Grass Lake Luther Pass, California Wes Christensen Graham Fogg - PowerPoint PPT Presentation

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 km 2 ) – Local scale hydrology (~100 m 2 ) • 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

Stream Flow (y-axes different scale) Grass Lake Outlet First Creek W Freel Meadows Creek 80 12 6.00 10 5.00 2010 2010 Flow (cfs) Flow (cfs) 70 8 4.00 2011 2011 6 3.00 2010 4 2.00 60 2011 2 1.00 0 0.00 4/20 5/20 6/19 7/19 8/18 9/1710/17 4/20 5/20 6/19 7/19 8/18 9/1710/17 50 Date Date Flow (cfs) Waterhouse Creek Freel Meadows Creek 40 4.5 35 4 30 3.5 2010 2010 25 30 Flow (cfs) Flow (cfs) 3 2011 2011 20 2.5 2 15 1.5 20 10 1 5 0.5 0 0 10 4/20 5/20 6/19 7/19 8/18 9/1710/17 4/20 5/20 6/19 7/19 8/18 9/1710/17 Date Date • Base flow ~ same in small creeks 0 – 4/20 5/20 6/19 7/19 8/18 9/17 10/17 WH Ck and WFM Ck slightly longer recession Date • 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

Vertical Hydraulic Gradients (+ is upward flow) • Gradient requires surf water 0.4 0.4 2011 (N) 2010 (N) • Hydraulic Gradient (m/m) Hydraulic Gradient (m/m) (GW-SW)/(DEPTH) 0.3 0.3 • High gradient over short 0.2 0.2 vertical distances 0.1 0.1 • Gradient drives flow from 0.0 0.0 hillslope/confined aquifer through the peat -0.1 -0.1 4/20 6/19 8/18 10/17 4/20 6/19 8/18 10/17 • Date Gradient S > Gradient N Date 2010 Date 2011 • N = road N1 N2 N3 N4 N5 N7 N8 N9 N10 N11 N12 N13 N14 N15 • S = glacial deposits 0.4 0.4 2010 (S) 2011 (S) Hydraulic Gradient (m/m) Hydraulic Gradient (m/m) 0.3 0.3 0.2 0.2 0.1 0.1 0.0 0.0 -0.1 -0.1 4/20 6/19 8/18 10/17 4/20 6/19 8/18 10/17 Date 2010 Date S1 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15

Confined Head Contours Fall 2010 Piezometric Head Spring 2011 Piezometric Head • “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

Specific Conductivity • North GW >> (South GW ~ Streams) • (South GW > South SW) ~ Streams • Dilution of GW and SW from snowmelt SC: Streams 2011 SC: Piezometers 2011 1st Creek WFM Creek FM Creek WH Creek Outlet N11 N5 S8 S3 N11 SW N5 SW S8 SW S3 SW 180.0 180.0 160.0 Specific Concutivity (uS/cm) 160.0 140.0 140.0 120.0 120.0 100.0 100.0 80.0 80.0 60.0 60.0 40.0 40.0 20.0 20.0 0.0 0.0 2/26 4/17 6/6 7/26 9/14 11/3 4/17 6/6 7/26 9/14 11/3 Date 2011 Date 2011

Parameter Estimation • Vertical hydraulic gradients sensitive to K sat (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 (K sat and thickness)

Vertical Hydraulic Conductivity from Temperature Vertical GW flow distorts propagation of surface heat changes into subsurface Harsh Winter Conditions • TidBit temperature loggers Deep Snow • various depths in piez Metal Piezometers • shallow outside in peat

Temperature Observations N7: T(t) at different depths 25 20 Temperature (C) 15 10 5 r=0cm, z=10.8cm r=22.4cm, z=10.2cm air T 0 7/26/11 7/27/11 7/28/11 7/29/11 7/30/11 7/31/11 8/1/11 8/2/11 Date • 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

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 T inside ~ 4°C higher than T outside • Max T inside earlier than Max T o utside • 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 Grass Lake Peat Retention Curves 2.00 PC1 PC2 1.50 PC3 suction (m) PC4 1.00 0.50 0.00 0.00 0.50 1.00 volumetric water content (%)

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