THERMAL AND PILOT TEM STUDIES IN SUNSHINE VALLEY, NEW MEXICO

Temperature-depth profiles were measured in 3 wells in Sunshine Valley, an area located east of the Rio Grande between Questa and Costilla, NM. The wells were logged to identify groundwater movement in Sunshine Valley aquifers. The deepest logged borehole (147 m) is the Cerro North well, which has 3 screened intervals. The geothermal gradient in the upper part of the well is low, ranging from 0.6–7.8 °C/km. There is a small jump in gradient at 68–69 m, suggesting an influx of slightly warmer water near the base of the upper screen. The gradient in the bottom 11 m increases dramatically to an average of 86°C/km. Slightly warmer water (0.95°C higher than the temperatures above this point; ranging from 11.51–12.46 °C) appears to enter the well through the bottom screen. A shallow well (42 m deep) located 1.3 km northeast of the Cerro North well was also logged. This well is 0.2°C cooler compared to the same interval in the Cerro North well (the surface elevations are similar). The isothermal nature of the curve is indicative of groundwater flow, but the short interval measured precludes definitive distinction between downflow and lateral flow of cold water around the casing. Similarly, a well located about 8 km southwest of Costilla is isothermal and is hard to interpret, but overall this well is warmer than the wells to the south by 4.4°C. This warmer well is close to wells with high measured discharge temperatures near Costilla (23.8°C at 188 m, 16.4°C at 61 m), suggesting higher geothermal potential in this area. Chloride mass balance calculations show that more than half of the recharge to Sunshine Valley from the Sangre de Cristo Mountains to the east circulates deeply in the mountain block and some of this warm water discharges along faults near the mountain front. We ran a transient electromagnetic (TEM) survey near the southern wells to further characterize Sunshine Valley aquifers. The survey detected a flat resistive layer (100 to 200 ohm-m) at a depth of ~65 m that interestingly coincides with the upper screened interval in the Cerro North well. This resistive layer could mark the eastern extent of the Servilleta Basalt, but offset across the east-down Sunshine Valley fault zone mapped about 1 km west of the survey site suggests that the basalt should be deeper than 65 m. Alternatively, the layer may be a gravelly interval containing fresh water.