Using Seismic Refraction to Characterize the Urban Critical Zone in Eastern U.S. Watersheds
Abstract
The structure, depth, mineralogy, porosity, and permeability of critical zone (CZ) materials determine groundwater flow paths and influence subsurface solute reactions, fluxes, and delivery to streams. Anthropogenic solute transport is of particular interest because of the potential environmental hazards, and their influence on chemical weathering processes both near urban surfaces and at the depth of the saprolite-bedrock interface. To better understand subsurface solute transport and to identify the interfaces where focused chemical weathering occurs, we need to characterize the CZ structure. In this study, we use near-surface geophysics (seismic refraction) to image the CZ structure in three urban watersheds in the eastern United States (near Philadelphia, Baltimore, and Washington, D.C.). The watersheds are located in the Piedmont physiographic province, close to the Fall Zone. Seismic profiles were established normal to the stream channel and extended across the valley at multiple locations along the stream profile in all three watersheds. We observe a sharp gradient from low to high velocity within 10 m of the surface in all profiles, which we interpret as the boundary between saprolite and bedrock. Below this boundary, we observe differing seismic velocities among the three watersheds, suggesting a higher susceptibility to weathering (and thus lower velocities) in mafic igneous bedrock compared to metamorphic bedrock. The elevation of the inferred saprolite-bedrock boundary is observed to be highest under the channel and lowering with distance from the channel, regardless of the orientation of the seismic profile. This pattern appears even in relatively flat floodplains. Additionally, fresh bedrock is observed at shallower depths at downstream locations within the same watershed, which may have greater implications for watershed-scale patterns of weathering. These methods can constrain the structure of the CZ, through which groundwater and solutes can flow. This study demonstrates the efficiency of using seismic refraction to characterize broad-scale (tens of meters) structure of the CZ in urban settings.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFMNS35D0395W