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 observe changes. 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.
