GROUNDWATER TEMPERATURE FLUCTUATIONS DURING AQUIFER RECOVERY IN THE BUCKMAN MUNICIPAL WELL FIELD, SANTA FE, NEW MEXICO

During the past six years, students attending the Summer of Applied Geophysical Experience (SAGE) field program in Santa Fe have had a unique opportunity to collect repeat temperature-depth data in monitoring wells in the Buckman municipal well field (BMWF). The BMWF is one of many sources of residential water for the city of Santa Fe. The BMWF has a history of substantial (> 100 m) drawdown and inelastic subsidence, followed by significant water level recovery and surface rebound after 2003. Repeat measurements of thermal profiles in two monitoring piezometer nests, SF3 and SF4, showed little to no change between 2013 and 2014. Surprisingly, temperatures at the bottom of these wells rose by 0.28–0.37°C (±0.05 °C) between 2014 and 2018, with the most dramatic change occurring between 2014 and 2016. Artesian flow in four wells began in 2015 and water levels have risen 2.5–8 m over the period of observation. The upward flow of water in the wellbores has caused the upper parts of the wells to warm by as much as 0.8–1.3°C since 2014. In contrast, a deep piezometer located outside the zone of water level rise, SF6 (geothermal gradient of 35°C/km), showed no temperature change between 2013 and 2016. We speculate that when the BMWF was in high production before 2003, a significant cone of depression formed, creating hydraulic gradients that drew in water from shallow aquifers, thus cooling portions of the deeper semi-confined aquifer system. As production decreased after 2003, the cone of depression relaxed and vertical hydraulic gradients associated with the regional-scale groundwater flow system began to warm the aquifer.

Geothermal gradients in SF3 and SF4 prior to 2015 were 73 to 80°C/km; the elevated gradients are associated with a small fault near SF4. A deeper piezometer, SF2, located 300 m to the east of SF3, has a geothermal gradient of 45°C/km and a complex warming and cooling signal that has shifted up and down the wellbore through time. The fact that SF2 is well-connected to a nearby production well, Buckman 1, may explain the complicated signal. The abrupt change in geothermal gradient between SF3 and SF2 coincides with a north-trending stratigraphic discontinuity that is resolved by InSAR, water level, and aqueous geochemistry data.