Presentation Type
Oral Presentation
Category
STEM (science, technology, engineering, mathematics)
Abstract/Artist Statement
Controls on the magnitude of bedrock recharge in two paired, snow-dominated watersheds
Mountains in arid and semi-arid regions receive a disproportionately large amount of precipitation compared to their bounding valley aquifers due to orographic effects, but little is known about how precipitation is partitioned and transported within the mountain block. Recent research has illuminated dynamic interactions between soil and bedrock reservoirs, showing that bedrock permeability is potentially a major control on the volume of bedrock groundwater recharge. This study focuses on i) evaluating differences in bedrock effective porosity, and ii) estimating how recharge magnitude is influenced by bedrock effective porosity. Two paired watersheds underlain by different bedrock lithologies were studied in the Lubrecht Experimental Forest near Greenough, MT. One watershed is underlain by 1.4 Ga argillite, while the other is dominated by 65 Ma granite. Bedrock well hydrographs were used to estimate bedrock recharge at the hillslope scale from snowmelt and storm events. Bedrock permeability and effective porosity were estimated and compared using Mean Residence Time (MRT) estimates, core sample analysis, and outcrop fracture mapping. Annual bedrock recharge in the argillaceous catchment ranged between 400 and 850 mm, and between 15 and 50 mm in the granite catchment. Mean residence time for the argillite watershed is approximately 4.74 years, and 2.12 year in the granite watershed. Mean effective porosity is 1.11% for the watershed underlain by argillite and 0.45% for the granitic catchment. Bedrock well hydrographs in the granitic catchment exhibit a flashy response to input and slow recession, while hydrographs in the argillite catchment show rapid response and recession. The difference in the recharge magnitudes, inferred storage, bedrock permeability, and well recession between both catchments implies bedrock strongly influences each catchment’s subsurface flow dynamics. This research expands current knowledge of the magnitude and controls on recharge to the bedrock reservoir in mountainous terrain, demonstrating the importance of understanding the role that bedrock plays in partitioning and transmitting flow through mountain blocks.
Mentor Name
Payton Gardner
Personal Statement
I began working on this project in my senior year at U. Montana and decided to continue investigating recharge to mountain aquifers for my graduate thesis. Initially, I chose to continue this research because I truly enjoy the challenge of figuring out a small piece of the puzzle that is mountain system hydrology. From a broader perspective, my work is important because it adds to our collective knowledge of where the water we depend on comes from. Water is quickly becoming a precious, and increasingly scarce, resource, not only in Montana but many other places, as well. Aquifers that sustain major food supplies and people living on them, e.g. in eastern Montana, have been suffering consistent, intermittent droughts for several years to date. These drought conditions are projected to persist or worsen in the near future. My work is important because it takes a step toward elucidating how mountain precipitation is transmitted from mountain blocks to the valley aquifers that bound them. Mountain blocks have been aptly called “water towers” for their role as a source of a large proportion of aquifer water. My research ties into this by looking at the amount of water, annually, that recharges (i.e. infiltrates below the land surface and makes it to the water table) the mountain blocks themselves. By understanding how much water recharges mountain aquifers, the better we will understand the amount of water available for valley aquifer recharge later on. Additionally, with on-going monitoring, this research will allow us to see how changes in recharge patterns, as a result of changing precipitation patterns via climate change, will affect water availability in valley aquifers. My research is just a small building block, but as others after me expand on this work, a clearer picture of the importance of mountain blocks and mountain precipitation will form. This may lead to more efficient water usage practices in the future as a result of our better understanding.
Controls on the magnitude of bedrock recharge in two paired, snow-dominated watersheds
Controls on the magnitude of bedrock recharge in two paired, snow-dominated watersheds
Mountains in arid and semi-arid regions receive a disproportionately large amount of precipitation compared to their bounding valley aquifers due to orographic effects, but little is known about how precipitation is partitioned and transported within the mountain block. Recent research has illuminated dynamic interactions between soil and bedrock reservoirs, showing that bedrock permeability is potentially a major control on the volume of bedrock groundwater recharge. This study focuses on i) evaluating differences in bedrock effective porosity, and ii) estimating how recharge magnitude is influenced by bedrock effective porosity. Two paired watersheds underlain by different bedrock lithologies were studied in the Lubrecht Experimental Forest near Greenough, MT. One watershed is underlain by 1.4 Ga argillite, while the other is dominated by 65 Ma granite. Bedrock well hydrographs were used to estimate bedrock recharge at the hillslope scale from snowmelt and storm events. Bedrock permeability and effective porosity were estimated and compared using Mean Residence Time (MRT) estimates, core sample analysis, and outcrop fracture mapping. Annual bedrock recharge in the argillaceous catchment ranged between 400 and 850 mm, and between 15 and 50 mm in the granite catchment. Mean residence time for the argillite watershed is approximately 4.74 years, and 2.12 year in the granite watershed. Mean effective porosity is 1.11% for the watershed underlain by argillite and 0.45% for the granitic catchment. Bedrock well hydrographs in the granitic catchment exhibit a flashy response to input and slow recession, while hydrographs in the argillite catchment show rapid response and recession. The difference in the recharge magnitudes, inferred storage, bedrock permeability, and well recession between both catchments implies bedrock strongly influences each catchment’s subsurface flow dynamics. This research expands current knowledge of the magnitude and controls on recharge to the bedrock reservoir in mountainous terrain, demonstrating the importance of understanding the role that bedrock plays in partitioning and transmitting flow through mountain blocks.