (primarily nitrate and ammonium).
In the spring and summer, nitrogen concentrations decline in the surface layer as available light
increases and phytoplankton consume nutrients. This productivity often depletes nitrogen before
phosphorus in the surface layer as phytoplankton consume the available inorganic nitrogen (see
Figure 5 for examples of observations of the decline in surface nitrate in Lynch Cove). The
nitrate concentrations in surface waters are substantially lower than those at deeper levels during
strong phytoplankton growth. Differences in density due to variations in salinity and
temperature through the water column lead to stratification (increased salinity and density of
water with increased depth), which limits the amount of bottom-water inorganic nitrogen that
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replenishes the surface layer. In this circumstance, when the natural nitrogen supply is limiting
growth, any human-caused nitrogen released to the euphotic zone will contribute to higher-than-
natural phytoplankton levels. HCDOP concluded from productivity tests that nitrogen supply is
the critical factor affecting phytoplankton growth in Hood Canal (Newton et al., 2012), which is
typical for marine environments.
Paulson et al. (2006) also noted that internal recycling of dissolved inorganic nitrogen (DIN)
appears to influence water column concentrations. They reported that DIN concentrations in
Lynch Cove at depth (420 ug/L) were significantly higher than concentrations at the entrance of
Hood Canal (280 ug/L). They attributed the difference (140 ug/L) to internal recycling within
Hood Canal.
Paulson et al. (2006) also examined water quality samples and conducted isotopic analysis of
water samples from various locations in Hood Canal and Lynch Cove. USGS found a similar
isotopic signature in upper layer organic matter and lower layer nitrate, and they surmised that
nutrient-rich, saline bottom water was largely responsible for sustaining the productivity of the
phytoplankton in the upper layer.
Circulation
Figure 9 shows a generalized depiction of the two-layer circulation pattern that is common in
estuaries. At the boundary between the fresher surface layer and saltier bottom layer, the out-
flowing freshwater entrains some of the bottom water, and this transports nitrogen to the surface
layer. The data for Hood Canal indicate a similar overall pattern, and this process is noted in the
science primer offered on the HCDOP website (Newton, undated).
Figure 10 shows the more complex patterns in parts of Puget Sound from the Strait of Juan de
Fuca moving south through Central and South Puget Sound. Consistent with the idealized
circulation in Figure 9, freshwater generally moves seaward to the Strait of Juan de Fuca from
the inland bays, and marine water moves into the Sound at depth from the sea. The pattern of
shallow sills and deeper basins contributes to variable mixing between layers.
Other factors play an important role in mixing. Tidal energy affects mixing in the interior of
Hood Canal, entraining nutrients into the surface layer. Tidal mixing may be significant in
Lynch Cove, where the mean tidal range (2-3 meters) is a significant fraction of the mean depth
(18 meters) (Brett, personal communication, 2012). Wind also plays a role in mixing and
entrainment of bottom-water nutrients in the interior bays. Finally, circulation in Hood Canal
shows strong seasonal variability, including an intrusion of denser, low-oxygen marine water at
depth in August and September (see Figure 11). This variability strongly influences the oxygen
distribution.
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Figure 9: Simplified two-layer estuarine circulation.
KILOMETERS ALONG CHANNEL
100
I I I I
200
300 -
Figure 10: Idealized Puget Sound circulation.
Letters refer to regions in the source publication. Source: Ebbesmeyer et al. (1984).
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01/06
04/06
07/06
10/06
10
n
30
28
-26
01/06
04/06
07/06
10/06
Figure 11: Observed seasonal variation of salinity with depth at Hoodsport.
Fall ocean water intrusion appears as higher-density (psu) water. Source: Kawase andBahng (2010).
HCDOP researchers also identified a previously unrecognized circulation pattern in the
Hoodsport area. Based on current velocity measurements recorded near the Hoodsport ORCA
buoy, Mickett et al. (2011) identified a subsurface seaward outflow near the middle of the
channel that appears in August/September and coincides with periods of low dissolved oxygen.
The source of this low dissolved oxygen water has not been isolated; the data used to identify the
phenomenon bound the source at less than 50-meter water depths landward of Hoodsport but
cannot resolve the source further. This seaward outflow adds complexity to circulation patterns
around Hoodsport and is discussed further in Question 3.
While this review focuses on potential impacts associated with human-caused nitrogen releases,
human development can also alter circulation patterns in an estuary or fjord. Circulation, in turn,
can alter dissolved oxygen directly by changing advective transport of oxygenated water or
indirectly by altering biological processes that affect dissolved oxygen. These circulation
influences are discussed only briefly, because studies to date have not assessed dissolved oxygen
impacts associated with changes in circulation (see discussion of factors that have not been
explored to date in the Uncertainty section).
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Fish Kills
While low dissolved oxygen is a concern in many areas of Puget Sound, Hood Canal is a primary
location of interest because of multiple fish kill events that were attributed to extremely low
dissolved oxygen levels. These events have generally occurred in September on the
southwestern shore of Central Hood Canal from Lilliwaup to Potlatch, which is substantially
deeper than Lynch Cove.
HCDOP researchers successfully identified the mechanisms leading to major episodic fish kills.
Dense marine water enters Hood Canal and pushes water with low oxygen levels toward the
water surface. As river inflows decline during the dry season, the freshwater cap at the surface,
which normally prevents the denser water from surfacing, becomes thinner. Southwest wind
events push this thin cap to the north, which allows low-oxygen water beneath it to surface
rapidly (Figure 12). In a matter of hours, oxygen levels rapidly decrease in the sensitive
nearshore regions of southwestern Hood Canal, which causes the fish kills (see Newton et al.,
201 Ic and Kawase and Bahng, 2011). The proximate cause of the episodic fish kills is a natural
event initiated by wind.
Wind
"Cap" of Higher DO at Surface
Low Oxygen
Water
Fall Intrusion
Centra! Hood Canal
Lynch Cove
Figure 12: Factors contributing to episodic fish kills.
Page 24
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In addition to episodic fish kills, low dissolved oxygen can significantly impact aquatic biota
and habitat over a large area. Dissolved oxygen is extremely low at depth in Hood Canal and
Lynch Cove in the late summer and early fall. Diver video surveys by the Skokomish Tribe and
others have documented the presence of bacterial mats and dead crab in the Lynch Cove area
(Newton et al., 201 la).
The human influence on fish kills is defined by the impact humans are having on the already-low
oxygen levels in various parts of Hood Canal. Later in this report (see Question 3), we
summarize and interpret the available estimates for these human impacts.
Review Scope and Approach
With this background, we pose the three questions related to human impacts on hypoxia in Hood
Canal, provide answers based on available information, and describe the uncertainties around
those answers. The sections below balance the need to communicate key components of a
complicated system with the need to remain concise. Each source of information mentioned
briefly in this document delves into far greater detail than represented in this summary
document.
This report includes both qualitative discussions of dominant processes and quantitative analyses
of impacts. In summary tables, numeric estimates are expressed as single values or ranges.
These are "best estimates" from a particular analysis. Substantial uncertainty surrounds many of
these estimates. Each section of the report includes qualitative discussions of key areas of
uncertainty related to data limitations, system complexity, and analytical methodology. We have
also constructed a quantitative, probabilistic analysis of uncertainty in the estimates of human
impact in Lynch Cove using simple aggregated models.
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This page is purposely left blank
Page 26
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Question 1: How Much Nitrogen Do Humans and
Other Sources in the Watershed Contribute to
Hood Canal?
This first question requires a variety of information sources such as measurements, land cover,
models, literature, and assumptions to estimate nitrogen releases to Hood Canal from the
surrounding watershed. Subsections summarize inputs from a wastewater treatment plant
(Alderbrook Resort), tributaries (watershed), groundwater and shoreline on-site sewage systems
(OSS), and atmospheric deposition (see Figure 13).
Atmospheric
Deposition
Marine Upwelling
Background
Figure 13: General nitrogen sources to Hood Canal.
Page 27
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Measured Forms of Nitrogen
Researchers estimate Hood Canal nitrogen loading in a variety of forms. Figure 14 presents the
definitions and relationships among the forms used in different source documents.
Total Nitrogen
Total Dissolved Nitrogen
Dissolved Inorganic Nitrogen
Dissolved Organic
Nitrogen
Particulate Organic
Nitrogen
_i
Nitrate • Nitrite H Ammonium
Total Kjeldahl Nitrogen
Figure 14: Forms of nitrogen and relationships among variables.
In this review, we compile nitrogen values in units reported in the Hood Canal source
documents. Some studies focus on dissolved inorganic nitrogen (DIN) as the predominant and
biologically available form of nitrogen. Because DIN estimates do not include the dissolved
organic fraction of nitrogen (DON), the use of DIN values will lead to lower loading estimates
than analysis of total dissolved nitrogen (TDN, equal to DIN plus DON). However, best
available information suggests that DON is a minor fraction in annual loading (Mohamedali
et al., 2011), although it may be more significant in freshwater during individual storm events
(Ward etal., 2012).
Alderbrook Resort Wastewater Treatment Plant
The only permitted wastewater discharge to the marine waters of Hood Canal is the Alderbrook
Resort. The National Pollutant Discharge Elimination System (NPDES) permit requires
monitoring of nitrogen concentrations (nitrate plus nitrite, ammonium, and total Kjeldahl
nitrogen). The plant conducted monthly monitoring from February 2009 through January 2010
then decreased frequency to quarterly monitoring (Ecology, 2008).
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Paulson et al. (2006) quantified this source as 1 metric ton (MT) per year DIN based on typical,
but not plant-specific, wastewater concentrations. Paulson et al. (2006) also cited a previous
estimate of 0.17 to 1.8 MT/yr by Fagergren et al. (2004), also based on typical wastewater
treatment plant effluent concentrations. No other published information quantified this source of
nitrogen to Hood Canal. However, based on data available from the Washington State Permit
and Reporting Information System (PARIS), we estimate that the Alderbrook Resort produces
0.13 MT/yr of nitrogen (0.011 mgd, 8.9 mg/L total nitrogen). This concentration is lower than
that produced by large plants in the Puget Sound region but typical of smaller plants that achieve
greater nitrogen reduction as a result of high plant capacity or operational factors (Mohamedali et
al., 2011).
Total Tributary Loadings (Natural plus Human)
Prior to the creation of the HCDOP, a study of nutrient loadings to Puget Sound was conducted
by USGS (Embrey and Inkpen, 1998). While not focused on Hood Canal, this study analyzed
monitoring data for the larger tributaries to the Canal, including the Skokomish, Hamma
Hamma, Dewatto, Dosewallips, and Duckabush Rivers. The DIN concentrations in Hood Canal
rivers ranked among the lowest in the Puget Sound tributaries analyzed in the study. The study
was focused on total loadings rather than the human-caused fraction of loadings. Embrey and
Inkpen (1998) estimate 304 MT/yr of total nitrogen to Hood Canal from major rivers. Load
estimates did not account for the smaller watershed areas adjacent to Hood Canal.
USGS (Paulson et al., 2006) summarized available data between 1959 and 2002 for DIN
concentrations in major tributaries to Hood Canal. The median concentrations ranged from
40 ug/L (Duckabush River) to 370 ug/L (Union River). The Skokomish, the largest and most
frequently sampled river, contained DIN ranging from non-detectable to 720 ug/L, with a median
value of 90 ug/L. Monthly DIN loads for monitored and un-monitored subbasins were
calculated using mean streamflow, the distribution of the monthly streamflow over the annual
cycle, and the mathematical relationship between DIN concentration and streamflow. The
annual DIN load from tributaries to Central Hood Canal (south of the Hood Canal bridge) and
Lynch Cove (east of Sisters Point) combined was an estimated 493 MT/yr. Monthly totals are
shown in Figure 15 below, along with estimated regional groundwater and atmospheric loads.
Paulson et al. (2006) did not estimate the human contribution within the total tributary loadings.
More recently, Steinberg et al. (2010) used two years (2005-2006) of monthly data collected by
HCDOP from 43 Hood Canal tributaries; these analyses were also presented in Richey et al.
(2010). Steinberg et al. (2010) directly calculated the nitrogen loadings entering Hood Canal
from monitoring data at the mouths of tributaries. They also constructed statistical models to
estimate the fraction of the measured tributary loading that is attributable to natural contributions
(conifer forest) and human contributions (OSS, other residential activities, and a portion of alder
forests) in each sampled watershed. The statistical model did not change the total loads, which
were calculated from monitoring data, but apportioned the total among different sources as
described under Question 2.
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« 120
e
o
t 100
§
40
Q
I
P
• Atmospheric input
I I Shallow subsurface flow
CH Regional ground water
I I Surface water
OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT
MONTH
Figure6. Monthly loadings from four sources of dissolved inorganic nitrogen to the surface waters of Hood Canal, western Washington.
1971-2002
Figure 15: USGS estimates of monthly DIN loads to Hood Canal.
Source: USGS (Paulson et al, 2006).
Steinberg et al. (2010) reported that approximately 700 MT/yr of TDN enter Hood Canal from
tributaries. This is higher than the value in Paulson et al. (2006), which calculated DIN loading.
Steinberg et al. (2010) estimated that 21-34% of the total nitrogen from tributaries was dissolved
organic nitrogen (147 - 238 MT/yr). This indicates that the Paulson et al. (2006) estimate of
493 MT/yr of DIN is generally consistent with the later estimates of 700 MT/yr of TDN by
Steinberg et al. (2010) and reported in Richey et al. (2010).
Figure 16 shows the measured flows and concentrations, and calculated loadings, from Steinberg
et al. (2010) and Richey et al. (2010). The figure also distinguishes the source contributions
based on the statistical model. In the upper left panel of the figure, the low nitrogen
concentrations in the mainstem (central) Hood Canal align with the conclusion of Embrey and
Inkpen (1998) that the rivers along Central Hood Canal carry some of the lowest concentrations
in Puget Sound. The figure also shows that TDN concentrations in Lynch Cove tributaries are
2 to 3 times higher than concentrations in tributaries to Central Hood Canal, and these TDN
concentrations are more typical of watersheds with greater development levels (Mohamedali
et al., 2011). The greater level of human activity, including OSS, other residential development,
and a portion of red alders in the Lynch Cove area has contributed to the increased nitrogen
concentrations in local tributaries.
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Maiustem Hood Canal
250
200-
150-
100
50
0
400 700
600-
500-
400-
300-
200-
100-
0 0
Lynch Cove
-300
-200
-100
TDX
Flow
-10
- 8
- 6
- 4
- 2
0
,175-
16-
Figure 16: Monthly TDN in tributaries to the main arm of Hood Canal and Lynch Cove.
Source: Steinberg et al. (2010) and Rickey et al. (2010).
There is significant seasonal variation in nitrogen loading from tributaries. A large fraction of
the nitrogen loading to Hood Canal occurs between the months of November and January, when
dissolved oxygen concentrations in Hood Canal are recovering from low levels in the late
summer. Higher tributary flows and loadings in the fall and winter increase the estuarine
circulation, replenishing the area with marine water with higher dissolved oxygen (Brett, 2012).
All of the studies provide credible, measurement-based estimates of the nitrogen entering Hood
Canal from tributaries, but the more recent estimates (Steinberg et al., 2010; Richey et al., 2010)
are likely more robust because they are supported by the larger monitoring database for more
tributaries collected by HCDOP in 2005-2006.
Human Contributions within Tributary Loadings
The statistical model developed in Steinberg et al. (2010) and reported in Richey et al. (2010)
apportioned the total tributary loads among natural and human sources. Based on analysis of
numerous potential watershed characteristics, Steinberg et al. (2010) concluded that 3 parameters
were the best-estimators of nitrogen loading: conifer forest area (i.e., natural contributions),
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mixed deciduous forest area, and population density. There was no further breakdown to
individual source contributions within these categories. The mixed deciduous forest loading
would account for the contribution from red alders, and the population loading would account for
OSS loading and other residential activities such as fertilizer application.
Steinberg et al. (2010) reported total loadings for Hood Canal, but Lynch Cove loadings were
reported as percentages of a total loading value shown only in a figure. From this figure
(see lower right panel of Figure 16), we visually estimate that the total Lynch Cove loading is
approximately 60 MT/yr. Table 2 applies the percent contributions among sources to this
estimate of the total loading.
Table 2: Tributary loads to Hood Canal and Lynch Cove by source.
Steinberg etal, 2010; Rickey etal, 2010.
Source
TON Loading
(MT/yr)
Fraction of Total
Tributary Loading3
Hood Canal
Natural (background)
Mixed Forest (Red Alder)
Human Population
Total
251
354
95
7002
35%
51%
14%
Lynch Cove
Natural (background)
Mixed Forest (Red Alder)
Human Population
Total
8
34
18
60U
13%
56%
31%
1 Estimated by visual inspection of Figure 16.
2 Steinberg et al. (2010) calculated the Figure 16 loads from measurements of flow and concentration
at tributary mouths and extrapolated to unsampled catchments using unit-area loadings.
3 Percent contribution estimated using statistical model.
Red alders are a natural component of riparian forests but are more prevalent now than occurred
historically (Collins and Montgomery, 2002; Davis, 1973; Brandenberger et al., 2008). Red
alder root nodules fix atmospheric nitrogen and produce leaves with higher nitrogen than other
plant species (Roberts and Bilby, 2009). Nitrogen leaches from the leaves during rain events.
Leaves and other plant materials fall to the ground seasonally, and decomposition processes
release nitrogen from atmospheric sources to the soil and water in red alder forests. Roberts and
Bilby (2009) concluded that red alder contributed to a 54% increase in nitrogen delivery to small
streams, and Volk (2004) found that this red alder contribution results in higher stream nutrient
concentrations. No study prior to Steinberg et al. (2010) estimated red alder contributions to
Hood Canal tributaries.
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Richey et al. (2010) also employed a mechanistic modeling approach using DSEM, composed of
a watershed model (DHSVM) linked to a solute export model (SEM), to derive estimates of
tributary loading for comparison to the statistical approach. Richey et al. (2010) reported model
results, which indicated a higher natural loading and lower population and red alder contribution
compared to the statistical model. However, Richey et al. (2010) provided only a general
description of the DSEM model. A manuscript is pending, and no details of the model were
available for review.
Overview of Methods to Estimate Loadings from
Groundwater and Shoreline On-site Sewage Systems
Tributary estimates do not account for all of the inputs of nitrogen to Hood Canal from the
watershed. Most upland OSS discharge nitrogen to groundwater within a tributary catchment, so
these loadings are captured by monitoring at the tributary mouths. In contrast, shoreline OSS
discharge to groundwater in close proximity to the shoreline and are not captured by tributary
monitoring. A significant fraction of the population in the Hood Canal watershed is located in
these shoreline areas. Based on water balance calculations and thermal images, a proportion of
groundwater in the watershed discharges directly to marine waters, and an unknown fraction
discharges to deep aquifers (see thermal images of shoreline plumes in Sheibley et al., 2010).
We used two methods to estimate shoreline groundwater contributions: (1) loading calculations
using monitoring data for shoreline seeps multiplied by groundwater discharge, and (2) per
capita loading estimates using population data along the shoreline outside of the tributary
monitoring areas. Estimates from both methods are described in the following sections.
Uncertainty is greater in estimates of nitrogen loading from shoreline groundwater than tributary
loading due to sampling limitations and complicated groundwater hydrology. A commensurate
monitoring program to capture all groundwater sources would require subsurface sampling along
the entire shoreline. While complete sampling is infeasible, an assessment using available field
measurements can provide reasonable estimates of the shoreline groundwater loading. Total
groundwater flows can be estimated using a water budget developed from tributary flow and
precipitation data, and nitrogen concentrations in groundwater can be estimated from field
sampling of shallow groundwater and shoreline springs. We analyzed the available flow and
nitrogen data to estimate the shoreline nitrogen loading below.
