Monitoring potential subsurface temperature changes resulting from anthropogenic influence and climate change
This study, part of the Intensively Managed Landscapes-Critical Zone Observatory project (http://criticalzone.org/iml/), was 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 and potentially interfere with the natural geothermal regime; and (3) whether agriculture practices affect the thermal conditions in the subsurface hydrologic system. A fiber-optic distributed temperature sensing (FO-DTS) system was installed in two adjacent boreholes, one cased and the other uncased. The deeper, uncased borehole was advanced through all the unconsolidated glacial sediments lying above the bedrock, penetrating multiple aquifers in the glacial sediments, including the Mahomet aquifer. The cased borehole was drilled only partway through the sediments, and a groundwater monitoring well was screened in the shallower Upper Glasford aquifer. During the growing season, water was pumped for irrigation from the Mahomet aquifer (about 4,000 gallons/minute) from wells located within two miles of the site. Present monitoring of groundwater levels at the test site, in a third borehole completed in the 1990s, indicates the aquifer was drawn down several feet when the irrigation wells were 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 Late Wisconsinan glaciation, and older Illinoian and pre-Illinoian glaciations bury a deeply dissected bedrock surface. A prominent feature on the bedrock surface, the Mahomet Bedrock Valley, underlies the test site. The bedrock valley, approximately 15.5 miles (25 kilometers) wide and almost 328.1 feet (100 meters) deep, is partially filled with the oldest deposits of glacial sand and gravel, which form an aquifer that is part of a regional groundwater system known as the Mahomet aquifer. A fiber-optic cable was lowered along the entire length of the borehole and sealed against the sidewall with grout. In the second, shallow borehole, only 131.2 feet (40 meters) deep, a fiber-optic cable was lowered down the borehole and attached along the outside of the casing.
Temperature measurements with 3.3-foot (1-meter) and 0.1 °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 131.2 feet (40 meters) show that the temperature variations differ by amplitude and trend 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 the shallow subsurface is correlated with climate change, artificial conduits (wells), and agricultural practices on a larger temporal scale.