Recipient Organization
UNIV OF MASSACHUSETTS
(N/A)
AMHERST,MA 01003
Performing Department
Geosciences
Non Technical Summary
Global climate change is altering the Earth's natural cycling of water from the ground to the air and back again, what is known as the hydrologic cycle. In New England, climate change is predicted to increase temperatures and increase the frequency and strength of rain events. The increased temperatures will result in less snow accumulation in the winter and an increased need for irrigation in the hotter summer as evapo-transpiration increases. This will alter significantly the recharge/extraction cycle. Will less water enter groundwater aquifers because of reduced snow fall? Will enough water recharge the aquifers to offset the amount extracted in the summer for irrigation?Certainly the timing of recharge will change.These changes will require a better understanding of recharge rates and a better characterization of groundwater aquifers; the volume of water present and its availability. Understanding the seasonal timing and rates of groundwater recharge is critical to maintaining a sustainable water supply. Importantly, how will these changes in the hydrolgical cycle effect sustainable agricultural practices?The Earth's Critical Zone is the thin veneer of the planet from the top of the tree canopy to the base of the aquifer, somewhere in the crystalline,fractured rock in New England. The flux of water through soil and rock influences the rate at which nutrients such as nitrogen cycle through the system and the rates of physical and chemical weathering of geologic material. In New England, glacial materials, primarily glacial till, overlie the bed rock. Both the glacial till and the fractured bedrock serve as aquifers in many regions of New England.In spite of its importance, our understanding of the complex Critical Zone is often limited by the lack of spatially extensive and time intensive data. The characterization of the Critical Zone requires abundant and informative data and near surface geophysics can play a major role in this direction.To better understand the physical processes in the Critical Zone, I will acquire Ground Penetrating Radar (GPR), electrical resistivity (ERT), self-potential (SP), and seismic data to characterize the subsurface. Each geophysical method measures different or complementary physical properties of the subsurface. I will acquire these data over several years during different seasons to observechanges. As water moves through the Critical Zone, it alters the physical properties of the subsurface. For example, water might replace air in pore spaces in the soil. This change in pore fluid in turn changes the speed of electromagnetic energy propagating through the ground. Thus, time-lapse imaging can link geophysical properties to hydrological properties. Further, small changes between time-lapse images are often easier to see, leading to better observations of subsurface physical processes.Commonly acquired point-based methods, such as well logs, do not allow the investigation of the spatial distribution of physical properties.Remote sensing generally penetrates only a few centimeters into the subsoil and their probing of the subsurface is hindered by vegetation.Ground-based, non-invasive geophysical techniques can be applied at different scales to image static and dynamic characteristics of the subsoil in response of hydrological stresses.The National Science Foundation currently funds several Critical Zone Observatories (CZO). The Boulder Creek, CO CZO, Shale Hills, PA CZO, and Luquillo, Puerto Rico CZO sites are on fractured rock, but these sites do not have a glacial component to them. Much of New England is covered with glacial deposits adding more complexity to understanding hydrological processes in the region. Studies at the current CZOs will not answer many questions that are unique to the New England region. The MacLeish Field Station in Whately, MA adds another component to Critical Zone investigations and provides a geological and hydrological environment similar to a wide area in New England. Since the MacLeish Field Station is local, we have easy access to it and can quickly deploy equipment to measure events as they are occurring or soon after they happen. This rapid response capability could be critical if unexpected or unique events occur,such as an event similar to tropical storm Irene.
Animal Health Component
50%
Research Effort Categories
Basic
50%
Applied
50%
Developmental
(N/A)
Goals / Objectives
We must characterize basic physical propertiessuch as the thickness and extent of the Critical Zone; how the Critical Zonethickness varies within the watershed; what controls this thickness variation;how much regolith exists; how thick and extensive isthe fractured bedrock; what role does this fractured bedrock play inthe hydrologic cycle; how do Critical Zone processes create andmaintain subsurface porosity?We need to know the geologic framework of the Critical Zone \citep{Befus2011}.A critical region in the subsurface is the contact between the overlying glacialmaterial and the underlying bedrock. Understanding the physical nature of thisburied interface will help determine the recharge rates to the underlyingfractured bedrock aquifer.I will collect ground penetrating radar (GPR), electrical resistivity, self-potential (SP),and seismic data at several times throughout the year to characterizethe subsurface at the MacLeish Field Station. These time-lapse measurementswill help to determine a variety of physical properties. Assuming thatthe geologic materials of the near surface are constant at these short time scales, changesin the geophysical response can be correlated to changes in water content of theCritical Zone.I will compare this geophysical data with precipitation measurements, soil moisturemeasurements and water level measurements in the wells.This research will correlate geophysical changes with changes in the other data sets.This comparison will help determine rates of aquifer recharge from precipitation into the glacialtill and fractured bedrock aquifer.Time-lapse data can provide better resolution and detection of geological processes;it is often easier to see \textit{changes} between imagescollected at different times than to see a feature in an image acquired only once.The field work will also establish a base line for any later studies.Statistical characterization based on pastobservations may no longer represent current conditions. We are not in a period ofstationary statistical measures. We need new measurements to develop long-term forecasts.We know that climate is changing. Thus, past studies may not be representativeof the future conditions.
