Eastern Washington
Stormwater Effectiveness Studies
Technical Evaluation Report
Bioretention Soil Media Thickness Study
Study Classification: Structural BMP
Study Objective(s):
 Evaluate Effectiveness
 Compare Effectiveness
 Develop Modified BMP
December 2021
Prepared For:
Spokane County
Public Works Department
1116 W. Broadway Avenue
Spokane, Washington 99260
(509)477-3600
Prepared By:
Osborn Consulting, Inc.
101 S Stevens St.
Spokane, Washington 99201
(509)867-3654
TECHNICAL EVALUATION REPORT
BIORETENTION SOIL MEDIA THICKNESS STUDY
December 2021
Page | ii
QAPP and TER Publication Information
The project Quality Assurance Project Plan (QAPP) and Technical Evaluation Report are stored
and accessible to the public on the Spokane County’s website at the following link:
https://www.spokanecounty.org/918/Stormwater-Utility.
For
questions
regarding
either
document, please contact Matt Zarecor by email [email protected] or phone (509)
477-7255.
TER Authors and Contact Information
Aimee Navickis-Brasch, P.E., Ph.D.
Engineering Manager
Osborn Consulting, Inc.
101 S Stevens St.
Spokane, WA 99201
[email protected]
(509) 867-3654 Ext. 301
Taylor Hoffman-Ballard, P.E.
Stormwater Engineer
Osborn Consulting, Inc.
101 S Stevens St.
Spokane, WA 99201
[email protected]
(509) 867-3654 Ext. 302
Nicole Chen, E.I.T.
Project Engineer
Osborn Consulting, Inc.
1402 3rd Avenue, Suite 415
Seattle, WA 98101
[email protected]
(206)628-9133 Ext. 232
QC Contact Information
Mark Maurer, PE, PLS
Senior Stormwater Engineer
Osborn Consulting, Inc.
101 S. Stevens Street, Suite 103
Spokane, WA 99201
[email protected]
TECHNICAL EVALUATION REPORT
BIORETENTION SOIL MEDIA THICKNESS STUDY
December 2021
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Document History
The study was conducted following the QAPP which can be accessed at the link on the previous
page. The study started in September 2018 and the last data was collected in January 2021. Sample
collection continued through the spring 2021 however due to an unusually dry spring, there were
no qualifying events. The draft Technical Evaluation Report (TER) was presented and submitted
to the Technical Advisory Group (TAG) in October 2021 for review and comment. Appendix E
of the TER contains a summary of the TAG’s comments along with a summary of responses to
the comments including how they were addressed in the document. The final TER was completed
in December 2021.
TECHNICAL EVALUATION REPORT
BIORETENTION SOIL MEDIA THICKNESS STUDY
December 2021
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Distribution List
Name, Title
Organization
Contact Information:
Email, Telephone
Matt Zarecor
Assistant County Engineer
Spokane County
[email protected]
509.477.7255
Bill Gale
Stormwater Utility Manager
Spokane County
[email protected]
509.477.7261
Ethan Murnin
Project Manager
Spokane County
[email protected]
509.477.7420
Jack Wells
Natural Resources Specialist
Yakima County
[email protected]
509.574.2350
Chad Philips
Stormwater Engineer
City of Spokane Valley
[email protected]
509.720.5018
Seth Walker
Walla Walla County
Participating Entity
TAG Member6
[email protected]
509.524.2715
Shilo Sprouse
Stormwater Services Program
Manager
City of Pullman
[email protected]
509.432.9052
Randy Meloy
Surface Water Engineer
City of Yakima
[email protected]
509.576.6781
Bill Aukett, Stormwater
Program Manager
City of Moses Lake
[email protected]
509.764.3792
Brittany Whitfield
Senior Engineer
City of Pasco
[email protected]
509.544.3080
Karen Dinicola
Water Quality Project Manager
Department of Ecology
[email protected]
360.407.6550
Doug Howie
Water Quality Project Manager
Department of Ecology
[email protected]
360.407.6444
Brandi Lubliner
Water Quality Project Manager
Department of Ecology
[email protected]
360.407.7140
Amanda Mars
WQ Program – ERO
Department of Ecology
[email protected]
509.329.3554
Aimee Navickis-Brasch
Engineering Manager
Osborn Consulting, Inc.
