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

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Grass Lake Outlet

2010

2011

Stream Flow (y-axes different scale)

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First Creek

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2011

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Waterhouse Creek

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W Freel Meadows Creek

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2011

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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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lic G

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t (m

/m)

Date 2010

2010 (S)

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2011 (S)

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Date 2011

2011 (N)

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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

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cifi

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(u

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)

Date 2011

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

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

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su

cti

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(m

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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