Groundwater Storage and Flow in an
Unconfined Pumice Aquifer,
Antelope Unit, Chemult Ranger
District, Winema-Fremont National
Forest, Oregon
Michael L. Cummings
Department of Geology
Portland State University
Outline
• Overview of geologic evolution
• Coupled hydrologic, geomorphic, and biologic
evolution
• Recharge, flow paths, and discharge
• Applications to fen sites
Landscape Evolution (bedrock)
• ~5+ Ma-3 Ma basic architecture of volcanic
sequence established.
– Larger basaltic andesite centers (Walker
Mountain, Sugarpine Mountain)
– Basaltic tuff cones (Tea Table Mountain, God
Butte)
– Olivine basalt lava flows (locally erupted)
– Pyroclastic flows (Cascade source?)
– Volcaniclastic sedimentation (relatively minor)
Landscape Evolution (erosion)
• ~3 Ma –
– Onset of erosion by Jack Creek and tributaries
– Onset of regional faulting
Landscape Evolution (bedrock)
• ~3 Ma - ~1 Ma
– Early phase of intracanyon flows – inverted
topography (modern landscape)
– Olivine basalt flows and small cinder cones
continue to erupt as erosion of Jack Creek and
tributaries continues
Landscape Evolution (Mazama)
• ~7000 years ago
– Cataclysmic eruption of Mount Mazama
– Deposition of 2 to 3 m of pumice in Jack Creek
drainage
• Lower pumice unit (~ 0.5 cm diameter, well sorted,
phenocrysts and lithics common)
• Upper pumice unit (up to 6 cm, moderately to poorly
sorted)
Coupled Hydrologic, Geomorphic, and
Biologic Response (Phase 1)
• Erosion patterns guided by pre-eruption
topography
• Surface water dominated
• Erosion through entire pumice section, erosion
through upper pumice unit, or erosion within the
upper pumice unit
• Coarse-grained upper pumice prone to erosion
due to buoyancy
• Fine-grained lower pumice less subject to erosion
Coupled Hydrologic, Geomorphic, and
Biologic Response (Phase 1)
• Groundwater accumulates in pumice section
through surface infiltration and seepage from
pre-eruption groundwater flow paths
• Seepage from evolving groundwater system
into evolving valley system
• Vegetation colonizes areas of seepage
• Areas of seepage and colonization less prone
to erosion allowing asymmetry to develop in
valleys
Coupled Hydrologic, Geomorphic, and
Biologic Response (Phase 2)
• Aggradation (glassy silt, crystals, lithics) within
valleys cut into pumice deposits
– Glassy silt deposited over either lower or upper
pumice units where exposed in valley walls
• Aggradation (glassy silt mostly) over complete
pumice section
– Alluvial fan deposits over pumice deposits in
valley bottoms of pre-eruption landscape
119 cm reworked
No iron staining
73 cm reworked
Strong iron cementation at 47-73 cm
Coupled Hydrologic, Geomorphic, and
Biologic Evolution (Phase 2)
• Evolution of valley segments where
colonization started early
– Peat accumulation where groundwater seepage
occurs on valley walls throughout summer
– Local relief continues to evolve where vegetation
protects areas of seepage and valley floor
continues to erode into pumice deposits –
asymmetry of valley increases through time
Coupled Hydrologic, Geomorphic, and
Biologic Evolution (Phase 2)
• Evolution of valley bottoms with full section of
pumice overlain by alluvium
– Vegetation reflects inundation in spring, high
water table in early summer, greater than about 1
m below surface in late summer
– Iron mottling common near interface between
alluvium and upper pumice deposit
28-August-2007
Recharge of Groundwater System
• Annual precipitation (snow in winter)
• Infiltration into pumice deposits to recharge
unconfined, perched pumice aquifer
• Deeper infiltration to regional, bedrock-hosted
groundwater system or to intermediate depth,
bedrock-hosted confined aquifers in zones of
higher porosity
Flow paths
• Shallow within the perched pumice aquifer
• Intermediate within permeable zones in
bedrock
• Deep within the regional groundwater system
Discharge
• Pumice aquifer discharges to fens and streams
– drain-out system (dominant)
• Intermediate flow paths discharge to fens and
streams along stratigraphic or structural zones
of higher permeability (subordinate)
• Deep flow paths discharge through regional
system to Klamath and/or Deschutes basins
Applications
• Dry Meadow
– Long narrow valley – pumice-filled at south end,
multiple fen areas, alluvium over paleosol and
welded tuff at north end
– Drill cores from near northern fen indicates low
permeability volcanic breccia to at least 100 feet
– Groundwater alternately enters and exits the
pumice aquifer as it moves down valley
Applications
• Johnson Site
– Extensive area of water seeping from lower
pumice unit into the northeast-southwest
trending main valley
– Valley directly east of Johnson Guard Station
contains 0.8 m of fill above lower pumice unit (0.8
to 1.1 m) – iron staining near contact (water table
0.5 m on 30-June-2010)
– Pre-eruption material is plastic silt with thin sandy
interlayers (1.1 to E.O.H. at 1.7 m)
Lower Johnson
• 0 – 33 cm dark brown to black, organic-rich
silt with charcoal pieces
• 33-61 cm upper pumice unit (poorly sorted)
– water table from 31 cm to 51 cm (10-July to
2-October)
• 61-205 cm lower pumice unit (~0.5 cm)
• 205-233 cm (E.O.H.) dark brown, low plasticity
silty sand
Temperature
• Groundwater temperature at lower Johnson
site
– 5.9° C at 13 cm below water table (10-July)
– 6.8° C at 91 cm below water table (12-September)
– 7.6° C at 32 cm below water table (2-October)
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Instrument site:
10.0°C at 70 cm; 6.7° C at 130 cm (11 July)
11.8° C at 70 cm; 9.0° C at 130 cm (12 August)
11.6° C at 70 cm, 9.6° C at 130 cm (26 August)
9.4° C at 70 cm, 8.5° C at 130 cm (3 October)
Applications
• Section 5
– Lodgepole pine forest – no surface water during
summer months
– Area of low relief with ~1.8 m of pumice overlying
a paleosol
– Water table approximately 0.8 to 1.0 m below
ground surface (13-August to 2-October – 2010)
– Well installed to ~15 m (lower 9 m was cored)
Poorly sorted, silty sand to clay-rich volcaniclastic sediment
2.4 to 5.7 m
Basalt hydroclastic 5.7 to 10.6 m
Basalt hydroclastic
Olivine basalt flow 10.6 to 15.2 m (E.O.H.)
Conclusions
• Water storage in the landscape is significantly
greater than prior to eruption of Mount
Mazama – unconfined pumice aquifer
• Geomorphic evolution since Mazama eruption
significantly influences distribution of
groundwater flow paths and storage
• Groundwater in the pumice aquifer is
primarily from snow melt
Conclusions
• Flow paths and rate of discharge from the pumice
aquifer is influenced by iron precipitation at
boundaries between pumice and reworked
deposits
• Groundwater discharges from and re-enters the
pumice aquifer multiple times as it moves down
valley
• Fens are transient features related to
configuration of pumice deposits within preeruption valleys
Conclusions
• Bedrock units are generally characterized by
low porosity (hydrovolcanics) and
permeability (lava flows)
• Groundwater movement in bedrock units is
probably highly localized
• Discharge from aquifers in bedrock
(permeable zones and fracture zone) are being
investigated by groundwater temperature
Questions and Comments