Shoreline OSS loadings have also been estimated by several researchers based on per capita
loading calculations, using shoreline population, wastewater nitrogen content, and an assumed
loss of nitrogen in groundwater prior to discharge to marine water. Since these OSS loading
estimates are based on a population extrapolation and not measured groundwater nitrogen
concentrations, it is important to evaluate whether the per capita estimates are consistent with
available monitoring information.
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Shoreline OSS Loading based on Groundwater Data
Groundwater Flow
Paulson et al. (2006) produced the only available estimates of groundwater inflow to Hood Canal
and Lynch Cove based on a detailed water budget. After developing tributary flow estimates for
both sampled and unsampled watersheds, they calculated the groundwater flow as the difference
between annual rainfall (minus evapotranspiration) and tributary flow. The resulting annual
average groundwater flow into Hood Canal was 7.3 m3/sec (258 cfs). Based on the estimated
average flow in tributaries of 142 m3/sec (5,013 cfs), they estimated that groundwater is 5% of
the annual average freshwater inflows to Hood Canal.
Paulson et al. (2006) did not explicitly report the Lynch Cove groundwater flow, but they applied
the water budget to Lynch Cove watersheds and reported the estimated monthly DIN loading.
Based on the loading estimate and assumed DIN concentrations in groundwater (both discussed
below), the estimated groundwater flow to Lynch Cove was 1.1 m3/sec. The analysis assumed
that groundwater flow was constant over the year with no seasonal variation. Paulson et al.
(2006) did not analyze the distribution of rainwater recharge and groundwater flow between
shallow and deep aquifers. In our analysis of human-caused groundwater nitrogen loadings, we
have conservatively assumed that the entire groundwater flow enters the euphotic zone of Hood
Canal (approximately the top 10 meters of the water column) to estimate human-caused nitrogen
loadings (see Question 2 below).
Simonds et al. (2008) also estimated groundwater flows for Lynch Cove, but some of the
methods resulted in very high flows (e.g., 22 m3/s for one method) that are not supported by the
water budget analysis of Paulson et al. (2006). These high fluxes were also reported in a journal
article (Swarzinski et al., 2007) that is no longer supported by best available information. Based
on adjustments in methodology and additional field data, USGS researchers indicate that their
most recent estimates for the total groundwater discharge to Lynch Cove marine waters range
from 0.5 to 1.9 m3/s (Sheibley and Paulson, pers. comm., 2011).
Brett (201 Id) used a measurement-based nitrogen flux calculation in Lynch Cove as a check on
estimates for shoreline OSS loadings. Rather than extrapolate a total water budget for the Lynch
Cove watershed as in Paulson et al. (2006), Brett (201 Id) used a simpler approach to estimate a
residual flow (0.5 m3/s) based on the annual precipitation minus evapotranspiration over land
areas not captured by tributary sampling. This "unsampled flow" could enter Lynch Cove via
surface water or shallow groundwater, where relative contributions are unknown. Given that
these areas do not have defined streams, much of this flow likely enters Lynch Cove as shallow
groundwater.
Shoreline Groundwater Nitrogen Concentrations and Loading
Paulson et al. (2006) estimated "regional groundwater" loading to Hood Canal and Lynch Cove
based on nitrogen concentrations in well samples and flow estimates. They found highly
variable concentrations, ranging from 60 to 1,000 ug/L DIN, potentially reflecting different
levels of human influences along with natural variation. They selected a mid-range value of
Page 34
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600 ug/L DIN to estimate groundwater loading. Coupled with the flow estimate, this led to an
estimated DIN loading of 138 MT/yr from regional groundwater to Hood Canal. For Lynch
Cove, they estimated that regional groundwater contributes a DIN loading of 1.7 MT/month in
September and October.
Simonds et al. (2008) estimated total groundwater loading to Lynch Cove using measured
nitrogen concentrations in wells, springs, and piezometer samples. The average total nitrogen
concentration in 23 samples was reported as 330 ug/L, and the average nitrate concentration was
reported as 310 ug/L. We found apparent errors in the reported means based on the raw data
provided in Simonds et al. (2008), computing an average total nitrogen concentration of
220 ug/L and nitrate concentration of 200 ug/L. Assuming negligible particulate nitrogen in
groundwater, the small difference in these two values (20 ug/L or 9% of the total nitrogen) is
attributed to ammonium and dissolved organic nitrogen. We used 220 ug/L to represent the
Simonds et al. (2008) estimates because it accounts for all DIN components but also includes
dissolved organic nitrogen. Based on this information, we assume the organic fraction of
nitrogen in groundwater is negligible and DIN is equivalent to TDN.
For nitrogen concentrations, Brett (201 Id) cited the previous compilation of sampling
information by Paulson et al. (2006) and noted similar data compilations by Atieh et al. (2008)
and Kitsap County (Banigan, 2008). From these sources, Brett (201 Id) reported that the average
groundwater DIN concentration in drinking water wells and unsampled areas appeared to fall in
the range of 500-600 ug/L.
Richey et al. (2010) used the DSEM model to estimate a "shoreline groundwater" loading of
0.5 MT/month to Lynch Cove for the period of June through September. The loads are presented
as the sum of 0.2 MT/month from natural sources (conifer forest), 0.2 MT/month from mixed
deciduous forest (alder), and 0.1 MT/month from population (e.g., OSS, fertilizer). Richey et al.
(2010) does not describe how these values were estimated or how they represent groundwater
discharge to marine waters. As noted earlier, the DSEM model has not been fully documented,
and no explanation of these values has been published.
Mason County has developed the best available sampling data for nearshore groundwater
discharges to southern Hood Canal (Georgeson et al., 2008). The County has sampled all
observed discharges along the shoreline for fecal coliform bacteria, including groundwater seeps,
springs, small streams, and bulkhead discharges. In select areas, samples were also analyzed for
nitrogen (see Figure 17). The monitoring program captured all seasons of the year.
Kitsap County also collected samples along the east shorelines of Hood Canal (Banigan, 2008).
Salinity information is not available for the Kitsap County data, which is needed to distinguish
freshwater discharges from marine or brackish water pushed into shoreline soils by the tides.
Brackish or marine samples could dilute the freshwater nitrogen contributions. Therefore, these
data have not been included in these estimates of freshwater discharges.
James (pers. comm., 201 la, 201 Ib, 2012) obtained the Mason County data and analyzed only
low-salinity samples (562 samples with salinity <1 ppt) to remove the influence of marine
nitrogen. This subset of Mason County samples has a distribution of DIN concentrations that is
Page 35
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skewed by a small number of very high concentrations, and James (201 Ib) recommended use of
the median value to represent the central tendency of the data. The median DIN concentration
for this subset of Mason County samples was 120 ug/L.
USGS (Sheibley and Paulson, pers. comm., 2011) also analyzed the Mason County data, but they
narrowed the dataset to include only seep samples with low salinity levels. They selected this
subset of data based on concerns that the full dataset included numerous samples from small
streams and bulkhead conduits discharging water from unknown sources. The seep samples
offer a more representative estimate of the shallow groundwater nitrogen concentrations adjacent
to Hood Canal and Lynch Cove. Like James (201 Ib), Sheibley and Paulson (2011) noted the
effect of high outliers in the data and recommended use of the median value for the loading
analysis. The median DIN concentration for all of the 382 seep samples in Hood Canal was
250 ug/L. The median DIN concentration was also 250 ug/L in Lynch Cove, where there are
significantly fewer nitrogen samples (25 samples, with none along the eastern shore at Belfair).
There are two reasons for the limited sampling in Lynch Cove. First, the County only analyzed
nitrogen in samples in the Lynch Cove area during follow-up sampling of discharges with
elevated fecal coliform concentrations (> 900 FC/lOOmL; see Figure 17). Second, the County
could not access the tideflats in the Belfair area safely (Mason County, 2013).
Because of Mason County's sampling strategy, the Sheibley and Paulson (2011) dataset includes
multiple sample results for problematic seeps (high fecal coliform and nitrogen). These seeps
were sampled multiple times by the County to determine if corrective action on failing OSS had
a notable effect on shoreline seep concentrations (Georgeson et al., 2008). For the 25 Lynch
Cove seeps with elevated fecal coliform concentrations, the County confirmed OSS failure at
7 monitoring locations and suspected failure at 14 others. Some of the high DIN seeps showed
substantial reductions in samples following OSS repair. For example, the seep with highest
value (65,000 ug/L in the summer) was re-sampled the following summer, and the DIN
concentration had dropped to 250 ug/L (Georgeson et al., 2008).
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MCPH Elevated Fecal Coliform and Nutrient Results
Figure 17: Mason County shoreline sampling areas and results of targeted sampling for nutrients
in discharges with elevated fecal coliform concentrations.
Nutrient results in mg/L. Source: Mason County (2013).
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To reduce the bias from multiple sampling of seeps, we reviewed the Sheibley and Paulson
(2011) dataset and reduced the distribution to a single value per seep with preference given to the
most recent summer sample value. This reduced the number of values from 382 to 325. The
median was almost unchanged, decreasing from 250 to 230 ug/L, but the mean dropped from
844 to 650 ug/L, illustrating the strong influence of the few high values on the mean. Figure 18
presents the distribution of measured groundwater concentrations in seep samples.
Mason County provided updated data summaries for all discharges, not only seeps (Georgeson,
2013). The median and mean DIN values in all discharges are similar to the median and mean
DIN in the USGS dataset for shoreline seeps (Sheibley and Paulson, 2011).
90 -i
80 -
70 -
60 -
S" 50
E
OJ
3
IT
OJ
£ 40
30 -
20 -
10 -
120%
100%
- 80%
- 60%
- 40%
- 20%
I Frequency
•Cumulative%
0%
DIN (ug/L)
Figure 18: DIN concentration in freshwater seep samples along Mason County shoreline areas
of Hood Canal.
Source: Sheibley and Paulson (2011), reduced to one value per seep (325 samples) by EPA/Ecology.
Page 38
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Table 3 lists the available groundwater flow and DIN concentration and load estimates for
Hood Canal and Lynch Cove. None of the available studies provide an estimate of natural
concentrations of nitrogen in groundwater. We conservatively attribute the total shoreline load
to human sources.
Table 3: Available estimates for flow, nitrogen concentration, and nitrogen loading for
groundwater discharges to marine waters.
Waterbody
Period
Study/Analysis
Flow
(mVsec)
DIN
(ug/L)
DIN load
MT/year
Hood Canal
Annual
Paulson et al. (2006)
7.3
600
138
MT/month
Lynch Cove
Lynch Cove
Lynch Cove
(Mason County
shore)
Lynch Cove
(Mason County
shore)
Lynch Cove
Lynch Cove
Sept - Oct
June - Sept
NA
NA
June - Sept
NA
Paulson et al. (2006)
Brett (20 lid)
Sheibley and Paulson
(pers. comm., 2011)
James
(pers. comm., 2012)
Richeyetal. (2010)
Simonds et al. (2008)
l.l1
0.52
0.5- 1.93
NA
NA
NA7
600
500 - 600
2304
1206
NA
2208
1.7
0.7-0.81
0.3-1.31
NA
0.55
NA
1 This value was not reported in the referenced document but is readily calculated using the other two values in this
row.
2 Estimated flow from unsampled watershed area in analysis of Steinberg et al. (2010).
3 No documentation available for these values at this time.
4 Median of 325 unique seep samples from Mason County shoreline. The median is 190 ug/L for 21 unique seep
samples in Lynch Cove.
5 Based on DSEM modeling analysis with no documentation at this time.
6 Median of all Mason County low-salinity data for Hood Canal/Lynch Cove shoreline.
7 Flow values under subsequent review by USGS as they are not bounded by a water balance analysis as conducted
in Paulson et al. (2006).
8 Mean total nitrogen value in same sample set. These are corrected values (see discussion of error in text).
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The independent review panel recommended use of the mean DIN concentration rather than
the median value in the loading calculations (PSI, 2012). As noted earlier, other researchers
(James, 2011; Sheibley and Paulson, 2011) recommended use of the median DIN concentration.
Because of the influence of a few high DIN samples, the difference between median and mean
concentration in the groundwater samples is substantial. For the seep samples from Mason
County (Sheibley and Paulson, 2011, reduced to single values per seep), the median DIN was
230 ug/L and the mean was 650 ug/L. For 21 Lynch Cove samples, the median was 190 ug/L
and the mean was over 2,700 ug/L due to a small number of high nitrogen concentrations. The
Lynch Cove sampling for nitrogen was restricted to discharges with elevated fecal coliform
concentrations. Because of the inherent bias in this sampling program, nitrogen results,
particularly the mean values, are likely over-estimated.
Flow volumes associated with failing OSS are likely low in comparison to regional groundwater.
Preferential flow paths due to groundwater hydrogeology likely produce higher flows and lower
concentrations indicative of regional groundwater compared with seeps of effluent from failing
OSS (Brett, 2012a). Therefore, the mean is not necessarily more indicative of total loading
without concomitant spatially varying groundwater flows and would bias load estimates high.
Rather than select a single value to present loadings, we incorporated the entire seep DIN
variability into a Monte Carlo uncertainty analysis that is described in our dissolved oxygen
assessment (see Question 3 below). For summary tables of watershed loading, we present the
median and mean DIN concentrations from the distribution to provide a range of loading
estimates. This approach was recommended by Brett (2012a) in response to the independent
panel report. For the lower bound of the loading range in Table 4, we combine the median DIN
concentration with the low-end groundwater flow estimate documented in Brett (201 Id). For the
upper bound, we combine the mean DIN concentration with the high-end flow estimated in
Paulson et al. (2006). The high-end flow estimate of Sheibley and Paulson (pers. comm., 2011)
was not used because documentation for this estimate is not available.
Table 4: Best estimates of range of seep DIN concentrations, groundwater flow, and shoreline
groundwater loadings for Lynch Cove.
Shoreline Seep DIN
(ug/L)1
Shoreline Groundwater Flow2
Shoreline Groundwater DIN Loading
(MT/mo)
Lower
Bound
230
0.5
0.3
Upper
Bound
650
1.1
1.9
1 Sheibley and Paulson (2011) analysis of data from Georgeson et al. (2008). Dataset reduced to one value
per seep by EPA/Ecology. Lower and upper bound are median and mean sample values, respectively.
2 Lower and upper bound from Brett (20 lid) and Paulson et al. (2006), respectively.
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Shoreline OSS Loadings based on Per Capita Calculations
Three research groups [Paulson et al. (2006), Steinberg et al. (2010), and Richey et al. (2010)]
estimated nitrogen loadings from shoreline OSS using per capita calculations. These studies
employed a range of estimates for population, per capita nitrogen loading, and loss of nitrogen in
soils between OSS discharge and the marine waters of Hood Canal.
Shoreline OSS loadings to Hood Canal
Paulson et al. (2006) estimated the annual loading from shoreline OSS to Hood Canal and Lynch
Cove. The analysis defined shoreline OSS contributions as all homes located within 150 meters
of the shoreline. Using 2000 census data and aerial photographs, they estimated an October
through May population of 6,400 in the Hood Canal watershed. For the number of residences
(4,900), this equated to an occupancy of 1.3 persons per house. This low occupancy represented
the offseason population in the Hood Canal watershed, where many vacation properties are not
continuously occupied. To account for higher seasonal population, Paulson et al. (2006) used the
number of housing units and a summer occupancy rate of 2.5 people/housing unit to obtain a
June through September population estimate of 12,200. We found a minor error in this part of
the report. The text indicated that an occupancy assumption of 2.2 people per house was used in
the calculation, but the actual value used in the calculation was 2.5. A value of 2.2 was clearly
the intended value, and this was the value used in the population estimates for Lynch Cove
(described below).
Paulson et al. (2006) then used literature values for per capita flow (60 gallons/day) and TDN
concentration from septic systems, assumed the organic fraction is 25% and is removed in soils,
and finally assumed a 10% denitrification rate. This resulted in a per capita loading that reaches
Hood Canal of 2.95 kg/yr, and an annual loading of DIN to Hood Canal from the shoreline
population of 26 MT/yr. This loading was expressed as DIN but also represented the TDN
loading, because it was assumed that none of the organic fraction reached Hood Canal.
Steinberg et al. (2010) did not estimate loadings from shoreline OSS. However, they presented a
hypothetical, "worst-case" calculation as an upper bound to make the point that OSS loads from
the entire watershed would be insignificant compared to marine nitrogen fluxes for the entirety
of Hood Canal. They assumed a population of 45,300 for the entire watershed (not only the
shoreline) and a per capita nitrogen release to the Canal of 4 to 5 kg per year. They reported that
the resulting load (not reported but readily calculated as 204 MT/yr) constituted 0.5% of the
marine load for the entire Hood Canal.
Shoreline OSS loadings to Lynch Cove
Using the assumptions for per capita contributions in the total Hood Canal OSS load estimates,
Paulson et al. (2006) calculated an average September/October loading of 0.8 MT/month to
Lynch Cove from shoreline OSS. The September contribution was calculated with a higher
summer occupancy rate and population (2.2 persons per house, 4,250 people total). This equates
to a monthly loading of 1.0 MT/mo for the estimated summer population, higher than for the
combined September and October time period.
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Paulson et al. (2006) reported shoreline OSS loads as a distinct loading to Lynch Cove,
independent of the 1.7 MT/mo loading from "regional groundwater" discussed previously. The
rationale was that "regional groundwater" estimates were based on DIN concentrations observed
in wells upland from the shoreline, and DIN concentrations in the upland wells are not
influenced by shoreline septic influences. Thus, the Paulson et al. (2006) analysis assumed that
any impact from shoreline OSS would be seen downstream of these well samples (Sheibley and
Paulson, 2011). This work pre-dated the sampling program by Mason and Kitsap Counties,
which provided direct measurement of groundwater nitrogen concentrations at the shoreline.
Steinberg et al. (2010) estimated the annual shoreline OSS loading to Lynch Cove and the Great
Bend. They estimated a "worst-case scenario" as a population of 4,500-5,000 in the area outside
sampled watersheds. Using the same per capita nitrogen value as other studies (4-5 kg per year),
they calculated a TDN loading from OSS to Lynch Cove/Great Bend of 21 MT/yr, equivalent to
1.8 MT/mo. The population estimate was based on analysis of both residential parcel and census
block data (Brett, pers. comm., 201 le).
Richey et al. (2010) considered the USGS (Paulson et al., 2006) study and used a similar
calculation method to estimate shoreline OSS loading. They focused on Lynch Cove and
examined the June through September period only. Richey et al. (2010) offered 4 new shoreline
OSS estimates based on different combinations of population and per capita loading, alongside
the Paulson et al. (2006) estimate of 0.8 MT/mo. Per capita nitrogen loading estimates were
based on published values of 4.5 kg/yr or 2.95 kg/yr (4.5 kg/yr with 35% nitrogen loss rate in
subsurface as used by Paulson et al. (2006); see above)1. The estimates for monthly loading
ranged from 1.3 to 3.9 MT/mo to Lynch Cove in the summer. The mid-range load (2.6 MT/mo
calculated using a shoreline population of 10,505 people and 2.95 kg per person per year) forms
the basis of several subsequent calculations of human contributions to dissolved oxygen
decreases. In their report and subsequent discussions, Richey et al. (2010) have emphasized that
their estimates were highly uncertain and additional analysis was warranted.
Richey et al. (2010) used significantly higher population estimates than the other researchers.
Their report does not identify the buffer width used to estimate shoreline houses and population;
however, this was subsequently confirmed to be 1,000 meters (Richey, pers. comm., 2011),
which is much larger than the 150-meter buffer used by Paulson et al. (2006). This buffer width
incorporates many upland OSS in tributary catchments that are already captured in the tributary
loading calculations. As a result, the methodology double-counts a significant number of OSS as
both shoreline and tributary sources.
