The Critical Zone (CZ) is the region between fresh bedrock, meters to tens of meters below ground surface, and the tree canopy. The thickness, structure, and material properties of this subsurface zone govern how water is stored and moves in the ecosystem. These in turn hold essential connections to ecosystem response and the role of water and soil and their connections with climate change. However, direct measurement of the Critical Zone via drilling, especially on hillslopes, is not trivial and could be expensive. Near surface geophysics (NSG) is a relatively low-cost set of methods that provide 2D to 3D mapping of subsurface structures.
UMD expertise in NSG provides an opportunity to obtain important information about subsurface processes and the Critical Zone structure. Communication with MGS-DNR and USGS about needs related to the Grand Challenge grant supporting this effort yielded a request for help in making the case for colocation of shallow groundwater wells and stream gauges at the newly installed mesonet sites because the present level of monitoring is not adequate to address the enable assessment all components of the water budget. This request ties to this target (#1 Near surface geophysics: water, soil, and land use) and also the next target (#2 Floods, droughts and Hydrological Processes).
Near-surface geophysics (NSG), including active source seismology, electrical resistivity, ground penetrating radar, gravimeter, etc. It is efficient to probe the shallow subsurface structure in tens of meters depth range such as the Critical Zone.. It is particularly useful on steep hillslopes and other challenging landscapes where direct excavation through drilling is impractical. As direct borehole drilling is much more expensive and not accessible everywhere in steep landscape, near-surface geophysics can provide non-direct measurements of the subsurface. We are attempting to develop and utilize a joint inversion technique that takes both active source seismic refraction and electrical resistivity, to simultaneously constrain porosity and moisture content of regolith and bedrock for the shallow surface (0-30 m depth). In addition to estimating geologic structure and water storage potential of the site, we plan to survey the site regularly to document changes of shallow moisture content in response to rainfall events and farming on the land. We established collaboration with the Maryland Department of Natural Resources and the Clarksville Facility at UMD, to conduct a series of geophysics and hydrology surveys from spring 2024 and onwards. We also propose to expand our study sites to locations such as the Mesonet sites nearby Clarksville. Several graduate students have participated in this research, and the team expects 1-2 REU students to join this project in summer 2024.
Site
The Central Maryland Research and Education Center (Clarksville Facility) is a UMD owned research center managed by the College of Agriculture and Natural Resources. It is located in Howard County, ~50 km from UMD (Figure 1a). It is home to the University’s dairy cattle and equine research herds. Regional climate here is warm and humid summers and mild winter. The coldest weather occurs in late January to early February. Prior to this study, the U.S. Geological Survey drilled 9 shallow and deep wells in the farm to characterize bedrock lithology and depth and groundwater level (Figure 1b). The Maryland Department of Natural Resources has been survey groundwater levels regularly, and the data can be found at the USGS site.
Methods
We develop a joint active source seismic refraction and electrical resistivity surveys to simultaneously constrain material strength (as porosity) and moisture. We plan to deploy several 2-D arrays along and across the main channel of a watershed in the Clarksville Facility at University of Maryland College of Agriculture and Natural Resources. We will repeat the survey lines in different seasons and before, during, and after a significant rainfall event, to characterize change of moisture content at different depth and different elevation along a watershed. Our stakeholder, Maryland Department of Natural Resources will take complementary measurements at 9 shallow to deep groundwater wells during the NSG surveys as a ground truth. Information such as depth to bedrock and rock type were well documented prior to the project, so they will serve as groundwater truth for the NSG.
Stakeholders
We are collaborating with the UMD Clarksville Facility, Maryland Department of Natural Resources, Coastal and Environmental Geology Program.
