Board Item 6b_2 May 2019_South Y FS handouts5/1/2019
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SOUTH TAHOE PUBLIC UTILITY DISTRICT
Regular Board Meeting
Board Agenda Item 6.b.
South Y Feasibility Study
May 2, 2019
I. Bergsohn, PG, HG
SOUTH Y
AREA
2018 PCE
•Predesign Investigation
•Shallow (< 100 ‘)
•<0.5 - 66 µg/l
•Deep (> 100’)
•LBWC 5 : 58 - 60 µg/L
•TKWC 2: 15 – 24 µg/L
•TKWC 1: 2.5 µg/L
•LTLW Phase 1
•Shallow (<100’)
•<0.5 – 1,680 µg/L
•LTLW GW Monitoring
•Shallow (<100’)
•<0.5 – 1,580 µg/L
PCE MCL = 5 µg/L
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FEASIBILITY STUDY
Screen Modeled Alternatives for
Engineering Evaluation
(6 Alternatives)
1) Define Infrastructure Needs
2) Develop Life Cycle Cost
Estimates
3) Complete Initial Study
Checklists/Cost of Mitigation
(3 Alternatives)
Select and Develop
Recommended
Alternative
(1 Alternative)
Predict the fate and transport of PCE (movement and concentration)Simulation Period – 2018 to 2068 (50 Years)
Bookend Conditions
Best Case – No Mass Recharge +Decay (PCE minimum)Worst Case – Mass Recharge + No Decay (PCE maximum)
Management Scenarios
Base In-situ Treatment (not Modeled)Extraction Wells
Replacement Municipal Wells
Targeted Pumping
90% of Treatment Capacity
90% of Well CapacityEvaluation Criteria
Total PCE Mass Removed
PCE Mass Removal Rate
Clean-Up Time
GROUNDWATER MODEL
EVALUATION
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SCENARIO PCE MASS
REMOVED
(lbs.)
Clean-Up Time
(Years After 2018)
LBWC 5 TKWC 2 TKWC 1
Base 267 –
6,516
7 –
29
11-
>50
0-
>50
Extraction
Wells
547 –
7,746
5 –
34
22 –
>50
0-
>50
Replacement
Wells
192 -
4,068
5 –
27
20 –
> 50
0-
>50
Targeted
Pumping
728 –
14,344
8 –
26
11 –
46
0 –
42
MODEL FINDINGS
CONSIDERATIONS
SCENARIO POSITIVE NEGATIVE
Base •No water quality Impacts
to neighboring District
Wells
•No added costs above
planned operations
•Lengthy clean-up time
•Limited Funding
•Possible consolidation
Extraction Wells •No water quality Impacts
to neighboring District
Wells
•Minimal PCE reductions at
Municipal Wells
•Extraction Well Capital and
O&M costs
•Extended clean-up times
Replacement Wells •Additional pumping helps
to retard Plume Migration
•Plume spreading
•Well Site Uncertainty
Targeted Pumping •Greatest amount of mass
removal
•Reduces Plume Extent
•Protects neighboring
Municipal Wells
•Added O&M
•Excess treated water
•Reuse/Disposal
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Base Treatment –maintain pumping at wells sufficient
to meet LBWC and TKWC water system demands
Targeted Pumping – Rely on wells equipped with
treatment to remove PCE and meet water system
demands
Surface Water Supply – gradually convert from
groundwater to surface water-based system
INTERIM REMEDIAL ALTERNATIVES
BASE TREATMENT
•LBWC & TKWC
•Continue current
operations
•Pump wells to satisfy
water system demand
•Active Wells pumped
in Lead
•Active wells with
treatment pumped in
Lag
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TARGETED PUMPING
•LBWC & TKWC
•Modify operations
•Pump wells to satisfy
water system demand
•Active Wells with
treatment pumped in
Lead
•Active wells pumped in
Lag
SURFACE WATER
•LBWC & TKWC
•Gradual conversion to
surface water
•Continue current well
operations in interim
•District to secure
surface water rights
•Enter into Agreement
with LBWC and TKWC
for provision of Surface
Water.
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Feasibility Study
Define Infrastructure Needs
Determine Life-cycle costs
Identify mitigation requirements
Recommended Alternative
Conceptual Design
Implementation Schedule
Project Finance
Governance Plan
NEXT STEPS
DISCUSSION