Using distributed temperature sensing to monitor potential subsurface temperature changes in an intensively managed landscape

This study, part of the Intensively Managed Landscapes-Critical Zone Observatory, is designed to investigate: (1) how the ambient ground temperature fluctuates with diurnal and seasonal changes; (2) whether a well can act as a temperature conduit to potentially interfere with the natural geothermal regime; and (3) whether agriculture practices impact the thermal conditions in the subsurface hydrologic system. A fiber-optic distributed temperature sensing (FO-DTS) system has been installed in two adjacent boreholes, one cased and the other uncased. Irrigation pumping (est. total 4000 gal/min) takes place during the growing season from wells located within two miles of the site. Current monitoring of groundwater levels at the test site indicates there is drawdown in the deepest aquifer when wells are being pumped. The test site is located in the complex glacial landscape of east-central Illinois. Deposits of clayey glacial till and gravelly sand of the most recent Wisconsinan and Illinoian glaciations bury a deeply dissected bedrock surface. A prominent feature, the Mahomet Bedrock Valley underlies the test site. The valley, almost 100 m deep, is partially filled with older deposits of glacial sand and gravel that form an aquifer that are part of a regional groundwater system, known as the Mahomet aquifer. The deepest, uncased borehole was drilled through the unconsolidated sediments above bedrock. A fiber-optic cable was lowered along the entire length and sealed against the sidewall with grout. A second borehole, only 40 m deep, was drilled to install a casing for a groundwater monitoring well screened in the shallower Upper Glasford aquifer. Fiber-optic cable in this borehole was attached along the outside of the casing. Temperature measurements with 1-m and 0.1 degrees C resolutions have been collected at various temporal scales, ranging from 30-minute to 2-week intervals, since June 2015. The initial data from the top 40 m show that the temperature variations in amplitudes and trends are different in each borehole. By collecting data continuously, we will be able to identify any temperature fluctuations when irrigation is occurring. We hope to understand how the geothermal regime in shallow subsurface is correlated with climate change, artificial conduits (wells), and agricultural practices on a larger temporal scale.