THERMAL RESPONSE TO INTERMITTENTLY-PUMPED WELLS AT DIFFERENT DISTANCES FROM A STREAM

Temperature fluctuations in response to high-frequency intermittent pumping were observed from February 2014 to August 2016 in two wells at different distances from a stream. The groundwater temperature observed in these pumping wells displayed two alternating signals shifting instantaneously between pump-on and pump-off periods. Pump-off temperatures varied seasonally, while pump-on temperature shifts depend on the extent of thermal circulations, seepage velocity, and aquifer diffusivity. Groundwater was warm in winter and cold in summer, but pumped water was of lower temperature in winter and higher in summer than the groundwater. However, groundwater temperature was of higher in the well close to stream (N4) than one far from the stream (N5). Groundwater temperature lagged surface water temperature such that N4 has shorter lag time than N5. The short-term temperature shifts indicate that pumping always introduces water of different temperature to the wells than that of ambient groundwater. The pumping-induced temperature shifts in both wells were highest in summer and winter and lowest in spring and fall. However, groundwater temperature and pumping-induced temperature shifts were higher for N4 than N5 throughout the year. Heat transfer rate in both wells was more than one order magnitude higher during pumping than that during recovery periods. Farther the wells from the stream, higher the heat transfer rate and drawdown. These results are interpreted to indicate that stream exfiltration is a major component of the water budget to these wells. Thermal method has benefit over hydraulic method, especially where temperature fluctuates significantly even though water level remains unchanged, and works best when groundwater temperature variation is seasonally large.