Itm 2_Pages from South Y PCE_Jun19_TAC_KJ-061819June 21, 2019 10 AM – 2 PM
South Tahoe Public Utility District Board Room
South Y Feasibility Study
TECHNICAL ADVISORY
COMMITTEE MEETING
#5
AGENDA ITEM 2.
FEASIBILITY STUDY
SCREENING/INTERIM REMEDIAL
ALTERNATIVES
Initial Model
Runs
•Baseline
•Mid-Plume Remediation
Sensitivity
Analysis
•Recharge Concentration
•Biogenic Degradation
•Source Term
RECAP OF TAC MEETING 4
SCENARIO DEVELOPMENT – ITERATIVE
APPROACH
Review
Effective-ness of
Runs
Develop
Additional Scenarios
Perform
Model Runs of
Scenarios
•PCE removal
•Use of new vs. existing
infrastructure
•Implementation cost
(relative order of
magnitude)
•Agency input:
•Future pumping rate
assumptions
•Replacement well
locations
RECAP OF TAC MEETING 4
SCENARIO DEVELOPMENT – OVERVIEW
•Scenario 1: Baseline (No action)
•Mid-plume remediation: Scenario 2: Extraction Wells in line with
EW-1
•Mid-plume remediation: Scenario 3: Insitu remediation (iron filing
wall) in line with LBWC 4
•Scenario 4: Replacement wells to move pumping from TKWC1
and to replace LBWC4
•Scenario 5: Increased pumping at TKWC2 and LBWC5 up to
90% of GAC treatment capacity, Reduced pumping from South
Tahoe PUD wells
RECAP OF TAC MEETING 4
ALTERNATIVES CONSIDERED BUT MODELING
NOT PURSUED
•Mid-range values of source and biodegradation
•Mid-plume remediation
•Reduced # of extraction wells at higher pumping rates
•Additional site-specific remediation
•Replacement well locations within plume
•Alternatives that were determined to be less effective than existing model runs were not run
•Future GW Model refinements for Operational Evaluation
•High pumping at TKWC2 and LBWC5 AND replacement wells
•Short term (5-7 years) of high pumping at TKWC2 and LBWC5, then use replacement wells in 5+ years)
•Additional pumping at TKWC2 to well capacity (vs GAC capacity)
RECAP OF TAC MEETING 4
PRELIMINARY ALTERNATIVES SCREENING
•RAOs
•Allow GW production w/o treatment
•Plume containment
•Water supply at < 50 ppb to avoid 97-005 and mass removal
•Target GW < 100’ bgs/replacement wells outside of plume
•Preserve cost recovery/grant funding
•Prefer beneficial reuse
•Reduce costs to retail customers
•Actions by Water Agencies that do not expose to future liability
• Source Area/Mid-Plume Remediation screened out
•Review Summary of Model Results To Inform Alternatives Screening
INITIAL ALTERNATIVES
•Based on the Model Scenarios:
•Do Nothing
•Alternative Water Supply: Replacement wells in deeper zone
•Containment/Capture: Pumping extraction/supply wells for
plume capture/interception
•In-situ treatment/remediation (at source or mid-plume)*
*Not Carried Forward
ALTERNATIVES PRESENTED TO WATER AGENCIES
FOR INPUT
•Alternatives Presented to LBWC/TKWC Representatives on
April 9, 2019:
•Do Nothing
•Replacement wells outside of plume (Alternative Water Supply)
•Enhanced pumping of TKWC2 and LBWC 5 to 90% of GAC
capacity (Containment/Capture of PCE)
•STPUD Board Meeting on May 2, 2019 to Discuss Water
Agency Input on Alternatives:
•Base Treatment (Do Nothing)
•Targeted Pumping (Containment/Capture of PCE)
•Surface Water Conversion (Alternative Water Supply)
OVERVIEW OF SCREENED ALTERNATIVES FOR
MORE DETAILED DEVELOPMENT
•Based on Input Provided by Water Agencies:
•Alternative 1 – Base Treatment (Do Nothing)
•Continue Existing Pumping/Treatment Operations
•Alternative 2 – Targeted Pumping (Containment/Capture of PCE)
•Increase Pumping of Wells with GAC Treatment
•Alternative 3 – Conversion to Surface Water Supply (Alternative
Water Supply)
•Replace Groundwater Supply with Surface Water Supply
•Other Recommended Activities
OVERVIEW OF ALTERNATIVE 1 – BASE
TREATMENT
•Operations:
•Operate LBWC 1 as lead, LBWC 5 (with GAC) as lag, LBWC to purchase remaining supply to meet demand from STPUD
•Operate TKWC 3 and lead, TKWC 2 (with GAC) as lag, TKWC 1 as lag-lag to meet demand
•District to provide emergency water to LBWC/TKWC in accordance with Mutual Aid Agreement
•New Permitting: None
•Relative Cost: No additional cost
•Benefits:
•Lowest implementation cost
•On-going PCE mass removal (347 – 2573 lbs.)
