SPATIAL AND TEMPORAL PATTERN MONITORING ON GROUNDWATER AND SURFACE WATER INTERACTIONS USING FIBER-OPTIC DISTRIBUTED TEMPERATURE SENSING

Fiber-optic distributed temperature sensing (FO-DTS) was used to monitoring stream bed temperature patterns over 1 km to identify groundwater discharge zones of the Upper Sangamon River, as part of the Intensely Managed Landscapes Critical Zone Observatory. This ongoing study is designed to identify the spatial and temporal patterns of groundwater and surface water interactions. A 1-km fiber optic cable was installed on the river bed in the upstream of the Sangamon River. The cable, connected to an Oryx DTS device, was deployed in a duplexed single-ended configuration with 1 m spatial resolution and 1-hour interval time. The first test for recording temperature measurements on this project was completed over 14 days in summer 2017.

The data collection was calibrated by ice and hot water baths in the first ~55 m of the cable. Temperature-Distance-Time (TDT) map of the river bed shows that temperature changed uniformly, and was associated with diurnal atmospheric temperature fluctuations. There was an obvious time-delay effect that between the atmospheric and riverbed temperatures that is controlled by thermal radiation. The downward-trending curve of riverbed temperatures is consistent with a decrease in average air temperature. But, measurements in a ~25 m downstream area show an abnormal "double-delayed" offset in the TDT. These effects continued throughout the entire test period, and was not impacted by atmospheric or hydrologic dynamics. One possibility is that groundwater is discharging to the river bed in that area.

To understand the thermal dynamics of groundwater discharge in various spatiotemporal scales, the ongoing research will incorporate the contributions from (1) changes in river velocity and depth over time, (2) local precipitation data, and (3) stratigraphic and lithologic changes across the river bed. Furthermore, a hydrological-thermal hybrid model, validated by FO-DTS measurements, could better represent the spatial and temporal variations so such interactions could be studied in different hydrologic settings.