Time-frequency analysis of fiber-optic temperature data on groundwater-surface water interactions

Accurate characterization of the hydrogeological framework and groundwater-surface water interactions in particular, is crucial to understanding contaminant transport between aquifers and rivers. Fiber-optic distributed temperature sensing (FODTS) is a novel tool that offers unprecedented resolution for characterizing groundwater-surface water interaction. Time-frequency analysis of FODTS data provides a means to distinguish between a wide range of hydrologic processes at various scales. This work demonstrates the use of the S-Transform to analyze FODTS time series and synchronous river stage time series. Our study focused on improving understanding of the dynamics of groundwater-surface water interaction at the Hanford 300-Area Richland, WA. We used ~1.5 km long fiber optic cable to continuously monitor real time temperature variations along the hyporheic corridor at ~1 m spatial resolution and 5 minutes temporal resolution. It is generally recognized that contaminated groundwater discharge in Hanford site 300-Area is highly controlled by fluctuations in the Columbia River stage. Using S-Transform analysis of the temperature data along the river corridor we investigated in more detail the contributions of periodicity in the river stage to focused groundwater discharge. This time-frequency analysis of the DTS data uniquely identified areas of stage-controlled enhanced exchange along the hyporheic corridor. Dam operations upstream of the site were identified as a factor controlling stage fluctuations with relatively long periods (4 - 8 days). S-Transform analysis was used to map areas of high amplitude in these diagnostic periods indicative of stage-driven exchange along the river corridor.