Influence of Metal Piezometers on Subsurface...
Transcript of 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
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Overview
• Site description
– Geology
– Stream flows
– Hydraulic gradients
– Specific conductivity
• Modeling
– Parameter estimations
and measurements
– Geomorphic basis for
geologic unit thickness
• Watershed model
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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
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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
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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
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Grass Lake Outlet
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Stream Flow (y-axes different scale)
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Freel Meadows Creek
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• Base flow ~ same in small creeks
– WH Ck and WFM Ck slightly longer recession
• Outlet recession different for 2010 & 2011
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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
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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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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
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Specific Conductivity
• North GW >> (South GW ~ Streams)
• (South GW > South SW) ~ Streams
• Dilution of GW and SW from snowmelt
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SC: Streams 2011
1st Creek WFM Creek FM Creek WH Creek Outlet
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Date 2011
SC: Piezometers 2011
N11 N5 S8 S3
N11 SW N5 SW S8 SW S3 SW
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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)
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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
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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
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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)
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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
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su
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volumetric water content (%)
Grass Lake Peat Retention Curves
PC1
PC2
PC3
PC4
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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
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Watershed Model
Hydrogeosphere
Fully coupled SW-GW flow
1m of surface recharge
Drain for 6 month
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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