1 Richey et al. (2010) incorrectly reported the basis for the per capita loading rate used by Paulson et al. (2006).
This rate (2.95 kg per person per year) was calculated by reducing the per capita source rate at the residence
(4.5 kg per person per year) by 35% based on assumed removal of nitrogen in soils. Richey etal. (2010) listed
the Paulson et al. (2006) value but asserted that "other evidence suggests that a more accurate loading rate is
4.5 kg/person-day," in fact, the Paulson et al. (2006) rate was based on 4.5 kg/person-day but incorporated a loss
term. This misinterpretation created confusion in the comparisons between loading estimates in Richey et al.
(2010). For example, they stated that "all of these calculations assume that the total N loading is transferred
directly to the Canal, with no loss", when three of the calculations utilize the Paulson et al. (2006) loading rate
(2.95 kg/person-day) that assumes a 35% loss of the per capita nitrogen loading in soils.
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In addition to the buffer width selection, Richey et al. (2010) employed other assumptions that
increased the population. For example, they assumed that the July/August population is twice
the September/October population in an adjustment to the estimate by Paulson et al. (2006).
Additionally, for their mid-range OSS estimates, Richey et al. (2010) assumed that the
population is an average of 7,004 people in June and 14,007 people in July/August. The higher
population in July/August translates to a very high occupancy rate (3.5 people per house at all
times).
Richey et al. (2010) also calculated a total OSS loading estimate (2.9 MT/mo) to Lynch Cove as
the sum of shoreline OSS (2.6 MT/mo), OSS contributions to tributaries (0.2 MT/mo), and
"shoreline groundwater" (0.1 MT/mo). This "shoreline groundwater" value is derived from the
DSEM model. Richey et al. (2010) did not document how the DSEM loading estimates were
derived in terms of the assumed groundwater characteristics and whether these estimates include
areas that were captured in the tributary estimates.
A more recent analysis (Horowitz and Peterson, pers. comm., 2011) identified 979 shoreline
OSS within a 30-meter buffer. Based on the close proximity to the shoreline of these OSS, a
number of these drain fields are tidally inundated. As a worst-case estimate, if one were to
assume that all of the nitrogen from the 979 nearshore OSS enters Lynch Cove, and use the
assumptions of Paulson et al. (2006) for per capita loading without loss in soils (4.5 kg/yr TDN)
and occupancy (2.2 persons per house), the estimated loading from the residences within 30
meters of shore would be 0.8 MT/mo TDN.
The assumptions and results for the three available studies of shoreline OSS loadings to Lynch
Cove (Paulson et al., 2006; Steinberg et al., 2010; and Richey et al., 2010) are summarized in
Tables 5 and 6. The studies differ markedly in their geographic scope and OSS assumptions. In
addition to differences in study area (Hood Canal or Lynch Cove) and the selected time frame
(summer or annual), the primary factors contributing to the variation are differences in estimates
for population, per capita loading, and subsurface nitrogen loss.
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Table 5: Estimated shoreline OSS nitrogen contributions based on per capita calculations.
Study
Paulson et al.
(2006)
Steinberg et al.
(2010)
Richey et al.
(2010)
Annual Shoreline
OSS Loading
to Hood Canal
(metric tons/year)
26
NA2
NA
Seasonal Shoreline
OSS Loading
to Lynch Cove
(metric tons/month)
0.8
(Sept - Oct)
1.81
1.3-3.9
(June - Sept)
1 This study reported annual "conceivable" loading without any attenuation of 21 MT/yr, equivalent to listed
value of 1.8 MT/mo if distributed uniformly throughout the year.
2 This study included hypothetical estimates for OSS loadings from population (45,300) in the entire watershed.
Table 6: Assumptions used in per capita estimation of summer shoreline OSS loading to Lynch
Cove.
Study
Paulson et al.
(2006)
Steinberg et al.
(2010)
Richey et al.
(2010)
Horowitz and
Petersen
(pers. comm.)6
Shoreline
Width
Buffer
(meter)
150
NA2
10004
30
Homes
(#)
1,934
NA
3,952
979
Occupancy
(persons per
home)
2.2
NA
2.7
2.26
Shoreline Buffer
Population
(persons)
and
(time frame)
4,250
(Sept)
4,500-5,000
(June - Sept)
10,505
(June - Sept)
21546
Per Capita
Nitrogen
(kg/yr)
2.95
4.5
2.95
4.56
Summer
Loading to
Lynch Cove
(MT/month
TON)
l.O1
1.83
2.65
0.86
This value was calculated based on Paulson et al. (2006) assumptions for summer population. Paulson et al. (2006)
report a September-October loading (0.8 MT/mo), which is a blend of summer and fall population values.
Study assumed 4,500 - 5,000 residents in unsampled shoreline areas based on unpublished HCDOP data.
Study analyzed annual loadings only. Listed value is the annual value (21 MT/yr) divided by 12.
Richey (pers. comm.)
Richey et al. (2010) referred to this as the "most plausible" value reported from a range of 1.3 to 3.9 MT/mo.
This study only estimated parcels within 30 meters of the shoreline; the remaining values are derived from that
parcel count and best available values for occupancy per housing unit and per capita nitrogen contributions.
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A major contributor to the differences in study results is the assumed distance from shore (buffer
width) used in the shoreline population estimation. As noted above, Paulson et al. (2006) used a
150-meter buffer, and Richey et al. (2010) used a 1,000-meter buffer. Steinberg et al. (2010)
used the entire watershed as a bounding calculation and also assessed the unsampled areas.
These are shown in Figure 19.
\
Atmospheric
Deposition
\
Marine Upwelling
30m
150m
1000m
Figure 19: Watershed areas defined in terms of buffer distances, unsampled area delineations,
and tributary catchments.
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The selected buffer width directly affects the estimated number of residences along the shoreline
that contribute nitrogen to marine groundwater loading. The selection of 30-, 150-, and 1,000-
meter buffers led to estimates of 979, 1934, and 3952 housing units, respectively. The available
studies do not clearly articulate the basis for selecting a particular buffer. In the course of this
review, Brett (pers. comm., 201 If) suggested that assessments should differentiate between those
residences where (1) the drain field is close enough to the marine boundary that tidal flushing of
the drain field occurs, (2) the drain field is not tidally influenced but are not far above saturated
soils, and (3) the drain field is several meters above saturated soils. Future estimates would be
improved by a sub-categorization of OSS along these lines as an alternative to calculations that
employ a single buffer width. These distinctions are important because research conducted on
OSS drain fields in the Hood Canal watershed (Atieh et al., 2008) show that in cases where the
drain field is several meters above the saturated zone, nitrogen discharges are stored in the soils
until the late fall storms saturate the soils and mobilize stored nitrogen (see also Steinberg et al.,
2010). Conversely, for tidal-influenced drain fields, nitrogen transport from the septic tank to
the marine waters can be very rapid (hours to days).
At greater distances inland from shore, an important factor is the degree to which nitrogen levels
attenuate in the subsurface, between the OSS and the location where the groundwater surfaces.
A study conducted in the Hood Canal watershed (Atieh et al., 2008) and compilations from
New England watersheds with similar geology (Valiela et al., 1997; Daley et al., 2010) indicate a
central tendency of 35-50% loss of nitrogen in effluent plumes down gradient from leach fields.
Kitsap County (undated report) summarized published studies in other watersheds in Washington
that found high nitrogen removal rates ranging from 79% to 89% on average. Paulson et al.
(2006) and the mid-range estimate for Richey et al. (2010) assumed a 35% reduction in total
nitrogen in the groundwater prior to release to Hood Canal.
Synthesis of Shoreline OSS Estimates
We reviewed two approaches to estimate shoreline OSS loadings to Lynch Cove. Using
shoreline seep monitoring data and groundwater discharge from a flow balance, the
measurement-based approach resulted in shoreline loading estimates ranging from 0.3 MT/mo to
1.9 MT/mo. The per capita approach employed by several researchers yielded loadings ranging
from 0.8 based on 30- or 150-meter buffers to 2.6 MT/mo (based on a 1,000-meter buffer).
Measurement-based ranges represent the best available estimates of shoreline OSS loading. The
per capita loading estimates fall within this range for 30-m or 150-m buffer widths, which are
less likely to double-count OSS contained within the tributary loading estimates. These values
[1.0 MT/mo and 0.8 MT/mo, derived from Paulson et al. (2006) and Horowitz and Petersen
(pers. comm., 2011), respectively] fall near the midpoint (1.1 MT/mo) in the range of
measurement-based loadings. Nevertheless, we emphasize that the per capita estimates are not
based on measured conditions. For this reason, we rely on the measurement-based approach as
the best available and consider the per capita estimates as an additional line of evidence.
This section presents best available estimates for shoreline OSS as a value or a range; however,
shoreline groundwater loading is an important component in the analysis of human impacts on
dissolved oxygen impacts. Therefore, the analysis of human impacts described in Question 3
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draws from the full distribution of seep nitrogen concentrations rather than a single concentration
value or range. The Monte Carlo analysis described in Question 3 moves beyond point estimates
or low/high bounds and employs random sampling of the seep DIN data to capture the full range
of uncertainty in shoreline OSS loadings.
Atmospheric Deposition (including Rainfall)
Wet Deposition
Paulson et al. (2006) observed that prevailing winds along Hood Canal are from the southwest,
so nitrogen in rainwater is generally not affected by urban areas. They estimated a DIN loading
of 30 MT/year from direct precipitation to the water surface of the entire Hood Canal based on
the National Atmospheric Deposition Program's Olympic National Park monitoring station.
Paulson also estimated a rainfall DIN loading of 272 MT/yr to the entire Hood Canal watershed
(water and land). As noted earlier, they estimated a tributary DIN export of 493 MT/yr, and the
rainfall load to land would be captured in the tributary export load. Paulson et al. (2006) also
estimated a direct precipitation loading of 0.1 MT/month for Lynch Cove in September and
October.
Steinberg et al. (2010) noted that nitrogen concentrations in rainwater in the Hood Canal
watershed are roughly 20 times lower than concentrations observed in the rainwater in the
Eastern United States. At the same time, the estimated concentrations (70 ug/L DIN) are similar
to tributary concentrations in the most undeveloped watersheds, suggesting that rainwater is
probably a significant component of natural background loadings in tributaries. Steinberg et al.
(2010) estimated TON loadings of 47 MT/yr to the surface of Central Hood Canal and 2.9 MT/yr
to the water surface of Lynch Cove.
Dry Deposition
Steinberg et al. (2010) analyzed available monitoring data for dry deposition of nitrogen.
Based on the distance-weighted average from three regional monitoring stations in western
Washington, they estimated loadings of 2.4 MT/yr to Central Hood Canal and 0.2 MT/yr to the
surface of Lynch Cove.
Summary of Watershed Loadings
The geographical area and time frame for each estimate are critical factors when interpreting the
results of the studies of nitrogen loading from the Hood Canal watershed. Table 7 summarizes
the various estimates by location and time of year. Each of the studies provided uncertainty
ranges, but these are omitted in this summary. We discuss uncertainty later in this report.
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Table 7: Watershed nitrogen loads to Hood Canal and Lynch Cove in available studies.
Study Area
(Time Frame)
Reference
Data Period
Hood Canal
(Annual)
Paulson et al.
(2006)
1959-2002
Steinberg et al.
(2010)
2005-2006
Parameter
Units
DIN
MT/year
TON
MT/year
Tributaries
Natural
Red Alder5
Population
493
-
-
-
700
251
354
95
Shoreline OSS
26
NA1
Atmospheric
Wet
Dry
30
30
-
49
47
2
Regional
Groundwater
Point Source
138
1
-
-
Total
688
749
Lynch Cove
(Sept - Oct)
Paulson et al.
(2006)
2004
Lynch Cove
(June - Sept)
Richey et al.
(20 10)2
Richey et al.
(20 10)3
2005-2006
DIN
MT/month
TON
MT/month
0.9
-
-
-
1.4
0.7
0.5
0.24
1.2
0.2
0.6
0.4
0.8
2.6
-
0.1
0.1
-
-
-
-
-
-
-
1.7
-
0.54
-
-
-
3.5
4.5
-
1 Steinberg et al. (2010) did not include a realistic shoreline OSS estimate but rather estimated a hypothetical
"worst-case" loading from all OSS using the population of the entire Hood Canal watershed (45,300). The OSS
load with these assumptions is 204 MT/yr (29% of the sampled watershed load).
2 Richey et al. (2010) presents several estimates. This column lists values described as "most plausible" loadings.
Tributary loadings are based on DSEM modeling analysis (limited documentation).
3 Richey et al. (2010) presents several estimates. This column lists seasonal loadings estimates for tributaries from
the statistical model documented in Steinberg et al. (2010).
4 The tributary loading from populated areas (0.2 MT/mo) was attributed to OSS. A fraction of the groundwater
loading (0.1 MT/mo) was also attributed to OSS. Therefore, the total estimated OSS loading was 2.9 MT/mo,
comprising loadings from shoreline OSS (2.6 MT/mo) and tributary/groundwater OSS (0.3 MT/mo).
5 Some portion of alder is associated with past human activities, but the relative contribution between natural alder
and human-enhanced alder has not been determined.
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Table 8 and Figure 20 present our best professional judgment of the potential range of loadings
to Lynch Cove based on the supporting information described in this report. The total tributary
values are measured loadings based on HCDOP monitoring data, while the source contributions
are based on the statistical model described in Steinberg et al. (2010) and repeated in Richey
et al. (2010). These source contribution estimates were preferred over the DSEM model values
reported in Richey et al. (2010), because the statistical model is more thoroughly documented
and peer reviewed. Groundwater and shoreline OSS estimates are carried forward from the
groundwater section. There is substantial uncertainty in each single value or range, and we
include the full range of concentration data in human impacts described under Question 3.
The total human contribution includes the population contribution within the tributary loadings
(0.4 MT/mo) and the shoreline OSS (0.3 to 1.9 MT/mo), while the natural component of the
tributary loadings (0.2 MT/mo) is associated with native conifer forests. Some portion of alder
that influences the mixed deciduous forest contribution is associated with past human activities,
but the relative contribution between natural alder and human-enhanced alder has not been
determined. For dissolved oxygen impact analysis, the entire alder contribution (0.6 MT/mo) is
assumed to be a human contribution as a conservative assumption.
Table 8: Potential range of watershed nitrogen loads to Lynch Cove in summer (June through
September) based on review and synthesis of available studies.
Source
Tributaries
Natural
(Conifer)
Mixed Deciduous
(Red Alder)
Population
(OSS, fertilizer)
Shoreline
(OSS, groundwater)
Atmospheric Deposition
(Combined Wet and Dry)
Total
Study
Steinberg etal. (2010);
seasonal estimates reported
in Richey etal. (2010)
Synthesis of multiple studies1
Paulson et al. (2006);
Steinberg etal. (2010)
Low Range
High Range
TON (MT/month)
1.2
(0.2)
(0.6)
(0.4)
0.3
<0.1
1.5
1.2
(0.2)
(0.6)
(0.4)
1.9
<0.1
3.1
1 Groundwater flow from Brett (201 Id) and Paulson et al. (2006). Median and mean DIN concentration in shoreline
seeps [from Georgeson et al. (2008) and Sheibley and Paulson (pers. comm, 2011)]. Per capita estimates from
Paulson et al. (2006). Nearshore housing units from Horowitz and Petersen (pers. comm., 2011).
Page 49
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Low bound
High bound
-------
Question 2: How Much Nitrogen Do Humans
Contribute to the Surface Layer of Hood Canal
Compared to Marine Sources of Nitrogen?
The surface layer dynamics in Hood Canal are key to linking nitrogen sources to dissolved
oxygen impacts. The nitrogen supply in the surface layer drives phytoplankton growth and
strongly affects oxygen conditions in the water column. In Question 1, we analyzed nitrogen
entering the surface layer from the watershed. The focus of Question 2 is the relative
contribution of this watershed nitrogen loading compared to the natural loading of oceanic
nitrogen mixing into the surface layer of Hood Canal.
The nitrate concentration at depth in the Great Bend/Lynch Cove area (400 ug/L) is
approximately four times higher than nitrogen concentrations in the Skokomish River [median of
90 ug/L reported in Paulson et al. (2006)], confirming the importance of marine nitrogen sources.
As noted earlier, the net circulation in the Canal transports deep water landward into the Canal.
Mixing processes transport nitrogen and other solutes from the bottom waters into the surface
layer of the Canal and the surface layer flows seaward. The transport into the surface layer has
both advective (upward) and turbulent mixing (bi-directional) components. The relative strength
of the advection (also called "upwelling") and mixing (also called "eddy diffusion") processes
varies over space and time, complicating efforts to distinguish between them analytically.
Three Hood Canal studies, Paulson et al. (2006), Steinberg et al. (2010), and Devol et al.
(201 la), evaluated the relative contribution of watershed discharges and marine advection to the
surface layer. These studies used a variety of methods as described below. A fourth study,
Kawase and Bahng (2012), evaluated the advective and diffusive flux of nitrogen to the surface
layer using a three-dimensional, biogeochemical model. The following sections describe
estimates derived from observations, aggregated models, and the three-dimensional ROMS
model.
Estimates based on Observations
USGS (Paulson et al., 2006) described the characteristic two-layer circulation of Hood Canal.
Based on current meter measurements described in Noble et al. (2006), they estimated the
landward transport of inorganic nitrogen in the deep waters of Hood Canal and Lynch Cove
using current meter data. Compared to an estimated 688 metric tons per year (MT/yr) flowing
into the surface layer from the entire watershed, they estimated that 10,100 to 34,000 MT/yr of
dissolved inorganic nitrogen (DIN) enters the Canal from Admiralty Inlet at depth. They also
narrowed the analysis to Lynch Cove in September and October, where the marine flux
contributed an estimated 132 MT/month of DIN to Lynch Cove, compared to 3.6 MT/month
entering the surface layer from the watershed, including atmospheric deposition.
Devol et al. (201 la) reported a mean sub-tidal velocity in the summer of 0.005 m/s at the
Twanoh buoy in Lynch Cove. Devol et al. (201 la) did not report the advective flux of nitrogen
based on this current speed. Multiplying the mean velocity by the cross-sectional area below the
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r\
pycnocline at this location (38,960 m in Devol et al., 201 la), we calculate an advective flux of
water of 195 m3/sec. Using the mean nitrate in the lower layer of 333 ug/L from Devol et al.
(201 la), we calculate a nitrate flux of 168 MT/month of nitrate from the deep layer to the surface
layer. The DIN value would be slightly higher after accounting for the ammonia component.
If the buoy data represent the cross-sectional average current velocity, the calculated transport
above could be the best available estimate. If the buoy is located where current velocities are
highest, then the calculated transport is overestimated. It is unclear whether either assumption is
valid based on the available studies. In addition, Noble et al. (2006) presents information
indicating the uncertainty in the measured current velocities in that study. This information
suggests that measurement error/uncertainty in the current meter data is an important
consideration when interpreting the data and calculating fluxes.
Estimates based on Aggregated Models
Two studies estimated the flux of nitrogen from deeper waters to the surface layer of Hood Canal
and Lynch Cove using "aggregated" or "box" models (Steinberg et al., 2010, and Devol et al.,
201 la). These aggregated models are generalized representations of the system that focus on
phytoplankton, nutrient, and oxygen relationships and variation in the vertical dimension only.
Aggregated models assume these relationships are homogeneous throughout the waterbody
segment. The aggregated models were used to estimate nitrogen loadings in a two-step process,
beginning with estimates of vertical advective flows to the surface layer. These flows were then
paired with measured nitrogen concentrations in the water column to calculate nitrogen loading
to the surface layer.
Vertical Advection of Marine Water
Prior to recent efforts to estimate nitrogen flux to surface waters in Hood Canal, Babson et al.
