New Mexico Geological Society Annual Spring Meeting — Abstracts

The Buckman well field (BWF) experienced substantial drawdowns and ground subsidence recorded by InSAR between 1993 and 2000. Subsequent changes in well field management resulted in dramatic recovery of water levels starting in 2007. Newly processed InSAR data from 2007-2010 show ground uplift across a 12 km² area in the field, with uplift rates as high as 50 mm/yr. Modeling using an inverse Mogi method suggests that a net increase of volume of ca. 4 x 10-4 km³ (324 acre-ft. equivalent) at a depth of 1200 m can explain the observed signal; this is approximately the depth of crystalline basement. This uplift is interpreted as an elastic response of rock units to increased pore pressures, and its magnitude is unexpected. Also imaged is a bimodal pattern of both subsidence and uplift that is most pronounced during the period 2009 – 2010. The sharp discontinuity observed between the two phases in the 2007-2010 data was previously imaged in the 1993-2000 data.

This discontinuity is also recorded in repeat temperature measurements in the BWF. Students attending the Summer of Applied Geophysical Experience (SAGE) field school had a unique opportunity to measure temperature profiles in monitoring wells in the BWF between 2013 and 2015. Repeat measurements of thermal profiles and well discharge temperatures indicate that aquifer temperatures are dynamic in some areas, while aquifer temperatures elsewhere remain fairly constant.

Here we focus on data from three monitoring wells (SF2b, SF3a, SF4a) and three production wells (Buckman 1, 6 and 8) near the Rio Grande. SF2b has a geothermal gradient of 38°C/km above 120 m and 54.4°C/km below 120 m; this increase of gradient with depth likely corresponds to a change from sandy deposits at shallow levels to finer-grained deposits at depth. The temperature in SF2b between 200 and 235 m has been warming gradually since 2008. The 2008 temperature was 0.5°C lower than our 2013 measurement and the 2015 temperature was 0.2°C higher than the 2013 measurement. The elevated thermal gradients measured in SF3a and SF4a of 72 and 79 C°/km are only 300 m west of SF2b; repeat thermal profiles measured in both wells between 2013 and 2014 are identical. Discharge temperatures and water levels in Buckman 1 and 6 have slowly been rising since 2008. Analysis of discharge temperatures collected during the summer months in Buckman 1 reveals that the well draws in cooler water during times of high and prolonged production. The discharge temperatures in Buckman 8 show no temporal trend.

Differences in thermal gradient, water chemistry and the response of water levels to pumping at Buckman 1 indicate a discontinuity between the aquifers sampled by the SF2 and the SF3/4 piezometer nests. The exact reason remains a mystery. Possibilities include (1) changes in aquifer thermal properties related to ground subsidence across the discontinuity imaged by InSAR; (2) thermal changes caused by long-term production in the well field; and (3) elevation of gradients by upwelling of warm fluids by the regional-scale flow system.