Since 2007, the sparsely populated San Agustin Plains has been a controversial basin: a company applied for a permit to pump 54,000 acre-feet per year and to pipe that water to a region outside of the Plains. In 2009, the neighboring watershed to the south, Alamosa Creek — the only perennial stream in the region — faced similar pressure with a mining company exploring for beryllium. In response to these pressures and questions about the hydrogeology of this area, the NM Bureau of Geology began an integrated geologic and hydrologic study of the basins in 2009.
We are in the process of completing a final set of reports of this study. The geology of the San Agustin Plains is dominated by the erosion of the surrounding mountains made of volcanic and sandstones originally sourced from volcanic rocks into two basins, the Eastern and Western San Agustin Plains. The Eastern San Agustin Plains is formed by three distinct sub-basins: the North graben, the C-N graben and the White Lake graben. Datil, the proposed development and the VLA are in the North graben. Our initial results show
- The primary aquifer of the San Agustin Plains is made of basin-fill deposits (gravels, sands and muds) that become less and less transmissive further into the basin. In the surrounding mountains, thin alluvial aquifers (sands and gravels) and variably fractured volcanic rocks are the primary aquifers.
- The San Agustin Plains drains out of its southwestern corner, not into upper Alamosa Creek; therefore, the San Agustin Plains is not hydrologically connected to the Rio Grande watershed.
- Based on detailed examination of the geology from two wells, we estimate that the ground may subside by as much as 6 inches for every 10 ft of groundwater drawdown due to pumping.
- The Plains as a whole are hydrologically in steady-state: the region is draining as much as it is recharging groundwater.
- Between the east and west San Agustin Plains, the groundwater moves from east to west and southwest, with very low gradient, showing slow flow.
- Groundwater recharge comes from
- groundwater flow out of thin aquifers in canyons that spill into the basins, and by rare stream flow of these ephermeral arroyos. The groundwater recharged in this way is generally 10s to a few thousand years old.
- diffuse mountain block recharge flowing through thin (10s of feet) fractured buried volcanic aquifers. This recharge mechanism is observed around the edges of the basin where groundwater is generally 8,000 to 15,000 years old.
- The North graben, the location of the proposed well field and pumping, shows little to no evidence of groundwater connection to the south: it may be hydrologically isolated, implying that pumping in this basin may impact only the North graben.
Funded by the Aquifer Mapping Program, the Bureau of Geology, the National Cooperative Geologic Mapping Program (STATEMAP), Healy Foundation and the NMOSE.
For more information, please contact:
Dr. Alex Rinehart, Hydrogeologist, NMT EES
- Rinehart, Alex; Koning, Daniel; Timmons, Stacy, 2107 Hydrogeology of the San Agustin Plains(presentation slides), 62nd Annual New Mexico Water Conference, August 15-16, 2017, New Mexico Tech Socorro, NM.
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Koning, Daniel J.; Rinehart, Alex, 2015, Preliminary analysis of the geologic structure of the eastern San Agustin Plains, N.M.in: 2015 New Mexico Geological Society Annual Spring Meeting, April 24, 2015, Macey Center, New Mexico Tech campus, Socorro, NM, p. 29.
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