(2006) developed a box model of Puget Sound circulation that divides the estuary into seven
basins, with two vertical layers in each basin. The model provides a time-series estimate of
vertical upwelling from the lower layer to the surface layer of each basin. Hood Canal is
represented as two connected basins in the model, with the southern basin representing the area
encompassing Central Hood Canal (south of the sill) and Lynch Cove. For this area, the model
predicts an annual average advective flow from the lower layer (depth greater than 13 meters) to
the surface layer of 2496 m3/sec. Since Central Hood Canal and Lynch Cove were combined in
a single basin, the model does not provide an estimate for Lynch Cove alone.
Steinberg et al. (2010) used a two-layer mass balance of salinity, also referred to as Knudsen's
relationship, to estimate the vertical advective flow into the surface layers of Hood Canal and
Lynch Cove. This analysis focused on annual average flows. The estimated advective flow in
Hood Canal was 3025 m3/sec, which was generally consistent with the results of the model of
Babson et al. (2006). The Lynch Cove vertical advection to the surface layer was 35.8 m3/sec.
Devol et al. (201 la) employed multiple methods to estimate vertical flow to the surface layer in
the summer in Lynch Cove. The methods included the two-layer method employed by Steinberg
et al. (2010) but applied to the summer season rather than the entire year (Method A). Method B
also represented a Knudsen salt balance, but only for the lower layer. The approach accounted
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for vertical diffusive mixing in addition to advection. Method C employed a nitrogen mass
balance to estimate vertical advection, including estimates for denitrification and primary
productivity processes. Method D was a tidally-averaged model with several horizontal boxes
and vertically resolved layers to describe estuarine circulation. The range of advective flow
estimated using these methods was 20.2 to 73.2 m3/sec.
Vertical Advective Flux of Nitrogen in Hood Canal
Steinberg et al. (2010) estimated the relative contribution of watershed and marine nitrogen
loadings to the surface layer of Lynch Cove and Hood Canal as a whole. The estimated marine
upwelling flows were paired with bottom layer average TDN concentrations to derive an annual
average loading to the surface layer, which Steinberg et al. (2010) defined as the top 5 meters in
Lynch Cove and 9 meters in the rest of Hood Canal. The TDN loading for Central Hood Canal
(39,000 MT/yr) generally agreed with the upper range estimates of Paulson et al. (2006).
Steinberg et al. (2010) then compared this marine loading to the loadings for watershed
discharges, rainfall, and dry deposition. The annual average contribution of marine upwelling to
surface layer TDN loadings was estimated at 98% for Central Hood Canal, similar to the upper
range of Paulson et al. (2006). Using conservative estimates, they estimated that OSS loadings
contribute at most 0.5% of the total nitrogen loading to the surface layer of Central Hood Canal.
Vertical Advective Flux of Nitrogen in Lynch Cove (Annual Average)
Steinberg et al. (2010) also conducted a separate analysis for Lynch Cove. They estimated a
marine upwelling load of 438 MT/yr of TDN, compared to a watershed and atmospheric load of
58 MT/yr and shoreline OSS load of 21 MT/yr. While they estimated that over 80% of the total
nitrogen loading from the tributaries to Lynch Cove was attributable to altered forests and human
development, the combined watershed and shoreline OSS loading contributed 15% of the total
nitrogen loading to the surface layer of the water column, with marine nitrogen representing the
remainder.
Vertical Advective Flux of Nitrogen in Lynch Cove (Seasonal)
Steinberg et al. (2010) did not calculate a seasonal flux of nitrogen into Lynch Cove, but they
acknowledged the importance of seasonal factors in the overall comparisons of marine and
watershed loading. For example, the summertime contributions of residential nitrogen loadings
are likely to be higher-than-average due to the increased summer population. These increased
residential contributions occur during a period of diminishing tributary flows and associated red
alder influences. In turn, diminished tributary flows result in diminished marine flows required
to maintain the salt balance that drives estuarine circulation. These seasonal changes lead to
higher shoreline human contributions relative to tributary and marine inputs in the summer
months.
Devol et al. (201 la) estimated the relative loading of nitrogen in the summer months from
marine and human sources to the surface layer in Lynch Cove. This analysis employed a 3-layer
model for Lynch Cove (Figure 21), in contrast to the 2-layer model used by Steinberg et al.
(2010). The top layer or "surface mixed layer" is above the pycnocline, which Devol et al.
Page 53
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(201 la) define as surface to 6 m. The middle layer (6 to 12 m depth) is the area below the
pycnocline but still within the euphotic zone. The lower layer defined by Devol et al. (201 la)
(12 m to bottom) is the area of minimal primary productivity and low dissolved oxygen.
Various methods produce a range of estimated vertical fluxes to the base of the euphotic zone
(11-57 MT/month) and the base of the pycnocline (5-19 MT/month).
septic
Q
Figure 21: Conceptual model used to define layers.
See Devol et al., 201 la, for an explanation of terms.
Devol et al. (201 la) analyzed human and natural marine fluxes of nitrogen to the top two layers
as well as the mixing processes that affect oxygen conditions. There are several important
assumptions in the 3-layer analysis of Devol et al. (201 la), including:
1. Surface layer algae productivity produces paniculate organic matter that settles to the second
and third layers. Oxygen production in the surface layer does not transmit oxygen to second
or third layers because the pycnocline is a barrier to mixing.
2. In mathematical terms, oxygen and nitrogen fluxes are fully balanced in the middle layer,
meaning that the productivity associated with natural marine nitrogen sources in the middle
layer (due to the vertical marine flux of nitrogen) produces and consumes oxygen in equal
measure and does not cause a net depletion of dissolved oxygen in the lower two layers. This
means that the only cause of dissolved oxygen depletion in the bottom layer occurs as a result
of net organic matter from the surface mixed layer passing through the middle layer to the
bottom layer.
Devol (2012a) noted that, conceptually, some of the surface production is oxidized in the
middle layer, and some escapes along with any unoxidized production from the middle layer.
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3. In mathematical terms, based on the assumptions above, the net observed oxygen depletion
in the bottom layer is caused entirely by the productivity in the surface layer. Therefore, the
fraction of human-to-marine nitrogen loading to the surface layer defines the proportional
human impact on the bottom two layers.
4. Fluxes are based on concentrations at the interfaces between the defined layers instead of
layer averages. This results in lower marine fluxes to middle and surface layers than would
be calculated using layer averages, because the nitrogen concentrations decrease at shallower
depths.
These assumptions have been the subject of debate among Hood Canal researchers. While a
3-layer construct is consistent with observations of phytoplankton productivity below the
pycnocline, the application of a 3-layer model to estimate natural and human-caused oxygen
impacts is not a commonly used approach. The researchers held a common understanding of the
3-layer construct, captured in a hypothetical diagram (Figure 22) of the natural and impacted
condition in Lynch Cove. However, the researchers were unable to reach consensus on the
assumptions used by Devol et al. (201 la) despite additional examination (Devol, pers. comm.,
201 lc). While it was agreed that the nitrogen loading to the surface layer impacts the oxygen
concentrations in the bottom layer, there was a difference of opinion on the potential effect of
marine nitrogen loading to the middle layer.
Brett (pers. comm., 201 Ig) offered the following alternative assumptions:
1. Particulate organic carbon (POC) generated by phytoplankton productivity and zooplankton
fecal matter in the surface and middle layers have similar settling velocity distributions and
mineralization rates.
2. POC settling velocity is much higher than the mineralization rate and much higher than the
downward diffusion/mixing of dissolved oxygen. Therefore, most decomposition occurs in
the bottom layer where dissolved oxygen is also substantially depleted.
3. The fractional mineralization of POC produced within and outside each layer should be
considered. For illustration, Brett (pers. comm., 201 Ig) hypothesized the following:
20% of the POC generated by production in the surface layer is mineralized in the first layer,
20% is mineralized in the middle layer, and 60% is decomposed in the bottom layer. In
contrast, 20% of the organic matter generated in the middle layer is mineralized there and
80% is mineralized in the bottom layer.
4. These conditions would produce a surface layer that has dissolved oxygen in equilibrium
with the atmosphere (due to gas exchange at the air-water interface), a middle layer that is
somewhat supersaturated with dissolved oxygen, and a bottom layer that is very depleted of
oxygen.
In contrast to Devol et al. (201 la), these assumptions would not restrict comparisons of human
and marine nitrogen loading to the surface layer alone, because the marine loading to the middle
layer is assumed to have a net detrimental effect on oxygen in the bottom layer.
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\,
\
Pycnocline
Human N
Marine N
to surface
layer
Marine N to
euphotic
zone
DO Concentration
Figure 22: Hypothetical Lynch Cove dissolved oxygen profiles for a 3 layer flux analysis.
The lack of consensus on the 3-layer model assumptions is important because the estimate of
human impact to dissolved oxygen (Question 3 below) is significantly affected by the
assumptions of the 3-layer model. Devol et al. (201 la) found that phytoplankton depletes the
nitrate concentration at the interface between the surface layer and the middle layer to one fourth
of the concentration in the bottom layer. As a result, the marine nitrogen loadings to the middle
layer (from the bottom layer) and surface layer (from the middle layer) differ by a factor of four.
This difference carries into the oxygen impact calculations in Devol et al. (201 la) and Brett
(2010a). As will be discussed under Question 3, Brett (2010a) adapted the 2-layer approach and
marine nitrogen loading of Steinberg et al. (2010) in subsequent oxygen depletion calculations
for Lynch Cove. Because the top layer of the 2-layer model extends to a depth that is similar to
the middle layer depth in the 3-layer model of Devol et al. (201 la), Brett (2010a) used a much
higher nitrogen concentration for the marine loading than the surface layer value used in
Devol et al. (201 la). This, in turn, led to a higher marine loading and a concomitantly lower
relative human impact when compared to the estimates of Devol et al. (201 la). The oxygen
impact calculations are discussed under Question 3 below.
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The independent review panel identified several fundamental limitations of each of the
approaches (PSI, 2012). The panel discouraged the use of Methods B, C, and D of Devol et al.
(201 la) and preferred Method A but only after re-analyzing the layer salinities used to calculate
exchanges using the vertical profiles of horizontal velocity multiplied by salinity. The panel also
identified the lack of tidal dispersion or any other time-varying contribution to salt and nitrate
fluxes as flaws in the analyses. They recommended that the uncertainties in the values should be
estimated. Finally, the panel did not agree with the approach to estimate the nitrogen flux from
the salt balance flux multiplied by a layer or interface concentration. The panel argued that the
analysis should account for the horizontal advection of nitrogen in the conservation of mass
calculations and pointed to the buoy data to explore the vertical variation in the vertical velocity.
No re-analysis has been conducted to address the technical points raised by the independent
review panel.
The capabilities and limitations of these aggregated models should be explored further before
relying on results for regulatory actions. As noted, these tools are highly simplified
representations of a complex, dynamic system. Brett (pers. comm., 201 Ih) noted the
complicated vertical structure of chlorophyll-a and nitrogen concentrations in the water column,
and this variation raises questions about the simple assumptions of the aggregated models
(e.g., depiction in Figure 22 above). For example, vertical plots from different time frames show
phytoplankton peaks and nitrogen depletion occurring both above and below the pycnocline.
Devol (2012a) noted that the use of monthly averages of around 300 profiles provide a
representative structure of mean condition, recognizing that there is variability around the mean.
Estimates based on ROMS Water Quality Model
The ROMS biogeochemical model of Hood Canal was also used to estimate vertical flux of
nitrogen in Lynch Cove in the summer (Kawase and Bahng, 2012). Additional model details are
described later in this section. The depths where fluxes were reported varied between Devol
et al. (201 la) and Kawase and Bahng (2012). Specifically, the ROMS modeling analysis used a
10-meter depth for the marine flux calculation without referencing the 3-layer construct and
6-meter depth of the surface layer in Devol et al. (201 la). Kawase and Bahng (2012) did note
that nitrogen in the model is depleted by phytoplankton to this depth, deeper than was observed.
In response to our requests to compare the ROMS and aggregated models, Kawase (pers. comm.,
201 le) indicated that the ROMS-estimated marine loadings were most representative of the
loadings to the middle layer estimated in the box model analysis of Devol et al. (201 la).
Unlike the steady-state aggregated models, the ROMS model provides a continuous simulation
of both advective and diffusive flux for the summer period. Two simulations were conducted.
The first was a "climatological" scenario representing a long-term average condition. The
second was a simulation of 2006 conditions. For the period July 1 to September 1, 2006, the
average summer flux at 10 meters was 78 MT/month of nitrogen, composed of 66 MT/mo of
nitrate and 12 MT/mo of ammonia. Approximately half the flux was diffusive (34 MT/mo
nitrate and 6 MT/mo ammonia). For the climatological scenario, the model predicted a total
nitrogen flux of 28 MT/mo and did not provide a breakdown of component fractions. There was
considerable variability in the magnitude of the flux over time within the summer season for both
scenarios.
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Kawase (pers. comm., 201 le) noted that the 2006 scenario result for nitrate advective flux
(32 MT/mo) was generally consistent with the range of estimates (11 to 57 MT/mo) in
Devol et al. (201 la) for advective flux of nitrate into the middle layer of the 3-layer model.
Kawase (pers. comm., 201 le) compared flux estimates for nitrate, because the ammonia flux was
not included in the Devol et al. (201 la) calculations for the middle layer. While ROMS and
aggregated model estimates were comparable for the flux of nitrate to the middle layer,
Devol et al. (201 la) did not use the flux to the middle layer in the analysis of oxygen impacts.
For reasons described above, Devol et al. (201 la) used the flux to the surface layer for this
analysis. The ROMS analysis (Kawase and Bahng, 2011) did not include flux estimates for
shallower depths to compare to the surface flux range (5-19 MT/mo) in Devol et al. (201 la).
Summary of Aggregated Model and ROMS Model Estimates
of Marine Nitrogen
Table 9 summarizes the various estimates of nitrogen loads to the surface mixed layer or surface
plus middle layer (euphotic zone) of Hood Canal as a whole and Lynch Cove as a subset. These
include both annual and seasonal values from the box models and ROMS model.
Synthesis of Marine Flux Estimates and Limitations
The marine flux calculations for Lynch Cove were a focus of the independent review
(PSI, 2012). The independent review panel raised several issues with the methodology and
documentation in the source material, including:
• Method B (lower layer salt balance) of Devol et al. (201 la) was discouraged because it relies
on an estimate of eddy diffusivity without corroboration.
• Method C (nitrogen mass balance) of Devol et al. (201 la) was problematic because it
involved several uncertain rate parameters such as denitrification and primary productivity.
• Method D (tidally averaged estuarine circulation model) of Devol et al. (201 la) had
insufficient documentation and the panel recommended it be used as a consistency check
only.
• Method A (Knudsen salt balance) of Devol et al. (201 la) and Steinberg et al. (2010) was
recommended because it relies on fewer estimated parameters. However, the panel
recommended a re-analysis of layer salinity that incorporates the vertical profile of both
horizontal velocity and salinity.
• The Knudsen salt balance neglects tidal dispersion, which could be significant.
• Steady-state analyses neglect time-varying components that could induce significant
exchanges.
• ORCA buoy data were not exploited to evaluate uncertainty in marine flux.
• The dispersive flux of nitrogen to the euphotic zone was not included in aggregated model
estimates but was included in ROMS model estimates.
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• Potential horizontal advective flux into the lower euphotic zone below pycnocline was
missed in the nitrogen mass balance.
• The method used to identify the pycnocline depth based on ORCA buoy data was not
documented.
• The method used to extrapolate near-surface salinity values from buoy data was not
documented.
The independent panel also recommended use of a consistent 2-layer model construct for all
calculations related to the marine nitrogen flux rather than the combination of 2- and 3-layer
models used by Devol et al. (201 la). Review panel comments indicated a preference for the
euphotic zone to calculate the marine nitrogen flux (PSI, 2012).
EPA and Ecology concur with the independent review panel's concerns with methodology and
documentation, but follow-up discussion with Hood Canal researchers failed to produce a
consensus on the path forward. The goal of this summary is to identify the "best estimates" for
calculation of human impacts. It is clear from the independent review report that all of the
available estimates for marine flux could be substantially improved through additional analysis
and peer review. The panel recommended a full re-analysis of the marine nitrogen flux. While
EPA and Ecology recommend re-analysis in the future, this report focuses on existing research
only rather than any planned new analyses. We have identified the currently available estimates
in this report while providing appropriate caveats and concerns about methodology and
documentation from our review and the independent panel review.
For analysis of dissolved oxygen impacts in Question 3, we conducted a Monte Carlo analysis of
the vertical nitrogen flux to the euphotic zone that includes uncertainty estimation. In light of the
methodology issues identified by the independent panel, we present a wide range of potential
marine nitrogen fluxes as best available information, as summarized in Figure 23.
The low end of the range is the advection-only flux to the surface mixed layer of Lynch Cove
based on a standard salt balance (Method A) in Devol et al. (201 la). This value is set as the low
bound, because it does not include mixing due to tidal dispersion. For the high bound, we used
the ROMS model estimate for 2006 in Kawase and Bahng (2011). The ROMS simulation
includes dispersive fluxes, but it may over-estimate the total flux due to excess vertical mixing
noted in the calibration, and discussed further under Question 3. The range based on these
methods is 5 to 78 MT/month.
Current velocity measurements from buoys in Lynch Cove suggest that the flux could be higher
than our upper bound of 78 MT/mo (Table 9). The large differences between measurement-
based estimates, ROMS model estimates, and aggregated model estimates have not been
analyzed and resolved at this time.
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Table 9: Estimates for nitrogen loads to the full euphotic zone and surface layer of Hood Canal and Lynch Cove.
Study
Time Frame
Geo-
graphic
region
Marine
Loading
Watershed
Loading
Total Loading
to Surface
%
Watershed
Human
Loading 8
%
Human
Shoreline
OSS
Loading
%
Shoreline
OSS
Hood Canal - Annual
Paulson et al. (2006)
Steinberg etal. (2010)
2004
2005-06
Hood
Canal
Hood
Canal
10,100-
34,000 MT/yr
39,000 MT/yr
688 MT/yr
750 MT/yr
10,788 -
3 1,688 MT/yr
39,750 MT/yr
2% - 6%
2%
NA
449 MT/yr
NA
1%
26 MT/yr
NA1
<1%
<1%
Lynch Cove - Annual
Steinberg etal. (2010)
2005-06
Lynch
Cove
438 MT/yr
79 MT/yr
5 17 MT/yr
15%
73 MT/yr
14%
21 MT/yr
4%
Lynch Cove - Summer (loading to euphotic zone)
Calculation from
Twanoh buoy data
Paulson et al. (2006)
Kawase and Bahng
(2012)
Kawase and Bahng
(2012)
Devoletal. (201 la)
July - Sept
2006
Sept - Oct
2004
July - Sept
2006
Long term
average
July - Sept
2005-2006
Lynch
Cove
Lynch
Cove
Lynch
Cove
Lynch
Cove
Lynch
Cove
168 MT/mo4
132 MT/mo4
78 MT/mo3
34 MT/mo3
11.1-57.3
MT/mo5
NA
3. 6 MT/mo
NA9
NA9
4.5 MT/mo6
NA
135.6 MT/mo
NA
NA
15.6-61.8
MT/mo6
NA
3%
NA
NA
7% - 29%
NA
NA
NA
NA
3.6 MT/mo6
NA
NA
NA
NA
6% - 23%
NA
0.8 MT/mo
NA
NA
2.6 MT/mo6
NA
<1%
NA
NA
4% -17%
Lynch Cove - Summer (loading to surface layer above pycnocline)
Devoletal. (201 la)
July - Sept
2005-2006
Lynch
Cove
5.3-19.2
MT/mo2
4.5 MT/mo7
9.8 -23.6
MT/mo7
19% -46%
3.6 MT/mo7
15% -37%
2.6 MT/mo7
11% -27%
Notes:
1 Steinberg et al. (2010) includes only a hypothetical worst-case OSS calculation for entire watershed population indicating that OSS is <1% of marine loading
2 Loading based on observed nitrogen concentrations at 6 m depth (interface between the top layer and middle layer in 3 layer construct). Advective flux only.