Project Methods
A key to this work will be observing and measuring changes in the subsurface. GPR provides high-resolution images of the stratigraphy and is also highly sensitive to water content. The dielectric permittivity of water is about 80. The dielectric permittivity of air is 1. For typical sediments and granitic bedrock, the dielectric permittivity ranges from around 3 to around 10. As the soil moisture of the ground changes, the image recorded with GPR will change (Clement and Ward, 2008). For example, the travel time to specific imaged reflections will change because changes in water content effect the electromagnetic velocity of the subsurface. We can use the changes in the image to estimate changes in water content. This estimate can be compared to measured soil moisture changes recorded by the permanently installed soil moisture probes at the site. Importantly, we can use GPR to provide wider, 3-D coverage of soil moisture than can a single point measurementElectrical resistivity tomography (ERT) maps the electrical conductivity of the subsurface. Geological materials often have different electrical conductivities. Silts and clay-rich sediments typically have a relatively high electrical conductivity. Granites and felsic igneous rocks tend to be less conductive (more resistive). The electrical conductivity is sensitive to pore water and to its salinity. At the MacLeish Field Station, fractured bedrock would likely have a higher conductivity than un fractured bedrock. We can use the conductivity images to map the distribution and depth of fractures at the Mac Leish Field Station. Also,ERT is sensitive to the presence of pore water. We can interpret changes in acquired ERT images, especially in the upper till layer, as changes in the saturation level. ERT complements GPR; GPR can estimate porosity through the dielectric permittivity and Topp's equation. ERT can estimate porosity and pore water salinity through electrical conductivity and Archie's law. These are two electromagnetic physical properties. The third, magnetic permeability, is usually considered 1 for typical, non-magnetic geologic materials.A technique gaining popularity is the spontaneous or self potential method. SP measures naturally occurring electrical potential differences caused by moving electrical charges. Groundwater contains ions. Groundwater flow thus creates an electric current as these charges flow with the water. Typically, the potential difference is measured between a stationary (base station) electrode and another electrode placed at many positions in a grid. These values are then contoured to provide a map of SP anomalies over the grid surface. For horizontal groundwater flow, the SP voltages would increase in the direction of flow. The SP method is capable of directly measuring groundwater flow.Seismic data will provide velocity information as well as images of the subsurface. Although seismic images are lower resolution compared to GPR, seismic energy penetrates deeper into the subsurface. I will acquire seismic refraction data to determine the velocity structure of the subsurface. In the seismic case, velocity relates to rock composition and strength. I will acquire shear wave data as well. The ratio of P-wave vleocity to S-wave velocity gives Poisson's ratio, a more discriminating measure of composition. Additionally, S-waves are anisotropic; the velocity depends on the direction of propagation. Anisotropy can be used to infer fracture orientation. I will use the refraction data to determine the thickness of the overlying glacial till and perhaps changes in strength of bedrock which might relate to fracture depth.An important aspect of this work is complementing and adding to the current work at the MacLeish Field Station. As noted, the MacLeish Field Station has kept records of various hydrological parameters for several years. This research will use these measurements to develop relations between the geophysically measured physical properties and the measured hydrological properties. According to Parsekian et al. (2014), "a key weakness is that it is challenging at best to reliably convert geophysical properties to the physical properties that control [Critical Zone] processes, particularly at the larger investigation scales." This study will link geophysical measurements with changes in water level and soil moisture content measured at the MacLeish Field Station. By directly correlating geophysical properties to hydrological properties, we hope to better understand the link between these properties. The available recorded hydrological data at the MacLeish Field Station will help to provide the link between geophysical and hydrological properties.Two other key points are "water content and fluid chemistry often drive a contrast in geophysical properties, and therefore, hydrologic CZ processes are often most easily resolved in a time-lapse sense," and "a key strength of geophysical measurements is spatially rich data sets that can be collected over time". Our proximity to the MacLeish Field Station will allow us to easily collect such time lapse data and the ongoing hydrological research at the site will help with correlating geophysical and hydrological measurements to better understand the processes occuring in the Critical Zone.To continue the long term monitoring at the MacLeish Field Station, we will install pressure transducers in the two MacLeish Field Station wells to record water level changes. The timing and magnitude of water level changes will help determine important hydrological properties.