TITLE:
Ground- and Surface-Water Interactions of a Pumice Aquifer in a Headwaters Watershed: Round Meadow, Fremont-Winema National Forest, Oregon, USA
AUTHORS:
Jonathan M. Weatherford, Michael L. Cummings
KEYWORDS:
Pumice Aquifer, Perched Aquifer, Oregon, Mount Mazama
JOURNAL NAME:
Journal of Water Resource and Protection,
Vol.8 No.11,
October
26,
2016
ABSTRACT: Plinian
pumice fall from the Holocene eruption of Mount Mazama in the Cascade volcanic
arc is an unconfined, perched aquifer in south-central Oregon. The pumice
aquifer provides near-surface groundwater storage that maintains biologically
diverse wetland environments. Wetland environments reflect post-eruption
disruption of the once uniform pumice blanket by fluvial and lacustrine
processes operating within the template of the pre-eruption landscape. In the
8.6 km2 Round Meadow watershed the pumice aquifer interacts with a
seasonally flooded meadow, fen, springs, and perennial stream. The laterally
uniform, isotropic pumice aquifer is disrupted by flat-bottomed ephemeral
stream valleys that drain to the seasonally flooded meadow. Surface water
levels in the seasonally flooded meadow are controlled by a knickpoint
developed on bedrock. The underlying aquifer is confined by a layer of
glass-rich diatomaceous silt grading upward to organic-rich silt. Here, the
aquifer is comprised of remnants of the pumice deposit, lag sand, and reworked
pumice. The water level in the confined aquifer is maintained by recharge from
the unconfined pumice aquifer following flow pathways beneath ephemeral stream
valleys. The fen is developed on a down-thrown block of welded tuff and
pre-eruption diatomaceous silt. Water levels in the fen are sensitive to
inter-annual variations in precipitation. Low discharge, low temperature (5.0°C
to 6.5°C), and low conductivity (30 to 50 μS/cm) springs appear to be fracture
controlled and rising through welded tuff. Spring discharge and seepage through
pumice from the welded tuff support perennial flow in the creek that also
carries discharge from the seasonally flooded meadow when water levels are high
enough to cross the knickpoint.