3 Loading based on simulated conditions at 10 m depth. Sum of advective and diffusive fluxes. Advective flux was 37 MT/mo in 2006 (not reported for long term average simulation).
4 Not explicitly characterized as loading to euphotic zone but as loading into Lynch Cove at depth (based on measured horizontal current velocities)
5 Loading based on observed nitrogen concentrations at 12 m depth (interface between the middle layer and bottom layer in 3 layer construct). Advective flux only.
6 Watershed and OSS loading assumed in Devol et al. (201 la) are drawn from Richey et al. (2010). As described in Question 1, the OSS loads used in these calculations (2.6 MT/mo) exceed
the range supported by seep measurements and groundwater flow estimates. Substituting a midrange value of 1.0 MT/mo from Table 8 produces the following values:
Total watershed loading = 2.9 MT/mo Human loading = 2.0 MT/mo (3% -14% of the total loading)
Total loading (marine + watershed) = 14.0 - 60.2 MT/mo Shoreline OSS = 1.0 MT/mo (2% - 7% of the total loading)
7 See footnote 6. Substituting a value of 1.0 MT/mo for shoreline OSS produces the following values:
Total watershed loading = 2.9 MT/mo Human loading = 2.0 MT/mo (9% - 24% of the total loading)
Total loading (marine + watershed) = 8.2 - 22.1 MT/mo Shoreline OSS = 1.0 MT/mo (5% -12% of the total loading)
8 All listed human loadings are calculated based on the assumption that all red alder contributions are human-caused. In reality, some fraction of these loadings are naturally occurring.
9 The ROMS model application (Kawase and Bahng, 2012) included analysis of dissolved oxygen impact based on estimated nitrate concentrations in tributaries under natural and current
conditions (80 ug/L and 150 ug/L, respectively). The nitrogen loadings for these simulations were not reported. See Question 3 below for further discussion.
-------
WATERSHED
Natural
Human
MARINE
All values in MT/mo
O
(N
03
-t-i
OJ
~0
OJ
Q
O
(N
OJ
O
(N
03
-t-i
OJ
s?
OJ
_Q
C
'dJ
4-J
LO
(N
O
(N
00
03
CO
C
03
OJ
l/l
03
03
Range Used in Monte Carlo Analysis
03
4-J
03
Q
O
CO
_c
O
c
03
c
O
OJ
03
CO
1 Study did not estimate monthly loadings. Listed value (37 MT/mo) is 1712th of annual estimate (438 MT/mo).
Figure 23: Range of marine nitrogen vertical flux estimates for Lynch Cove.
The range of estimates for marine nitrogen loadings to the surface waters of Lynch Cove are
provided in Table 10. Based on these loadings, we calculate the relative contribution to the
surface layer from marine sources and watershed loading. Within the total watershed loading,
we also distinguish the (a) total human contributions (watershed population, mixed deciduous
forest, shoreline OSS and groundwater) and (b) only the shoreline (OSS and groundwater)
contributions. Question 3 uses the range of marine loading estimates (5-78 MT/mo) in a
Monte Carlo analysis of dissolved oxygen impacts.
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Table 10: Potential range of marine nitrogen loading to surface water in Lynch Cove and relative
contribution of human loadings. Synthesis of available information for 2005-2006 summer conditions.
Human
Impact
Scenario
Lower
Bound
Upper
Bound
Marine
Loading
Watershed
Loading
Total
Loading to
Surface
Layer
Watershed
Human
Loading
TON1
(MT/mo)
782
51
1.53
3.13
79.5
8.1
1.34
2.94
Fraction of
Total Loading
from Human
Sources
%
2%
36%
Shoreline
OSS
Loading
TON
(MT/mo)
0.35
1.95
Fraction of
Total Loading
from Shoreline
OSS
%
<1%
23%
1 Value for mixed surface layer using Method A in Devol et al. (201 la).
2 Value for 2006 euphotic zone loading from ROMS model in Kawase and Bahng (2012).
Tributary estimates from statistical model in Steinberg et al. (2010) and repeated in Richey et al. (2010).
Total tributary loading is 1.2 MT/mo (1.0 MT/mo from human sources). Lower and upper bound defined by
shoreline OSS range of 0.3 to 1.9 MT/mo.
4 The total mixed deciduous forest (red alder) and human population contribution in watershed based on
statistical model in Steinberg et al. (2010) and Richey et al. (2010) is 1.0 MT/mo, plus shoreline OSS loading
of 0.3 to 1.9 MT/mo.
5 From synthesis in Table 8.
The answers to Questions 1 (watershed nitrogen loadings) and 2 (marine nitrogen loadings)
provide estimates of the relative contribution of human nitrogen loadings to the total nitrogen
mass entering the surface layer of Hood Canal or Lynch Cove. Researchers used a variety of
methods and estimated a range of magnitudes for both marine inputs and local human sources.
All analyses suggest that the predominant overall source of nitrogen to Hood Canal and Lynch
Cove is natural nitrogen entering from the Pacific Ocean at depth and entraining into the surface
layer of Hood Canal through vertical mixing. The dominant contribution of marine nitrogen
holds throughout the year and throughout Central Hood Canal and Lynch Cove. Nevertheless,
the state water quality standards require further analysis of the impact of human-caused loadings.
Where natural conditions cause dissolved oxygen levels to fall below threshold concentrations
(7.0 mg/L in Hood Canal), the total of all human sources cannot deplete oxygen levels by more
than 0.2 mg/L for biologically relevant spatial and temporal scales. Estimates of oxygen
depletion are assessed under Question 3 below.
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Question 3: What Is the Impact of Human
Nitrogen Contributions on Dissolved Oxygen in
Hood Canal?
This question is the most important and also the most difficult to answer, because a rigorous
estimate requires reliable and comprehensive data on marine and terrestrial nutrient loading,
oxygen concentration data over space and time, and a dynamic, two- or three-dimensional model
of Hood Canal that is well tested against observations and shown to have good skill in simulating
oxygen variability. Before delving into the modeling challenges, we summarize monitoring data
that characterize long-term oxygen conditions in Hood Canal, including sediment core data and
dissolved oxygen trend analyses. This is followed by a summary of impacts estimated using
aggregated models and the three-dimensional water quality model. Finally, we describe
conditions leading to fish kill events and the hydrodynamic and water quality connectivity
between Lynch Cove and Hoodsport.
Background - Sediment Core Study
The NOAA Coastal Hypoxia Research Program funded a study by the Pacific Northwest
National Laboratory and other organizations to collect and analyze sediment cores from Hood
Canal and other locations around Puget Sound. Sediment cores were dated using isotopic lead.
Redox-sensitive metals were used as indicators of historical oxygen conditions. In addition, the
cores were analyzed for microfossils to identify phytoplankton assemblages. This work resulted
in a report (Brandenberger et al., 2008) and journal article (Brandenberger et al., 2011). Key
findings include:
1. Sediment cores indicate that hypoxia occurred in Hood Canal before European settlement.
2. Overall oxygen levels were lower before 1900 than between 1900 and 2005, contrary to the
patter expected if population increases and land cover changes strongly influenced dissolved
oxygen levels.
3. Human-caused forces since 1900 have substantially increased the organic matter fluxes
entering Hood Canal and Puget Sound. Recent trends in the late 20* century indicate a
progression toward pre-industrial patterns in organic matter.
4. The cores show a strong shift from predominantly marine organic matter and lower oxygen
conditions prior to 1900 to more terrestrial organic matter with more oxygenated conditions
between 1900 and 2005.
In contrast to what was anticipated, Brandenberger et al. (2008) found that major land use
changes in Puget Sound and Hood Canal watersheds did not coincide with decreasing marine
oxygen conditions. On the contrary, low oxygen conditions prevailed in a decadal pattern prior
to 1900 and reflected climate influences such as the Pacific Decadal Oscillation (PDO).
Oxygen-rich conditions generally occurred mid-century.
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PDO Index
Mo/AI Ratio (hypoxia)
PDO Index
Mo/AI Ratio (Oxygenated)
CO
-2.0
1600
1700
1800
1900
2000
Figure 24: Sediment core hypoxia indicator and Pacific Decadal Oscillation indicator for the
period 1600-2005.
Source: Brandenberger et al. (2008).
Brandenberger et al. (2008) provides evidence of shifts in phytoplankton communities since
1900. The microfossil reconstructions for diatom and foraminifera were too limited to support
statistically valid conclusions. However, overall abundance of diatom assemblages declined
over time since circa 1900, with the most pronounced decline appearing after the 1950s (see
Figure 39 of Brandenberger et al, 2008; HC-5 core). This trend is opposite to expected results if
cultural eutrophication drives dissolved oxygen concentration. However, the number of genera
identified in the samples declined circa 1950s as planktonic species increased. These
relationships may indicate a recent trending toward eutrophic conditions in the last several
decades. Alternatively, the decrease in diatom abundance may confirm other paleoecological
markers (organic markers, biogenic silica, and redox-sensitive metals) that indicate a decline in
all proxies of water column productivity starting in the early 1900s and reaching a minimum in
the 1950s.
While sediment core analysis is extremely valuable in providing information about long-term
trends in water quality, the core samples cannot provide information about the most recent
decades of hypoxia in Hood Canal. The cores also cannot determine whether human activities
have exacerbated the naturally low oxygen levels in Central Hood Canal and the shallower
waters of Lynch Cove.
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Trends in Dissolved Oxygen Measurement Data
A long-term downward trend in dissolved oxygen over time would be one line of evidence
indicating that population growth could be impacting Hood Canal dissolved oxygen
concentrations. Dissolved oxygen in Hood Canal and elsewhere in Puget Sound and the Straits
has been measured periodically since the 1950s.
Bassin et al. (2011) analyzed trends in the dissolved oxygen data for five locations in Puget
Sound for the period from 1932 to 2009 using a seasonal Kendall test applied to decadal data.
The five locations were the Strait of Juan de Fuca; Admiralty Inlet, the traditional northern
boundary to define Puget Sound; Point Jefferson in the main basin, also called Central Puget
Sound; Central Hood Canal; and Lynch Cove. The analysis did not include Ecology's ambient
monitoring data collected in the 1970s and 1980s, although data exist for four of the five
locations. The analysis focused on dissolved oxygen concentrations at 100 to 200 meters depth
from the first three stations, an oxygen inventory below 20 meters across six stations in the main
arm of Hood Canal (Warner, 201 la), and concentrations at a depth of 20 meters in Lynch Cove.
The Lynch Cove site is shallower than the other stations.
Bassin et al. (2011) found a statistically significant decreasing trend in dissolved oxygen
concentrations from 1950 to 1999 in the Hood Canal (main arm) inventory (Figure 25). Because
the Strait of Juan de Fuca and Admiralty Inlet also exhibited declining dissolved oxygen for the
period 1950-2009, Bassin et al. (2011) did not rule out Pacific Ocean or other influences that are
common to all stations. All locations are subject to human influences as well. The trends vary
substantially depending on the time period used in the analyses. In fact, the Hood Canal oxygen
inventory increased for the period 2000-2009, and Lynch Cove exhibited the fastest increasing
rate for the same period. Temperature trends influence, but do not account for, the magnitude of
dissolved oxygen change. There is no evidence of a unique, negative trend in the Central Hood
Canal oxygen concentrations compared with other sites in Puget Sound.
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Predicted Change
^3
-1 -::;
1930 1940 1S50 196(1 1970 19BO 1990 2000 2010 I
'-'"^ ..
•"••>--:;'-------+
~
I
3
-a
c 1930 1940 U50 1960 1970 1960 1990 ?000 2010
a
*••
I 1
1
D --.
S, -1
?
O 2
Tr 1930 1940 1S50 1960 1970 19BO 19M 2000 201D
_>
a
I
-1
_l 1_
1930 1940 1950 1960 1970 19BO 1990 2000 2010
1540
1960 1980
Date
o
t:
Figure 25: Statistically significant trends (seasonal Kendall) in dissolved oxygen at five
locations in Puget Sound and the Strait of Juan de Fuca.
Source: Bassin et al. (2011).
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Many factors influence these trends, including the time period used for the analysis. As noted in
Brandenberger et al. (2008), the 1950s data-collection period coincided with a cool-phase Pacific
Decadal Oscillation (PDO), which is associated with higher oxygen levels. More recent
conditions include warm-phase PDOs associated with lower oxygen levels. Comparing data
from 1930-1966 with data over the last decade does not account for climate cycles. Because of
this cyclic pattern, differences between the 1950s and present day cannot be solely attributed to
humans causing a downward trend. In fact, the increasing trend in dissolved oxygen at Lynch
Cove, where human contributions are expected to have the largest influence, runs counter to
population growth.
Based on review of the available information, we have found no compelling evidence that
humans have caused decreasing trends in dissolved oxygen in Hood Canal. Climate cannot be
ruled out, the trends are not unique to Hood Canal, and dissolved oxygen in Lynch Cove, where
relative human influences are the largest, has increased and not decreased.
Oxygen Depletion Estimates Based on Aggregated Models
Both aggregated models (Devol et al., 201 la; Brett, 2010a) and the ROMS biogeochemical
model discussed in the next section were used to estimate dissolved oxygen impacts. The work
to date has focused primarily on estimating average summer impacts in Lynch Cove as a whole
rather than maximum impacts at specific locations and times. There has also been a strong focus
on OSS impacts and less emphasis on overall human impacts from the watershed as a whole.
Brett (2010a) and Devol et al. (201 la) used aggregated models to estimate the proportion of total
nitrogen loadings entering Lynch Cove due to human activity, and then applied this proportion to
the total observed dissolved oxygen deficit in Lynch Cove to estimate human impacts. The
calculations do not account for all processes affecting dissolved oxygen and represent
simplifications of complex systems. For example, circulation is represented as a simple two-
layer system, whereas current meter data show a more complex pattern (e.g., Figure 5 in Devol
et al., 201 Ib). Similarly, seasonal time frames that focus on June through September to represent
peak population and algae growth may or may not account for all influences on the system. For
example, the aggregated models do not assess whether spring nitrogen loadings affect summer
conditions and estimated impacts. Nevertheless, aggregated models can provide reasonable first
approximations of human impacts.
Total Observed Dissolved Oxygen Deficit in Lynch Cove
Both Brett (2010a, 201 li) and Devol et al. (201 la) estimated the total dissolved oxygen deficit
in Lynch Cove based on the data collected at the ORCA buoys at Twanoh and Hoodsport.
Brett (2010a) found that average annual dissolved oxygen concentration below 11 meters depth
was 1.1 mg/L less at the Twanoh buoy in Lynch Cove than at the Hoodsport and Duckabush
buoys in Central Hood Canal from 2005 to 2009. Figure 8 presents these data. If Lynch Cove is
compared to Hoodsport alone, the average difference was 0.7 mg/L (Brett, pers. comm., 201 li).
In contrast, Devol et al. (201 la) estimated a deficit of 1.6 to 2.3 mg/L based on summer
conditions at the Twanoh and Hoodsport buoys.
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Brett (pers. comm., 201 li) identified the significant disparity in the Lynch Cove deficit estimates
in Brett (2010a) and Devol et al. (201 la) despite the use of a common dataset from the ORCA
buoys. Based on additional data comparisons, Brett (201 li) concluded that the biggest factor
causing the disparity was the depth range used in the data analyses. Central Hood Canal is
significantly deeper (100 m at the Hoodsport buoy) than Lynch Cove (30 m at the Twanoh
buoy). The choice of the depth range was based on differing assumptions about the connectivity
of the deeper waters of Hoodsport (30 m to 100 m) to Lynch Cove (10 to 30 m). Brett (2010a,
201 la) used a depth range from 11 meters to the bottom, while Devol et al. (201 la) excluded
values from 30 meters to the bottom at Hoodsport.
Devol (pers. comm., 201 Id) analyzed the assumptions for mixing between Hoodsport and
Lynch Cove. He noted that ocean water flows freely along isopycnal (equal-density) surfaces,
and it takes much more energy to mix or flow across isopycnal zones. During the early and
mid-summer, prior to the late-summer ocean intrusion, the density of water at 25 meters is
approximately equal at Hoodsport and Twanoh (Lynch Cove). However, the density increases as
depth increases at Hoodsport. It requires energy to move this dense water up to a shallower
depth. In the absence of an external force, Devol noted, it is unlikely that water at 20-30 meters
depth in Lynch Cove either originates from or mixes with higher-density water at greater depths
at Hoodsport. For this reason, he excluded oxygen values below 30 meters depth at Hoodsport.
The independent review panel recommended that the deficit calculation focus on oxygen
conditions along similar densities (termed "isopycnal lines") between Hoodsport and Twanoh.
Devol (pers. comm., 201 Id) noted that this was the approach taken in Devol et al. (201 la). He
elaborated that the isopycnal lines are horizontal between Hoodsport and Twanoh for most of the
summer, and therefore common depths between the two locations should be used. This density
pattern would call for exclusion of dissolved oxygen data from 30 meters to the bottom at
Hoodsport from the comparison to Twanoh concentrations.
The aggregated modeling approach assumes that circulation conditions are constant for June
through September. For most of this period, it is reasonable to assume that mixing is constrained
within isopycnal zones. However, the intrusion of denser, low-oxygen marine water at depth in
September is a significant external forcing mechanism that alters the mid-summer balance. As
Devol (201 Id) notes, energy is required to overcome isopycnal mixing, and the September
intrusion provides an influx of mixing energy. It is plausible that the early-summer dissolved
oxygen deficit is best evaluated using the Devol et al. (201 la) approach, while the September
deficit may be better characterized by the Brett (201 li) approach.
Devol et al. (201 la) filtered data using a 25-day lag to address the travel time between the
Hoodsport and Twanoh buoys. It is not clear whether it is appropriate to incorporate a fixed time
lag into the analysis, because the travel time likely varies significantly with the onset of the
September intrusion. It is also difficult to discern a clear lag in time-series comparisons such as
in Figure 8. Furthermore, it appears that the 25-day travel time calculated by Devol et al.
(201 la) was based on circulation conditions during the September intrusion of saline ocean
water, when current speeds are unusually high compared to mid-summer conditions. We back-
calculated the velocity from the advective flow from Devol et al. (201 la) using the salt balance
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method. The resulting value (0.0005 m/s) is an order of magnitude lower than the velocity used
in the travel time calculation by Devol et al. (201 la).
For the Monte Carlo analysis of aggregated model estimates described below, we adopted the
oxygen deficit estimates of Devol et al. (201 la) that reflect stable isopycnal conditions in early
summer (June through early August). Based on concerns about the appropriateness of the fixed
lag assumption of 25 days, we selected the non-lagged values in Devol et al. (201 la). We used
the 20-30 meter depth range, where the mean deficit was 1.6 mg/L with a standard deviation
of 0.5 mg/L for 2006-2010. The deficit for the full depth range below the euphotic zone
(12-30 meters) was not estimated in Devol et al. (201 la).
OSS Impacts on Dissolved Oxygen
Steinberg et al. (2010) used a two-layer salt balance to quantify the relative contribution of
watershed and marine nitrogen loading on an annual basis, as described in Question 2. That
analysis found that shoreline and tributary OSS contributed at most 0.5% of the total annual
nitrogen loading to the main arm of Hood Canal and at most 8% of the annual loading to Lynch
Cove based on the per capita estimates. Steinberg et al. (2010) did not estimate dissolved
oxygen impacts associated with these human nitrogen contributions.
