New Mexico Geological Society Annual Spring Meeting — Abstracts

We present a compilation of water chemistry and groundwater level measurements from the eastern San Agustin Plains and the upper Alamosa Creek, NM. The San Agustin Plains forms a closed surface water basin with poorly constrained groundwater connections to neighboring basins. This basin is surrounded by the Datil Mountains to the northwest, the Gallinas Mountains to the northeast, Tres Montosa to the east and the northern San Mateo Mountains to the southeast. The C-N embayment of the eastern San Agustin Plains extends between the Luera and northern San Mateo Mountains. A low surface water divide separates the C-N embayment and the upper Alamosa Creek drainage. We define upper Alamosa Creek from its headwaters at the surface divide with the C-N embayment to just beyond the Monticello Box upstream of the village of Monticello.Both the eastern San Agustin Plains and upper Alamosa Creek occupy normal-fault bounded basins. The uppermost units of the San Agustin Plains consist of sandy basin fill, with thick playa-lacustrine deposits in the C-N embayment. The primary aquifers of Alamosa Creek are shallow alluvial aquifers. Uplands surrounding these basins are underlain by a series of ignimbrites, volcaniclastic sediment and lava flows associated with the Mogollon-Datil volcanic field, which locally may have high transmissivity fractured zones. Nested calderas are present near the boundary of the C-N embayment and upper Alamosa Creek.We sampled groundwater levels, major ion and trace metal chemistry, and environmental tracers including stable isotopes (O and H), carbon-14 and tritium. Groundwater levels have very low gradients in the eastern San Agustin Plains. However, between the C-N embayment and upper Alamosa creek, there is a steep southward groundwater gradient. In the upper Alamosa Creek, groundwater levels are generally shallow and mimic topography. The transition suggests a very low transmissivity zone in the aquifer under the upper Alamosa Creek and the C-N embayment boundary.Trace metal and major ion chemistry suggests there is little connection between the San Agustin Plains and the Alamosa Creek basins. Major ion chemistry results have dominant Ca to Na and HCO₃ to SO₄ concentrations with slightly lower Ca and higher Na concentrations in the eastern San Agustin Plains, relative to the Alamosa Creek drainage. Concentrations of trace metals appear to be spatially correlated with faults and caldera margins in the eastern San Agustin Plains. They have generally low concentrations both in the central basin and in the upper Alamosa Creek.

The ¹⁴C groundwater ages show relatively “old” water (>10,000 years on average) in the eastern San Agustin Plains, while the waters in upper Alamosa Creek are dominantly younger. This is further supported by the presence of tritium in the upper Alamosa Creek waters (groundwater ages <50 y) and the general lack of tritium from water samples from the eastern San Agustin Plains. The isolation of the shallow aquifers of the eastern San Agustin Plains from Alamosa Creek is further supported by snow-dominated lighter δ¹⁸O values in the San Agustin Plains groundwater, and heavier δ¹⁸O values in the Alamosa Creek groundwater.

pp. 53