Heat flow into intermittently-pumped wells within unconfined alluvial aquifers

Abstract

Moving water can transport temperature signals of surface water into groundwater or vice-versa. We analyzed the temperature signals in the atmosphere, river, and pumped water to elucidate source water contributions to the pumping wells from the ambient groundwater and the river. Temperature fluctuations in response to high-frequency intermittent pumping were observed from February 2014 to August 2016 in 14 wells within three shallow unconfined alluvial aquifers along the Ohio River. The groundwater temperature observed in pumping wells displayed two alternating signals between pump-on and pump-off periods. The difference in groundwater temperatures between pump-on and pump-off periods was defined as a temperature shift. Temperature shifts indicate that pumping always introduces water of different temperature to the wells than that of the ambient groundwater. While seasonal pump-off temperatures varied up to 3.2 °C, the short-term temperature shifts ranged from 0.2 to 2.5 °C. Groundwater was generally warm in winter and cold in summer, but pumped water was of lower temperature in winter and higher in summer than the ambient groundwater. However, the pumping-induced temperature shifts were highest in summer and winter and lowest in spring and fall because groundwater temperature lagged that of surface water by approximately six months. The thermal transfer velocity in groundwater was more than one order magnitude higher during pumping than that during the recovery periods. High-frequency pumping changed produced-water temperature and increased the thermal transfer velocity in the groundwater. These results indicate that stream exfiltration is a major component of the water budget to a number of these wells. Thermal analysis reveals that the temperature of pumped water can trace source water to the pumping wells. This thermal approach for estimating the source of pumped water works best when groundwater temperature variation is seasonally large.