Subsequent analyses by Brett (2010a) used the estimates of the human contribution of nitrogen to
the surface layer from OSS, along with dissolved oxygen data for Hood Canal, to estimate the
human impact on dissolved oxygen from all OSS in the entire watershed. First, Brett (2010a)
found that average annual dissolved oxygen concentration below 11 meters depth was 1.1 mg/L
less in Lynch Cove based on the Twanoh buoy than in Central Hood Canal based on the
Hoodsport and Duckabush buoys from 2005 to 2009. Comparing to the Hoodsport buoy data
alone, the difference was 0.7 mg/L (Brett, 201 li). Figure 8 presents these data. Brett (2010a)
estimated OSS impacts of <0.07 mg/L in Lynch Cove by multiplying the dissolved oxygen
deficit (1.1 mg/L ) by the ratio of human OSS nitrogen to total annual nitrogen that reaches the
surface layer (4 to 8% from Steinberg et al., 2010).
Devol et al. (201 la) also employed aggregated models to estimate dissolved oxygen impacts,
but the analytical approach differed from that used by Brett (2010a). First, as detailed under
Question 2, Devol et al. (201 la) used a 3-layer model construct with different nitrogen loading
assumptions than Brett (2010a). In addition, Devol et al. (201 la) used higher estimates for
population and associated OSS loading from Richey et al. (2010). Because of these differences,
the associated fraction of total nitrogen loading to the surface water from OSS was significantly
higher in Devol et al. (201 la) than in Brett (2010a).
Devol et al. (201 la) also compared dissolved oxygen concentrations at Hoodsport and Lynch
Cove, but they narrowed the time frame to the summer months (June through September) and
restricted the comparison to water depths less than the maximum 30-meter depth of Lynch Cove
rather than the entire depth at Hoodsport. Devol et al. (201 la) also incorporated a 25-day lag
time in the analysis to account for travel time between Hoodsport and Twanoh. This analysis
resulted in estimated summer dissolved oxygen deficits in Lynch Cove, relative to Hoodsport,
ranging from 1.2 to 2.6 mg/L for water depths of 6 to 30 m.
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Devol et al. (201 la) calculated the human dissolved oxygen impact in Lynch Cove using an
estimated total dissolved oxygen deficit of 2 mg/L. Devol et al. (201 la) estimated OSS impacts
of 0.24 to 0.60 mg/L by multiplying the 2 mg/L deficit by the OSS fraction of total nitrogen
entering the surface layer of Lynch Cove in summer. The OSS fraction was 12 to 30% based on
OSS load of 2.9 metric tons per month (MT/mo) from Richey et al. (2011).
In a separate analysis, Devol et al. (201 la; see also Newton and Devol, pers. comm., 2011)
calculated the potential dissolved oxygen impact from total OSS loading to Lynch Cove using
stoichiometric ratios in phytoplankton. The authors considered this a check rather than a
prediction of impact (Newton and Devol, pers. comm., 2011). Assuming all of the OSS loading
produces phytoplankton biomass that sinks to the lower water column, Newton and Devol
(pers. comm., 2011) calculated that the respiration of 2.9 MT/mo attributed to total OSS nitrogen
(from Richey et al., 2010) would result in an oxygen impact of 0.3 mg/L. However, the
2.9 MT/mo includes a shoreline OSS estimate (2.6 MT/mo) based on a 1000-m buffer that
double-counts contributions also covered in tributary estimates.
Table 11 lists the available estimates of OSS impacts on Lynch Cove dissolved oxygen
concentrations in the summer. The disparity in the estimated oxygen impacts among researchers
is the result of differing methodologies and assumptions for estimation of OSS loadings, marine
nitrogen flux, and oxygen deficits in Lynch Cove.
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Table 11: Estimates of OSS impacts on dissolved oxygen in Lynch Cove in summer.
All values are means for June through September.
Study
Brett (20 lOa) and
Steinberg etal. (20 10)
Devoletal. (20 11 a)
Devoletal. (20 11 a)
and Newton and Devol
(pers. comm., 2011)
Method
Proportion of
Observed O2 Deficit
Proportion of
Observed O2 Deficit
Stoichiometric
Analysis of Biomass
Production/Decay
Proportion of
Surface Nitrogen
Loading from OSS
Sources (%)
4% - 8%2
12% - 30%3
NA
Lynch Cove
Total Dissolved
Oxygen Deficit
(mg/L)
1.1
2.0
NA
OSS Impact
to Dissolved
Oxygen Deficit1
(mg/L)
0.072
0.2-0.63
0.33
1 Combined shoreline and upland OSS contributions. These studies focused solely on impacts of OSS loadings rather
than total human impacts. Total impacts would include population-related loadings and the human-caused portion
of the red alder loadings in tributaries.
2 Dividing the annual estimates in Steinberg et al. (2010) by 12, the uniform monthly loadings associated with these
estimates are a sampling-based watershed loading of 1.45 MT/mo (3.5% of 496 MT/yr), anunsampled shoreline
OSS loading of 0 to 1.75 MT/mo (maximum 21 MT/yr; see table 6), and marine nitrogen flux of 37 MT/mo to the
full euphotic zone.
3 These values were calculated using a shoreline OSS loading of 2.6 MT/mo, additional watershed OSS loading of
0.3 MT/mo (see footnote 4 in Table 7), a marine nitrogen flux of 5 to 19 MT/mo to the pycnocline, and a total flux
of 9.5 to 23.5 MT/mo.
Probabilistic Estimation Using Aggregated Model Results
None of the studies published to date have attempted to compile a probabilistic assessment of the
impact of human nutrient loading on marine dissolved oxygen. The preceding sections describe
the "best estimates" of loadings and oxygen deficits in Lynch Cove, and generally describe the
uncertainty in those estimates. To supplement the available work, we estimated dissolved
oxygen impacts using the aggregated model coupled with a Monte Carlo approach. The
following equation represents the aggregated model calculation for human impact to dissolved
oxygen:
Y = (Nhuman/Ntotal ) *
where,
Y = Human impact on dissolved oxygen (mg/L)
Nhuman = Human loading of TDN to Lynch Cove surface layer (MT/mo) from all
watershed sources (shoreline plus tributary loadings)
Ntotai = Total loading of TDN to Lynch Cove surface layer (MT/mo) from all watershed
and marine sources
D = Dissolved oxygen deficit in Lynch Cove (mg/L)
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This can be expanded to identify the individual TDN loading components estimated by
researchers to date:
Y = ((NtnbH + N0ss)/( NtnbH + NtnbN + NOss + Nmarme)) * D
where,
NtribH = Human loading of TDN in tributaries (MT/mo) including watershed OSS and
mixed deciduous forest
NtribN = Natural loading of TDN in tributaries (MT/mo)
NOSS = Shoreline OSS loading (MT/mo)
Nmarine = Marine loading (MT/mo)
The Monte Carlo approach employs a parameter distribution for each of the terms on the right
side of this equation. Different sources and types of uncertainty are represented by the selected
distributions, but these distributions do not account for all of the sources and types of
uncertainty. Nevertheless, this approach provides an initial estimate of the potential range of
uncertainty. In general, when considering multiple sources of uncertainty, we adopted the largest
range. For example, for nutrient loadings from tributaries, we considered potential analytical
error in measurements of nutrients and tributary flow as well as the correlation error of the
statistical model used to estimate the origin of the nutrients (e.g., humans vs. natural sources). In
this case, we selected a conservative estimate of uncertainty in laboratory analyses for nutrients,
because this uncertainty was higher than the uncertainty in flow and model correlation (which is
also influenced by laboratory analysis error). The distribution characteristics based on best
professional judgment are listed in Table 12.
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Table 12: Assumed distributions for the uncertainty analysis.
Para-
meter
NtribH
NtnbN
Noss
(Flow
term)
Noss
(TON
term)
-L> marine
D
Distribution
Normal
Normal
Uniform
Random
sampling
of all data
Uniform
Normal
Units
MT/mo
MT/mo
m3/sec
mg/L
MT/mo
mg/L
Mean
1.0
0.2
NA
NA
NA
1.6
Std
Dev
0.20
0.04
NA
NA
NA
0.5
Range
NA
NA
0.5-1.1
NA
5-78
NA
Uncertainty Represented
General estimate of
analytical error for nutrient
concentrations
General estimate of
analytical error for nutrient
concentrations
Range of groundwater flow
estimates [Brett (201 Id) and
Paulson et al. (2006)]
Variability of DIN
concentration in 325 samples
of shoreline seeps in Hood
Canal (Sheibley and Paulson,
pers. comm., 2011)
Range of estimates based on
available models [Devol et al.
(201 la), Kawase and Bahng
(2012)]
Mean and variance of deficit
estimates in Devol et al.
(201 la)
Notes
(1)
(1)
(2)
(2)
(3)
(4)
Notes:
(1) For the tributary estimates, the mean value is selected from the statistical model of Steinberg et al. (2010) for
summer loadings. We selected a standard deviation value of 20% around the mean. This is a conservative
approximation of the error in laboratory analyses for nutrients (Mathieu, 2006). This choice is based on the
assumption that the analytical variation in nutrient concentration is the primary source of uncertainty in the nutrient
loading estimation, rather than the uncertainty in flow estimates.
(2) For the OSS loading, the flow distribution is uniform, with the range based on estimates of Brett (201 Id) and
Paulson et al. (2006). For TON concentration, there are sufficient data in the Hood Canal shoreline sampling data
(325 seep samples) to randomly sample from the data instead of using a parameterized distribution. The Hood
Canal data was selected over the data from Lynch Cove sites based on the scarcity of Lynch Cove data (23 samples)
and the similarity in the median values of the two datasets.
(3) For the marine flux term, we have noted the significant differences among researchers in assumptions and
methods as well as concerns about methodology identified by the independent review panel (PSI, 2012). For this
analysis, we identified the full range of available estimates and assumed a uniform distribution with an equal
probability of occurrence of any value in the range.
(4) For the Lynch Cove oxygen deficit term, we have used the average and standard deviation of multiple-year
estimates in Devol et al. (201 la) derived from spatial and temporal patterns in marine dissolved oxygen
concentrations.
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The Monte Carlo simulation was run with 5000 random samples drawn from the distributions
and groundwater seep data. The resulting distribution for the human impact estimate is shown in
th
Figure 26. The median predicted impact is 0.06 mg/L and the 10 and 90 rank percentile
predictions are 0.03 mg/L and 0.23 mg/L, respectively.
1400
1200
1000
> soo -
c
cu
3
CT
CU
i 600
400 -
200 -
Frequency
Cumulative%
120%
100%
80%
60%
40%
20%
0%
dddd dddd dddd dddd
DO Impact (mg/L)
Figure 26: Lynch Cove impacts based on Monte Carlo analysis.
Sensitivity of Monte Carlo Results to Marine Flux Range
The results are strongly dependent on the estimated range of uncertainty in the vertical flux of
marine nitrogen into Lynch Cove surface waters (5 - 78 MT/mo). This sensitivity is evident
when we apply the Monte Carlo simulation with fixed values for the marine flux at the low and
high bounds. A side-by-side comparison of these two sensitivity tests is shown in Figure 27.
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3000
2500
2000
c
0)
cr
-------
In summary, each of the aggregated model calculations relies on key assumptions and data
derivations that contribute uncertainty to the resulting estimates. The researchers' approaches
employ an averaging of system characteristics over long time spans (seasonal/annual) and large
spatial scales (Lynch Cove/Hood Canal and different portions of the water column), so they
provide screening-level estimates of mean impact. Researchers did not reach consensus on how
to quantify human impacts, and the different aggregated model approaches led to large variations
in the impact estimates. The Monte Carlo analysis accounts for the wide range of values used for
key model parameters. However, the available estimates have also been questioned by the
independent review panel, and researchers have not addressed the panel recommendations for
additional analysis. Notwithstanding these concerns, our current analysis of aggregated models
for Lynch Cove predicts a mean, summer human impact of 0.06 mg/L and an uncertainty range
of 0.3 to 0.23 mg/L. These results are inconclusive with respect to compliance with water
quality standards because they span the threshold for defining violations and the methodology
used by researchers must be reevaluated.
Oxygen Depletion Estimates Based on ROMS Model
Predictions
Because of the complex processes that affect dissolved oxygen conditions in marine waterbodies,
researchers and regulators often use water quality models to estimate human impacts to ambient
conditions. The most sophisticated models provide continuous simulations of the natural
processes that affect nutrients, algae, and dissolved oxygen in a water body. The use of spatially
explicit numerical models is very common in supporting management actions under the Clean
Water Act. Puget Sound, by virtue of its complex bathymetry, presents a unique challenge for
the model developer. Unlike a river, which can be effectively analyzed using a one-dimensional,
steady-state model, Puget Sound analysis requires development and application of a two- or
three-dimensional, time-varying model.
The ROMS model is the most comprehensive analytical tool used to date to estimate dissolved
oxygen impacts in Hood Canal. ROMS can incorporate all major processes that affect oxygen,
including boundary conditions, seasonal variability, vertical and horizontal mixing,
phytoplankton dynamics, and nitrogen loading, fate and transport. A water quality model
essentially consists of two models in one, because it must simulate both water movement
(circulation) and water quality. The two sub-models are typically developed in sequence,
starting with the circulation model.
Kawase (2007) and Kawase and Bahng (2010) documented the performance and limitations of
the ROMS hydrodynamic model. Kawase and Bahng (2012) describe the water quality model
of Hood Canal and a set of initial model scenarios. This report is a reasonable first step in
documenting model performance and application to initial model scenarios. However,
insufficient information is provided to complete a full review of important model components for
regulatory decision-making.
While the current documentation is limited, the report does include a number of plots comparing
observed and simulated water quality constituents, computed error statistics, and discussion of
model capabilities and limitations. In general, this information indicates that the model
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simulates the major processes affecting dissolved oxygen and provides plausible predictions of
both long-term average ("climatological") and 2006 water quality conditions for constituents that
affect dissolved oxygen. At the same time, the report highlights areas where the low model skill
is cause for concern. For example, higher-than-measured salinity in the surface layer and oxygen
at mid-range depths (see Figure 28 below) could indicate problems with circulation and mixing.
Kawase and Banng (2012) state that these errors would generally lead to an under-prediction of
dissolved oxygen impacts. The root mean square error for dissolved oxygen predictions ranged
from 0.4 mg/L (Hoodsport at depth) to 1.7 mg/L (Lynch Cove surface layer). In general, model
error decreased with depth.
Average Oxygen 4-1 Om, Hoodsport. ORCA
i I I I I I I I I I
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Average Oxygen 10-50m. Hoodsport ORCA
Observed, 2006
Observed, 2007
Observed, 2008
Observed. 2009
Modeled. 2005
Jan Feb Mar Apr May Jun
Aug Sep Oct Nov Dec
Average Oxygen below 50m, Hoodsport ORCA
_| L
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 28: Comparison of predicted and measured dissolved oxygen conditions at Hoodsport for
2006.
Source: Kawase and Bahng (2012).
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Kawase and Bahng (2012) ran three model scenarios using the long-term average boundary
conditions to evaluate the sensitivity of Hood Canal to changes in the nitrogen concentration of
the tributaries. These simulations do not explicitly include OSS loadings as boundary inputs.
All tributaries throughout Central Hood Canal and Lynch Cove were set to a uniform, fixed
concentration for nitrate-nitrite. The three selected concentrations were 80, 500, and 1,000 ug/L.
The lowest value (80 ug/L) represented a pristine tributary, while 500 ug/L represented impacted
tributaries. Since most tributaries are well below 500 ug/L, including large tributaries like the
Skokomish River, this second scenario is arguably well beyond worst case for the Canal as a
whole. The third scenario sets all tributaries to 1,000 ug/L, representing a severely impacted
condition. It is highly unlikely that all tributaries throughout Hood Canal could reach this
concentration in the foreseeable future.
Kawase and Bahng (2012) analyzed the results of the three tributary scenarios and determined
that the change in the predicted oxygen concentration was linearly proportional to the tributary
nitrate concentration. This is consistent with the assumptions used in the proportional
calculations and aggregated models (Devol et al., 201 la; Steinberg et al., 2010; and Brett 2010a).
This relationship enabled Kawase and Bahng (2012) to estimate impacts from any nitrate
concentration between 80 and 1,000 ug/L without re-running the model. In order to provide a
more realistic estimate of current impacts to Hood Canal, they applied linear interpolation to the
model output data to estimate the oxygen conditions that would occur with tributaries
discharging at 150 ug/L, based on the annual, flow-weighted average nitrate concentration for
tributaries to Hood Canal (Brett 201 Ib). After identifying the model grid cell with the highest
impact, which was located on the southeast corner of Lynch Cove, they calculated a 10-day
average impact at this location (Kawase, pers. comm., 201 Ig). Using this methodology,
Kawase and Bahng (2012) estimated that the maximum dissolved oxygen impact was 0.14 mg/L
in Lynch Cove.
Kawase (pers. comm., 201 la) provided additional modeling information for Lynch Cove to
clarify findings presented in Kawase and Bahng (2012). This focused on model outputs for the
difference in dissolved oxygen between the pristine scenario (tributaries set to 80 ug/L nitrate-
nitrite) and the impacted scenarios (tributaries at 150 ug/L and 500 ug/L nitrate-nitrite). To
facilitate comparison with the aggregated model results described above, Kawase (pers. comm.,
201 la) averaged the differences for all Lynch Cove model cells for depths 10 meters to the
bottom for each month. Table 13 presents these model outputs.
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Table 13: ROMS model predictions for average dissolved oxygen impact in Lynch Cove at
depths greater than 10 meters.
Values refer to differences in dissolved oxygen compared with the 80 ug/L run and are compiled to
compare with aggregated model results. Source: Kawase, pers. comm., 201 la.
Tributary
Loading
Scenario
150 ug/L
500 ug/L
June through September
Mean Dissolved Oxygen
Impact (mg/L)
0.04
0.3
The 150 ug/L scenario likely underestimates the actual impact in Lynch Cove for two reasons.
First, the model simulations did not explicitly include OSS discharges, which contribute to
human nitrogen loading in the summer in Lynch Cove. Second, Lynch Cove tributaries average
about 300 ug/L in the summer months (Figure 16), higher than the 150 ug/L scenario.
The 500 ug/L scenario likely overestimates the actual impact to Lynch Cove by setting the
tributaries to a significantly higher concentration than current conditions (Figure 16). The
scenario also sets the Central Hood Canal tributaries, including the nearby Skokomish River, to
500 ug/L. This is far higher than current concentrations of approximately 100 ug/L (Figure 16),
and would overestimate dissolved oxygen depletion at depth in Central Hood Canal.
Considering current tributary concentrations and OSS loads, the 150 and 500 ug/L scenarios
likely bracket the current watershed contributions. Based on available information, the 80 ug/L
scenario represents a reasonable natural background condition. Because dissolved oxygen
impact increases linearly with differential nitrogen load, the model scenario results indicate that
the average summer impact on Lynch Cove dissolved oxygen lies within the range of 0.04 to
0.30 mg/L.
The ROMS model was not developed to a level of refinement sufficient to resolve the date,
location, and magnitude of the maximum human impact. The model domain did not include
portions of Lynch Cove with a depth less than 10 meters, and this truncated a substantial area of
the Cove (Kawase and Bahng, 2010). Lynch Cove extends further east of the model grid
boundary and becomes gradually shallower. This tapering will change the pattern of oxygen
concentrations and impacts. Nevertheless, the model did provide general insights about mean
and maximum impacts. The model predicted that June was the month with the highest monthly
average impact from humans compared to a natural condition. The predicted impacts in
June were approximately 50% higher than the average impact for June through September
(Kawase, pers. comm., 201 la). In addition, model predictions indicated that the maximum
impact occurred at the eastern end of Lynch Cove, which was the area with the model grid
limitations noted above (Kawase, pers. comm., 201 Ib). If maximum impacts are assessed in the
future, the modeling work should focus on constructing a more extensive and refined model
representation of this area of Lynch Cove.