[email protected]
509.867.3654
Taylor Hoffman
Stormwater Research Engineer
Osborn Consulting, Inc.
[email protected]
509.867.3654
Kathy Sattler, Laboratory
Project Manager
Anatek Laboratories
[email protected]
509.838.3999
Medhanie Tecle
Engineering Manager
Materials Testing &
Consulting, Inc.
[email protected]
360.534.9777
TECHNICAL EVALUATION REPORT
BIORETENTION SOIL MEDIA THICKNESS STUDY
December 2021
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1.0
Table of Contents
1.0
TABLE OF CONTENTS …………………………………………………………………………………………………………….V
2.0
EXECUTIVE SUMMARY ………………………………………………………………………………………………………….. 1
3.0
INTRODUCTION ……………………………………………………………………………………………………………………… 4
3.1
INTRODUCTION TO THE STRUCTURAL BMP …………………………………………………………………………………….. 4
3.2
PROBLEM DESCRIPTION …………………………………………………………………………………………………………………. 5
3.3
PROJECT GOALS AND OBJECTIVES …………………………………………………………………………………………………… 6
3.4
PROJECT OVERVIEW ………………………………………………………………………………………………………………………. 6
3.5
STUDY LOCATION …………………………………………………………………………………………………………………………. 6
4.0
SAMPLING PROCEDURES ……………………………………………………………………………………………………… 9
4.1
TYPES OF DATA COLLECTED ………………………………………………………………………………………………………….. 9
4.2
SAMPLE COLLECTION PROCESS ………………………………………………………………………………………………… 11
4.2.1
SOP Overview ……………………………………………………………………………………………………………………. 11
4.2.2
Audit Overview ………………………………………………………………………………………………………………….. 11
4.2.3
MONITORING EQUIPMENT OVERVIEW ………………………………………………………………………………………. 11
5.0
DATA QUALITY ASSESSMENT ……………………………………………………………………………………………. 15
5.1
DATA VERIFICATION …………………………………………………………………………………………………………………… 15
5.2
DATA USABILITY ASSESSMENT ……………………………………………………………………………………………………… 17
5.2.1 TAPE Qualifying Event and Pollutant Concentration Criteria ……………………………………………………… 18
6.0
RESULTS AND DISCUSSION ……………………………………………………………………………………………….. 23
6.1
WATER QUALITY DATA ANALYSIS ……………………………………………………………………………………………….. 23
6.1.1
STORM REPORTS ……………………………………………………………………………………………………………………… 23
6.1.2
STATISTICAL ANALYSIS ……………………………………………………………………………………………………………. 25
6.1.3
POLLUTANT REMOVAL EFFICIENCY ………………………………………………………………………………………….. 26
6.2
COMPARISON OF RESULTS TO TAPE TREATMENT PERFORMANCE GOALS ………………………………………… 29
6.3
INFILTRATION PERFORMANCE ……………………………………………………………………………………………………… 36
7.0
FUTURE ACTION RECOMMENDATIONS …………………………………………………………………………… 41
8.0
CONCLUSIONS ………………………………………………………………………………………………………………………. 42
9.0
REFERENCES ………………………………………………………………………………………………………………………….. 44
10.0
APPENDICES ………………………………………………………………………………………………………………………….. 46
APPENDIX A. LABORATORY ANALYTICAL REPORTS ……………………………………………………………………………….. 47
Appendix A.2 Bioretention Soil Media Reports ……………………………………………………………………………………. 49
APPENDIX B. STORM REPORTS ………………………………………………………………………………………………………………. 51
APPENDIX C. DATA QUALITY ASSESSMENT ……………………………………………………………………………………………. 52
Appendix C.1 Quality Objectives ………………………………………………………………………………………………………. 54
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Appendix C.2 Quality Assurance Worksheets ……………………………………………………………………………………… 62
Appendix C.3 Field Forms ………………………………………………………………………………………………………………… 63
Appendix C.4 Field Audit …………………………………………………………………………………………………………………. 66
Appendix C.5 Deviations from QAPP ………………………………………………………………………………………………… 67
Appendix C.6 Identification of Quality Assurance Issues & Recommended Solutions ………………………………. 70
APPENDIX D. STATISTICAL AND DATA ANALYSIS …………………………………………………………………………………… 74
Appendix D.1 Statistical Comparison of Influent and Effluent ……………………………………………………………… 75
Appendix D.2 Pollutant Effluent Concentrations and Removal Efficiencies ……………………………………………. 76
Appendix D.3 Ecology Bootstrapping Method …………………………………………………………………………………….. 77
APPENDIX E. TAG COMMENT RESPONSES …………………………………………………………………………………………….. 78
TECHNICAL EVALUATION REPORT
BIORETENTION SOIL MEDIA THICKNESS STUDY
December 2021
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2.0
