Influence of Metal Piezometers on Subsurface...
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Hydrogeology of Grass Lake Luther Pass, California Wes Christensen Graham Fogg University of California, Davis Department of Geology
Transcript of Influence of Metal Piezometers on Subsurface...
Hydrogeology of
Grass Lake Luther Pass, California
Wes Christensen
Graham Fogg
University of California, Davis
Department of Geology
Overview
• Site description
– Geology
– Stream flows
– Hydraulic gradients
– Specific conductivity
• Modeling
– Parameter estimations
and measurements
– Geomorphic basis for
geologic unit thickness
• Watershed model
Physically Based Modeling • Explore the potential response of a
groundwater sustained peatland (Grass Lake) to predicted changes in climate
– Earlier snow melt
– Less snow, more rain on snow
• Small watershed scale (~10 km2)
– Local scale hydrology (~100 m2)
• Physical parameters governing groundwater flow and storage
– Hydraulic conductivity
– Storage coefficients
– Thicknesses of geologic units
• Protected “Research Natural Area”
– NO tracer tests
– NO pumping
– Minimal disturbance
– Natural T signal
Grass Lake Geology
• Weathered granodiorite
• Tertiary volcanics
• Tahoe glaciation (145 ka)
– Recessional and lateral moraines
• Tioga glaciation (19 ka)
– Terminal and cirque moraines
• Alluvium
• Peat
Grass Lake Streams
• Well defined outlet stream
• 4 perennial streams
• 4 intermittent streams
entering Grass Lake
– Associated with Tioga
age glacial cirques
• Intermittent /ephemeral
channels in upper WS
0
10
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60
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4/20 5/20 6/19 7/19 8/18 9/17 10/17
Flo
w (
cfs
)
Date
Grass Lake Outlet
2010
2011
Stream Flow (y-axes different scale)
0
2
4
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Flo
w (
cfs
)
Date
First Creek
2010
2011
0
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Flo
w (
cfs
)
Date
Waterhouse Creek
2010
2011
0.00
1.00
2.00
3.00
4.00
5.00
6.00
4/20 5/20 6/19 7/19 8/18 9/1710/17
Flo
w (
cfs
)
Date
W Freel Meadows Creek
2010
2011
0
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20
25
30
35
4/20 5/20 6/19 7/19 8/18 9/1710/17
Flo
w (
cfs
)
Date
Freel Meadows Creek
2010
2011
• Base flow ~ same in small creeks
– WH Ck and WFM Ck slightly longer recession
• Outlet recession different for 2010 & 2011
Groundwater • 30 piezometers near shore
• Screened in sediment below peat
• Gradient = (GW-SW)/(DEPTH)
– Limited to presence of SW
• Often artesian flow (upward)
GW SW
Date
N1 N2 N3 N4 N5 N7 N8N9 N10 N11 N12 N13 N14 N15
Vertical Hydraulic Gradients
(+ is upward flow)
• Gradient requires surf water
• (GW-SW)/(DEPTH)
• High gradient over short
vertical distances
• Gradient drives flow from
hillslope/confined aquifer
through the peat
• Gradient S > Gradient N
• N = road
• S = glacial deposits
S1 S3 S4 S5 S6 S7 S8
S9 S10 S11 S12 S13 S14 S15
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0.0
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Hyd
rau
lic G
rad
ien
t (m
/m)
Date 2010
2010 (S)
-0.1
0.0
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Hyd
rau
lic G
rad
ien
t (m
/m)
Date
2011 (S)
-0.1
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Hyd
rau
lic G
rad
ien
t (m
/m)
Date 2011
2011 (N)
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Hyd
rau
lic G
rad
ien
t (m
/m)
Date 2010
2010 (N)
Confined Head Contours
• “Shadow” effect from bedrock
• Stream influence
• Larger change along N than S
– 0.1 to 0.9m change in N
– 0.1 to 0.3m change in S
Fall 2010 Piezometric Head Spring 2011 Piezometric Head
Specific Conductivity
• North GW >> (South GW ~ Streams)
• (South GW > South SW) ~ Streams
• Dilution of GW and SW from snowmelt
0.0
20.0
40.0
60.0
80.0
100.0
120.0
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Spe
cifi
c C
on
cuti
vity
(u
S/cm
)
Date 2011
SC: Streams 2011
1st Creek WFM Creek FM Creek WH Creek Outlet
0.0
20.0
40.0
60.0
80.0
100.0
120.0
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Date 2011
SC: Piezometers 2011
N11 N5 S8 S3
N11 SW N5 SW S8 SW S3 SW
Parameter Estimation
• Vertical hydraulic gradients sensitive to Ksat(peat) – Can be determined using vertical T profiles and head
• Recession of Outlet flow sensitive to peat water retention
• Recession of stream flows sensitive to hillslope transmissivity and storage (Ksat and thickness)
Harsh Winter Conditions
Deep Snow
Metal Piezometers
Vertical GW flow distorts propagation of surface heat changes into subsurface
• TidBit temperature loggers
• various depths in piez
• shallow outside in peat
Vertical Hydraulic Conductivity
from Temperature
Temperature Observations
• Maximum T is delayed in both piez and peat relative to air T (~7-10 hours)
• Minimum T in piez is delayed relative to minimum air T (~1 hour)
• Minimum T in peat is delayed relative to air T and piez T (~5 hours)
• Obvious difference between T signal in piezometer and in peat
N7: T(t) at different depths
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Date
Tem
pera
ture
(C
)
r=0cm, z=10.8cmr=22.4cm, z=10.2cmair T
Effects of Metal on Temperature • Thermal Conductivity of metal 16 W m-1 K-1
• Thermal conductivity of peat < 0.5 W m-1 K-1
• At ~10 cm Tinside ~ 4°C higher than Toutside
• Max Tinside earlier than Max Toutside • Significantly affects parameter estimates
r (m) r (m)
Peat Water
Retention • Hanging water column
• Spec suction head to 1.5m
• Saturated water content ~80%
• Water content at 0.5m ~60%
• PC4 was the most
decomposed sample
0.00
0.50
1.00
1.50
2.00
0.00 0.50 1.00
su
cti
on
(m
)
volumetric water content (%)
Grass Lake Peat Retention Curves
PC1
PC2
PC3
PC4
Shallow Subsurface Thickness • Down cutting of streams in upper WS in response to glaciers
– 80+m thick weathered bedrock (grus)
• Projection of glaciated bedrock surface – 5 to 40m thick glacial till
• Electrical Resistivity Imaging (Doug Clark, unpub.) – 80m thick valley fill (peat surface to bedrock)
• Probes and ERI
– 0 to 10m thick peat
• Lidar Data was
INDESPENSIBLE
- Provided by
Tahoe Regional Planning Agency
http://dx.doi.org/10.5069/G9PN93H2
Watershed Model
Hydrogeosphere
Fully coupled SW-GW flow
1m of surface recharge
Drain for 6 month
Acknowledgements
• Ida Fischer
• Sherry Devenberg
• Caleb Kesling
• LTBMU
– David Immeker
– Sarah Howell
– Shana Gross
• Fogg Lab
– Nick Newcomb
– Nick Engdahl
– Dylan Boyle
– Ehsan Rasa
– Charlie Paradis