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Synthesis of Human Dissolved Oxygen Impact Estimates
The available estimates of human impact to dissolved oxygen in Hood Canal and Lynch Cove
have been produced using both aggregated box model calculations and the ROMS model. If all
methods are reasonably implemented, these methods should be in general agreement on overall
scale of impact (e.g., long-term average impact over large areas).
Table 14 provides a comparison of "best estimates" from aggregated model results and ROMS
model predictions for the human impact to dissolved oxygen in Lynch Cove. While the
aggregated and ROMS model analyses were not fully coordinated in terms of watershed loads,
spatial areas, or time periods, the results are in general agreement.
Table 14: Comparison of predicted human impact on average summer1 dissolved oxygen
concentration in Lynch Cove below the euphotic zone2.
Analytical Tool
Monte Carlo Analysis
of Aggregated Models
ROMS Model
Estimated Human Impact
(mg/L DO)
Median
0.06
NA
Range
0.03-0.23
0.04-0.3
1 June through September.
2 Depth of euphotic zone varies among researchers.
In summary, ROMS model simulations and aggregated models coalesce around an average
summer impact ranging from 0.03 to 0.3 mg/L. As noted earlier, the Washington water quality
standards restrict human impacts to 0.2 mg/L or less. The available models and analyses have
limitations that preclude a reliable estimate of impacts. Therefore, there is insufficient
information to conclusively evaluate compliance with water quality standards at this time.
Analysis of Episodic Fish Kill Events
A fundamental question that drove research was the degree to which humans contribute to the
episodic fish kills that have occurred near Hoodsport. The aggregated models were only applied
to Lynch Cove and were not used to assess the human contribution to low dissolved oxygen
where fish kills occur. However, the ROMS model was applied in Central Hood Canal as well
as Lynch Cove.
Kawase and Bahng (2012) provide impact estimates for the area in Central Hood Canal between
Hamma Hamma and Annas Bay that correspond to the zone for episodic fish kills. The
maximum impact from human sources in this area was 0.04 mg/L, which occurred in June.
Fish kills have occurred in September when the dissolved oxygen impact from human sources
was less than 0.04 mg/L. Kawase and Bahng (2012) concluded that outside of Lynch Cove, the
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"impact of nutrient loading from impacted streams is still minuscule and is not a significant
factor in issues such as fish kills in comparison with natural processes."
Other analyses of episodic fish kills in September 2006 and September 2010 provide strong
evidence that southwesterly wind events cause sudden surfacing of low-oxygen water to the
surface layer in Central Hood Canal, and that these wind events are the proximate cause of the
fish kills (Kawase, 2007; Devol et al., 2011; Newton, 2010; Newton et al., 201 la; Kawase and
Bahng, 2012). The hypothesis is supported by ROMS model simulations for September 2006
that predicted reduced dissolved oxygen at the surface of Central Hood Canal at the fish kill
location under a southwesterly wind (see Figure 29).
,10s
Surface Cwygen (mg/L): September 19, 2006
5.31
5.3
5.29
§5.28
5.27
5.26
5.25
| | | |
,10
4.9 4.95 5 5.05 5.1 5.15 5.2 5.25 5.3
UTM 10 Easting (m) x105
Figure 29: Water quality model prediction of surface oxygen during the 2006 fish kill.
Source: Kawase and Bahng (2012).
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None of the research to date has addressed the potential for fish kills due to chronically low
dissolved oxygen at any location in Hood Canal or Lynch Cove.
Connectivity between Hoodsport and Lynch Cove
Lynch Cove is the part of Hood Canal where human contributions have the greatest influence on
dissolved oxygen levels, and the most recent analyses of human impacts have focused on this
region. Three analyses (Paulson et al., 2006; Steinberg et al., 2010; and Kawase and Bahng,
2012) found no substantive link between human nutrient contributions and episodic fish kills
near Hoodsport. However, Mickett et al. (2011) documented a subsurface seaward outflow that
potentially connects the region where humans have the greatest influence on dissolved oxygen
(Lynch Cove) to the area and season of the episodic fish kills at Hoodsport. This has been
hypothesized as a factor contributing to fish kills.
Three processes potentially influence water column oxygen levels at Hoodsport: (1) near-bottom
dissolved oxygen declines over the summer and fall due to sediment and lower water column
processes, (2) mid-depth dissolved oxygen declines due to water column respiration, and (3) low
dissolved oxygen water at mid-depth is transported from areas landward by the subsurface
seaward outflow from August through October.
First, ORCA buoy data confirm that near-bottom processes draw down dissolved oxygen
throughout Hood Canal over the summer. However, Mickett et al. (2011) discounted local near-
bottom processes, since the near-bottom waters were not as low in dissolved oxygen as the
minimum values at mid-depths at Hoodsport. However, this does not rule out near-bottom water
from other regions transported seaward at mid-depths, including transport associated with the
intrusion of dense ocean water in late August/early September.
Second, water-column respiration depletes oxygen levels immediately below the euphotic zone.
As described in Devol et al. (201 la), primary productivity in the euphotic zone produces organic
matter that decomposes lower in the water column, with concentrations declining by 2 mg/L over
4 months. Mickett et al. (2011, and personal communication) ruled out respiration as causing the
dissolved oxygen minimum at 20 meters depth, based on bounding calculations that have not
been published. Mickett (2012) noted that Hoodsport ORCA data show that the lowest dissolved
oxygen concentrations are correlated with seaward flow. Also, given the relatively short
distance from Hoodsport to Lynch Cove (-15 km) and high current speeds in the subsurface
outflow (~ 5 cm/s), the time for respiration to act on this water during transport from Lynch
Cove to Hoodsport would be relatively short (-3.5 days). While respiration cannot be ruled out
as a process occurring within the seaward flow, the short travel time is a factor that reduces the
influence of respiration on the dissolved oxygen minima observed at Hoodsport.
The subsurface seaward outflow represents the third potential factor influencing the dissolved
oxygen minimum at Hoodsport. Three aspects of this phenomenon are described below:
(1) evidence from current velocity data, (2) source of water transported, and (3) volume of water
transported seaward at Hoodsport.
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Current velocity data indicate a subsurface seaward outflow at Hoodsport, corroborated by
patterns found at the Twanoh buoy and thalweg transects between the buoys conducted in 2006
and 2007. This subsurface seaward outflow appears seasonally in August, varies from year to
year, and intensifies at weekly time scales. Mickett et al. (2011) links this subsurface seaward
outflow to intrusions of high-density ocean water entering Hood Canal over the sill near Bangor.
Density varies with coastal upwelling conditions, which affects how the intrusion propagates
landward. Some proportion of the intrusion may propagate into Lynch Cove where it entrains
water, rises to a specific density layer, then moves seaward via a subsurface outflow (July 26,
2011 meeting with Cope, Roberts, Mickett, Devol, Warner, Brett, and Newton). The timing
coincides with low oxygen levels at Hoodsport, but it also occurs at other times of the year when
fish kills and low dissolved oxygen do not occur. Kawase (pers. comm., 201 If) reports a
persistent outflow at depth around Hoodsport simulated by the ROMS model.
The source of the lowest-oxygen water cannot be identified definitively based on the information
available but is inconsistent with Lynch Cove density at the time that minimum dissolved oxygen
levels occur. Current velocity and density data indicate that the water source is landward of
Hoodsport and from depths shallower than 50 m (Mickett et al., 2011) but cannot isolate the
region more specifically. The densities associated with the initial dissolved oxygen minimum at
Potlatch in August 2006 propagated there from the landward direction and could have originated
in Lynch Cove itself (Mickett et al., 2011 and Warner, pers. comm., 201 Ib). However, by
September 2006, the density was greater than that measured at Sisters (Mickett et al., 2011;
Warner, pers. comm., 201 Ib). Therefore, the water source feeding the subsurface seaward
outflow identified at Hoodsport changed as dissolved oxygen continued to decline and was
consistent with a shift to upwelled deep water (Warner, pers. comm., 201 Ic). In 2007, the
minimum dissolved oxygen at Potlatch also indicated a denser, more seaward source than the
water at Sisters Point.
Estimates of the volume of water transported by the subsurface seaward outflow varied widely
among researchers. These were expressed as the ratio of low dissolved oxygen water at
Hoodsport to water transported by the subsurface seaward outflow. Mickett et al. (2011)
estimated a ratio of 3 or 4 to 1, meaning the volume of water transported from landward of
Hoodsport is 33% or 25% of the volume of low dissolved oxygen water at Hoodsport. Details of
the calculation were not presented. More recently, Mickett (2012) revised this estimate to 1 or 2
to 1, meaning the transport volume represents 100% or 50% of the low dissolved oxygen water
at Hoodsport. The revised calculation is based on estimates of the entire volume of water
landward of the Twanoh ORCA mooring (3.9xl08 m3) and the volume of water associated with
the outflow at Hoodsport (10m thick x 2000 m wide x 20000 m long, or 4xl08 m3). The higher
2 to 1 ratio was based on a higher assumed thickness of 20 m (Mickett, 2012).
Brett (pers. comm., 201 Ij) estimated the ratio of Lynch Cove water volume to the volume of
near-bottom low dissolved oxygen water seaward of Sisters into the Central Hood Canal as 25 to
1 or 4%. However, this estimate included more than just the water volume associated with the
subsurface seaward outflow. Finally, Warner (pers. comm., July 26, 2011 meeting) estimated
the ratio of entrained low dissolved oxygen water from landward of Sisters to the low dissolved
oxygen water identified at Hoodsport as 7 to 1 or 14%; the details have not been published.
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Researchers estimated that the subsurface seaward outflow is capable of transporting 4 to 100%
of the low dissolved oxygen water observed at Hoodsport. This range does not resolve whether
the subsurface seaward outflow is a major or minor contributor to the concentration and volume
of low dissolved oxygen water. Researchers could not reach consensus on interpretation. If
Mickett (2011, 2012) is correct, and Lynch Cove water is the dominant source of low dissolved
oxygen water contributing to fish kills at Hoodsport, then the human contribution to fish kills is
at most the human impact on Lynch Cove dissolved oxygen. A detailed hydrodynamic modeling
assessment would be needed to better understand the overall patterns of water exchange between
Lynch Cove and Hoodsport, as well as the effect of the fall intrusion on that exchange.
The proximity of the subsurface seaward outflow to the location of episodic fish kills has also
been questioned. Brett (pers. comm., 201 li) noted that the subsurface seaward outflow mainly
occurred mid-thalweg and not in nearshore areas where fish kills have been observed. Mickett
(pers. comm., 2012) noted that the exact location offish mortality is unknown and that
assumptions that mortality occurs in the nearshore may be erroneous. However, diver surveys
have documented gasping marine life in shallow and deep water near the southwest margin of
Central Hood Canal at Hoodsport.
Finally, the influence of human nitrogen contributions in Lynch Cove on episodic fish kills at
Hoodsport cannot be greater than the human impact on Lynch Cove dissolved oxygen. Our
synthesis of available information indicates that this impact ranges from 0.03 to 0.3 mg/L on
average over the summer. This magnitude of human impact remains inconclusive based on
analytical issues described earlier. Also, no research has addressed whether a decrease of 0.03 to
0.3 mg/L would change the occurrence of a fish kill.
Impacts would be lower in the specific time frame of the September fish kills because of the
increased marine nitrogen entering the system through the fall intrusion. The estimated impacts
would also be lower in the Hoodsport area, due to the increased marine nitrogen flux closer to
the sill and lower nitrogen concentrations in the tributaries in the Olympic range, including the
Skokomish River, compared with Lynch Cove tributaries. The ROMS model predicted a
maximum human impact of 0.04 mg/L in the Hoodsport vicinity. However, this maximum
impact was predicted for the month of June, so predicted impacts for September were lower.
Even if a fraction of the subsurface seaward outflow originates in Lynch Cove, it is unlikely that
this water is degraded enough by human activities to affect the episodic fish kills in the
Hoodsport area.
In summary, the current velocity data indicate that an episodic subsurface seaward outflow
occurs from August through October. Some instances of the outflow coincide in time and space
with lowest dissolved oxygen conditions. The source of the low dissolved oxygen water cannot
be identified definitively. While this phenomenon was hypothesized as a linkage between
human nitrogen loadings to Lynch Cove and episodic fish kills at Hoodsport, several analyses
contradict this:
• The source water for the subsurface seaward flow has not been conclusively assessed, and
density patterns suggest that the source water changes between August and September due to
the inflowing marine waters of the fall intrusion. While Lynch Cove water may have
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contributed to the Hoodsport dissolved oxygen minimum in August, density signatures were
consistent with upwelled near-bottom water and not Lynch Cove water by September. This
indicates that Lynch Cove water did not influence dissolved oxygen conditions at Hoodsport
at the time of September fish kills.
• Researchers were unable to reach consensus on how much water the subsurface seaward
outflow is capable of transporting, ranging from minor amounts to the entire volume of low
dissolved oxygen water at Hoodsport. A detailed modeling study would be needed to
understand these water exchanges.
• Even if the source water is Lynch Cove, and it represents the bulk of the low dissolved
oxygen water at Hoodsport, then the relative contribution from humans is at most the human
impact on dissolved oxygen in Lynch Cove. This remains inconclusive, but best available
estimates are 0.03 to 0.3 mg/L. No researcher has addressed whether this decrease would
change the occurrence of a fish kill.
• Even if a fraction of the subsurface seaward outflow originates in Lynch Cove, available
information indicates that the oxygen concentration in the outflow is minimally impacted by
human-released nitrogen.
• The subsurface seaward outflow affects the thalweg, or center, of the channel, and not the
nearshore regions where fish mortality has been observed.
Based on these considerations, we concur with the independent review panel conclusion that
"the evidence that low DO (dissolved oxygen) water from LC (Lynch Cove) makes a
contribution to episodic fish kills at Hoodsport is weak." By extension, there is no compelling
evidence that human nitrogen releases to Lynch Cove significantly contribute to fish kills at
Hoodsport.
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Uncertainty
Overview
The complex linkage between nutrient loadings and phytoplankton growth is a fundamental
source of uncertainty in the analysis of nutrient effects on hypoxia. While researchers agree on
the basic linkage of nitrogen loading, phytoplankton production, and dissolved oxygen depletion,
phytoplankton productivity is highly complex and variable. The independent review panel
highlighted this fundamental uncertainty in its report (PSI, 2012):
Phytoplankton community enumeration data from the Hood Canal (Pacific Shellfish Institute,
2008, 2009 progress reports) indicate that the phytoplankton biomass responses to
environmental factors (including N inputs) are dominated by episodic blooms. These blooms
do not always quantitatively track N inputs, but rather appear to be a response to several
environmental factors that contemporaneously control their formation. These physical
controlling factors include light, temperature, flushing and residence times, and vertical
mixing, in addition to grazing. Therefore, even though N is likely to be the main nutrient that
controls (limits) phytoplankton production, it is difficult to develop a strong and predictable
direct relationship between N inputs and phytoplankton growth/bloom responses, especially
over the time frame (days to weeks) that blooms typically develop, die and sink into bottom
waters where they could fuel hypoxia.
The effort to quantify the impact of human-caused nitrogen loadings on dissolved oxygen must
be conducted with an acknowledgement of the complexity and variability of the aquatic
environment.
Aggregated and Three-Dimensional Marine Models
All of the estimates of human impact are derived from quantitative water quality models of
Hood Canal or Lynch Cove. These models range from the steady-state aggregated models of
Devol et al. (201 la) to the high-resolution, three-dimensional water quality model that simulates
nitrogen, associated phytoplankton biomass, and dissolved oxygen concentrations (Kawase and
Bahng, 2012). Both simple and complex models play an important role in diagnosing and
analyzing water quality impacts.
Aggregated Models
Aggregated box models are highly simplified representations of water quality processes,
circulation patterns, and seasonal variation. The various aggregated models developed for Hood
Canal are screening-level tools appropriate as first approximations. Aggregated models employ
an averaging of system characteristics over long time spans (seasonal/annual), large areas
(Lynch Cove/Hood Canal), and portions of the water column (surface mixed layer). By
definition, they will provide only estimates of mean impact over large areas and seasonal time
frames.
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In addition to the inherent limitations of aggregated models, this report and the independent
review (PSI, 2012) identified several methodology problems in the aggregated models for Lynch
Cove. These problems included problematic model assumptions and mathematical formulation,
gaps in the data analyses, and inadequate documentation. No re-analysis has been conducted to
address the technical points raised by the independent review panel. This report continues to rely
on the available information, but the aggregated models should be re-analyzed before adopting
the results in a regulatory decision. The uncertainty associated with the model formulations, as
well as the widely varying estimates of marine nitrogen flux, renders the Lynch Cove aggregated
model results inconclusive.
ROMS Water Quality Model
Prediction error occurs in all water quality models, but it is particularly difficult to calibrate the
nutrient, phytoplankton, zooplankton, and dissolved oxygen interactions in a dynamic, three-
dimensional model such as the ROMS application to Hood Canal. Typically, the model is run
repeatedly with varying input parameters in an attempt to reduce error in predicting measured
values. Once this process has run its course, constrained by project goals, resources, and
schedule, modelers decide to close the calibration process and run predictive scenarios with the
model. In calibrating the biogeochemistry model, Kawase and Bahng (2012) explored sensitivity
to a large number of model parameters; however, the exploration was by no means exhaustive or
complete. Different combinations of parameters can give rise to nearly identical results
(Kawase, pers. comm., 201 Id).
If additional time and funding are available, supplemental data collection may improve model
performance. Kawase and Bahng (2012) noted the absence of particulate biomass data at both
the model boundary and locations within the model domain. This data gap contributes to
uncertainty in the model's total nitrogen budget, and it has hindered calibration of parameters
pertaining to the behavior of particulate matter in the model (Kawase, pers. comm., 201 Id).
Once the model application matures to the point where model skill is sufficient to address
specific management questions, the method of evaluating the questions can also introduce
uncertainty. The scenarios run for Hood Canal were highly idealized. Rather than defining
tributary inflows to the model domain individually and assigning nitrogen levels using measured
conditions, a uniform and constant concentration was applied to all tributaries. Another issue is
the absence of explicit groundwater inflows and shoreline OSS loads to marine waters in both
the calibration runs and scenarios.
More thorough documentation and review of the model is needed before recommending its use
for regulatory action. Improvements can also be made in quantitative accuracy and scenario
construction. Nevertheless, the model is a very important analytical tool for dissolved oxygen
analysis, because it simultaneously and continuously accounts for processes that affect dissolved
oxygen at a level of spatial resolution that captures the variable bathymetry of Hood Canal. All
of the other approaches simplify the system to estimate marine loadings of nitrogen and
dissolved oxygen impacts.
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Uncertainty in the behavior of Lynch Cove also limits the application of the ROMS model to
Lynch Cove for regulatory purposes. However, even given the uncertainty related to model
performance, the ROMS model generally corroborates the findings from the aggregated models
that humans have a negligible impact on Central Hood Canal (Steinberg et al., 2010) and an
impact on the order of 0.03 to 0.3 mg/L in Lynch Cove.
Uncertainty in Nitrogen Loading Estimates
Based on the range of estimates in the studies reviewed and independent review comments, it is
evident that there is substantial uncertainty in the reported estimates of both human and natural
nitrogen loading. Each section of this review discusses uncertainty. The Monte Carlo analysis
quantified the uncertainty in oxygen impact estimates due to uncertainties in the marine and
human loading estimates.
It is not practical to list all of the assumptions and data gaps that contribute to the uncertainty in
loading estimates here, but we offer the following examples of important areas of uncertainty:
• Uncertainty in estimation of marine nitrogen flux to the euphotic zone. Factors include
natural variability and limitations in methodology identified in this report and by the
independent review panel.
• Uncertainty in estimation of groundwater and OSS loading. Factors include variability in
shoreline nitrogen samples, assumptions of the basin water budget, and fate and transport of
loadings at variable distance and depth relative to the receiving water.
