2016 NGWA Groundwater Summit

Aspect Controls on Rock Weathering and Permeability within the Boulder Creek Critical Zone Observatory

Monday, April 25, 2016
Confluence Ballroom Foyer (The Westin Denver Downtown)
Aaron Bandler , Hydrologic Science and Engineering, Colorado School of Mines, Golden, CO
Charles Magill , USA Environment, Golden, CO
Stacy Hendricks , Geology, Rocky Mountain College, Billings, MT
Kamini Singha , Colorado School of Mines, Golden, CO

We investigate the controls of slope aspect on rock weathering and permeability within the Boulder Creek Critical Zone Observatory by studying the orientation and density of subsurface fracturing. Based on a series of seismic refraction surveys, we compare the seismic anisotropy of the subsurface soil, saprolite (highly weathered, porous rock), and bedrock with the distribution of fractures observed in 7 borehole logs. In rocky mountainous terrain, weathering typically occurs fastest along rock fractures, which we examine on north- and south-facing slopes via the relationship between fracture orientation and seismic velocity. The south-facing slope demonstrates pronounced seismic anisotropy 2-9m below ground surface, with p-wave velocities of up to 2,000 m/s in the E-W direction, and approximately 1,000 m/s in the N-S direction. By contrast, the north-facing aspect demonstrates slower p-wave velocities and less significant anisotropy in this zone, with velocities ranging from approximately 800-1,500 m/s 2-9m below ground surface. We identify this zone as saprolite and observe its upper and lower boundaries to be consistent on both slopes. In the uppermost 2m of both slopes (soil), p-wave velocities are approximately 500 m/s in all measured directions. Below 9m (bedrock), p-wave velocities are approximately 3,500-4,500 m/s in all measured directions. Borehole logs show conjugate sets of fractures on both slopes striking in a general E-W direction, consistent with the direction of anisotropy observed in the south-facing saprolite. Based on current weathering models, we interpret the slower and more isotropic velocities of the north-facing saprolite to be the result of more intense weathering triggered by a colder average surface temperature and more intense spring runoff than the south-facing slope. Assuming that both slopes experience similar tectonic stress, we interpret the differences in the saprolite velocities of the two slopes as evidence that slope aspect controls rock weathering and permeability.

Aaron Bandler, Hydrologic Science and Engineering, Colorado School of Mines, Golden, CO
Aaron has a B.A. in Geology from Colorado College (2011) and is currently working on an M.S. in Hydrology at Colorado School of Mines. He is interested in using combined geological and geophysical methods to explore groundwater questions. Previous research has included using Electrical Resistivity to examine groundwater in a vineyard, and using earthquake locations to delineate fault planes in Argentina. Aaron has also spent years working with kids and teens in urban, rural, and wilderness environments.

Charles Magill, USA Environment, Golden, CO
Charles Magill holds a Geology B.A. from Western State College of Colorado and a Hydrology M.S. from Colorado School of Mines. He is currently a hydrogeologist at the USA Environment office in Golden, Colorado, working on hazardous waste site remediation, groundwater monitoring, and engineering design. He has been involved in environmental projects ranging from stormwater modeling for open-pit mining sites, to construction of permeable reactive barriers for groundwater treatment.

Stacy Hendricks, Geology, Rocky Mountain College, Billings, MT
Growing up in Western Nebraska, Stacy has been interested in the Earth and how it really works from a very young age. With the help of some very enthusiastic teachers throughout the years, she had learned to embrace her burning questions about the Earth and its processes. She is continuing her higher education with particular interest in hydrology and climate change.

Kamini Singha, Colorado School of Mines, Golden, CO
Kamini Singha, Ph.D., is a professor in the Department of Geology and Geological Engineering and the associate director of the Hydrologic Science and Engineering Program at the Colorado School of Mines. She worked at the U.S. Geological Survey Branch of Geophysics from 1997 to 2000, and was a member of the faculty at The Pennsylvania State University from 2005 to 2012. She earned her B.S. in geophysics from the University of Connecticut in 1999 and her Ph.D. in hydrogeology from Stanford University in 2005.