•Enables potable reuse of treated water
•Disadvantages:
•South Y Plume may persist for at least 20+ years
•Burden of added treatment costs may not be sustainable
•Potential significant costs of water system(s) consolidation ($millions)
OVERVIEW OF ALTERNATIVE 2 – TARGETED
PUMPING
•Operations:
•Operate LBWC 5 (with GAC) as lead, LBWC 1 as lag, LBWC to purchase remaining supply to meet demand from STPUD
•Operate TKWC 2 (with GAC) as lead, TKWC 3 as lag, TKWC 1 as lag-lag to meet demand
•District to provide emergency water to LBWC/TKWC in accordance with Mutual Aid Agreement
•New Permitting: None •Relative Cost: Upsize Conveyance, Additional O&M for pumping and treatment (GAC changeout)
•Benefits:
•Optimizes PCE mass removal (497 - 3265 lbs.) •Added plume containment protective of TKWC1
•Optimizes potable reuse of treated water.
•Disadvantages:
•Added implementation costs •increased operational complexity •Likely need for secondary discharge of treated water
OVERVIEW OF ALTERNATIVE 3 – CONVERSION
TO SURFACE WATER SUPPLY
INTERMEDIATE OPERATIONS (UP TO 15 YEARS)
•Operations:
•Operate LBWC 1 as lead, LBWC 5 as lag, LBWC to purchase
remaining supply to meet demand from STPUD
•Operate TKWC 3 and lead, TKWC 2 as lag, TKWC 1 as lag-lag
to meet demand
•District to provide emergency water to LBWC/TKWC in
accordance with Mutual Aid Agreement
•New Permitting: None
•Relative Cost: No additional cost (similar to Alternative 1)
OVERVIEW OF ALTERNATIVE 3 – CONVERSION
TO SURFACE WATER SUPPLY
FULL IMPLEMENTATION
•Operations:
•Surface water replaces LBWC/TKWC groundwater supply to meet demand
•LBWC 1 as backup supply for LBWC •TKWC 3 and 1 as backup supplies for TKWC •New Permitting: Water Rights Modification, WTP permitting, NPDES permit for disposal (if needed) •Relative Cost:
•Capital for intake, pipelines, pumping, disposal, storage (if needed)
•Destruction of TKWC 2/LBWC 5 •Additional O&M for treatment, intake, pipelines, pumping, disposal, storage (if needed) •Benefits:
•Switch to reliable (PCE-free) surface water supply •Water purveyor shared development and operational costs; •Disadvantages:
•Limited additional PCE mass removal (232 – 1418 lbs.)
•TKWC and LBWC reliance on District for future water supply.
OTHER RECOMMENDED ACTIVITIES
•Properly Destroy LBWC 4
•Complete vertical profiling of TKWC 2
•Develop monitoring program/Construct sentinel wells upgradient of LBWC 5/TKWC 2 to alert to increasing PCE levels
•Private well survey to identify potential conduits for vertical migration
•Investigate other potential vertical migration conduits
•Encourage expedited investigations and remedial actions to:
•identify/characterize PCE source areas
•Remove source area contamination
•Contain source area plume migration
SCREENED MODELING ASSUMPTIONS
Alt
System
Annual
Average
Demand*
LBWC 1
(MGY)
LBWC 5
(MGY)
TKWC 1
(MGY)
TKWC 2
(MGY)
TKWC 3
(MGY)
STPUD
South Y
Wells
(MGY)
1
(50-year
simulation)
1,974.94 84.10 19.25 51.35 117.59 126.99 1,502.90
2
(50-year
simulation)
1,974.94 19.25 84.10 51.35 126.99 117.59 1,502.90
3
Intermediate
(15-year
simulation)
1,974.94 84.10 19.25 51.35 126.99 117.59 1,502.90
*Average Annual Demands for 2009-2018
TAC DISCUSSION RE ALTERNATIVES OVERVIEW