• Uncertainty in the loading estimates from the statistical model used to analyze tributary
loadings. Factors include selection of model parameters and model prediction error.
These uncertainties stem from a multitude of challenges in the overall analysis. Some sources of
uncertainty can be quantified in a straightforward manner, such as the variability of sampling
data. Some uncertainties, however, are more nuanced and difficult to quantify, such as the
uncertainty associated with the omission of a potential nutrient sources or use of estimation
methods that are over-simplified.
The available studies have not attempted to aggregate the uncertainty of individual component
analyses (e.g., groundwater loading, watershed loading, marine fluxes, oxygen deficits) into an
aggregate uncertainty range around the estimates for the dissolved oxygen impact. In this
review, we have primarily focused on identifying all relevant information and reviewing the
appropriateness of key analytical methods. These areas took priority over quantification of
aggregate uncertainty. At the same time, the simplicity of the aggregated model approach
allowed us to apply a simple Monte Carlo analysis to estimate the range of uncertainty in the
estimates of human impact to dissolved oxygen in Lynch Cove.
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Factors Not Explored in Available Documents
Several other topics are relevant to the question of dissolved oxygen in Hood Canal but were not
addressed in any published documentation.
Future Population Growth
All available studies of human-caused nitrogen loading are based on existing residential
development and forest conditions. To inform management decisions, future studies should
account for potential changes in the watershed in the coming decades.
Potential Effects of Sediment Enrichment
Lynch Cove sediments likely have a substantial influence on dissolved oxygen conditions and
nutrient dynamics in this shallow area of Hood Canal. Over the long term (years to decades),
sediments can be enriched if human releases of nutrients cause a significant increase in algae
biomass, and this enrichment increases the sediment oxygen demand (SOD) on the overlying
water. In addition, sediment enrichment affects the release of dissolved nutrients from sediments
and denitrification in low-oxygen waters near the bottom. No Lynch Cove or other sediment
observations are provided in the studies reviewed.
Some of the available studies provide preliminary analysis of sediment-related processes. The
ROMS model (Kawase and Bahng, 2012) included SOD as a model parameter but did not
document the sensitivity of predictions to the selected input value for SOD. The model also
included a function that assumed that particulate nitrogen reaching the bottom was remineralized
as ammonium. Engstrom et al. (2009) analyzed denitrification and anaerobic ammonium
oxidation in the sediments at sites within Hood Canal. Several researchers have noted that low
oxygen conditions at the bottom of Lynch Cove are conducive to denitrification of groundwater
seeps. Sheibley (pers. comm., 2011) describes denitrification at the groundwater-marine
interface as a major loss mechanism, with rates of 1480 MT/yr (0 to 4800 MT/yr) for Lynch
Cove. However, results of the USGS flask studies have not yet been published.
Future studies should consider the potential for long-term enrichment of sediments, particularly
in Lynch Cove, and the effects of enrichment on water quality.
Potential Effects of the Hood Canal Bridge and Skokomish River Diversion
Located at the entrance to the Canal, the Hood Canal bridge rests on large floating pontoons in
the surface layer of the Canal. Natural circulation is characterized by the seaward flow of
freshwater in the surface layer. A recent paper, published after the independent and stakeholder
review processes, evaluated the potential impact of the bridge on circulation (Khangaonkar and
Wang, 2013). Based on preliminary results from an FVCOM hydrodynamic model of Hood
Canal, Khangaonkar and Wang (2013) concluded that the presence of the floating bridge may
increase the residence times in the basin by 8 to 13%. This effect on circulation may also affect
water quality, but additional work would be needed to quantify effects on dissolved oxygen.
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Management of the Skokomish River may also impact circulation (Newton et al., 2010). While
the annual freshwater flow to Hood Canal from the river would be unchanged, the timing and
location of flows from the Lake Cushman reservoir and hydroelectric facility could alter local,
seasonal circulation. Similarly, Bremerton draws its water supply from the upper Union River
watershed, which could affect streamflows. This potential influence was not described in any
current Hood Canal materials.
The effects of the bridge and river management can be examined using mathematical models of
circulation and water quality. It would difficult with existing data alone to tease apart these
influences through data analysis because of the complex interplay of climate and human
contributions, as well as data limitations from the period prior to construction of these structures.
The Hood Canal bridge was built in 1961 and the Skokomish River dams even earlier. Any
reductions to circulation due to these factors also coincide with cyclic climate influences.
Additional analysis would be warranted before ruling in or ruling out an influence of these
factors on Hood Canal dissolved oxygen.
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Hood Canal Science Review Summary
This review and interpretation incorporates information from published documents including
draft and final reports, journal articles, and memoranda. Supplemental information was provided
by Hood Canal researchers in meetings, conference calls, and email communications. Based on
the best available information, we conclude the following:
1. Sediment cores establish that hypoxia occurred before European settlement, and oxygen
levels in Hood Canal were lower before 1900 than between 1900 and 2005. Shifts in organic
matter associated with human watershed activities did not coincide with shifts in low oxygen
conditions in Hood Canal. On the contrary, prior to the 1900s, low oxygen conditions
prevailed in a decadal pattern that is consistent with climate influences (Brandenberger et al.,
2008 and 2011).
2. There is no compelling evidence that humans have caused decreasing long-term trends in
dissolved oxygen in Hood Canal. Climate effects cannot be ruled out, the decreasing trends
are not unique to Hood Canal, and dissolved oxygen in Lynch Cove, where relative human
influences are the largest, has increased and not decreased (Bassin et al., 2011).
a. While the Hood Canal dissolved oxygen inventory shows a decline between the 1950s
and 2000s, this pattern is also found in the Strait of Juan de Fuca and in other parts of
Puget Sound and is not unique to Hood Canal. The decline is consistent with the
decades-to-centuries climatic pattern that Brandenberger et al. (2008 and 2011) found in
sediment cores.
b. Lynch Cove, the area east of Sisters Point, is the part of Hood Canal where human
contributions are most likely to influence dissolved oxygen concentrations. Lynch Cove
dissolved oxygen concentrations show a statistically significant increase in
concentrations between the 1950s and 2000s. This pattern is counter to the decline in
the Hood Canal dissolved oxygen inventory, declines in other areas of Puget Sound, and
expected patterns if human contributions strongly influence dissolved oxygen levels.
3. Hood Canal phytoplankton growth is likely nitrogen-limited, so nitrogen mass loading to the
euphotic zone directly affects dissolved oxygen conditions. The predominant overall source
of nitrogen to Hood Canal is Pacific Ocean nitrogen entering as deep waters and entraining in
the surface layer of Hood Canal. This dominant contribution of marine nitrogen holds
throughout the year and throughout the main arm of Hood Canal and into Lynch Cove
(Steinberg et al., 2010; Paulson et al., 2006; Devol et al., 201 la; and Kawase and Bahng,
2012).
4. Human activities have increased nitrogen loadings to portions of Hood Canal from tributaries
and shoreline groundwater compared to natural conditions.
a. Most of the tributary loading occurs between November and May, whereas severe
hypoxia and fish kills have occurred in the late-summer months. Humans add to natural
nitrogen loads directly through residential development, primarily on-site sewage systems
(OSS), and indirectly via red alders. Red alders fix nitrogen from the atmosphere and
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exist today in higher numbers than in past years due to logging activities and slow
regrowth of conifers.
i. In Lynch Cove, tributaries contribute 40% to 80% of the total nitrogen loading from
the watershed in the summer months. Red alders and residential development
contribute approximately 50% and 33% of the tributary loading, respectively
(Steinberg et al., 2010; Richey et al., 2010).
b. Nitrogen loadings entering Central Hood Canal and Lynch Cove from groundwater and
shoreline OSS are difficult to quantify because of variability in the natural and human
environment, limited sampling information, and uncertainties in fate and transport of
subsurface wastewater.
i. Based on the best available estimates, loadings from shoreline OSS discharges in
Lynch Cove range from 20% to 60% of the total nitrogen loadings from the
watershed in the summer (see synthesis of Paulson et al., 2006; Georgeson et al.,
2008; and Sheibley and Paulson, pers. comm., 2011).
Lynch Cove is the part of Hood Canal where human contributions have the greatest relative
influence on marine dissolved oxygen. For waterbodies with naturally low dissolved oxygen
concentrations, Washington State water quality standards allow a maximum impact of
0.2 mg/L due to human activities. The available studies for Lynch Cove are inconclusive on
compliance with this standard due to substantial uncertainty in the methods, marine nitrogen
loadings, and averaging over time and space.
a. For Lynch Cove, aggregated models and three-dimensional model simulations coalesce
around an average summer impact ranging from 0.03 to 0.3 mg/L (see synthesis of
Steinberg et al., 2010; Richey et al., 2010; Paulson et al., 2006; Georgeson et al., 2008;
Sheibley and Paulson, pers. comm., 2011; Devol et al., 201 la; and Kawase and Bahng,
2012).
b. Substantial uncertainty remains in the estimates of human impact to dissolved oxygen,
particularly in estimates of marine nitrogen loadings to Lynch Cove. Other sources of
uncertainty include the inability of simple models to represent complexity of the system,
insufficient spatial and temporal data, and factors omitted from analysis. In addition, the
maximum impact cannot be addressed quantitatively in Lynch Cove due to limitations of
currently available models. Due to the uncertainty and analysis limitations, it is not
possible to definitively assess with the available information whether the water quality
standards are currently violated by human-caused nitrogen loads.
c. Variability in shoreline groundwater nitrogen concentrations is a key area of uncertainty
and concern. Groundwater quality is generally good, but a small number of seeps have
extremely high nitrogen concentrations. These seeps have a disproportional influence on
the overall human loading of nitrogen. Pollution Identification Program (PIC) efforts
have shown the ability to drastically reduce these concentrations through improved OSS
management.
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Researchers have unraveled the conditions that lead to episodic fish kill events in Central
Hood Canal. These fish kills occur due to a cascade of natural events. Dense marine water
enters Hood Canal and pushes water with low oxygen levels toward the water surface. As
river inflows decline during the dry season, the freshwater cap on the surface, which
normally prevents the denser water from surfacing, becomes thinner. Southwest wind events
push this thin cap to the north, which allows low-oxygen water beneath it to surface rapidly.
In a matter of hours, oxygen levels rapidly decrease in the sensitive nearshore regions of
southwestern Hood Canal (Newton et al., 201 Ic; Kawase and Bahng, 2012), which causes
the episodic fish kills at Sund Rock near Hoodsport.
a. Because of the large-scale nitrogen loadings from marine waters, human-caused nitrogen
discharges contribute to less than 1% of the nitrogen loading that fuels low dissolved
oxygen in Central Hood Canal. In the main arm of Hood Canal from Lilliwaup to
Potlatch, where fish kills have been reported, initial model simulations indicate very
small impacts to dissolved oxygen from human-caused nitrogen loadings (Kawase and
Bahng, 2012). The maximum impact was 0.04 mg/L in June and lower in September
when fish kills occur.
b. There is no compelling evidence that the subsurface seaward outflow has a significant
effect on fish kills at Hoodsport. Density signatures are consistent with upwelled near-
bottom water and not Lynch Cove water when fish kills occur in September.
Additionally, regardless of the source of the water in the seaward outflow, the oxygen
concentration within the outflow is no more than the impact of human nitrogen sources
on Lynch Cove dissolved oxygen, which cannot be determined conclusively with the
available information.
c. In addition to causing highly publicized fish kills, chronic low dissolved oxygen observed
over large areas of Hood Canal and Lynch Cove can stress or kill marine organisms
(particularly bottom-dwelling organisms). This was not directly addressed by available
studies, although the Lynch Cove dissolved oxygen analysis is applicable.
d. Long-term, natural variability in dissolved oxygen conditions identified in sediment cores
(Brandenberger et al., 2008) indicates a potential for lower dissolved oxygen conditions
and higher biological impacts in the future compared to the available monitoring record.
These conditions could be caused by more severe natural background conditions and/or
higher population and development in the watershed. No studies quantitatively assessed
effects of potential future nitrogen loadings on dissolved oxygen.
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Recommendations for Future Technical Work
Given the substantial limitations and uncertainties in the work to date, additional study of human
impacts to Lynch Cove may assist planning and regulatory activities in the watershed in the
future. A separate effort documents available regulatory options at the state and federal level
(Baldi and Eaton, 2012). Some options do not require that technical uncertainties be resolved
before taking management actions. The management actions should inform which technical
uncertainties are addressed in future technical efforts.
We recommend the following actions to improve the estimates of human impact on dissolved
oxygen:
1. Develop Refined 3D Biogeochemical Model of Hood Canal and Lynch Cove
We recommend development of a biogeochemical model that more fully represents the
physical characteristics of Lynch Cove (e.g., shallow areas, wetting/drying) and reasonably
predicts the observed vertical structure in dissolved oxygen, nitrogen, and chlorophyll a.
This may require additional observations to describe key processes such as sediment-water
interactions. The modeling should also include estimates of the effects of the Hood Canal
bridge and future nitrogen loadings on circulation and oxygen conditions.
2. Organize, Archive, and Share Hood Canal Dissolved Oxygen Program (HCDOP) Data
The extensive raw data from tributary and marine sampling collected by HCDOP is not
readily available to researchers outside HCDOP. These data should be organized and
archived on CDs and/or a publicly available website so that other organizations and
individuals can conduct additional modeling and analysis.
3. Re-analyze Marine Nitrogen Flux
While development of a refined biogeochemical model is the top priority, re-analysis of the
nitrogen flux calculations for the aggregated model of Lynch Cove would provide valuable
screening-level information for comparison to estimates of a new 3D model recommended
above. This task was recommended by the independent review panel based on concerns
about the methodology used to date.
4. Continue Shoreline Seep/Groundwater Monitoring
Shoreline sampling of seeps provides important information about the quality of groundwater
entering Lynch Cove from the shoreline. Continued investigation of the sources of high
nitrogen seeps will improve estimates of shoreline on-site sewage systems (OSS) loadings
and identify failing or poorly managed OSS.
5. Analyze Sediment-Water Nitrogen Fluxes
Several researchers have noted that low oxygen conditions at the bottom of Lynch Cove are
conducive to denitrification of groundwater seeps. This process may be mitigating the
Page 95
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nitrogen flux to Lynch Cove from shoreline OSS. A better understanding of the role of
denitrification in the Lynch Cove nutrient budget would help estimate the overall budget of
nitrogen sources and sinks in the watershed.
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Glossary, Acronyms, and Abbreviations
Term
Advection
Aggregated
model
Ammonium
Biota
cfs
Clean Water
Act
DIN
Dissolved
oxygen
DON
DSEM
Ecology / ECY
Eddy diffusion
EPA
Estuarine
circulation
Euphotic zone
Fish kills
HCCC
HCDOP
Hood Canal
Hypoxia
Isopycnal
Loading
Lower
Hood Canal
Lynch Cove
m3/s
Marine water
mg/d
mg/L
mo
Definition
The transport of water due to the bulk movement of water parcels, such as through tidal
exchanges.
Simplified representation of complex system that is used for screening-level
assessments.
Form of nitrogen present in natural waters and effluent from wastewater treatment
systems. Part of the dissolved inorganic nitrogen (DIN). Ammonium is the form of
ammonia present at pH values typical of natural waters and wastewater effluent.
Flora (plants) and fauna (animals)
Cubic feet per second
Federal act passed in 1972 that contains provisions to restore and maintain the quality of
the nation's waters. Section 303(d) of the Act establishes the Total Maximum Daily
Load (TMDL) program.
Dissolved inorganic nitrogen, which is the sum of nitrate, nitrite, and ammonium. The
forms of nitrogen most readily available for biological processes.
A measure of the amount of oxygen dissolved in water
Dissolved organic nitrogen
DHSVM watershed model (Distributed Hydrology Soil Vegetation Model) linked to a
solute export model (SEM)
Washington State Department of Ecology
The mixing of substances within waters due to turbulent diffusion processes.
U.S. Environmental Protection Agency
Water circulation pattern that results from a net inflow of marine water near the bottom
of the water column coupled with a net outflow of fresher water near the surface.
Surface water layer where light fuels photosynthesis
Observed fish mortality events caused by natural and/or human factors
Hood Canal Coordinating Council
Hood Canal Dissolved Oxygen Program
Marine waters south of Foulweather Bluff and Tala Point. Sometimes defined by the sill
or Hood Canal floating bridge.
Low dissolved oxygen
Line or surface of constant density, which in marine waters includes the effects of both
salinity and temperature.
The input of pollutants into a waterbody
Portion of Hood Canal generally defined as the region east of Sisters Point.
Portion of Hood Canal defined as either the region east of Sisters Point (equivalent to
Lower Hood Canal) or the shallowest portions of Lower Hood Canal.
Cubic meters per second
Salt water
Million gallons per day
Milligrams per liter, equivalent to parts per million
Month
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Term
MT
NA
Nitrate
Nitrite
Nonpoint
sources
NPDES
Nutrient
ORCA
OSS
PDO
PIC
PNNL
Point
sources
ppt
Puget Sound
Pycnocline
ROMS
Sediment core
Seeps
Sill
Sinks
SOD
TON
TN
Total Kjeldahl
nitrogen
ug/L
Definition
Metric tons, equivalent to 1,000 kg
Not applicable
Form of nitrogen present in natural waters and effluent from wastewater treatment
systems. Part of the dissolved inorganic nitrogen (DIN).
Transitional form of nitrogen present in raw wastewater but not typically found in
natural waters or effluent from wastewater treatment systems. Part of the dissolved
inorganic nitrogen (DIN).
Pollution that enters any waters of the state from any dispersed land-based or water-
based activities, including but not limited to atmospheric deposition, surface-water
runoff from agricultural lands, urban areas, or forest lands, subsurface or underground
sources, or discharges from boats or marine vessels not otherwise regulated under the
NPDES program. Generally, any unconfmed and diffuse source of contamination.
Legally, any source of water pollution that does not meet the legal definition of "point
source" in section 502(14) of the Clean Water Act.
National Pollutant Discharge Elimination System. National program for issuing,
modifying, revoking, and reissuing, terminating, monitoring, and enforcing permits, and
imposing and enforcing pretreatment requirements under the Clean Water Act. The
NPDES program regulates discharges from wastewater treatment plants, large factories,
and other facilities that use, process, and discharge water back into lakes, streams, rivers,
bays, and oceans.
Substance such as carbon, nitrogen, and phosphorus used by organisms to live and grow.
Too many nutrients in the water can promote algal blooms and rob the water of oxygen
vital to aquatic organisms.
Oceanic Remote Chemical-optical Analyzer
On-site sewage systems, including but not limited to septic systems
Pacific Decadal Oscillation
Pollution Identification and Correction
Pacific Northwest National Laboratory
Sources of pollution that discharge at a specific location from pipes, outfalls, and
conveyance channels to a surface water. Examples of point source discharges include
municipal wastewater treatment plants, municipal stormwater systems, industrial waste
treatment facilities, and construction sites that clear more than 5 acres of land.
Part per thousand, equivalent to mg/L
The marine waters south of Admiralty Inlet
The vertical marine layer where the density changes the most quickly. Generally the
pycnocline is shallower than the euphotic zone.
Rutgers Ocean Model
Vertical sediment sample
Places where small flows of water exit the ground or other solid surface.
A relatively shallow area of the seabed
Physical and biological structures in the environment that accumulate and store
substances for an indefinite period
Sediment oxygen demand
Total dissolved nitrogen
Total nitrogen
Form of nitrogen that includes ammonium and dissolved organic nitrogen (DON) if the
sample is filtered, but also particulate organic nitrogen if the sample is not filtered.
Micrograms per liter, equivalent to parts per billion
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Term
USGS
UW
Watershed
yr
Definition
U.S. Geological Survey
University of Washington
A drainage area or basin in which all land and water areas drain or flow
collector such as a stream, river, or lake at a lower elevation.
toward a central
Year
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