Executive Summary
The focus of this study is to evaluate the effectiveness of bioretention cells containing an 18-inch
bioretention soil media (BSM) layer in comparison to a 12-inch BSM layer. Bioretention cells
are shallow landscaped depressions which are designed to capture and treat stormwater runoff
from small contributing areas. The cells utilize an engineered BSM containing compost (40%)
and sand (60%) to remove total suspended solids (TSS), dissolved metals, and oils from
stormwater as runoff infiltrates through the media. Research studying BSM has suggested that
removal of these pollutants primarily occurs in the top 6 inches of the media layer. Further
research has shown that BSM containing compost leaches nutrients and that higher content of
compost in the bioretention cell is associated with higher concentrations of nutrients being
leached from the media. This study will compare the treatment performance of 18-inch media
depth required by the Department of Ecology Eastern Washington (EWA) Stormwater
Management Manual (SWMMEW) to a 12-inch media depth. The goal for this study is to justify
a modified bioretention BMP which uses the existing BSM to a depth of 12-inches (rather than
the current required 18-inch depth) for providing treatment of TSS and dissolved copper and
zinc.
The goal for this study was achieved by conducting field testing of two bioretention cells each
containing BSM depths of 18-inches and 12-inches. The field testing was conducted at a site
located on Gonzaga University’s campus in Spokane, Washington. The test site was constructed
in 2014, automated monitoring equipment was installed in the fall of 2017, and field testing was
conducted from Fall 2018 to Spring 2021. Automated monitoring equipment installed at the test
site was used to collect composite influent and effluent water quality samples, flow rate,
temperature, and precipitation data. The water quality parameters tested included the required
and screening parameters defined in the 2011 Technology Assessment Protocol Ecology (TAPE)
for basic, dissolved metals, and oil treatment. The physiochemical properties of the BSM were
also analyzed. The media infiltration rate and saturated hydraulic conducting were measured
using the effluent flow rate data and a modified falling head test.
Samples were collected from a total of 29 storm events. For each of these events, a storm report
was created, and the data was evaluated to determine whether the storm met the TAPE qualifying
storm criteria and sample collection criteria. These criteria define the minimum storm depth and
duration, storm antecedent and post storm dry period, minimum number of aliquots, sample
event coverage, and minimum number of samples. The evaluation results determined that 9
storm events met the TAPE criteria for qualifying conditions, 8 storm events met all except for
one to two of the TAPE criteria for qualifying conditions (potentially qualifying), and 12 storms
did not meet three or more of the TAPE criteria (non-qualifying).
The collected data was analyzed for qualifying and potentially qualifying events to determine the
effectiveness of the 12-inch BSM depth compared to the 18-inch BSM depth. The specific
objectives completed to meet the study goals are summarized below along with a summary of the
results.
TECHNICAL EVALUATION REPORT
BIORETENTION SOIL MEDIA THICKNESS STUDY
December 2021
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Objective 1: Determine the pollutant removal efficiency of the BSM mix at a depth of 18-inches
compared to 12-inches.
No statistically significant difference was noted between the treatment performance of the 18-
inch BSM depth compared to the 12-inch depth for TSS, dissolved copper, or dissolved zinc. A
statistically significant difference was measured for leaching of TP; the 18-inch BSM depth
leached more TP (-381%) on average than the 12-inch depth…