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Silver Lake Sediment Cap Five‐Year Monitoring
Citizens Coordinating Council
March 28, 2019
Summary of Remediation 2012 through 2013
• Removal and off‐site disposal of the following: o 400 cubic yards of sediment near a former
GE outfall. o 2,900 cubic yards of soil, sediment and
debris from the lower bank. o 9,200 cubic yards of bank soil and debris
• Bank restoration with clean fill and native vegetation.
• Sediment capped with a sand‐topsoil mix with a minimum total organic carbon (TOC) content of 0.5%.
Photos obtained and provided by Avatar Environmental
17
monitoring locations for cap thickness
monitoring locations for cap thickness AND sediment PCB levels
near‐shore cap monitoring locations (construction only, now covered with stone)
Silver Lake
Silver Lake‐ Cap Monitoring 5 Years After Cap Completion
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6” Bioturbation Layer
6” Chemical Isolation Layer
2” Sacrificial Mixing Layer
On‐going natural sediment Lake deposition
Provides a layer for aquatic organisms and prevents mixing of the bioturbation & isolation layers
Controls migration of PCBs from under‐lying sediments to surface water
Prevents contamination of chemical Isolation layer during cap placement
Native Lake Sediment
Silver Lake Sediment Cap Design In‐lake Portion: No Geotextile or Armor Stone
Example core samples: Silver Lake cap and underlying sediment
M. Argue photo
Representative Silver Lake Core for Processing
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• .A
• • ► ~ 4 •
• 4 ► 1,, 1 4 ► 14 • • ► . • ~
Cap Th
ickn
ess, inches
Silver Lake Sediment Cap Thickness Annual Data from 2013 to 2018 40
2013 2014 2015 2016 2017 2018 Average of Six Years
35 12‐inch cap thickness
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25
20
15
10
5
0
See Tables 1 and 4 and Figure 2 from GE's 2018 Monitoring of Cap Thickness and Integrity, Cap Isoloation Layer, and Deposition on Cap Surface Reportfor additional notes regarding core thickness measurement results.
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TOP
REM
Mixing
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Mixing
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Mixing
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Mixing
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Mixing
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Mixing
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Mixing
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Mixing
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Mixing
Sample ID SL‐CAP‐01 SL‐CAP‐04 SL‐CAP‐06 SL‐CAP‐08 SL‐CAP‐10 SL‐CAP‐12 SL‐CAP‐14 SL‐CAP‐16 SL‐CAP‐19 SL‐CAP‐20
Silver Lake Sediment Cap PCB Data Immediatley Following Cap Construction and 1 & 5‐Year Post Contruction
2013 Immediate
2014 (Year 1)
2018 (Year 5)
Total P
CB (m
g/kg
dry weigh
t)
PCB concentration in the Mixing Interval at SL‐CAP‐01 in 2013 was 51.8ppm. That full concentration is not shown on the Mixing Interval graph to reduce Y‐axis scale.
Non detected concentration represents half the associated reporting limit. For additonal notes see Tables 2 and 5 from GE's 2018 Monitoring of Cap Thickness and Integrity, Cap Isoloation Layer, and Deposition on Cap Surface Report.
1.4 Interval
TOP
ISO
TOP
ISO
TOP
ISO
TOP
ISO
TOP
ISO
TOP
ISO
TOP
ISO
TOP
ISO
TOP
ISO
TOP
ISO
Sample SL‐CAP‐01 SL‐CAP‐04 SL‐CAP‐06 SL‐CAP‐08 SL‐CAP‐10 SL‐CAP‐12 SL‐CAP‐14 SL‐CAP‐16 SL‐CAP‐19 SL‐CAP‐20
2013 Immediate
2014 (Year 1)
2018 (Year 5)
Non detected concentration representshalf the associated reporting limit. For additonal notes see Tables 2 and 5 from GE's 2018 Monitoringof Cap Thickness and Integrity, Cap Isoloation Layer, and Deposition on Cap Surface Report.
ISO is the Isolation Layer
Silver Lake Sediment Cap PCB Data Immediatley Following Cap Construction and 1 & 5‐Year Post Contruction
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1
0.8
0.6
0.4
0.2
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Total P
CB (m
g/kg
dry
weigh
t)
Sedimen
t PCB
con
centratio
n – pa
rts pe
r million
20,000
800
600
400
200
MAX*
Sediment PCB Levels Five Years After Capping
Average PCB levels in the top foot of the lake bottom are 99.9% LOWER with the sediment cap now in place.
AVG*
Pre‐capping AVG=0.05 MAX=0.24 AVG=0.26 MAX=0.99
*NOT including an outlier of 20,689 ppm Isolation Layer Top Layer
Next Steps • Continued long term monitoring and maintenance;
next cap sampling event scheduled for the Summer of 2023.
• Cap thickness and PCBs analysis will be assessed at the same 10 locations that PCB analysis has been performed at during the first 5 years of monitoring.
• Cap thickness will be assessed at 3 additional locations where the 2018 monitoring event showed the cap to be less than 14” thick (but still above 12”).
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Photos obtained and provided by Avatar Environmental
Questions?
