South Y PCE_6 Mar 2019_mtg_ DRI_2 Slides per Page3/7/2019
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GROUNDWATER MODELING
Susie Rybarski, Mark Hausner, Greg Pohll
WHAT IS GROUNDWATER?
•Liquid water in the
subsurface.
•Water occupies spaces
between sand, silt, and
gravel in fill; or fractures and
cavities in rocks.
•Water movement through
subsurface is governed by
aquifer properties.
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WHAT IS GROUNDWATER?
•Groundwater flows from
areas of recharge to areas
of discharge.
Aquifers exist where groundwater can
be developed to provide adequate
supply to wells.
Recharge
Well
Pumping
Lake Tahoe
HOW DO CONTAMINANTS MOVE IN
GROUNDWATER?
•Advection – contaminants migrate with the average flow
velocity of groundwater
•Dispersion – contaminants migrate faster or slower than
average, and/or spread laterally due to variations in flow path
and local velocities
•Decay – microorganisms break down contaminant into other
constituents
Advection Dispersion
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WHAT IS A GROUNDWATER MODEL?
•A simplified representation of an aquifer system which can be used to:
•Simulate past and existing conditions to gain insight into aquifer properties
•Predict future aquifer responses to modeled inputs (i.e. water level declines)
•Predict fate and transport of contaminants or other groundwater constituents
•The model presented here was created using MODFLOW (flow) and MT3DMS
(transport), two codes developed by the USGS.
HOW ARE GROUNDWATER MODELS CREATED?
•The aquifer is represented by a grid or mesh, which may be divided into layers
•Sources and sinks are applied to the model – these include groundwater recharge, well
pumping, and surface water.
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HOW ARE GROUNDWATER MODELS CREATED?
•Model parameters are adjusted until simulated water levels and concentrations closely
match measured values. This is known as model calibration.
•Once a model is calibrated, simulations can be run into the future to predict how the
plume will respond to potential management plans.
0.0001
0.001
0.01
0.1
1
10
100
1971 1985 1999 2012PCE (ug/l)Baseline Observed
0.0001
0.001
0.01
0.1
1
10
100
1971 1985 1999 2012PCE (ug/l)Simulated Observed
Bad Good
MODEL RESULTS 1971-2019
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MANAGEMENT SCENARIOS
•Scenario 1: No action (Baseline)
•Scenario 2: Extraction wells
•Row of 21 shallow extraction wells pumped at 40 gpm
•Scenario 4: Replacement wells
•Move pumping from TKWC 1 and LBWC 5 to two replacement wells east of the plume
•Scenario 5: Maximize current PCE treatment
•Increase pumping at TKWC 2 and LBWC 5 up to 90% of treatment capacity; reduce pumping at South Tahoe PUD wells
SCENARIO 1: NO ACTION (BASELINE)
Scenario 1, 2038 stress period, layer 1Scenario 1, 2018 stress period, layer 1
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SCENARIO 2: EXTRACTION WELLS
•Goal is to remove PCE mass
•Row of 21 shallow extraction
wells, each well pumping at
40 gpm
•All production wells pumping
at baseline rates
Scenario 2, 2018 stress period, layer 1
SCENARIO 2: EXTRACTION WELLS
Scenario 1, 2038 stress period, layer 1 Scenario 2, 2038 stress period, layer 1
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SCENARIO 2: EXTRACTION WELLS
CONCLUSIONS
•Approximately 135 kg of PCE is removed by extraction
wells and 113 kg of PCE is removed by production
wells.
•Declines in concentrations at production wells are
minimal compared to the ‘No Action’ scenario as
extraction wells tend to retard plume migration.
SCENARIO 4: REPLACEMENT WELLS
•Goal is to pump water from
an uncontaminated aquifer
zone
•Pumping at TKWC 1 moved
to RW-D
•Half of LBWC 5 pumping
moved to RW-F
•All other production wells
pumped at baseline rates
Scenario 4, 2018 stress period, layer 1
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SCENARIO 4: REPLACEMENT WELLS
Scenario 1, 2038 stress period, layer 1 Scenario 4, 2038 stress period, layer 1
SCENARIO 4: REPLACEMENT WELLS
CONCLUSIONS
•Little effect on PCE concentrations in TKWC 2 and LBWC 5.
•Plume migration is retarded by additional pumping; plume spreads to the east.
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SCENARIO 5: MAX SUPPLY WELL PUMPING
•Goal is to remove PCE mass
by pumping existing supply
wells within the plume to
90% of treatment capacity
(i.e. pump and treat).
•Increase pumping at LBWC 5
and TKWC 2.
•Excess water to be used by
South Tahoe PUD; pumping
at district wells decreased by
an equivalent rate.
Scenario 5, 2038 stress period, layer 1
SCENARIO 5: MAX SUPPLY WELL PUMPING
Scenario 1, 2038 stress period, layer 1 Scenario 5, 2038 stress period, layer 1
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SCENARIO 5: MAX SUPPLY WELL PUMPING
CONCLUSIONS
•Greatest amount of mass extraction over all scenarios
(330 kg)
•Increased pumping at LBWC 5 and TKWC 2 tends to
protect TKWC 1, reducing concentration at that well
•Plume footprint greatly reduced
CONCLUSIONS
•Scenario 2 (Extraction wells) tends to retard plume
migration away from production wells.
•Scenario 4 (Replacement wells) results in spreading of
the plume towards replacement wells.
•Scenario 5 (Max supply well pumping) removes the
greatest amount of mass and results in the largest
reduction of plume area.