Watershed
Sampling
and
Monitoring
Presentation
Council Chambers
March 3, 2026
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Watershed Sampling and
Monitoring
• A joint venture between the City of Napa, Napa
County, and Napa Resource Conservation
District (RCD)
• Goal of monitoring inflow to drinking water
reservoirs to protect the source waters
– establish baseline data as land use changes occur
within the watersheds
– detect water quality anomalies which may impact the
reservoirs and drinking water safety
– forecast water quality conditions in wet & dry years
– identify areas of concern so they can be addressed
through BMP modification
– inform potential necessary improvements to the
corresponding water treatment plants
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Project Development
Proposed
project in
Milliken
Watershed
raised concern
over land use
change in
pristine
watershed with
minimal water
treatment
2011
Discussions
for
monitoring
the
watersheds
begun
2017–2019
Joint City &
County
presentation to
Council on
current
sampling and
proposed new
Watershed
Master Plan
Project & MOU
March 19,
2019
City & County
enter into
MOU for
Water Quality
Monitoring
November
19, 2019
First limited
sampling
event
occurred
December
10, 2019
First multi-
agency
training &
sampling
events
January 28-
29, 2020
Project was
paused due
to COVID-19
Social
Distancing
restrictions
March 2020
Project
resumed with
COVID PPE &
policies in
place
February
23, 2021
Consistent
sampling
events
2022-
present
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Multi-Agency Coordination
• City of Napa staff
• Provide annual sample collection training
• Set sample collection events & dates
• Coordinate with the sub-laboratory for sample containers & data reports, verify/prepare coolers and reporting documentation • Determine the number of locations to be collected for each event (Flow based)
• Collect samples at limited & open access locations
• Verify & process data into the City’s database
• Provide data to Napa County & one participating property owner • Napa County Staff & RCD Staff
• Attend trainings
• Collect samples at open access locations
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Sampling Events
Each Event
• 27 established locations
• Occurs on the last
Wednesday of the month
• Occurs when 10 or more
locations have flowing
water
• Involves all three
agencies
• Collects ≈10 liters of
water from each location
• 141 individual analytes
for each location
Event Averages
• 17 samples collected per
event
• 4 sample collection
groups per event
• 48 hours of work per
event
• 7 separate events occur
each year
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Pre Sampling
Event Tasks
• Confirm the number of
sample locations with
flowing water
• Order sample containers
• Confirm sampling event to
County and RCD
participants
• Pick up, label, and sort
containers
• Prepare all required
documentation (COCs,
route maps, field logbooks)
• Morning of the Event
• Calibrate field analysis
equipment/meters
• Collate all sampling
equipment from storage
and stage for individual
agency pick up
• Fill up and transport ice
bags for sample
preservation.
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Sampling Locations
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Sampling Location
Comparison
Dry Winter Conditions
Wet Winter Conditions
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Custom
Sampling
Equipment
Designed and built by City
staff to enhance the safety,
accessibility, uniformity, and
speed of sample collection.
Tested with equipment
blanks to verify sample
results are not altered with
design.
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Field Analysis
Instrumentation
Used for analysis which
must be made in the field at
the time of sample
collection.
Analytes include:
Temperature
Dissolved Oxygen
pH
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Sample Containers
Includes:
Unpreserved Plastics VOAs Sterile Containers
Amber Glass Acidic Containers Specialty Containers
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Sampling Events
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Sampling Events
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Sampling Events
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Post Sampling
Event Tasks
• Delivering samples to
sub-laboratory
• Processing field
logbooks and COCs
• Sampling equipment
cleaning, maintenance,
and repair, if needed
• Data entry and report &
invoice processing
• Minor data analysis
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Findings To Date
• Distinct water profiles for each watershed
– 3 main contributing creeks (Hennessey)
– 1 main contributing creek for Milliken with uniform profile • Relatively stable water profiles
• Low level, transient detections of pesticides, herbicides, roadway contaminants, and plasticizers in the Hennessey Watershed
• Persistent detections of PFOA & PFAS compounds in
upper Conn Creek
• Used in post-LNU fire water quality monitoring for robust assessment of potential fire impacts
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Findings
To Date
• Detected 6 times at 5 locationsbis(2-Ethylhexyl) phthalate – plasticizer
• Detected 1 time at 1 locationdiethylphthalate
– plasticizer
• Detected 1 time at 1 location
Phenol
– plastics and consumer
products
• Detected 3 times at 3 locationsIndeno (1,2,3-cd)pyrene – combustion emission
• Detected 1 time at 1 locationDibenzo(a,h)anthracene
– combustion emission
• Detected 2 times at 2 locationsBenzo(g,h,i)perylene
– road tar byproduct
• Detected 1 time at 1 locationFluorene
– road tar byproduct
• Detected 1 time at 1 location2,4,6-Trichlorophenol
- pesticide
• Detected 1 time at 1 locationNaphthalene
– pesticide
• Detected 1 time at 1 locationPhenanthrene
– pesticide
Transient hits of 10 unregulated
compounds in 9 locations from 2020
- 2025.
No consecutive hits from one event
to the next event
(no demonstrated persistence > 30
days).
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Potential Sources of
Contaminants
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Conductivity in Raw Waters
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Conductivity in Raw Waters
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Total Organic Carbon in Raw Waters
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Total Organic Carbon in Raw Waters
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Total PFOA & PFAS Compounds in Raw Waters
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Total PFOA & PFAS Compounds in Raw Waters
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Watershed Master Plan
Project Summary
• Impeded Start in 2020 / Functional Start
in 2021
• 41 Sampling Events to date
• 680+ Individual Samples Collected
• Data justifies continued, ongoing
monitoring for contaminants
• Continued collaboration between the City,
County, and RCD is key to continued
success
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Future Plans
• Monitor incoming water quality to
optimize watershed & reservoir
management, invasive species suitability,
and potential watershed & water
treatment responses
• Continue sampling events to calibrate the
model for wet versus dry water year
comparisons – more data needed to
establish patterns
• Utilize the data collected so far to inform
BAT decisions for the Hennessey Water
Treatment Plant upgrade
→An ounce of prevention is worth a
pound of cure.
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Comments & Questions
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