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Groundwater Storage and Flow in an Unconfined Pumice Aquifer, Antelope Unit, Chemult Ranger District, Winema-Fremont National Forest, Oregon

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  • The study area lies north-northeast of Crater Lake National Park and is covered by 2 to 3 m of pumice deposited during the climactic eruption of Mount Mazama approximately 7700 years before present. The pumice deposit hosts unconfined, seasonally connected, perched aquifers that support groundwater dependent ecosystems at points of discharge in the 80 km2 study area. Sparse bedrock outcrops are dominated by basalt lava flows, but cores from groundwater monitoring wells at four sites contain abundant moderately to weakly indurated, interlayered basalt hydroclastic- and pyroclastic-flow deposits, and matrix-rich tuff breccia. Although some water may enter the unconfined pumice aquifer from flow paths within bedrock units, the lithologies encountered in wells and little to no water in piezometers screened in bedrock suggest little contribution to the unconfined aquifer from bedrock-hosted flow paths. Pre-Mazama surficial deposits are the local base or may locally augment storage in the unconfined aquifer. The distribution of these deposits is strongly influenced by the pre-eruption topography with poorly to moderately well sorted silt- and clay-rich sedimentary deposits common in pre-eruption valleys and shallow, bed-rock controlled depressions. Post-eruption landscape response included erosion of pumice with valley bottoms cut into pumice, pre-eruption surficial deposits, or bedrock. Where pumice is preserved, the coarser-grained upper pumice unit (moderately to poorly sorted coarse lapilli to blocks) has been removed and the erosion surface is cut into the finer-grained lower pumice unit (well-sorted, fine to coarse lapilli). This early-formed erosion surface is commonly buried by alluvium consisting of crystal-rich sand near the lower contact grading upward to rounded pumice-bearing glassy silt, silty sand, and pumice gravel. The contacts between the alluvium and valley walls cut into pumice deposits are commonly iron stained and locally intensely cemented by iron oxide. In some pre-eruption valley configurations, alluvial fans composed of glassy-silt, crystals, and rounded pumice extend across the valley bottom and overlie the complete pumice section. These deposits are 1.0 to 1.5 m thick in some fans. Recharge of the unconfined pumice aquifer occurs during spring snow melt from direct contribution by snow melting on valley floors and upland depressions, runoff from partially frozen ground, and shallow flow paths in the pumice blanket. Once in the unconfined pumice aquifer groundwater may infiltrate to deeper levels, be consumed by evapotranspiration, migrate through the aquifer along seasonally connected flow pathways, or return to the surface at fens and springs. Where the water table within the pumice is within approximately 1 m of the surface during the dry season, grasses and sedges are common in well-vegetated meadows. Where year-round discharge takes place, fens characterized by high biodiversity and peat deposits are present. The location of these discharge sites appears to reflect ongoing response of the landscape to the eruption of Mount Mazama. At the discharge points, water is consumed by evapotranspiration through lush and diverse vegetation communities, evaporates, or infiltrates back into the unconfined aquifer down valley. Groundwater temperature may provide an inexpensive way to define flow pathways in the unconfined pumice aquifer and to detect contribution of ground water contributed from deeper seated flow pathways in bedrock. Two monitoring sites, one at the Wilshire fen and the other at the Johnson Meadow fen, suggest cooler water entering the unconfined pumice aquifer in late summer.
  • Presented at The Oregon Water Conference, May 24-25, 2011, Corvallis, OR.
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